summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorOthmar Gsenger <otti@anytun.org>2008-04-07 20:24:28 +0000
committerOthmar Gsenger <otti@anytun.org>2008-04-07 20:24:28 +0000
commit1551994d578d6ff17eb3f2e5cd69b0e06ceef24c (patch)
treee1226ae161e442a8ea9230731739553c4b3ace4a
parentfixed ipv6 support (diff)
removed old documentation
-rw-r--r--doc/libsrtp-1.4.4.pdfbin364186 -> 0 bytes
-rw-r--r--doc/rfc3711-srtp.txt3139
2 files changed, 0 insertions, 3139 deletions
diff --git a/doc/libsrtp-1.4.4.pdf b/doc/libsrtp-1.4.4.pdf
deleted file mode 100644
index 94ad008..0000000
--- a/doc/libsrtp-1.4.4.pdf
+++ /dev/null
Binary files differ
diff --git a/doc/rfc3711-srtp.txt b/doc/rfc3711-srtp.txt
deleted file mode 100644
index ecc0648..0000000
--- a/doc/rfc3711-srtp.txt
+++ /dev/null
@@ -1,3139 +0,0 @@
-
-
-
-
-
-
-Network Working Group M. Baugher
-Request for Comments: 3711 D. McGrew
-Category: Standards Track Cisco Systems, Inc.
- M. Naslund
- E. Carrara
- K. Norrman
- Ericsson Research
- March 2004
-
-
- The Secure Real-time Transport Protocol (SRTP)
-
-Status of this Memo
-
- This document specifies an Internet standards track protocol for the
- Internet community, and requests discussion and suggestions for
- improvements. Please refer to the current edition of the "Internet
- Official Protocol Standards" (STD 1) for the standardization state
- and status of this protocol. Distribution of this memo is unlimited.
-
-Copyright Notice
-
- Copyright (C) The Internet Society (2004). All Rights Reserved.
-
-Abstract
-
- This document describes the Secure Real-time Transport Protocol
- (SRTP), a profile of the Real-time Transport Protocol (RTP), which
- can provide confidentiality, message authentication, and replay
- protection to the RTP traffic and to the control traffic for RTP, the
- Real-time Transport Control Protocol (RTCP).
-
-Table of Contents
-
- 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
- 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
- 2. Goals and Features . . . . . . . . . . . . . . . . . . . . . . 4
- 2.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 5
- 3. SRTP Framework . . . . . . . . . . . . . . . . . . . . . . . . 5
- 3.1. Secure RTP . . . . . . . . . . . . . . . . . . . . . . . 6
- 3.2. SRTP Cryptographic Contexts. . . . . . . . . . . . . . . 7
- 3.2.1. Transform-independent parameters . . . . . . . . 8
- 3.2.2. Transform-dependent parameters . . . . . . . . . 10
- 3.2.3. Mapping SRTP Packets to Cryptographic Contexts . 10
- 3.3. SRTP Packet Processing . . . . . . . . . . . . . . . . . 11
- 3.3.1. Packet Index Determination, and ROC, s_l Update. 13
- 3.3.2. Replay Protection. . . . . . . . . . . . . . . . 15
- 3.4. Secure RTCP . . . . . . . . . . . . . . . . . . . . . . . 15
-
-
-
-Baugher, et al. Standards Track [Page 1]
-
-RFC 3711 SRTP March 2004
-
-
- 4. Pre-Defined Cryptographic Transforms . . . . . . . . . . . . . 19
- 4.1. Encryption . . . . . . . . . . . . . . . . . . . . . . . 19
- 4.1.1. AES in Counter Mode. . . . . . . . . . . . . . . 21
- 4.1.2. AES in f8-mode . . . . . . . . . . . . . . . . . 22
- 4.1.3. NULL Cipher. . . . . . . . . . . . . . . . . . . 25
- 4.2. Message Authentication and Integrity . . . . . . . . . . 25
- 4.2.1. HMAC-SHA1. . . . . . . . . . . . . . . . . . . . 25
- 4.3. Key Derivation . . . . . . . . . . . . . . . . . . . . . 26
- 4.3.1. Key Derivation Algorithm . . . . . . . . . . . . 26
- 4.3.2. SRTCP Key Derivation . . . . . . . . . . . . . . 28
- 4.3.3. AES-CM PRF . . . . . . . . . . . . . . . . . . . 28
- 5. Default and mandatory-to-implement Transforms. . . . . . . . . 28
- 5.1. Encryption: AES-CM and NULL. . . . . . . . . . . . . . . 29
- 5.2. Message Authentication/Integrity: HMAC-SHA1. . . . . . . 29
- 5.3. Key Derivation: AES-CM PRF . . . . . . . . . . . . . . . 29
- 6. Adding SRTP Transforms . . . . . . . . . . . . . . . . . . . . 29
- 7. Rationale. . . . . . . . . . . . . . . . . . . . . . . . . . . 30
- 7.1. Key derivation . . . . . . . . . . . . . . . . . . . . . 30
- 7.2. Salting key. . . . . . . . . . . . . . . . . . . . . . . 30
- 7.3. Message Integrity from Universal Hashing . . . . . . . . 31
- 7.4. Data Origin Authentication Considerations. . . . . . . . 31
- 7.5. Short and Zero-length Message Authentication . . . . . . 32
- 8. Key Management Considerations. . . . . . . . . . . . . . . . . 33
- 8.1. Re-keying . . . . . . . . . . . . . . . . . . . . . . . 34
- 8.1.1. Use of the <From, To> for re-keying. . . . . . . 34
- 8.2. Key Management parameters. . . . . . . . . . . . . . . . 35
- 9. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
- 9.1. SSRC collision and two-time pad. . . . . . . . . . . . . 37
- 9.2. Key Usage. . . . . . . . . . . . . . . . . . . . . . . . 38
- 9.3. Confidentiality of the RTP Payload . . . . . . . . . . . 39
- 9.4. Confidentiality of the RTP Header. . . . . . . . . . . . 40
- 9.5. Integrity of the RTP payload and header. . . . . . . . . 40
- 9.5.1. Risks of Weak or Null Message Authentication. . . 42
- 9.5.2. Implicit Header Authentication . . . . . . . . . 43
- 10. Interaction with Forward Error Correction mechanisms. . . . . 43
- 11. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 43
- 11.1. Unicast. . . . . . . . . . . . . . . . . . . . . . . . . 43
- 11.2. Multicast (one sender) . . . . . . . . . . . . . . . . . 44
- 11.3. Re-keying and access control . . . . . . . . . . . . . . 45
- 11.4. Summary of basic scenarios . . . . . . . . . . . . . . . 46
- 12. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 46
- 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
- 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
- 14.1. Normative References . . . . . . . . . . . . . . . . . . 47
- 14.2. Informative References . . . . . . . . . . . . . . . . . 48
- Appendix A: Pseudocode for Index Determination . . . . . . . . . . 51
- Appendix B: Test Vectors . . . . . . . . . . . . . . . . . . . . . 51
- B.1. AES-f8 Test Vectors. . . . . . . . . . . . . . . . . . . 51
-
-
-
-Baugher, et al. Standards Track [Page 2]
-
-RFC 3711 SRTP March 2004
-
-
- B.2. AES-CM Test Vectors. . . . . . . . . . . . . . . . . . . 52
- B.3. Key Derivation Test Vectors. . . . . . . . . . . . . . . 53
- Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55
- Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 56
-
-1. Introduction
-
- This document describes the Secure Real-time Transport Protocol
- (SRTP), a profile of the Real-time Transport Protocol (RTP), which
- can provide confidentiality, message authentication, and replay
- protection to the RTP traffic and to the control traffic for RTP,
- RTCP (the Real-time Transport Control Protocol) [RFC3350].
-
- SRTP provides a framework for encryption and message authentication
- of RTP and RTCP streams (Section 3). SRTP defines a set of default
- cryptographic transforms (Sections 4 and 5), and it allows new
- transforms to be introduced in the future (Section 6). With
- appropriate key management (Sections 7 and 8), SRTP is secure
- (Sections 9) for unicast and multicast RTP applications (Section 11).
-
- SRTP can achieve high throughput and low packet expansion. SRTP
- proves to be a suitable protection for heterogeneous environments
- (mix of wired and wireless networks). To get such features, default
- transforms are described, based on an additive stream cipher for
- encryption, a keyed-hash based function for message authentication,
- and an "implicit" index for sequencing/synchronization based on the
- RTP sequence number for SRTP and an index number for Secure RTCP
- (SRTCP).
-
-1.1. Notational Conventions
-
- The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
- "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
- document are to be interpreted as described in [RFC2119]. The
- terminology conforms to [RFC2828] with the following exception. For
- simplicity we use the term "random" throughout the document to denote
- randomly or pseudo-randomly generated values. Large amounts of
- random bits may be difficult to obtain, and for the security of SRTP,
- pseudo-randomness is sufficient [RFC1750].
-
- By convention, the adopted representation is the network byte order,
- i.e., the left most bit (octet) is the most significant one. By XOR
- we mean bitwise addition modulo 2 of binary strings, and || denotes
- concatenation. In other words, if C = A || B, then the most
- significant bits of C are the bits of A, and the least significant
- bits of C equal the bits of B. Hexadecimal numbers are prefixed by
- 0x.
-
-
-
-
-Baugher, et al. Standards Track [Page 3]
-
-RFC 3711 SRTP March 2004
-
-
- The word "encryption" includes also use of the NULL algorithm (which
- in practice does leave the data in the clear).
-
- With slight abuse of notation, we use the terms "message
- authentication" and "authentication tag" as is common practice, even
- though in some circumstances, e.g., group communication, the service
- provided is actually only integrity protection and not data origin
- authentication.
-
-2. Goals and Features
-
- The security goals for SRTP are to ensure:
-
- * the confidentiality of the RTP and RTCP payloads, and
-
- * the integrity of the entire RTP and RTCP packets, together with
- protection against replayed packets.
-
- These security services are optional and independent from each other,
- except that SRTCP integrity protection is mandatory (malicious or
- erroneous alteration of RTCP messages could otherwise disrupt the
- processing of the RTP stream).
-
- Other, functional, goals for the protocol are:
-
- * a framework that permits upgrading with new cryptographic
- transforms,
-
- * low bandwidth cost, i.e., a framework preserving RTP header
- compression efficiency,
-
- and, asserted by the pre-defined transforms:
-
- * a low computational cost,
-
- * a small footprint (i.e., small code size and data memory for
- keying information and replay lists),
-
- * limited packet expansion to support the bandwidth economy goal,
-
- * independence from the underlying transport, network, and physical
- layers used by RTP, in particular high tolerance to packet loss
- and re-ordering.
-
- These properties ensure that SRTP is a suitable protection scheme for
- RTP/RTCP in both wired and wireless scenarios.
-
-
-
-
-
-Baugher, et al. Standards Track [Page 4]
-
-RFC 3711 SRTP March 2004
-
-
-2.1. Features
-
- Besides the above mentioned direct goals, SRTP provides for some
- additional features. They have been introduced to lighten the burden
- on key management and to further increase security. They include:
-
- * A single "master key" can provide keying material for
- confidentiality and integrity protection, both for the SRTP stream
- and the corresponding SRTCP stream. This is achieved with a key
- derivation function (see Section 4.3), providing "session keys"
- for the respective security primitive, securely derived from the
- master key.
-
- * In addition, the key derivation can be configured to periodically
- refresh the session keys, which limits the amount of ciphertext
- produced by a fixed key, available for an adversary to
- cryptanalyze.
-
- * "Salting keys" are used to protect against pre-computation and
- time-memory tradeoff attacks [MF00] [BS00].
-
- Detailed rationale for these features can be found in Section 7.
-
-3. SRTP Framework
-
- RTP is the Real-time Transport Protocol [RFC3550]. We define SRTP as
- a profile of RTP. This profile is an extension to the RTP
- Audio/Video Profile [RFC3551]. Except where explicitly noted, all
- aspects of that profile apply, with the addition of the SRTP security
- features. Conceptually, we consider SRTP to be a "bump in the stack"
- implementation which resides between the RTP application and the
- transport layer. SRTP intercepts RTP packets and then forwards an
- equivalent SRTP packet on the sending side, and intercepts SRTP
- packets and passes an equivalent RTP packet up the stack on the
- receiving side.
-
- Secure RTCP (SRTCP) provides the same security services to RTCP as
- SRTP does to RTP. SRTCP message authentication is MANDATORY and
- thereby protects the RTCP fields to keep track of membership, provide
- feedback to RTP senders, or maintain packet sequence counters. SRTCP
- is described in Section 3.4.
-
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 5]
-
-RFC 3711 SRTP March 2004
-
-
-3.1. Secure RTP
-
- The format of an SRTP packet is illustrated in Figure 1.
-
- 0 1 2 3
- 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
- |V=2|P|X| CC |M| PT | sequence number | |
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | timestamp | |
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | synchronization source (SSRC) identifier | |
- +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
- | contributing source (CSRC) identifiers | |
- | .... | |
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | RTP extension (OPTIONAL) | |
- +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | | payload ... | |
- | | +-------------------------------+ |
- | | | RTP padding | RTP pad count | |
- +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
- | ~ SRTP MKI (OPTIONAL) ~ |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | : authentication tag (RECOMMENDED) : |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | |
- +- Encrypted Portion* Authenticated Portion ---+
-
- Figure 1. The format of an SRTP packet. *Encrypted Portion is the
- same size as the plaintext for the Section 4 pre-defined transforms.
-
- The "Encrypted Portion" of an SRTP packet consists of the encryption
- of the RTP payload (including RTP padding when present) of the
- equivalent RTP packet. The Encrypted Portion MAY be the exact size
- of the plaintext or MAY be larger. Figure 1 shows the RTP payload
- including any possible padding for RTP [RFC3550].
