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+Network Working Group O. Gsenger
+Internet-Draft C. Pointner
+Expires: October 3, 2009 April 2009
+
+
+ secure anycast tunneling protocol (SATP)
+ draft-gsenger-pointner-secure-anycast-tunneling-protocol-01
+
+Status of this Memo
+
+ By submitting this Internet-Draft, each author represents that any
+ applicable patent or other IPR claims of which he or she is aware
+ have been or will be disclosed, and any of which he or she becomes
+ aware will be disclosed, in accordance with Section 6 of BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF), its areas, and its working groups. Note that
+ other groups may also distribute working documents as Internet-
+ Drafts.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ The list of current Internet-Drafts can be accessed at
+ http://www.ietf.org/ietf/1id-abstracts.txt.
+
+ The list of Internet-Draft Shadow Directories can be accessed at
+ http://www.ietf.org/shadow.html.
+
+ This Internet-Draft will expire on October 3, 2009.
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+Gsenger & Pointner Expires October 3, 2009 [Page 1]
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+Internet-Draft secure anycast tunneling protocol (SATP) April 2009
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+Abstract
+
+ The secure anycast tunneling protocol (SATP) defines a protocol used
+ for communication between any combination of unicast and anycast
+ tunnel endpoints. It allows tunneling of every ETHER TYPE protocol
+ (ethernet, ip ...). SATP directly includes cryptography and message
+ authentication based on the methods used by the Secure Real-time
+ Transport Protocol(SRTP) [RFC3711]. It can be used as an encrypted
+ alternative to IP Encapsulation within IP [RFC2003] and Generic
+ Routing Encapsulation (GRE) [RFC2784]. Both anycast receivers and
+ senders are supported.
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+Gsenger & Pointner Expires October 3, 2009 [Page 2]
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+Internet-Draft secure anycast tunneling protocol (SATP) April 2009
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+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
+ 1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 4
+ 2. Motivation and usage scenarios . . . . . . . . . . . . . . . . 5
+ 2.1. Usage scenarions . . . . . . . . . . . . . . . . . . . . . 5
+ 2.1.1. Tunneling from unicast hosts over anycast routers
+ to other unicast hosts . . . . . . . . . . . . . . . . 5
+ 2.1.2. Tunneling from unicast hosts to anycast networks . . . 6
+ 2.1.3. Redundant tunnel connection of 2 networks . . . . . . 6
+ 2.2. Encapsulation . . . . . . . . . . . . . . . . . . . . . . 7
+ 3. Using SATP on top of IP . . . . . . . . . . . . . . . . . . . 9
+ 3.1. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 9
+ 3.2. ICMP messages . . . . . . . . . . . . . . . . . . . . . . 9
+ 4. Protocol specification . . . . . . . . . . . . . . . . . . . . 10
+ 4.1. Header format . . . . . . . . . . . . . . . . . . . . . . 10
+ 4.2. sequence number . . . . . . . . . . . . . . . . . . . . . 10
+ 4.3. sender ID . . . . . . . . . . . . . . . . . . . . . . . . 10
+ 4.4. MUX . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
+ 4.5. payload type . . . . . . . . . . . . . . . . . . . . . . . 10
+ 4.6. payload . . . . . . . . . . . . . . . . . . . . . . . . . 11
+ 4.7. padding (OPTIONAL) . . . . . . . . . . . . . . . . . . . . 11
+ 4.8. padding count (OPTIONAL) . . . . . . . . . . . . . . . . . 11
+ 4.9. authentication tag (RECOMMENDED) . . . . . . . . . . . . . 11
+ 5. Cryptography . . . . . . . . . . . . . . . . . . . . . . . . . 12
+ 5.1. Basic Concepts . . . . . . . . . . . . . . . . . . . . . . 12
+ 5.1.1. Cryptographic Contexts . . . . . . . . . . . . . . . . 12
+ 5.1.2. SATP Packet Processing . . . . . . . . . . . . . . . . 13
+ 5.1.3. Key derivation . . . . . . . . . . . . . . . . . . . . 15
+ 5.2. Predefined Transforms . . . . . . . . . . . . . . . . . . 16
+ 5.2.1. Encryption . . . . . . . . . . . . . . . . . . . . . . 16
+ 5.2.2. Authentication and Integrity . . . . . . . . . . . . . 18
+ 5.2.3. Key Derivation Pseudo Random Functions . . . . . . . . 18
+ 5.3. Adding SATP Transforms . . . . . . . . . . . . . . . . . . 19
+ 6. Key Managment and Anycast Synchronization Considerations . . . 20
+ 7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
+ 7.1. Replay protection . . . . . . . . . . . . . . . . . . . . 21
+ 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
+ 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
+ 9.1. Normative References . . . . . . . . . . . . . . . . . . . 23
+ 9.2. Informational References . . . . . . . . . . . . . . . . . 23
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
+ Intellectual Property and Copyright Statements . . . . . . . . . . 26
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+Internet-Draft secure anycast tunneling protocol (SATP) April 2009
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+1. Introduction
+
+ SATP is a mixture of a generic encapsulation protocol like GRE
+ [RFC2784] and a secure tunneling protocol as IPsec [RFC2401] in
+ tunnel mode. It can be used to build redundant virtual private
+ network (VPN) connections. It supports peer-to-peer tunnels, where
+ tunnel endpoints can be any combination of unicast, multicast or
+ anycast hosts, so it defines a Host Anycast Service [RFC1546].