-
- None of the pre-defined encryption transforms uses any padding; for
- these, the RTP and SRTP payload sizes match exactly. New transforms
- added to SRTP (following Section 6) may require padding, and may
- hence produce larger payloads. RTP provides its own padding format
- (as seen in Fig. 1), which due to the padding indicator in the RTP
- header has merits in terms of compactness relative to paddings using
- prefix-free codes. This RTP padding SHALL be the default method for
- transforms requiring padding. Transforms MAY specify other padding
- methods, and MUST then specify the amount, format, and processing of
- their padding. It is important to note that encryption transforms
-
-
-
-Baugher, et al. Standards Track [Page 6]
-
-RFC 3711 SRTP March 2004
-
-
- that use padding are vulnerable to subtle attacks, especially when
- message authentication is not used [V02]. Each specification for a
- new encryption transform needs to carefully consider and describe the
- security implications of the padding that it uses. Message
- authentication codes define their own padding, so this default does
- not apply to authentication transforms.
-
- The OPTIONAL MKI and the RECOMMENDED authentication tag are the only
- fields defined by SRTP that are not in RTP. Only 8-bit alignment is
- assumed.
-
- MKI (Master Key Identifier): configurable length, OPTIONAL. The
- MKI is defined, signaled, and used by key management. The
- MKI identifies the master key from which the session
- key(s) were derived that authenticate and/or encrypt the
- particular packet. Note that the MKI SHALL NOT identify
- the SRTP cryptographic context, which is identified
- according to Section 3.2.3. The MKI MAY be used by key
- management for the purposes of re-keying, identifying a
- particular master key within the cryptographic context
- (Section 3.2.1).
-
- Authentication tag: configurable length, RECOMMENDED. The
- authentication tag is used to carry message authentication
- data. The Authenticated Portion of an SRTP packet
- consists of the RTP header followed by the Encrypted
- Portion of the SRTP packet. Thus, if both encryption and
- authentication are applied, encryption SHALL be applied
- before authentication on the sender side and conversely on
- the receiver side. The authentication tag provides
- authentication of the RTP header and payload, and it
- indirectly provides replay protection by authenticating
- the sequence number. Note that the MKI is not integrity
- protected as this does not provide any extra protection.
-
-3.2. SRTP Cryptographic Contexts
-
- Each SRTP stream requires the sender and receiver to maintain
- cryptographic state information. This information is called the
- "cryptographic context".
-
- SRTP uses two types of keys: session keys and master keys. By a
- "session key", we mean a key which is used directly in a
- cryptographic transform (e.g., encryption or message authentication),
- and by a "master key", we mean a random bit string (given by the key
- management protocol) from which session keys are derived in a
-
-
-
-
-
-Baugher, et al. Standards Track [Page 7]
-
-RFC 3711 SRTP March 2004
-
-
- cryptographically secure way. The master key(s) and other parameters
- in the cryptographic context are provided by key management
- mechanisms external to SRTP, see Section 8.
-
-3.2.1. Transform-independent parameters
-
- Transform-independent parameters are present in the cryptographic
- context independently of the particular encryption or authentication
- transforms that are used. The transform-independent parameters of
- the cryptographic context for SRTP consist of:
-
- * a 32-bit unsigned rollover counter (ROC), which records how many
- times the 16-bit RTP sequence number has been reset to zero after
- passing through 65,535. Unlike the sequence number (SEQ), which
- SRTP extracts from the RTP packet header, the ROC is maintained by
- SRTP as described in Section 3.3.1.
-
- We define the index of the SRTP packet corresponding to a given
- ROC and RTP sequence number to be the 48-bit quantity
-
- i = 2^16 * ROC + SEQ.
-
- * for the receiver only, a 16-bit sequence number s_l, which can be
- thought of as the highest received RTP sequence number (see
- Section 3.3.1 for its handling), which SHOULD be authenticated
- since message authentication is RECOMMENDED,
-
- * an identifier for the encryption algorithm, i.e., the cipher and
- its mode of operation,
-
- * an identifier for the message authentication algorithm,
-
- * a replay list, maintained by the receiver only (when
- authentication and replay protection are provided), containing
- indices of recently received and authenticated SRTP packets,
-
- * an MKI indicator (0/1) as to whether an MKI is present in SRTP and
- SRTCP packets,
-
- * if the MKI indicator is set to one, the length (in octets) of the
- MKI field, and (for the sender) the actual value of the currently
- active MKI (the value of the MKI indicator and length MUST be kept
- fixed for the lifetime of the context),
-
- * the master key(s), which MUST be random and kept secret,
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 8]
-
-RFC 3711 SRTP March 2004
-
-
- * for each master key, there is a counter of the number of SRTP
- packets that have been processed (sent) with that master key
- (essential for security, see Sections 3.3.1 and 9),
-
- * non-negative integers n_e, and n_a, determining the length of the
- session keys for encryption, and message authentication.
-
- In addition, for each master key, an SRTP stream MAY use the
- following associated values:
-
- * a master salt, to be used in the key derivation of session keys.
- This value, when used, MUST be random, but MAY be public. Use of
- master salt is strongly RECOMMENDED, see Section 9.2. A "NULL"
- salt is treated as 00...0.
-
- * an integer in the set {1,2,4,...,2^24}, the "key_derivation_rate",
- where an unspecified value is treated as zero. The constraint to
- be a power of 2 simplifies the session-key derivation
- implementation, see Section 4.3.
-
- * an MKI value,
-
- * <From, To> values, specifying the lifetime for a master key,
- expressed in terms of the two 48-bit index values inside whose
- range (including the range end-points) the master key is valid.
- For the use of <From, To>, see Section 8.1.1. <From, To> is an
- alternative to the MKI and assumes that a master key is in one-
- to-one correspondence with the SRTP session key on which the
- <From, To> range is defined.
-
- SRTCP SHALL by default share the crypto context with SRTP, except:
-
- * no rollover counter and s_l-value need to be maintained as the
- RTCP index is explicitly carried in each SRTCP packet,
-
- * a separate replay list is maintained (when replay protection is
- provided),
-
- * SRTCP maintains a separate counter for its master key (even if the
- master key is the same as that for SRTP, see below), as a means to
- maintain a count of the number of SRTCP packets that have been
- processed with that key.
-
- Note in particular that the master key(s) MAY be shared between SRTP
- and the corresponding SRTCP, if the pre-defined transforms (including
- the key derivation) are used but the session key(s) MUST NOT be so
- shared.
-
-
-
-
-Baugher, et al. Standards Track [Page 9]
-
-RFC 3711 SRTP March 2004
-
-
- In addition, there can be cases (see Sections 8 and 9.1) where
- several SRTP streams within a given RTP session, identified by their
- synchronization source (SSRCs, which is part of the RTP header),
- share most of the crypto context parameters (including possibly
- master and session keys). In such cases, just as in the normal
- SRTP/SRTCP parameter sharing above, separate replay lists and packet
- counters for each stream (SSRC) MUST still be maintained. Also,
- separate SRTP indices MUST then be maintained.
-
- A summary of parameters, pre-defined transforms, and default values
- for the above parameters (and other SRTP parameters) can be found in
- Sections 5 and 8.2.
-
-3.2.2. Transform-dependent parameters
-
- All encryption, authentication/integrity, and key derivation
- parameters are defined in the transforms section (Section 4).
- Typical examples of such parameters are block size of ciphers,
- session keys, data for the Initialization Vector (IV) formation, etc.
- Future SRTP transform specifications MUST include a section to list
- the additional cryptographic context's parameters for that transform,
- if any.
-
-3.2.3. Mapping SRTP Packets to Cryptographic Contexts
-
- Recall that an RTP session for each participant is defined [RFC3550]
- by a pair of destination transport addresses (one network address
- plus a port pair for RTP and RTCP), and that a multimedia session is
- defined as a collection of RTP sessions. For example, a particular
- multimedia session could include an audio RTP session, a video RTP
- session, and a text RTP session.
-
- A cryptographic context SHALL be uniquely identified by the triplet
- context identifier:
-
- context id = <SSRC, destination network address, destination
- transport port number>
-
- where the destination network address and the destination transport
- port are the ones in the SRTP packet. It is assumed that, when
- presented with this information, the key management returns a context
- with the information as described in Section 3.2.
-
- As noted above, SRTP and SRTCP by default share the bulk of the
- parameters in the cryptographic context. Thus, retrieving the crypto
- context parameters for an SRTCP stream in practice may imply a
- binding to the correspondent SRTP crypto context. It is up to the
- implementation to assure such binding, since the RTCP port may not be
-
-
-
-Baugher, et al. Standards Track [Page 10]
-
-RFC 3711 SRTP March 2004
-
-
- directly deducible from the RTP port only. Alternatively, the key
- management may choose to provide separate SRTP- and SRTCP- contexts,
- duplicating the common parameters (such as master key(s)). The
- latter approach then also enables SRTP and SRTCP to use, e.g.,
- distinct transforms, if so desired. Similar considerations arise
- when multiple SRTP streams, forming part of one single RTP session,
- share keys and other parameters.
-
- If no valid context can be found for a packet corresponding to a
- certain context identifier, that packet MUST be discarded.
-
-3.3. SRTP Packet Processing
-
- The following applies to SRTP. SRTCP is described in Section 3.4.
-
- Assuming initialization of the cryptographic context(s) has taken
- place via key management, the sender SHALL do the following to
- construct an SRTP packet:
-
- 1. Determine which cryptographic context to use as described in
- Section 3.2.3.
-
- 2. Determine the index of the SRTP packet using the rollover counter,
- the highest sequence number in the cryptographic context, and the
- sequence number in the RTP packet, as described in Section 3.3.1.
-
- 3. Determine the master key and master salt. This is done using the
- index determined in the previous step or the current MKI in the
- cryptographic context, according to Section 8.1.
-
- 4. Determine the session keys and session salt (if they are used by
- the transform) as described in Section 4.3, using master key,
- master salt, key_derivation_rate, and session key-lengths in the
- cryptographic context with the index, determined in Steps 2 and 3.
-
- 5. Encrypt the RTP payload to produce the Encrypted Portion of the
- packet (see Section 4.1, for the defined ciphers). This step uses
- the encryption algorithm indicated in the cryptographic context,
- the session encryption key and the session salt (if used) found in
- Step 4 together with the index found in Step 2.
-
- 6. If the MKI indicator is set to one, append the MKI to the packet.
-
- 7. For message authentication, compute the authentication tag for the
- Authenticated Portion of the packet, as described in Section 4.2.
- This step uses the current rollover counter, the authentication
-
-
-
-
-
-Baugher, et al. Standards Track [Page 11]
-
-RFC 3711 SRTP March 2004
-
-
- algorithm indicated in the cryptographic context, and the session
- authentication key found in Step 4. Append the authentication tag
- to the packet.
-
- 8. If necessary, update the ROC as in Section 3.3.1, using the packet
- index determined in Step 2.
-
- To authenticate and decrypt an SRTP packet, the receiver SHALL do the
- following:
-
- 1. Determine which cryptographic context to use as described in
- Section 3.2.3.
-
- 2. Run the algorithm in Section 3.3.1 to get the index of the SRTP
- packet. The algorithm uses the rollover counter and highest
- sequence number in the cryptographic context with the sequence
- number in the SRTP packet, as described in Section 3.3.1.
-
- 3. Determine the master key and master salt. If the MKI indicator in
- the context is set to one, use the MKI in the SRTP packet,
- otherwise use the index from the previous step, according to
- Section 8.1.
-
- 4. Determine the session keys, and session salt (if used by the
- transform) as described in Section 4.3, using master key, master
- salt, key_derivation_rate and session key-lengths in the
- cryptographic context with the index, determined in Steps 2 and 3.
-
- 5. For message authentication and replay protection, first check if
- the packet has been replayed (Section 3.3.2), using the Replay
- List and the index as determined in Step 2. If the packet is
- judged to be replayed, then the packet MUST be discarded, and the
- event SHOULD be logged.
-
- Next, perform verification of the authentication tag, using the
- rollover counter from Step 2, the authentication algorithm
- indicated in the cryptographic context, and the session
- authentication key from Step 4. If the result is "AUTHENTICATION
- FAILURE" (see Section 4.2), the packet MUST be discarded from
- further processing and the event SHOULD be logged.
-
- 6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for
- the defined ciphers), using the decryption algorithm indicated in
- the cryptographic context, the session encryption key and salt (if
- used) found in Step 4 with the index from Step 2.
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 12]
-
-RFC 3711 SRTP March 2004
-
-
- 7. Update the rollover counter and highest sequence number, s_l, in
- the cryptographic context as in Section 3.3.1, using the packet
- index estimated in Step 2. If replay protection is provided, also
- update the Replay List as described in Section 3.3.2.
-
- 8. When present, remove the MKI and authentication tag fields from
- the packet.
-
-3.3.1. Packet Index Determination, and ROC, s_l Update
-
- SRTP implementations use an "implicit" packet index for sequencing,
- i.e., not all of the index is explicitly carried in the SRTP packet.
- For the pre-defined transforms, the index i is used in replay
- protection (Section 3.3.2), encryption (Section 4.1), message
- authentication (Section 4.2), and for the key derivation (Section
- 4.3).
-
- When the session starts, the sender side MUST set the rollover
- counter, ROC, to zero. Each time the RTP sequence number, SEQ, wraps
- modulo 2^16, the sender side MUST increment ROC by one, modulo 2^32
- (see security aspects below). The sender's packet index is then
- defined as
-
- i = 2^16 * ROC + SEQ.
-
- Receiver-side implementations use the RTP sequence number to
- determine the correct index of a packet, which is the location of the
- packet in the sequence of all SRTP packets. A robust approach for
- the proper use of a rollover counter requires its handling and use to
- be well defined. In particular, out-of-order RTP packets with
- sequence numbers close to 2^16 or zero must be properly handled.
-
- The index estimate is based on the receiver's locally maintained ROC
- and s_l values. At the setup of the session, the ROC MUST be set to
- zero. Receivers joining an on-going session MUST be given the
- current ROC value using out-of-band signaling such as key-management
- signaling. Furthermore, the receiver SHALL initialize s_l to the RTP
- sequence number (SEQ) of the first observed SRTP packet (unless the
- initial value is provided by out of band signaling such as key
- management).
-
- On consecutive SRTP packets, the receiver SHOULD estimate the index
- as
- i = 2^16 * v + SEQ,
-
- where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32)
- such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC
- + s_l (see Appendix A for pseudocode).
-
-
-
-Baugher, et al. Standards Track [Page 13]
-
-RFC 3711 SRTP March 2004
-
-
- After the packet has been processed and authenticated (when enabled
- for SRTP packets for the session), the receiver MUST use v to
- conditionally update its s_l and ROC variables as follows. If
- v=(ROC-1) mod 2^32, then there is no update to s_l or ROC. If v=ROC,
- then s_l is set to SEQ if and only if SEQ is larger than the current
- s_l; there is no change to ROC. If v=(ROC+1) mod 2^32, then s_l is
- set to SEQ and ROC is set to v.
-
- After a re-keying occurs (changing to a new master key), the rollover
- counter always maintains its sequence of values, i.e., it MUST NOT be
- reset to zero.
-
- As the rollover counter is 32 bits long and the sequence number is 16
- bits long, the maximum number of packets belonging to a given SRTP
- stream that can be secured with the same key is 2^48 using the pre-
- defined transforms. After that number of SRTP packets have been sent
- with a given (master or session) key, the sender MUST NOT send any
- more packets with that key. (There exists a similar limit for SRTCP,
- which in practice may be more restrictive, see Section 9.2.) This
- limitation enforces a security benefit by providing an upper bound on
- the amount of traffic that can pass before cryptographic keys are
- changed. Re-keying (see Section 8.1) MUST be triggered, before this
- amount of traffic, and MAY be triggered earlier, e.g., for increased
- security and access control to media. Recurring key derivation by
- means of a non-zero key_derivation_rate (see Section 4.3), also gives
- stronger security but does not change the above absolute maximum
- value.
-
- On the receiver side, there is a caveat to updating s_l and ROC: if
- message authentication is not present, neither the initialization of
- s_l, nor the ROC update can be made completely robust. The
- receiver's "implicit index" approach works for the pre-defined
- transforms as long as the reorder and loss of the packets are not too
- great and bit-errors do not occur in unfortunate ways. In
- particular, 2^15 packets would need to be lost, or a packet would
- need to be 2^15 packets out of sequence before synchronization is
- lost. Such drastic loss or reorder is likely to disrupt the RTP
- application itself.
-
- The algorithm for the index estimate and ROC update is a matter of
- implementation, and should take into consideration the environment
- (e.g., packet loss rate) and the cases when synchronization is likely
- to be lost, e.g., when the initial sequence number (randomly chosen
- by RTP) is not known in advance (not sent in the key management
- protocol) but may be near to wrap modulo 2^16.
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 14]
-
-RFC 3711 SRTP March 2004
-
-
- A more elaborate and more robust scheme than the one given above is
- the handling of RTP's own "rollover counter", see Appendix A.1 of
- [RFC3550].
-
-3.3.2. Replay Protection
-
- Secure replay protection is only possible when integrity protection
- is present. It is RECOMMENDED to use replay protection, both for RTP
- and RTCP, as integrity protection alone cannot assure security
- against replay attacks.
-
- A packet is "replayed" when it is stored by an adversary, and then
- re-injected into the network. When message authentication is
- provided, SRTP protects against such attacks through a Replay List.
- Each SRTP receiver maintains a Replay List, which conceptually
- contains the indices of all of the packets which have been received
- and authenticated. In practice, the list can use a "sliding window"
- approach, so that a fixed amount of storage suffices for replay
- protection. Packet indices which lag behind the packet index in the
- context by more than SRTP-WINDOW-SIZE can be assumed to have been
- received, where SRTP-WINDOW-SIZE is a receiver-side, implementation-
- dependent parameter and MUST be at least 64, but which MAY be set to
- a higher value.
-
- The receiver checks the index of an incoming packet against the
- replay list and the window. Only packets with index ahead of the
- window, or, inside the window but not already received, SHALL be
- accepted.
-
- After the packet has been authenticated (if necessary the window is
- first moved ahead), the replay list SHALL be updated with the new
- index.
-
- The Replay List can be efficiently implemented by using a bitmap to
- represent which packets have been received, as described in the
- Security Architecture for IP [RFC2401].
-
-3.4. Secure RTCP
-
- Secure RTCP follows the definition of Secure RTP. SRTCP adds three
- mandatory new fields (the SRTCP index, an "encrypt-flag", and the
- authentication tag) and one optional field (the MKI) to the RTCP
- packet definition. The three mandatory fields MUST be appended to an
- RTCP packet in order to form an equivalent SRTCP packet. The added
- fields follow any other profile-specific extensions.
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 15]
-
-RFC 3711 SRTP March 2004
-
-
- According to Section 6.1 of [RFC3550], there is a REQUIRED packet
- format for compound packets. SRTCP MUST be given packets according
- to that requirement in the sense that the first part MUST be a sender
- report or a receiver report. However, the RTCP encryption prefix (a
- random 32-bit quantity) specified in that Section MUST NOT be used
- since, as is stated there, it is only applicable to the encryption
- method specified in [RFC3550] and is not needed by the cryptographic
- mechanisms used in SRTP.
-
- 0 1 2 3
- 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
- |V=2|P| RC | PT=SR or RR | length | |
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | SSRC of sender | |
- +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
- | ~ sender info ~ |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | ~ report block 1 ~ |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | ~ report block 2 ~ |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | ~ ... ~ |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | |V=2|P| SC | PT=SDES=202 | length | |
- | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
- | | SSRC/CSRC_1 | |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | ~ SDES items ~ |
- | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
- | ~ ... ~ |
- +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
- | |E| SRTCP index | |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
- | ~ SRTCP MKI (OPTIONAL) ~ |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | : authentication tag : |
- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
- | |
- +-- Encrypted Portion Authenticated Portion -----+
-
-
- Figure 2. An example of the format of a Secure RTCP packet,
- consisting of an underlying RTCP compound packet with a Sender Report
- and SDES packet.
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 16]
-
-RFC 3711 SRTP March 2004
-
-
- The Encrypted Portion of an SRTCP packet consists of the encryption
- (Section 4.1) of the RTCP payload of the equivalent compound RTCP
- packet, from the first RTCP packet, i.e., from the ninth (9) octet to
- the end of the compound packet. The Authenticated Portion of an
- SRTCP packet consists of the entire equivalent (eventually compound)
- RTCP packet, the E flag, and the SRTCP index (after any encryption
- has been applied to the payload).
-
- The added fields are:
-
- E-flag: 1 bit, REQUIRED
- The E-flag indicates if the current SRTCP packet is
- encrypted or unencrypted. Section 9.1 of [RFC3550] allows
- the split of a compound RTCP packet into two lower-layer
- packets, one to be encrypted and one to be sent in the
- clear. The E bit set to "1" indicates encrypted packet, and
- "0" indicates non-encrypted packet.
-
- SRTCP index: 31 bits, REQUIRED
- The SRTCP index is a 31-bit counter for the SRTCP packet.
- The index is explicitly included in each packet, in contrast
- to the "implicit" index approach used for SRTP. The SRTCP
- index MUST be set to zero before the first SRTCP packet is
- sent, and MUST be incremented by one, modulo 2^31, after
- each SRTCP packet is sent. In particular, after a re-key,
- the SRTCP index MUST NOT be reset to zero again.
-
- Authentication Tag: configurable length, REQUIRED
- The authentication tag is used to carry message
- authentication data.
-
- MKI: configurable length, OPTIONAL
- The MKI is the Master Key Indicator, and functions according
- to the MKI definition in Section 3.
-
- SRTCP uses the cryptographic context parameters and packet processing
- of SRTP by default, with the following changes:
-
- * The receiver does not need to "estimate" the index, as it is
- explicitly signaled in the packet.
-
- * Pre-defined SRTCP encryption is as specified in Section 4.1, but
- using the definition of the SRTCP Encrypted Portion given in this
- section, and using the SRTCP index as the index i. The encryption
- transform and related parameters SHALL by default be the same
- selected for the protection of the associated SRTP stream(s),
- while the NULL algorithm SHALL be applied to the RTCP packets not
- to be encrypted. SRTCP may have a different encryption transform
-
-
-
-Baugher, et al. Standards Track [Page 17]
-
-RFC 3711 SRTP March 2004
-
-
- than the one used by the corresponding SRTP. The expected use for
- this feature is when the former has NULL-encryption and the latter
- has a non NULL-encryption.
-
- The E-flag is assigned a value by the sender depending on whether the
- packet was encrypted or not.
-
- * SRTCP decryption is performed as in Section 4, but only if the E
- flag is equal to 1. If so, the Encrypted Portion is decrypted,
- using the SRTCP index as the index i. In case the E-flag is 0,
- the payload is simply left unmodified.
-
- * SRTCP replay protection is as defined in Section 3.3.2, but using
- the SRTCP index as the index i and a separate Replay List that is
- specific to SRTCP.
-
- * The pre-defined SRTCP authentication tag is specified as in
- Section 4.2, but with the Authenticated Portion of the SRTCP
- packet given in this section (which includes the index). The
- authentication transform and related parameters (e.g., key size)
- SHALL by default be the same as selected for the protection of the
- associated SRTP stream(s).
-
- * In the last step of the processing, only the sender needs to
- update the value of the SRTCP index by incrementing it modulo 2^31
- and for security reasons the sender MUST also check the number of
- SRTCP packets processed, see Section 9.2.
-
- Message authentication for RTCP is REQUIRED, as it is the control
- protocol (e.g., it has a BYE packet) for RTP.
-
- Precautions must be taken so that the packet expansion in SRTCP (due
- to the added fields) does not cause SRTCP messages to use more than
- their share of RTCP bandwidth. To avoid this, the following two
- measures MUST be taken:
-
- 1. When initializing the RTCP variable "avg_rtcp_size" defined in
- chapter 6.3 of [RFC3550], it MUST include the size of the fields
- that will be added by SRTCP (index, E-bit, authentication tag, and
- when present, the MKI).
-
- 2. When updating the "avg_rtcp_size" using the variable "packet_size"
- (section 6.3.3 of [RFC3550]), the value of "packet_size" MUST
- include the size of the additional fields added by SRTCP.
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 18]
-
-RFC 3711 SRTP March 2004
-
-
- With these measures in place the SRTCP messages will not use more
- than the allotted bandwidth. The effect of the size of the added
- fields on the SRTCP traffic will be that messages will be sent with
- longer packet intervals. The increase in the intervals will be
- directly proportional to size of the added fields. For the pre-
- defined transforms, the size of the added fields will be at least 14
- octets, and upper bounded depending on MKI and the authentication tag
- sizes.
-
-4. Pre-Defined Cryptographic Transforms
-
- While there are numerous encryption and message authentication
- algorithms that can be used in SRTP, below we define default
- algorithms in order to avoid the complexity of specifying the
- encodings for the signaling of algorithm and parameter identifiers.
- The defined algorithms have been chosen as they fulfill the goals
- listed in Section 2. Recommendations on how to extend SRTP with new
- transforms are given in Section 6.
-
-4.1. Encryption
-
- The following parameters are common to both pre-defined, non-NULL,
- encryption transforms specified in this section.
-
- * BLOCK_CIPHER-MODE indicates the block cipher used and its mode of
- operation
- * n_b is the bit-size of the block for the block cipher
- * k_e is the session encryption key
- * n_e is the bit-length of k_e
- * k_s is the session salting key
- * n_s is the bit-length of k_s
- * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix, a
- non-negative integer, specified by the message authentication code
- in use.
-
- The distinct session keys and salts for SRTP/SRTCP are by default
- derived as specified in Section 4.3.
-
- The encryption transforms defined in SRTP map the SRTP packet index
- and secret key into a pseudo-random keystream segment. Each
- keystream segment encrypts a single RTP packet. The process of
- encrypting a packet consists of generating the keystream segment
- corresponding to the packet, and then bitwise exclusive-oring that
- keystream segment onto the payload of the RTP packet to produce the
- Encrypted Portion of the SRTP packet. In case the payload size is
- not an integer multiple of n_b bits, the excess (least significant)
- bits of the keystream are simply discarded. Decryption is done the
- same way, but swapping the roles of the plaintext and ciphertext.
-
-
-
-Baugher, et al. Standards Track [Page 19]
-
-RFC 3711 SRTP March 2004
-
-
- +----+ +------------------+---------------------------------+
- | KG |-->| Keystream Prefix | Keystream Suffix |---+
- +----+ +------------------+---------------------------------+ |
- |
- +---------------------------------+ v
- | Payload of RTP Packet |->(*)
- +---------------------------------+ |
- |
- +---------------------------------+ |
- | Encrypted Portion of SRTP Packet|<--+
- +---------------------------------+
-
- Figure 3: Default SRTP Encryption Processing. Here KG denotes the
- keystream generator, and (*) denotes bitwise exclusive-or.
-
- The definition of how the keystream is generated, given the index,
- depends on the cipher and its mode of operation. Below, two such
- keystream generators are defined. The NULL cipher is also defined,
- to be used when encryption of RTP is not required.
-
- The SRTP definition of the keystream is illustrated in Figure 3. The
- initial octets of each keystream segment MAY be reserved for use in a
- message authentication code, in which case the keystream used for
- encryption starts immediately after the last reserved octet. The
- initial reserved octets are called the "keystream prefix" (not to be
- confused with the "encryption prefix" of [RFC3550, Section 6.1]), and
- the remaining octets are called the "keystream suffix". The
- keystream prefix MUST NOT be used for encryption. The process is
- illustrated in Figure 3.
-
- The number of octets in the keystream prefix is denoted as
- SRTP_PREFIX_LENGTH. The keystream prefix is indicated by a positive,
- non-zero value of SRTP_PREFIX_LENGTH. This means that, even if
- confidentiality is not to be provided, the keystream generator output
- may still need to be computed for packet authentication, in which
- case the default keystream generator (mode) SHALL be used.
-
- The default cipher is the Advanced Encryption Standard (AES) [AES],
- and we define two modes of running AES, (1) Segmented Integer Counter
- Mode AES and (2) AES in f8-mode. In the remainder of this section,
- let E(k,x) be AES applied to key k and input block x.
-
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 20]
-
-RFC 3711 SRTP March 2004
-
-
-4.1.1. AES in Counter Mode
-
- Conceptually, counter mode [AES-CTR] consists of encrypting
- successive integers. The actual definition is somewhat more
- complicated, in order to randomize the starting point of the integer
- sequence. Each packet is encrypted with a distinct keystream
- segment, which SHALL be computed as follows.
-
- A keystream segment SHALL be the concatenation of the 128-bit output
- blocks of the AES cipher in the encrypt direction, using key k = k_e,
- in which the block indices are in increasing order. Symbolically,
- each keystream segment looks like
-
- E(k, IV) || E(k, IV + 1 mod 2^128) || E(k, IV + 2 mod 2^128) ...
-
- where the 128-bit integer value IV SHALL be defined by the SSRC, the
- SRTP packet index i, and the SRTP session salting key k_s, as below.
-
- IV = (k_s * 2^16) XOR (SSRC * 2^64) XOR (i * 2^16)
-
- Each of the three terms in the XOR-sum above is padded with as many
- leading zeros as needed to make the operation well-defined,
- considered as a 128-bit value.
-
- The inclusion of the SSRC allows the use of the same key to protect
- distinct SRTP streams within the same RTP session, see the security
- caveats in Section 9.1.
-
- In the case of SRTCP, the SSRC of the first header of the compound
- packet MUST be used, i SHALL be the 31-bit SRTCP index and k_e, k_s
- SHALL be replaced by the SRTCP encryption session key and salt.
-
- Note that the initial value, IV, is fixed for each packet and is
- formed by "reserving" 16 zeros in the least significant bits for the
- purpose of the counter. The number of blocks of keystream generated
- for any fixed value of IV MUST NOT exceed 2^16 to avoid keystream
- re-use, see below. The AES has a block size of 128 bits, so 2^16
- output blocks are sufficient to generate the 2^23 bits of keystream
- needed to encrypt the largest possible RTP packet (except for IPv6
- "jumbograms" [RFC2675], which are not likely to be used for RTP-based
- multimedia traffic). This restriction on the maximum bit-size of the
- packet that can be encrypted ensures the security of the encryption
- method by limiting the effectiveness of probabilistic attacks [BDJR].
-
- For a particular Counter Mode key, each IV value used as an input
- MUST be distinct, in order to avoid the security exposure of a two-
- time pad situation (Section 9.1). To satisfy this constraint, an
- implementation MUST ensure that the combination of the SRTP packet
-
-
-
-Baugher, et al. Standards Track [Page 21]
-
-RFC 3711 SRTP March 2004
-
-
- index of ROC || SEQ, and the SSRC used in the construction of the IV
- are distinct for any particular key. The failure to ensure this
- uniqueness could be catastrophic for Secure RTP. This is in contrast
- to the situation for RTP itself, which may be able to tolerate such
- failures. It is RECOMMENDED that, if a dedicated security module is
- present, the RTP sequence numbers and SSRC either be generated or
- checked by that module (i.e., sequence-number and SSRC processing in
- an SRTP system needs to be protected as well as the key).
-
-4.1.2. AES in f8-mode
-
- To encrypt UMTS (Universal Mobile Telecommunications System, as 3G
- networks) data, a solution (see [f8-a] [f8-b]) known as the f8-
- algorithm has been developed. On a high level, the proposed scheme
- is a variant of Output Feedback Mode (OFB) [HAC], with a more
- elaborate initialization and feedback function. As in normal OFB,
- the core consists of a block cipher. We also define here the use of
- AES as a block cipher to be used in what we shall call "f8-mode of
- operation" RTP encryption. The AES f8-mode SHALL use the same
- default sizes for session key and salt as AES counter mode.
-
- Figure 4 shows the structure of block cipher, E, running in f8-mode.
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 22]
-
-RFC 3711 SRTP March 2004
-
-
- IV
- |
- v
- +------+
- | |
- +--->| E |
- | +------+
- | |
- m -> (*) +-----------+-------------+-- ... ------+
- | IV' | | | |
- | | j=1 -> (*) j=2 -> (*) ... j=L-1 ->(*)
- | | | | |
- | | +-> (*) +-> (*) ... +-> (*)
- | | | | | | | |
- | v | v | v | v
- | +------+ | +------+ | +------+ | +------+
- k_e ---+--->| E | | | E | | | E | | | E |
- | | | | | | | | | | |
- +------+ | +------+ | +------+ | +------+
- | | | | | | |
- +------+ +--------+ +-- ... ----+ |
- | | | |
- v v v v
- S(0) S(1) S(2) . . . S(L-1)
-
- Figure 4. f8-mode of operation (asterisk, (*), denotes bitwise XOR).
- The figure represents the KG in Figure 3, when AES-f8 is used.
-
-4.1.2.1. f8 Keystream Generation
-
- The Initialization Vector (IV) SHALL be determined as described in
- Section 4.1.2.2 (and in Section 4.1.2.3 for SRTCP).
-
- Let IV', S(j), and m denote n_b-bit blocks. The keystream,
- S(0) ||... || S(L-1), for an N-bit message SHALL be defined by
- setting IV' = E(k_e XOR m, IV), and S(-1) = 00..0. For
- j = 0,1,..,L-1 where L = N/n_b (rounded up to nearest integer if it
- is not already an integer) compute
-
- S(j) = E(k_e, IV' XOR j XOR S(j-1))
-
- Notice that the IV is not used directly. Instead it is fed through E
- under another key to produce an internal, "masked" value (denoted
- IV') to prevent an attacker from gaining known input/output pairs.
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 23]
-
-RFC 3711 SRTP March 2004
-
-
- The role of the internal counter, j, is to prevent short keystream
- cycles. The value of the key mask m SHALL be
-
- m = k_s || 0x555..5,
-
- i.e., the session salting key, appended by the binary pattern 0101..
- to fill out the entire desired key size, n_e.
-
- The sender SHOULD NOT generate more than 2^32 blocks, which is
- sufficient to generate 2^39 bits of keystream. Unlike counter mode,
- there is no absolute threshold above (below) which f8 is guaranteed
- to be insecure (secure). The above bound has been chosen to limit,
- with sufficient security margin, the probability of degenerative
- behavior in the f8 keystream generation.
-
-4.1.2.2. f8 SRTP IV Formation
-
- The purpose of the following IV formation is to provide a feature
- which we call implicit header authentication (IHA), see Section 9.5.
-
- The SRTP IV for 128-bit block AES-f8 SHALL be formed in the following
- way:
-
- IV = 0x00 || M || PT || SEQ || TS || SSRC || ROC
-
- M, PT, SEQ, TS, SSRC SHALL be taken from the RTP header; ROC is from
- the cryptographic context.
-
- The presence of the SSRC as part of the IV allows AES-f8 to be used
- when a master key is shared between multiple streams within the same
- RTP session, see Section 9.1.
-
-4.1.2.3. f8 SRTCP IV Formation
-
- The SRTCP IV for 128-bit block AES-f8 SHALL be formed in the
- following way:
-
- IV= 0..0 || E || SRTCP index || V || P || RC || PT || length || SSRC
-
- where V, P, RC, PT, length, SSRC SHALL be taken from the first header
- in the RTCP compound packet. E and SRTCP index are the 1-bit and
- 31-bit fields added to the packet.
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 24]
-
-RFC 3711 SRTP March 2004
-
-
-4.1.3. NULL Cipher
-
- The NULL cipher is used when no confidentiality for RTP/RTCP is
- requested. The keystream can be thought of as "000..0", i.e., the
- encryption SHALL simply copy the plaintext input into the ciphertext
- output.
-
-4.2. Message Authentication and Integrity
-
- Throughout this section, M will denote data to be integrity
- protected. In the case of SRTP, M SHALL consist of the Authenticated
- Portion of the packet (as specified in Figure 1) concatenated with
- the ROC, M = Authenticated Portion || ROC; in the case of SRTCP, M
- SHALL consist of the Authenticated Portion (as specified in Figure 2)
- only.
-
- Common parameters:
-
- * AUTH_ALG is the authentication algorithm
- * k_a is the session message authentication key
- * n_a is the bit-length of the authentication key
- * n_tag is the bit-length of the output authentication tag
- * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix as
- defined above, a parameter of AUTH_ALG
-
- The distinct session authentication keys for SRTP/SRTCP are by
- default derived as specified in Section 4.3.
-
- The values of n_a, n_tag, and SRTP_PREFIX_LENGTH MUST be fixed for
- any particular fixed value of the key.
-
- We describe the process of computing authentication tags as follows.
- The sender computes the tag of M and appends it to the packet. The
- SRTP receiver verifies a message/authentication tag pair by computing
- a new authentication tag over M using the selected algorithm and key,
- and then compares it to the tag associated with the received message.
- If the two tags are equal, then the message/tag pair is valid;
- otherwise, it is invalid and the error audit message "AUTHENTICATION
- FAILURE" MUST be returned.
-
-4.2.1. HMAC-SHA1
-
- The pre-defined authentication transform for SRTP is HMAC-SHA1
- [RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL
- be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to
- the session authentication key and M as specified above, i.e.,
- HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag
- left-most bits.
-
-
-
-Baugher, et al. Standards Track [Page 25]
-
-RFC 3711 SRTP March 2004
-
-
-4.3. Key Derivation
-
-4.3.1. Key Derivation Algorithm
-
- Regardless of the encryption or message authentication transform that
- is employed (it may be an SRTP pre-defined transform or newly
- introduced according to Section 6), interoperable SRTP
- implementations MUST use the SRTP key derivation to generate session
- keys. Once the key derivation rate is properly signaled at the start
- of the session, there is no need for extra communication between the
- parties that use SRTP key derivation.
-
- packet index ---+
- |
- v
- +-----------+ master +--------+ session encr_key
- | ext | key | |---------->
- | key mgmt |-------->| key | session auth_key
- | (optional | | deriv |---------->
- | rekey) |-------->| | session salt_key
- | | master | |---------->
- +-----------+ salt +--------+
-
- Figure 5: SRTP key derivation.
-
- At least one initial key derivation SHALL be performed by SRTP, i.e.,
- the first key derivation is REQUIRED. Further applications of the
- key derivation MAY be performed, according to the
- "key_derivation_rate" value in the cryptographic context. The key
- derivation function SHALL initially be invoked before the first
- packet and then, when r > 0, a key derivation is performed whenever
- index mod r equals zero. This can be thought of as "refreshing" the
- session keys. The value of "key_derivation_rate" MUST be kept fixed
- for the lifetime of the associated master key.
-
- Interoperable SRTP implementations MAY also derive session salting
- keys for encryption transforms, as is done in both of the pre-
- defined transforms.
-
- Let m and n be positive integers. A pseudo-random function family is
- a set of keyed functions {PRF_n(k,x)} such that for the (secret)
- random key k, given m-bit x, PRF_n(k,x) is an n-bit string,
- computationally indistinguishable from random n-bit strings, see
- [HAC]. For the purpose of key derivation in SRTP, a secure PRF with
- m = 128 (or more) MUST be used, and a default PRF transform is
- defined in Section 4.3.3.
-
-
-
-
-
-Baugher, et al. Standards Track [Page 26]
-
-RFC 3711 SRTP March 2004
-
-
- Let "a DIV t" denote integer division of a by t, rounded down, and
- with the convention that "a DIV 0 = 0" for all a. We also make the
- convention of treating "a DIV t" as a bit string of the same length
- as a, and thus "a DIV t" will in general have leading zeros.
-
- Key derivation SHALL be defined as follows in terms of <label>, an
- 8-bit constant (see below), master_salt and key_derivation_rate, as
- determined in the cryptographic context, and index, the packet index
- (i.e., the 48-bit ROC || SEQ for SRTP):
-
- * Let r = index DIV key_derivation_rate (with DIV as defined above).
-
- * Let key_id = <label> || r.
-
- * Let x = key_id XOR master_salt, where key_id and master_salt are
- aligned so that their least significant bits agree (right-
- alignment).
-
- <label> MUST be unique for each type of key to be derived. We
- currently define <label> 0x00 to 0x05 (see below), and future
- extensions MAY specify new values in the range 0x06 to 0xff for other
- purposes. The n-bit SRTP key (or salt) for this packet SHALL then be
- derived from the master key, k_master as follows:
-
- PRF_n(k_master, x).
-
- (The PRF may internally specify additional formatting and padding of
- x, see e.g., Section 4.3.3 for the default PRF.)
-
- The session keys and salt SHALL now be derived using:
-
- - k_e (SRTP encryption): <label> = 0x00, n = n_e.
-
- - k_a (SRTP message authentication): <label> = 0x01, n = n_a.
-
- - k_s (SRTP salting key): <label> = 0x02, n = n_s.
-
- where n_e, n_s, and n_a are from the cryptographic context.
-
- The master key and master salt MUST be random, but the master salt
- MAY be public.
-
- Note that for a key_derivation_rate of 0, the application of the key
- derivation SHALL take place exactly once.
-
- The definition of DIV above is purely for notational convenience.
- For a non-zero t among the set of allowed key derivation rates, "a
- DIV t" can be implemented as a right-shift by the base-2 logarithm of
-
-
-
-Baugher, et al. Standards Track [Page 27]
-
-RFC 3711 SRTP March 2004
-
-
- t. The derivation operation is further facilitated if the rates are
- chosen to be powers of 256, but that granularity was considered too
- coarse to be a requirement of this specification.
-
- The upper limit on the number of packets that can be secured using
- the same master key (see Section 9.2) is independent of the key
- derivation.
-
-4.3.2. SRTCP Key Derivation
-
- SRTCP SHALL by default use the same master key (and master salt) as
- SRTP. To do this securely, the following changes SHALL be done to
- the definitions in Section 4.3.1 when applying session key derivation
- for SRTCP.
-
- Replace the SRTP index by the 32-bit quantity: 0 || SRTCP index
- (i.e., excluding the E-bit, replacing it with a fixed 0-bit), and use
- <label> = 0x03 for the SRTCP encryption key, <label> = 0x04 for the
- SRTCP authentication key, and, <label> = 0x05 for the SRTCP salting
- key.
-
-4.3.3. AES-CM PRF
-
- The currently defined PRF, keyed by 128, 192, or 256 bit master key,
- has input block size m = 128 and can produce n-bit outputs for n up
- to 2^23. PRF_n(k_master,x) SHALL be AES in Counter Mode as described
- in Section 4.1.1, applied to key k_master, and IV equal to (x*2^16),
- and with the output keystream truncated to the n first (left-most)
- bits. (Requiring n/128, rounded up, applications of AES.)
-
-5. Default and mandatory-to-implement Transforms
-
- The default transforms also are mandatory-to-implement transforms in
- SRTP. Of course, "mandatory-to-implement" does not imply
- "mandatory-to-use". Table 1 summarizes the pre-defined transforms.
- The default values below are valid for the pre-defined transforms.
-
- mandatory-to-impl. optional default
-
- encryption AES-CM, NULL AES-f8 AES-CM
- message integrity HMAC-SHA1 - HMAC-SHA1
- key derivation (PRF) AES-CM - AES-CM
-
- Table 1: Mandatory-to-implement, optional and default transforms in
- SRTP and SRTCP.
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 28]
-
-RFC 3711 SRTP March 2004
-
-
-5.1. Encryption: AES-CM and NULL
-
- AES running in Segmented Integer Counter Mode, as defined in Section
- 4.1.1, SHALL be the default encryption algorithm. The default key
- lengths SHALL be 128-bit for the session encryption key (n_e). The
- default session salt key-length (n_s) SHALL be 112 bits.
-
- The NULL cipher SHALL also be mandatory-to-implement.
-
-5.2. Message Authentication/Integrity: HMAC-SHA1
-
- HMAC-SHA1, as defined in Section 4.2.1, SHALL be the default message
- authentication code. The default session authentication key-length
- (n_a) SHALL be 160 bits, the default authentication tag length
- (n_tag) SHALL be 80 bits, and the SRTP_PREFIX_LENGTH SHALL be zero
- for HMAC-SHA1. In addition, for SRTCP, the pre-defined HMAC-SHA1
- MUST NOT be applied with a value of n_tag, nor n_a, that are smaller
- than these defaults. For SRTP, smaller values are NOT RECOMMENDED,
- but MAY be used after careful consideration of the issues in Section
- 7.5 and 9.5.
-
-5.3. Key Derivation: AES-CM PRF
-
- The AES Counter Mode based key derivation and PRF defined in Sections
- 4.3.1 to 4.3.3, using a 128-bit master key, SHALL be the default
- method for generating session keys. The default master salt length
- SHALL be 112 bits and the default key-derivation rate SHALL be zero.
-
-6. Adding SRTP Transforms
-
- Section 4 provides examples of the level of detail needed for
- defining transforms. Whenever a new transform is to be added to
- SRTP, a companion standard track RFC MUST be written to exactly
- define how the new transform can be used with SRTP (and SRTCP). Such
- a companion RFC SHOULD avoid overlap with the SRTP protocol document.
- Note however, that it MAY be necessary to extend the SRTP or SRTCP
- cryptographic context definition with new parameters (including fixed
- or default values), add steps to the packet processing, or even add
- fields to the SRTP/SRTCP packets. The companion RFC SHALL explain
- any known issues regarding interactions between the transform and
- other aspects of SRTP.
-
- Each new transform document SHOULD specify its key attributes, e.g.,
- size of keys (minimum, maximum, recommended), format of keys,
- recommended/required processing of input keying material,
- requirements/recommendations on key lifetime, re-keying and key
- derivation, whether sharing of keys between SRTP and SRTCP is allowed
- or not, etc.
-
-
-
-Baugher, et al. Standards Track [Page 29]
-
-RFC 3711 SRTP March 2004
-
-
- An added message integrity transform SHOULD define a minimum
- acceptable key/tag size for SRTCP, equivalent in strength to the
- minimum values as defined in Section 5.2.
-
-7. Rationale
-
- This section explains the rationale behind several important features
- of SRTP.
-
-7.1. Key derivation
-
- Key derivation reduces the burden on the key establishment. As many
- as six different keys are needed per crypto context (SRTP and SRTCP
- encryption keys and salts, SRTP and SRTCP authentication keys), but
- these are derived from a single master key in a cryptographically
- secure way. Thus, the key management protocol needs to exchange only
- one master key (plus master salt when required), and then SRTP itself
- derives all the necessary session keys (via the first, mandatory
- application of the key derivation function).
-
- Multiple applications of the key derivation function are optional,
- but will give security benefits when enabled. They prevent an
- attacker from obtaining large amounts of ciphertext produced by a
- single fixed session key. If the attacker was able to collect a
- large amount of ciphertext for a certain session key, he might be
- helped in mounting certain attacks.
-
- Multiple applications of the key derivation function provide
- backwards and forward security in the sense that a compromised
- session key does not compromise other session keys derived from the
- same master key. This means that the attacker who is able to recover
- a certain session key, is anyway not able to have access to messages
- secured under previous and later session keys (derived from the same
- master key). (Note that, of course, a leaked master key reveals all
- the session keys derived from it.)
-
- Considerations arise with high-rate key refresh, especially in large
- multicast settings, see Section 11.
-
-7.2. Salting key
-
- The master salt guarantees security against off-line key-collision
- attacks on the key derivation that might otherwise reduce the
- effective key size [MF00].
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 30]
-
-RFC 3711 SRTP March 2004
-
-
- The derived session salting key used in the encryption, has been
- introduced to protect against some attacks on additive stream
- ciphers, see Section 9.2. The explicit inclusion method of the salt
- in the IV has been selected for ease of hardware implementation.
-
-7.3. Message Integrity from Universal Hashing
-
- The particular definition of the keystream given in Section 4.1 (the
- keystream prefix) is to give provision for particular universal hash
- functions, suitable for message authentication in the Wegman-Carter
- paradigm [WC81]. Such functions are provably secure, simple, quick,
- and especially appropriate for Digital Signal Processors and other
- processors with a fast multiply operation.
-
- No authentication transforms are currently provided in SRTP other
- than HMAC-SHA1. Future transforms, like the above mentioned
- universal hash functions, MAY be added following the guidelines in
- Section 6.
-
-7.4. Data Origin Authentication Considerations
-
- Note that in pair-wise communications, integrity and data origin
- authentication are provided together. However, in group scenarios
- where the keys are shared between members, the MAC tag only proves
- that a member of the group sent the packet, but does not prevent
- against a member impersonating another. Data origin authentication
- (DOA) for multicast and group RTP sessions is a hard problem that
- needs a solution; while some promising proposals are being
- investigated [PCST1] [PCST2], more work is needed to rigorously
- specify these technologies. Thus SRTP data origin authentication in
- groups is for further study.
-
- DOA can be done otherwise using signatures. However, this has high
- impact in terms of bandwidth and processing time, therefore we do not
- offer this form of authentication in the pre-defined packet-integrity
- transform.
-
- The presence of mixers and translators does not allow data origin
- authentication in case the RTP payload and/or the RTP header are
- manipulated. Note that these types of middle entities also disrupt
- end-to-end confidentiality (as the IV formation depends e.g., on the
- RTP header preservation). A certain trust model may choose to trust
- the mixers/translators to decrypt/re-encrypt the media (this would
- imply breaking the end-to-end security, with related security
- implications).
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 31]
-
-RFC 3711 SRTP March 2004
-
-
-7.5. Short and Zero-length Message Authentication
-
- As shown in Figure 1, the authentication tag is RECOMMENDED in SRTP.
- A full 80-bit authentication-tag SHOULD be used, but a shorter tag or
- even a zero-length tag (i.e., no message authentication) MAY be used
- under certain conditions to support either of the following two
- application environments.
-
- 1. Strong authentication can be impractical in environments where
- bandwidth preservation is imperative. An important special
- case is wireless communication systems, in which bandwidth is a
- scarce and expensive resource. Studies have shown that for
- certain applications and link technologies, additional bytes
- may result in a significant decrease in spectrum efficiency
- [SWO]. Considerable effort has been made to design IP header
- compression techniques to improve spectrum efficiency
- [RFC3095]. A typical voice application produces 20 byte
- samples, and the RTP, UDP and IP headers need to be jointly
- compressed to one or two bytes on average in order to obtain
- acceptable wireless bandwidth economy [RFC3095]. In this case,
- strong authentication would impose nearly fifty percent
- overhead.
-
- 2. Authentication is impractical for applications that use data
- links with fixed-width fields that cannot accommodate the
- expansion due to the authentication tag. This is the case for
- some important existing wireless channels. For example, zero-
- byte header compression is used to adapt EVRC/SMV voice with
- the legacy IS-95 bearer channel in CDMA2000 VoIP services. It
- was found that not a single additional octet could be added to
- the data, which motivated the creation of a zero-byte profile
- for ROHC [RFC3242].
-
- A short tag is secure for a restricted set of applications. Consider
- a voice telephony application, for example, such as a G.729 audio
- codec with a 20-millisecond packetization interval, protected by a
- 32-bit message authentication tag. The likelihood of any given
- packet being successfully forged is only one in 2^32. Thus an
- adversary can control no more than 20 milliseconds of audio output
- during a 994-day period, on average. In contrast, the effect of a
- single forged packet can be much larger if the application is
- stateful. A codec that uses relative or predictive compression
- across packets will propagate the maliciously generated state,
- affecting a longer duration of output.
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 32]
-
-RFC 3711 SRTP March 2004
-
-
- Certainly not all SRTP or telephony applications meet the criteria
- for short or zero-length authentication tags. Section 9.5.1
- discusses the risks of weak or no message authentication, and section
- 9.5 describes the circumstances when it is acceptable and when it is
- unacceptable.
-
-8. Key Management Considerations
-
- There are emerging key management standards [MIKEY] [KEYMGT] [SDMS]
- for establishing an SRTP cryptographic context (e.g., an SRTP master
- key). Both proprietary and open-standard key management methods are
- likely to be used for telephony applications [MIKEY] [KINK] and
- multicast applications [GDOI]. This section provides guidance for
- key management systems that service SRTP session.
-
- For initialization, an interoperable SRTP implementation SHOULD be
- given the SSRC and MAY be given the initial RTP sequence number for
- the RTP stream by key management (thus, key management has a
- dependency on RTP operational parameters). Sending the RTP sequence
- number in the key management may be useful e.g., when the initial
- sequence number is close to wrapping (to avoid synchronization
- problems), and to communicate the current sequence number to a
- joining endpoint (to properly initialize its replay list).
-
- If the pre-defined transforms are used, SRTP allows sharing of the
- same master key between SRTP/SRTCP streams belonging to the same RTP
- session.
-
- First, sharing between SRTP streams belonging to the same RTP session
- is secure if the design of the synchronization mechanism, i.e., the
- IV, avoids keystream re-use (the two-time pad, Section 9.1). This is
- taken care of by the fact that RTP provides for unique SSRCs for
- streams belonging to the same RTP session. See Section 9.1 for
- further discussion.
-
- Second, sharing between SRTP and the corresponding SRTCP is secure.
- The fact that an SRTP stream and its associated SRTCP stream both
- carry the same SSRC does not constitute a problem for the two-time
- pad due to the key derivation. Thus, SRTP and SRTCP corresponding to
- one RTP session MAY share master keys (as they do by default).
-
- Note that message authentication also has a dependency on SSRC
- uniqueness that is unrelated to the problem of keystream reuse: SRTP
- streams authenticated under the same key MUST have a distinct SSRC in
- order to identify the sender of the message. This requirement is
- needed because the SSRC is the cryptographically authenticated field
-
-
-
-
-
-Baugher, et al. Standards Track [Page 33]
-
-RFC 3711 SRTP March 2004
-
-
- used to distinguish between different SRTP streams. Were two streams
- to use identical SSRC values, then an adversary could substitute
- messages from one stream into the other without detection.
-
- SRTP/SRTCP MUST NOT share master keys under any other circumstances
- than the ones given above, i.e., between SRTP and its corresponding
- SRTCP, and, between streams belonging to the same RTP session.
-
-8.1. Re-keying
-
- The recommended way for a particular key management system to provide
- re-key within SRTP is by associating a master key in a crypto context
- with an MKI.
-
- This provides for easy master key retrieval (see Scenarios in Section
- 11), but has the disadvantage of adding extra bits to each packet.
- As noted in Section 7.5, some wireless links do not cater for added
- bits, therefore SRTP also defines a more economic way of triggering
- re-keying, via use of <From, To>, which works in some specific,
- simple scenarios (see Section 8.1.1).
-
- SRTP senders SHALL count the amount of SRTP and SRTCP traffic being
- used for a master key and invoke key management to re-key if needed
- (Section 9.2). These interactions are defined by the key management
- interface to SRTP and are not defined by this protocol specification.
-
-8.1.1. Use of the <From, To> for re-keying
-
- In addition to the use of the MKI, SRTP defines another optional
- mechanism for master key retrieval, the <From, To>. The <From, To>
- specifies the range of SRTP indices (a pair of sequence number and
- ROC) within which a certain master key is valid, and is (when used)
- part of the crypto context. By looking at the 48-bit SRTP index of
- the current SRTP packet, the corresponding master key can be found by
- determining which From-To interval it belongs to. For SRTCP, the
- most recently observed/used SRTP index (which can be obtained from
- the cryptographic context) is used for this purpose, even though
- SRTCP has its own (31-bit) index (see caveat below).
-
- This method, compared to the MKI, has the advantage of identifying
- the master key and defining its lifetime without adding extra bits to
- each packet. This could be useful, as already noted, for some
- wireless links that do not cater for added bits. However, its use
- SHOULD be limited to specific, very simple scenarios. We recommend
- to limit its use when the RTP session is a simple unidirectional or
- bi-directional stream. This is because in case of multiple streams,
- it is difficult to trigger the re-key based on the <From, To> of a
- single RTP stream. For example, if several streams share a master
-
-
-
-Baugher, et al. Standards Track [Page 34]
-
-RFC 3711 SRTP March 2004
-
-
- key, there is no simple one-to-one correspondence between the index
- sequence space of a certain stream, and the index sequence space on
- which the <From, To> values are based. Consequently, when a master
- key is shared between streams, one of these streams MUST be
- designated by key management as the one whose index space defines the
- re-keying points. Also, the re-key triggering on SRTCP is based on
- the correspondent SRTP stream, i.e., when the SRTP stream changes the
- master key, so does the correspondent SRTCP. This becomes obviously
- more and more complex with multiple streams.
-
- The default values for the <From, To> are "from the first observed
- packet" and "until further notice". However, the maximum limit of
- SRTP/SRTCP packets that are sent under each given master/session key
- (Section 9.2) MUST NOT be exceeded.
-
- In case the <From, To> is used as key retrieval, then the MKI is not
- inserted in the packet (and its indicator in the crypto context is
- zero). However, using the MKI does not exclude using <From, To> key
- lifetime simultaneously. This can for instance be useful to signal
- at the sender side at which point in time an MKI is to be made
- active.
-
-8.2. Key Management parameters
-
- The table below lists all SRTP parameters that key management can
- supply. For reference, it also provides a summary of the default and
- mandatory-to-support values for an SRTP implementation as described
- in Section 5.
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 35]
-
-RFC 3711 SRTP March 2004
-
-
- Parameter Mandatory-to-support Default
- --------- -------------------- -------
-
- SRTP and SRTCP encr transf. AES_CM, NULL AES_CM
- (Other possible values: AES_f8)
-
- SRTP and SRTCP auth transf. HMAC-SHA1 HMAC-SHA1
-
- SRTP and SRTCP auth params:
- n_tag (tag length) 80 80
- SRTP prefix_length 0 0
-
- Key derivation PRF AES_CM AES_CM
-
- Key material params
- (for each master key):
- master key length 128 128
- n_e (encr session key length) 128 128
- n_a (auth session key length) 160 160
- master salt key
- length of the master salt 112 112
- n_s (session salt key length) 112 112
- key derivation rate 0 0
-
- key lifetime
- SRTP-packets-max-lifetime 2^48 2^48
- SRTCP-packets-max-lifetime 2^31 2^31
- from-to-lifetime <From, To>
- MKI indicator 0 0
- length of the MKI 0 0
- value of the MKI
-
- Crypto context index params:
- SSRC value
- ROC
- SEQ
- SRTCP Index
- Transport address
- Port number
-
- Relation to other RTP profiles:
- sender's order between FEC and SRTP FEC-SRTP FEC-SRTP
- (see Section 10)
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 36]
-
-RFC 3711 SRTP March 2004
-
-
-9. Security Considerations
-
-9.1. SSRC collision and two-time pad
-
- Any fixed keystream output, generated from the same key and index
- MUST only be used to encrypt once. Re-using such keystream (jokingly
- called a "two-time pad" system by cryptographers), can seriously
- compromise security. The NSA's VENONA project [C99] provides a
- historical example of such a compromise. It is REQUIRED that
- automatic key management be used for establishing and maintaining
- SRTP and SRTCP keying material; this requirement is to avoid
- keystream reuse, which is more likely to occur with manual key
- management. Furthermore, in SRTP, a "two-time pad" is avoided by
- requiring the key, or some other parameter of cryptographic
- significance, to be unique per RTP/RTCP stream and packet. The pre-
- defined SRTP transforms accomplish packet-uniqueness by including the
- packet index and stream-uniqueness by inclusion of the SSRC.
-
- The pre-defined transforms (AES-CM and AES-f8) allow master keys to
- be shared across streams belonging to the same RTP session by the
- inclusion of the SSRC in the IV. A master key MUST NOT be shared
- among different RTP sessions.
-
- Thus, the SSRC MUST be unique between all the RTP streams within the
- same RTP session that share the same master key. RTP itself provides
- an algorithm for detecting SSRC collisions within the same RTP
- session. Thus, temporary collisions could lead to temporary two-time
- pad, in the unfortunate event that SSRCs collide at a point in time
- when the streams also have identical sequence numbers (occurring with
- probability roughly 2^(-48)). Therefore, the key management SHOULD
- take care of avoiding such SSRC collisions by including the SSRCs to
- be used in the session as negotiation parameters, proactively
- assuring their uniqueness. This is a strong requirements in
- scenarios where for example, there are multiple senders that can
- start to transmit simultaneously, before SSRC collision are detected
- at the RTP level.
-
- Note also that even with distinct SSRCs, extensive use of the same
- key might improve chances of probabilistic collision and time-
- memory-tradeoff attacks succeeding.
-
- As described, master keys MAY be shared between streams belonging to
- the same RTP session, but it is RECOMMENDED that each SSRC have its
- own master key. When master keys are shared among SSRC participants
- and SSRCs are managed by a key management module as recommended
- above, the RECOMMENDED policy for an SSRC collision error is for the
- participant to leave the SRTP session as it is a sign of malfunction.
-
-
-
-
-Baugher, et al. Standards Track [Page 37]
-
-RFC 3711 SRTP March 2004
-
-
-9.2. Key Usage
-
- The effective key size is determined (upper bounded) by the size of
- the master key and, for encryption, the size of the salting key. Any
- additive stream cipher is vulnerable to attacks that use statistical
- knowledge about the plaintext source to enable key collision and
- time-memory tradeoff attacks [MF00] [H80] [BS00]. These attacks take
- advantage of commonalities among plaintexts, and provide a way for a
- cryptanalyst to amortize the computational effort of decryption over
- many keys, or over many bytes of output, thus reducing the effective
- key size of the cipher. A detailed analysis of these attacks and
- their applicability to the encryption of Internet traffic is provided
- in [MF00]. In summary, the effective key size of SRTP when used in a
- security system in which m distinct keys are used, is equal to the
- key size of the cipher less the logarithm (base two) of m.
- Protection against such attacks can be provided simply by increasing
- the size of the keys used, which here can be accomplished by the use
- of the salting key. Note that the salting key MUST be random but MAY
- be public. A salt size of (the suggested) size 112 bits protects
- against attacks in scenarios where at most 2^112 keys are in use.
- This is sufficient for all practical purposes.
-
- Implementations SHOULD use keys that are as large as possible.
- Please note that in many cases increasing the key size of a cipher
- does not affect the throughput of that cipher.
-
- The use of the SRTP and SRTCP indices in the pre-defined transforms
- fixes the maximum number of packets that can be secured with the same
- key. This limit is fixed to 2^48 SRTP packets for an SRTP stream,
- and 2^31 SRTCP packets, when SRTP and SRTCP are considered
- independently. Due to for example re-keying, reaching this limit may
- or may not coincide with wrapping of the indices, and thus the sender
- MUST keep packet counts. However, when the session keys for related
- SRTP and SRTCP streams are derived from the same master key (the
- default behavior, Section 4.3), the upper bound that has to be
- considered is in practice the minimum of the two quantities. That
- is, when 2^48 SRTP packets or 2^31 SRTCP packets have been secured
- with the same key (whichever occurs before), the key management MUST
- be called to provide new master key(s) (previously stored and used
- keys MUST NOT be used again), or the session MUST be terminated. If
- a sender of RTCP discovers that the sender of SRTP (or SRTCP) has not
- updated the master or session key prior to sending 2^48 SRTP (or 2^31
- SRTCP) packets belonging to the same SRTP (SRTCP) stream, it is up to
- the security policy of the RTCP sender how to behave, e.g., whether
- an RTCP BYE-packet should be sent and/or if the event should be
- logged.
-
-
-
-
-
-Baugher, et al. Standards Track [Page 38]
-
-RFC 3711 SRTP March 2004
-
-
- Note: in most typical applications (assuming at least one RTCP packet
- for every 128,000 RTP packets), it will be the SRTCP index that first
- reaches the upper limit, although the time until this occurs is very
- long: even at 200 SRTCP packets/sec, the 2^31 index space of SRTCP is
- enough to secure approximately 4 months of communication.
-
- Note that if the master key is to be shared between SRTP streams
- within the same RTP session (Section 9.1), although the above bounds
- are on a per stream (i.e., per SSRC) basis, the sender MUST base re-
- key decision on the stream whose sequence number space is the first
- to be exhausted.
-
- Key derivation limits the amount of plaintext that is encrypted with
- a fixed session key, and made available to an attacker for analysis,
- but key derivation does not extend the master key's lifetime. To see
- this, simply consider our requirements to avoid two-time pad: two
- distinct packets MUST either be processed with distinct IVs, or with
- distinct session keys, and both the distinctness of IV and of the
- session keys are (for the pre-defined transforms) dependent on the
- distinctness of the packet indices.
-
- Note that with the key derivation, the effective key size is at most
- that of the master key, even if the derived session key is
- considerably longer. With the pre-defined authentication transform,
- the session authentication key is 160 bits, but the master key by
- default is only 128 bits. This design choice was made to comply with
- certain recommendations in [RFC2104] so that an existing HMAC
- implementation can be plugged into SRTP without problems. Since the
- default tag size is 80 bits, it is, for the applications in mind,
- also considered acceptable from security point of view. Users having
- concerns about this are RECOMMENDED to instead use a 192 bit master
- key in the key derivation. It was, however, chosen not to mandate
- 192-bit keys since existing AES implementations to be used in the
- key-derivation may not always support key-lengths other than 128
- bits. Since AES is not defined (or properly analyzed) for use with
- 160 bit keys it is NOT RECOMMENDED that ad-hoc key-padding schemes
- are used to pad shorter keys to 192 or 256 bits.
-
-9.3. Confidentiality of the RTP Payload
-
- SRTP's pre-defined ciphers are "seekable" stream ciphers, i.e.,
- ciphers able to efficiently seek to arbitrary locations in their
- keystream (so that the encryption or decryption of one packet does
- not depend on preceding packets). By using seekable stream ciphers,
- SRTP avoids the denial of service attacks that are possible on stream
- ciphers that lack this property. It is important to be aware that,
- as with any stream cipher, the exact length of the payload is
- revealed by the encryption. This means that it may be possible to
-
-
-
-Baugher, et al. Standards Track [Page 39]
-
-RFC 3711 SRTP March 2004
-
-
- deduce certain "formatting bits" of the payload, as the length of the
- codec output might vary due to certain parameter settings etc. This,
- in turn, implies that the corresponding bit of the keystream can be
- deduced. However, if the stream cipher is secure (counter mode and
- f8 are provably secure under certain assumptions [BDJR] [KSYH] [IK]),
- knowledge of a few bits of the keystream will not aid an attacker in
- predicting subsequent keystream bits. Thus, the payload length (and
- information deducible from this) will leak, but nothing else.
-
- As some RTP packet could contain highly predictable data, e.g., SID,
- it is important to use a cipher designed to resist known plaintext
- attacks (which is the current practice).
-
-9.4. Confidentiality of the RTP Header
-
- In SRTP, RTP headers are sent in the clear to allow for header
- compression. This means that data such as payload type,
- synchronization source identifier, and timestamp are available to an
- eavesdropper. Moreover, since RTP allows for future extensions of
- headers, we cannot foresee what kind of possibly sensitive
- information might also be "leaked".
-
- SRTP is a low-cost method, which allows header compression to reduce
- bandwidth. It is up to the endpoints' policies to decide about the
- security protocol to employ. If one really needs to protect headers,
- and is allowed to do so by the surrounding environment, then one
- should also look at alternatives, e.g., IPsec [RFC2401].
-
-9.5. Integrity of the RTP payload and header
-
- SRTP messages are subject to attacks on their integrity and source
- identification, and these risks are discussed in Section 9.5.1. To
- protect against these attacks, each SRTP stream SHOULD be protected
- by HMAC-SHA1 [RFC2104] with an 80-bit output tag and a 160-bit key,
- or a message authentication code with equivalent strength. Secure
- RTP SHOULD NOT be used without message authentication, except under
- the circumstances described in this section. It is important to note
- that encryption algorithms, including AES Counter Mode and f8, do not
- provide message authentication. SRTCP MUST NOT be used with weak (or
- NULL) authentication.
-
- SRTP MAY be used with weak authentication (e.g., a 32-bit
- authentication tag), or with no authentication (the NULL
- authentication algorithm). These options allow SRTP to be used to
- provide confidentiality in situations where
-
- * weak or null authentication is an acceptable security risk, and
- * it is impractical to provide strong message authentication.
-
-
-
-Baugher, et al. Standards Track [Page 40]
-
-RFC 3711 SRTP March 2004
-
-
- These conditions are described below and in Section 7.5. Note that
- both conditions MUST hold in order for weak or null authentication to
- be used. The risks associated with exercising the weak or null
- authentication options need to be considered by a security audit
- prior to their use for a particular application or environment given
- the risks, which are discussed in Section 9.5.1.
-
- Weak authentication is acceptable when the RTP application is such
- that the effect of a small fraction of successful forgeries is
- negligible. If the application is stateless, then the effect of a
- single forged RTP packet is limited to the decoding of that
- particular packet. Under this condition, the size of the
- authentication tag MUST ensure that only a negligible fraction of the
- packets passed to the RTP application by the SRTP receiver can be
- forgeries. This fraction is negligible when an adversary, if given
- control of the forged packets, is not able to make a significant
- impact on the output of the RTP application (see the example of
- Section 7.5).
-
- Weak or null authentication MAY be acceptable when it is unlikely
- that an adversary can modify ciphertext so that it decrypts to an
- intelligible value. One important case is when it is difficult for
- an adversary to acquire the RTP plaintext data, since for many
- codecs, an adversary that does not know the input signal cannot
- manipulate the output signal in a controlled way. In many cases it
- may be difficult for the adversary to determine the actual value of
- the plaintext. For example, a hidden snooping device might be
- required in order to know a live audio or video signal. The
- adversary's signal must have a quality equivalent to or greater than
- that of the signal under attack, since otherwise the adversary would
- not have enough information to encode that signal with the codec used
- by the victim. Plaintext prediction may also be especially difficult
- for an interactive application such as a telephone call.
-
- Weak or null authentication MUST NOT be used when the RTP application
- makes data forwarding or access control decisions based on the RTP
- data. In such a case, an attacker may be able to subvert
- confidentiality by causing the receiver to forward data to an
- attacker. See Section 3 of [B96] for a real-life example of such
- attacks.
-
- Null authentication MUST NOT be used when a replay attack, in which
- an adversary stores packets then replays them later in the session,
- could have a non-negligible impact on the receiver. An example of a
- successful replay attack is the storing of the output of a
- surveillance camera for a period of time, later followed by the
-
-
-
-
-
-Baugher, et al. Standards Track [Page 41]
-
-RFC 3711 SRTP March 2004
-
-
- injection of that output to the monitoring station to avoid
- surveillance. Encryption does not protect against this attack, and
- non-null authentication is REQUIRED in order to defeat it.
-
- If existential message forgery is an issue, i.e., when the accuracy
- of the received data is of non-negligible importance, null
- authentication MUST NOT be used.
-
-9.5.1. Risks of Weak or Null Message Authentication
-
- During a security audit considering the use of weak or null
- authentication, it is important to keep in mind the following attacks
- which are possible when no message authentication algorithm is used.
-
- An attacker who cannot predict the plaintext is still always able to
- modify the message sent between the sender and the receiver so that
- it decrypts to a random plaintext value, or to send a stream of bogus
- packets to the receiver that will decrypt to random plaintext values.
- This attack is essentially a denial of service attack, though in the
- absence of message authentication, the RTP application will have
- inputs that are bit-wise correlated with the true value. Some
- multimedia codecs and common operating systems will crash when such
- data are accepted as valid video data. This denial of service attack
- may be a much larger threat than that due to an attacker dropping,
- delaying, or re-ordering packets.
-
- An attacker who cannot predict the plaintext can still replay a
- previous message with certainty that the receiver will accept it.
- Applications with stateless codecs might be robust against this type
- of attack, but for other, more complex applications these attacks may
- be far more grave.
-
- An attacker who can predict the plaintext can modify the ciphertext
- so that it will decrypt to any value of her choosing. With an
- additive stream cipher, an attacker will always be able to change
- individual bits.
-
- An attacker may be able to subvert confidentiality due to the lack of
- authentication when a data forwarding or access control decision is
- made on decrypted but unauthenticated plaintext. This is because the
- receiver may be fooled into forwarding data to an attacker, leading
- to an indirect breach of confidentiality (see Section 3 of [B96]).
- This is because data-forwarding decisions are made on the decrypted
- plaintext; information in the plaintext will determine to what subnet
- (or process) the plaintext is forwarded in ESP [RFC2401] tunnel mode
- (respectively, transport mode). When Secure RTP is used without
-
-
-
-
-
-Baugher, et al. Standards Track [Page 42]
-
-RFC 3711 SRTP March 2004
-
-
- message authentication, it should be verified that the application
- does not make data forwarding or access control decisions based on
- the decrypted plaintext.
-
- Some cipher modes of operation that require padding, e.g., standard
- cipher block chaining (CBC) are very sensitive to attacks on
- confidentiality if certain padding types are used in the absence of
- integrity. The attack [V02] shows that this is indeed the case for
- the standard RTP padding as discussed in reference to Figure 1, when
- used together with CBC mode. Later transform additions to SRTP MUST
- therefore carefully consider the risk of using this padding without
- proper integrity protection.
-
-9.5.2. Implicit Header Authentication
-
- The IV formation of the f8-mode gives implicit authentication (IHA)
- of the RTP header, even when message authentication is not used.
- When IHA is used, an attacker that modifies the value of the RTP
- header will cause the decryption process at the receiver to produce
- random plaintext values. While this protection is not equivalent to
- message authentication, it may be useful for some applications.
-
-10. Interaction with Forward Error Correction mechanisms
-
- The default processing when using Forward Error Correction (e.g., RFC
- 2733) processing with SRTP SHALL be to perform FEC processing prior
- to SRTP processing on the sender side and to perform SRTP processing
- prior to FEC processing on the receiver side. Any change to this
- ordering (reversing it, or, placing FEC between SRTP encryption and
- SRTP authentication) SHALL be signaled out of band.
-
-11. Scenarios
-
- SRTP can be used as security protocol for the RTP/RTCP traffic in
- many different scenarios. SRTP has a number of configuration
- options, in particular regarding key usage, and can have impact on
- the total performance of the application according to the way it is
- used. Hence, the use of SRTP is dependent on the kind of scenario
- and application it is used with. In the following, we briefly
- illustrate some use cases for SRTP, and give some guidelines for
- recommended setting of its options.
-
-11.1. Unicast
-
- A typical example would be a voice call or video-on-demand
- application.
-
-
-
-
-
-Baugher, et al. Standards Track [Page 43]
-
-RFC 3711 SRTP March 2004
-
-
- Consider one bi-directional RTP stream, as one RTP session. It is
- possible for the two parties to share the same master key in the two
- directions according to the principles of Section 9.1. The first
- round of the key derivation splits the master key into any or all of
- the following session keys (according to the provided security
- functions):
-
- SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.
-
- (For simplicity, we omit discussion of the salts, which are also
- derived.) In this scenario, it will in most cases suffice to have a
- single master key with the default lifetime. This guarantees
- sufficiently long lifetime of the keys and a minimum set of keys in
- place for most practical purposes. Also, in this case RTCP
- protection can be applied smoothly. Under these assumptions, use of
- the MKI can be omitted. As the key-derivation in combination with
- large difference in the packet rate in the respective directions may
- require simultaneous storage of several session keys, if storage is
- an issue, we recommended to use low-rate key derivation.
-
- The same considerations can be extended to the unicast scenario with
- multiple RTP sessions, where each session would have a distinct
- master key.
-
-11.2. Multicast (one sender)
-
- Just as with (unprotected) RTP, a scalability issue arises in big
- groups due to the possibly very large amount of SRTCP Receiver
- Reports that the sender might need to process. In SRTP, the sender
- may have to keep state (the cryptographic context) for each receiver,
- or more precisely, for the SRTCP used to protect Receiver Reports.
- The overhead increases proportionally to the size of the group. In
- particular, re-keying requires special concern, see below.
-
- Consider first a small group of receivers. There are a few possible
- setups with the distribution of master keys among the receivers.
- Given a single RTP session, one possibility is that the receivers
- share the same master key as per Section 9.1 to secure all their
- respective RTCP traffic. This shared master key could then be the
- same one used by the sender to protect its outbound SRTP traffic.
- Alternatively, it could be a master key shared only among the
- receivers and used solely for their SRTCP traffic. Both alternatives
- require the receivers to trust each other.
-
- Considering SRTCP and key storage, it is recommended to use low-rate
- (or zero) key_derivation (except the mandatory initial one), so that
- the sender does not need to store too many session keys (each SRTCP
- stream might otherwise have a different session key at a given point
-
-
-
-Baugher, et al. Standards Track [Page 44]
-
-RFC 3711 SRTP March 2004
-
-
- in time, as the SRTCP sources send at different times). Thus, in
- case key derivation is wanted for SRTP, the cryptographic context for
- SRTP can be kept separate from the SRTCP crypto context, so that it
- is possible to have a key_derivation_rate of 0 for SRTCP and a non-
- zero value for SRTP.
-
- Use of the MKI for re-keying is RECOMMENDED for most applications
- (see Section 8.1).
-
- If there are more than one SRTP/SRTCP stream (within the same RTP
- session) that share the master key, the upper limit of 2^48 SRTP
- packets / 2^31 SRTCP packets means that, before one of the streams
- reaches its maximum number of packets, re-keying MUST be triggered on
- ALL streams sharing the master key. (From strict security point of
- view, only the stream reaching the maximum would need to be re-keyed,
- but then the streams would no longer be sharing master key, which is
- the intention.) A local policy at the sender side should force
- rekeying in a way that the maximum packet limit is not reached on any
- of the streams. Use of the MKI for re-keying is RECOMMENDED.
-
- In large multicast with one sender, the same considerations as for
- the small group multicast hold. The biggest issue in this scenario
- is the additional load placed at the sender side, due to the state
- (cryptographic contexts) that has to be maintained for each receiver,
- sending back RTCP Receiver Reports. At minimum, a replay window
- might need to be maintained for each RTCP source.
-
-11.3. Re-keying and access control
-
- Re-keying may occur due to access control (e.g., when a member is
- removed during a multicast RTP session), or for pure cryptographic
- reasons (e.g., the key is at the end of its lifetime). When using
- SRTP default transforms, the master key MUST be replaced before any
- of the index spaces are exhausted for any of the streams protected by
- one and the same master key.
-
- How key management re-keys SRTP implementations is out of scope, but
- it is clear that there are straightforward ways to manage keys for a
- multicast group. In one-sender multicast, for example, it is
- typically the responsibility of the sender to determine when a new
- key is needed. The sender is the one entity that can keep track of
- when the maximum number of packets has been sent, as receivers may
- join and leave the session at any time, there may be packet loss and
- delay etc. In scenarios other than one-sender multicast, other
- methods can be used. Here, one must take into consideration that key
- exchange can be a costly operation, taking several seconds for a
- single exchange. Hence, some time before the master key is
- exhausted/expires, out-of-band key management is initiated, resulting
-
-
-
-Baugher, et al. Standards Track [Page 45]
-
-RFC 3711 SRTP March 2004
-
-
- in a new master key that is shared with the receiver(s). In any
- event, to maintain synchronization when switching to the new key,
- group policy might choose between using the MKI and the <From, To>,
- as described in Section 8.1.
-
- For access control purposes, the <From, To> periods are set at the
- desired granularity, dependent on the packet rate. High rate re-
- keying can be problematic for SRTCP in some large-group scenarios.
- As mentioned, there are potential problems in using the SRTP index,
- rather than the SRTCP index, for determining the master key. In
- particular, for short periods during switching of master keys, it may
- be the case that SRTCP packets are not under the current master key
- of the correspondent SRTP. Therefore, using the MKI for re-keying in
- such scenarios will produce better results.
-
-11.4. Summary of basic scenarios
-
- The description of these scenarios highlights some recommendations on
- the use of SRTP, mainly related to re-keying and large scale
- multicast:
-
- - Do not use fast re-keying with the <From, To> feature. It may, in
- particular, give problems in retrieving the correct SRTCP key, if
- an SRTCP packet arrives close to the re-keying time. The MKI
- SHOULD be used in this case.
-
- - If multiple SRTP streams in the same RTP session share the same
- master key, also moderate rate re-keying MAY have the same
- problems, and the MKI SHOULD be used.
-
- - Though offering increased security, a non-zero key_derivation_rate
- is NOT RECOMMENDED when trying to minimize the number of keys in
- use with multiple streams.
-
-12. IANA Considerations
-
- The RTP specification establishes a registry of profile names for use
- by higher-level control protocols, such as the Session Description
- Protocol (SDP), to refer to transport methods. This profile
- registers the name "RTP/SAVP".
-
- SRTP uses cryptographic transforms which a key management protocol
- signals. It is the task of each particular key management protocol
- to register the cryptographic transforms or suites of transforms with
- IANA. The key management protocol conveys these protocol numbers,
- not SRTP, and each key management protocol chooses the numbering
- scheme and syntax that it requires.
-
-
-
-
-Baugher, et al. Standards Track [Page 46]
-
-RFC 3711 SRTP March 2004
-
-
- Specification of a key management protocol for SRTP is out of scope
- here. Section 8.2, however, provides guidance on the parameters that
- need to be defined for the default and mandatory transforms.
-
-13. Acknowledgements
-
- David Oran (Cisco) and Rolf Blom (Ericsson) are co-authors of this
- document but their valuable contributions are acknowledged here to
- keep the length of the author list down.
-
- The authors would in addition like to thank Magnus Westerlund, Brian
- Weis, Ghyslain Pelletier, Morgan Lindqvist, Robert Fairlie-
- Cuninghame, Adrian Perrig, the AVT WG and in particular the chairmen
- Colin Perkins and Stephen Casner, the Transport and Security Area
- Directors, and Eric Rescorla for their reviews and support.
-
-14. References
-
-14.1. Normative References
-
- [AES] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197,
- http://www.nist.gov/aes/
-
- [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
- Hashing for Message Authentication", RFC 2104, February
- 1997.
-
- [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
- Requirement Levels", BCP 14, RFC 2119, March 1997.
-
- [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for
- Internet Protocol", RFC 2401, November 1998.
-
- [RFC2828] Shirey, R., "Internet Security Glossary", FYI 36, RFC 2828,
- May 2000.
-
- [RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
- "RTP: A Transport Protocol for Real-time Applications", RFC
- 3550, July 2003.
-
- [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
- Video Conferences with Minimal Control", RFC 3551, July
- 2003.
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 47]
-
-RFC 3711 SRTP March 2004
-
-
-14.2. Informative References
-
- [AES-CTR] Lipmaa, H., Rogaway, P. and D. Wagner, "CTR-Mode
- Encryption", NIST, http://csrc.nist.gov/encryption/modes/
- workshop1/papers/lipmaa-ctr.pdf
-
- [B96] Bellovin, S., "Problem Areas for the IP Security
- Protocols," in Proceedings of the Sixth Usenix Unix
- Security Symposium, pp. 1-16, San Jose, CA, July 1996
- (http://www.research.att.com/~smb/papers/index.html).
-
- [BDJR] Bellare, M., Desai, A., Jokipii, E. and P. Rogaway, "A
- Concrete Treatment of Symmetric Encryption: Analysis of DES
- Modes of Operation", Proceedings 38th IEEE FOCS, pp. 394-
- 403, 1997.
-
- [BS00] Biryukov, A. and A. Shamir, "Cryptanalytic Time/Memory/Data
- Tradeoffs for Stream Ciphers", Proceedings, ASIACRYPT 2000,
- LNCS 1976, pp. 1-13, Springer Verlag.
-
- [C99] Crowell, W. P., "Introduction to the VENONA Project",
- http://www.nsa.gov:8080/docs/venona/index.html.
-
- [CTR] Dworkin, M., NIST Special Publication 800-38A,
- "Recommendation for Block Cipher Modes of Operation:
- Methods and Techniques", 2001.
- http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-
- 38a.pdf.
-
- [f8-a] 3GPP TS 35.201 V4.1.0 (2001-12) Technical Specification 3rd
- Generation Partnership Project; Technical Specification
- Group Services and System Aspects; 3G Security;
- Specification of the 3GPP Confidentiality and Integrity
- Algorithms; Document 1: f8 and f9 Specification (Release
- 4).
-
- [f8-b] 3GPP TR 33.908 V4.0.0 (2001-09) Technical Report 3rd
- Generation Partnership Project; Technical Specification
- Group Services and System Aspects; 3G Security; General
- Report on the Design, Specification and Evaluation of 3GPP
- Standard Confidentiality and Integrity Algorithms (Release
- 4).
-
- [GDOI] Baugher, M., Weis, B., Hardjono, T. and H. Harney, "The
- Group Domain of Interpretation, RFC 3547, July 2003.
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 48]
-
-RFC 3711 SRTP March 2004
-
-
- [HAC] Menezes, A., Van Oorschot, P. and S. Vanstone, "Handbook
- of Applied Cryptography", CRC Press, 1997, ISBN 0-8493-
- 8523-7.
-
- [H80] Hellman, M. E., "A cryptanalytic time-memory trade-off",
- IEEE Transactions on Information Theory, July 1980, pp.
- 401-406.
-
- [IK] T. Iwata and T. Kohno: "New Security Proofs for the 3GPP
- Confidentiality and Integrity Algorithms", Proceedings of
- FSE 2004.
-
- [KINK] Thomas, M. and J. Vilhuber, "Kerberized Internet
- Negotiation of Keys (KINK)", Work in Progress.
-
- [KEYMGT] Arrko, J., et al., "Key Management Extensions for Session
- Description Protocol (SDP) and Real Time Streaming Protocol
- (RTSP)", Work in Progress.
-
- [KSYH] Kang, J-S., Shin, S-U., Hong, D. and O. Yi, "Provable
- Security of KASUMI and 3GPP Encryption Mode f8",
- Proceedings Asiacrypt 2001, Springer Verlag LNCS 2248, pp.
- 255-271, 2001.
-
- [MIKEY] Arrko, J., et. al., "MIKEY: Multimedia Internet KEYing",
- Work in Progress.
-
- [MF00] McGrew, D. and S. Fluhrer, "Attacks on Encryption of
- Redundant Plaintext and Implications on Internet Security",
- the Proceedings of the Seventh Annual Workshop on Selected
- Areas in Cryptography (SAC 2000), Springer-Verlag.
-
- [PCST1] Perrig, A., Canetti, R., Tygar, D. and D. Song, "Efficient
- and Secure Source Authentication for Multicast", in Proc.
- of Network and Distributed System Security Symposium NDSS
- 2001, pp. 35-46, 2001.
-
- [PCST2] Perrig, A., Canetti, R., Tygar, D. and D. Song, "Efficient
- Authentication and Signing of Multicast Streams over Lossy
- Channels", in Proc. of IEEE Security and Privacy Symposium
- S&P2000, pp. 56-73, 2000.
-
- [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
- Recommendations for Security", RFC 1750, December 1994.
-
- [RFC2675] Borman, D., Deering, S. and R. Hinden, "IPv6 Jumbograms",
- RFC 2675, August 1999.
-
-
-
-
-Baugher, et al. Standards Track [Page 49]
-
-RFC 3711 SRTP March 2004
-
-
- [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukuhsima, H.,
- Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
- Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke,
- T., Yoshimura, T. and H. Zheng, "RObust Header Compression:
- Framework and Four Profiles: RTP, UDP, ESP, and
- uncompressed (ROHC)", RFC 3095, July 2001.
-
- [RFC3242] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
- (ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP ", RFC
- 3242, April 2002.
-
- [SDMS] Andreasen, F., Baugher, M. and D. Wing, "Session
- Description Protocol Security Descriptions for Media
- Streams", Work in Progress.
-
- [SWO] Svanbro, K., Wiorek, J. and B. Olin, "Voice-over-IP-over-
- wireless", Proc. PIMRC 2000, London, Sept. 2000.
-
- [V02] Vaudenay, S., "Security Flaws Induced by CBC Padding -
- Application to SSL, IPsec, WTLS...", Advances in
- Cryptology, EUROCRYPT'02, LNCS 2332, pp. 534-545.
-
- [WC81] Wegman, M. N., and J.L. Carter, "New Hash Functions and
- Their Use in Authentication and Set Equality", JCSS 22,
- 265-279, 1981.
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 50]
-
-RFC 3711 SRTP March 2004
-
-
-Appendix A: Pseudocode for Index Determination
-
- The following is an example of pseudo-code for the algorithm to
- determine the index i of an SRTP packet with sequence number SEQ. In
- the following, signed arithmetic is assumed.
-
- if (s_l < 32,768)
- if (SEQ - s_l > 32,768)
- set v to (ROC-1) mod 2^32
- else
- set v to ROC
- endif
- else
- if (s_l - 32,768 > SEQ)
- set v to (ROC+1) mod 2^32
- else
- set v to ROC
- endif
- endif
- return SEQ + v*65,536
-
-Appendix B: Test Vectors
-
- All values are in hexadecimal.
-
-B.1. AES-f8 Test Vectors
-
- SRTP PREFIX LENGTH : 0
-
- RTP packet header : 806e5cba50681de55c621599
-
- RTP packet payload : 70736575646f72616e646f6d6e657373
- 20697320746865206e65787420626573
- 74207468696e67
-
- ROC : d462564a
- key : 234829008467be186c3de14aae72d62c
- salt key : 32f2870d
- key-mask (m) : 32f2870d555555555555555555555555
- key XOR key-mask : 11baae0dd132eb4d3968b41ffb278379
-
- IV : 006e5cba50681de55c621599d462564a
- IV' : 595b699bbd3bc0df26062093c1ad8f73
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 51]
-
-RFC 3711 SRTP March 2004
-
-
- j = 0
- IV' xor j : 595b699bbd3bc0df26062093c1ad8f73
- S(-1) : 00000000000000000000000000000000
- IV' xor S(-1) xor j : 595b699bbd3bc0df26062093c1ad8f73
- S(0) : 71ef82d70a172660240709c7fbb19d8e
- plaintext : 70736575646f72616e646f6d6e657373
- ciphertext : 019ce7a26e7854014a6366aa95d4eefd
-
- j = 1
- IV' xor j : 595b699bbd3bc0df26062093c1ad8f72
- S(0) : 71ef82d70a172660240709c7fbb19d8e
- IV' xor S(0) xor j : 28b4eb4cb72ce6bf020129543a1c12fc
- S(1) : 3abd640a60919fd43bd289a09649b5fc
- plaintext : 20697320746865206e65787420626573
- ciphertext : 1ad4172a14f9faf455b7f1d4b62bd08f
-
- j = 2
- IV' xor j : 595b699bbd3bc0df26062093c1ad8f71
- S(1) : 3abd640a60919fd43bd289a09649b5fc
- IV' xor S(1) xor j : 63e60d91ddaa5f0b1dd4a93357e43a8d
- S(2) : 220c7a8715266565b09ecc8a2a62b11b
- plaintext : 74207468696e67
- ciphertext : 562c0eef7c4802
-
-B.2. AES-CM Test Vectors
-
- Keystream segment length: 1044512 octets (65282 AES blocks)
- Session Key: 2B7E151628AED2A6ABF7158809CF4F3C
- Rollover Counter: 00000000
- Sequence Number: 0000
- SSRC: 00000000
- Session Salt: F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 (already shifted)
- Offset: F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000
-
- Counter Keystream
-
- F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000 E03EAD0935C95E80E166B16DD92B4EB4
- F0F1F2F3F4F5F6F7F8F9FAFBFCFD0001 D23513162B02D0F72A43A2FE4A5F97AB
- F0F1F2F3F4F5F6F7F8F9FAFBFCFD0002 41E95B3BB0A2E8DD477901E4FCA894C0
- ... ...
- F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF EC8CDF7398607CB0F2D21675EA9EA1E4
- F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF00 362B7C3C6773516318A077D7FC5073AE
- F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF01 6A2CC3787889374FBEB4C81B17BA6C44
-
- Nota Bene: this test case is contrived so that the latter part of the
- keystream segment coincides with the test case in Section F.5.1 of
- [CTR].
-
-
-
-
-Baugher, et al. Standards Track [Page 52]
-
-RFC 3711 SRTP March 2004
-
-
-B.3. Key Derivation Test Vectors
-
- This section provides test data for the default key derivation
- function, which uses AES-128 in Counter Mode. In the following, we
- walk through the initial key derivation for the AES-128 Counter Mode
- cipher, which requires a 16 octet session encryption key and a 14
- octet session salt, and an authentication function which requires a
- 94-octet session authentication key. These values are called the
- cipher key, the cipher salt, and the auth key in the following.
- Since this is the initial key derivation and the key derivation rate
- is equal to zero, the value of (index DIV key_derivation_rate) is
- zero (actually, a six-octet string of zeros). In the following, we
- shorten key_derivation_rate to kdr.
-
- The inputs to the key derivation function are the 16 octet master key
- and the 14 octet master salt:
-
- master key: E1F97A0D3E018BE0D64FA32C06DE4139
- master salt: 0EC675AD498AFEEBB6960B3AABE6
-
- We first show how the cipher key is generated. The input block for
- AES-CM is generated by exclusive-oring the master salt with the
- concatenation of the encryption key label 0x00 with (index DIV kdr),
- then padding on the right with two null octets (which implements the
- multiply-by-2^16 operation, see Section 4.3.3). The resulting value
- is then AES-CM- encrypted using the master key to get the cipher key.
-
- index DIV kdr: 000000000000
- label: 00
- master salt: 0EC675AD498AFEEBB6960B3AABE6
- -----------------------------------------------
- xor: 0EC675AD498AFEEBB6960B3AABE6 (x, PRF input)
-
- x*2^16: 0EC675AD498AFEEBB6960B3AABE60000 (AES-CM input)
-
- cipher key: C61E7A93744F39EE10734AFE3FF7A087 (AES-CM output)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 53]
-
-RFC 3711 SRTP March 2004
-
-
- Next, we show how the cipher salt is generated. The input block for
- AES-CM is generated by exclusive-oring the master salt with the
- concatenation of the encryption salt label. That value is padded and
- encrypted as above.
-
- index DIV kdr: 000000000000
- label: 02
- master salt: 0EC675AD498AFEEBB6960B3AABE6
-
- ----------------------------------------------
- xor: 0EC675AD498AFEE9B6960B3AABE6 (x, PRF input)
-
- x*2^16: 0EC675AD498AFEE9B6960B3AABE60000 (AES-CM input)
-
- 30CBBC08863D8C85D49DB34A9AE17AC6 (AES-CM ouptut)
-
- cipher salt: 30CBBC08863D8C85D49DB34A9AE1
-
- We now show how the auth key is generated. The input block for AES-
- CM is generated as above, but using the authentication key label.
-
- index DIV kdr: 000000000000
- label: 01
- master salt: 0EC675AD498AFEEBB6960B3AABE6
- -----------------------------------------------
- xor: 0EC675AD498AFEEAB6960B3AABE6 (x, PRF input)
-
- x*2^16: 0EC675AD498AFEEAB6960B3AABE60000 (AES-CM input)
-
- Below, the auth key is shown on the left, while the corresponding AES
- input blocks are shown on the right.
-
- auth key AES input blocks
- CEBE321F6FF7716B6FD4AB49AF256A15 0EC675AD498AFEEAB6960B3AABE60000
- 6D38BAA48F0A0ACF3C34E2359E6CDBCE 0EC675AD498AFEEAB6960B3AABE60001
- E049646C43D9327AD175578EF7227098 0EC675AD498AFEEAB6960B3AABE60002
- 6371C10C9A369AC2F94A8C5FBCDDDC25 0EC675AD498AFEEAB6960B3AABE60003
- 6D6E919A48B610EF17C2041E47403576 0EC675AD498AFEEAB6960B3AABE60004
- 6B68642C59BBFC2F34DB60DBDFB2 0EC675AD498AFEEAB6960B3AABE60005
-
-
-
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 54]
-
-RFC 3711 SRTP March 2004
-
-
-Authors' Addresses
-
- Questions and comments should be directed to the authors and
- avt@ietf.org:
-
- Mark Baugher
- Cisco Systems, Inc.
- 5510 SW Orchid Street
- Portland, OR 97219 USA
-
- Phone: +1 408-853-4418
- EMail: mbaugher@cisco.com
-
-
- Elisabetta Carrara
- Ericsson Research
- SE-16480 Stockholm
- Sweden
-
- Phone: +46 8 50877040
- EMail: elisabetta.carrara@ericsson.com
-
-
- David A. McGrew
- Cisco Systems, Inc.
- San Jose, CA 95134-1706
- USA
-
- Phone: +1 301-349-5815
- EMail: mcgrew@cisco.com
-
-
- Mats Naslund
- Ericsson Research
- SE-16480 Stockholm
- Sweden
-
- Phone: +46 8 58533739
- EMail: mats.naslund@ericsson.com
-
-
- Karl Norrman
- Ericsson Research
- SE-16480 Stockholm
- Sweden
-
- Phone: +46 8 4044502
- EMail: karl.norrman@ericsson.com
-
-
-
-Baugher, et al. Standards Track [Page 55]
-
-RFC 3711 SRTP March 2004
-
-
-Full Copyright Statement
-
- Copyright (C) The Internet Society (2004). This document is subject
- to the rights, licenses and restrictions contained in BCP 78 and
- except as set forth therein, the authors retain all their rights.
-
- This document and the information contained herein are provided on an
- "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
- OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
- ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
- INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
- INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
- WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
-
-Intellectual Property
-
- The IETF takes no position regarding the validity or scope of any
- Intellectual Property Rights or other rights that might be claimed to
- pertain to the implementation or use of the technology described in
- this document or the extent to which any license under such rights
- might or might not be available; nor does it represent that it has
- made any independent effort to identify any such rights. Information
- on the procedures with respect to rights in RFC documents can be
- found in BCP 78 and BCP 79.
-
- Copies of IPR disclosures made to the IETF Secretariat and any
- assurances of licenses to be made available, or the result of an
- attempt made to obtain a general license or permission for the use of
- such proprietary rights by implementers or users of this
- specification can be obtained from the IETF on-line IPR repository at
- http://www.ietf.org/ipr.
-
- The IETF invites any interested party to bring to its attention any
- copyrights, patents or patent applications, or other proprietary
- rights that may cover technology that may be required to implement
- this standard. Please address the information to the IETF at ietf-
- ipr@ietf.org.
-
-Acknowledgement
-
- Funding for the RFC Editor function is currently provided by the
- Internet Society.
-
-
-
-
-
-
-
-
-
-Baugher, et al. Standards Track [Page 56]
-