+ Encryption is done per packet, so the protocol is robust against
+ packet loss and routing changes. To reduce header overhead,
+ encryption techniques similar to SRTP [RFC3711] are being used.
+
+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 [RFC2119].
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+Gsenger & Pointner Expires October 3, 2009 [Page 4]
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+Internet-Draft secure anycast tunneling protocol (SATP) April 2009
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+2. Motivation and usage scenarios
+
+ This section gives an overview of possible usage scenarios. Please
+ note that the protocols used in the figures are only examples and
+ that SATP itself does not care about either transport protocols or
+ encapsulated protocols. Routing is not done by SATP and each
+ implemetation MAY choose it's own way of doing this task (e.g. using
+ functions provided by the operating system). SATP is used only to
+ encapsulate and encrypt data.
+
+2.1. Usage scenarions
+
+2.1.1. Tunneling from unicast hosts over anycast routers to other
+ unicast hosts
+
+ An example of SATP used to tunnel in a unicast client - anycast
+ server model
+
+ --------- router -----------
+ / \
+ unicast ------+---------- router ------------+------ unicast
+ host \ / host
+ --------- router -----------
+
+ unicast | encrypted | anycast | encrypted | unicast
+ tunnel | communication | tunnel | communication | tunnel
+ endpoint | using SATP | endpoint | using SATP | endpoint
+
+ Figure 1
+
+ In this scenario the payload is encapsuleted into a SATP packet by a
+ unicast host and gets transmitted to one of the anycast routers.
+ After transmisson the packet gets decapsulated by the router. This
+ router makes a routing descision based on the underlying protocol and
+ transmits a new SATP package to one or more unicast hosts depending
+ on this decision.
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+Internet-Draft secure anycast tunneling protocol (SATP) April 2009
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+2.1.2. Tunneling from unicast hosts to anycast networks
+
+ An example of SATP used to encrypt data between a unicast host and
+ anycast networks
+
+ -------Router -+---- DNS Server
+ / \
+ / --- 6to4 Router
+ /
+ unicast -------+----------Router --+--- DNS Server
+ host \ \
+ \ --- 6to4 Router
+ \
+ -------Router -+---- DNS Server
+ \
+ --- 6to4 Router
+
+ unicast | encrypted | anycast | plaintext
+ tunnel | communication | tunnel | anycast
+ endpoint | using SATP | endpoint | services
+
+
+ Figure 2
+
+ When the unicast hosts wants to transmit data to one of the anycast
+ DNS servers, it encapsulates the data and sends a SATP packet to the
+ anycast address of the routers. The packet arrives at one of the
+ routers, gets decapsulated and is then forwarded to the DNS server.
+ This method can be used to tunnel between clients and networks
+ providing anycast services. It can also be used the other way to
+ virtually locate a unicast service within anycasted networks.
+
+2.1.3. Redundant tunnel connection of 2 networks
+
+ An example of SATP used to connect 2 networks
+
+ Router ----------- ---------------Router
+ / \ / \
+ Network - Router ------------x Network
+ A \ / \ / B
+ Router ----------- ---------------Router
+
+ | packets | packets | packets |
+ plaintext | get | take a | get | plaintext
+ packets | de/encrypted | random | de/encrypted | packets
+ |de/encapsulated| path |de/encapsulated|
+
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+Gsenger & Pointner Expires October 3, 2009 [Page 6]
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+ Figure 3
+
+ Network A has multiple routers which act as gateway/tunnel endpoints
+ to another network B. This way a redundant encrypted tunnel
+ connection between the two networks is built up. All tunnel
+ endpoints of network A share the same anycast address and all tunnel
+ endpoints of network B share another anycast address. When a packet
+ from network A is transmitted to network B, it first arrives on one
+ of network A's border routers. Which router is used is determined by
+ network A's internal routing. This router encapsulates the package
+ and sends it to the anycast address of network B's routers. After
+ arrival the SATP packet gets decapsulated and routed to its
+ destination within network B.
+
+2.2. Encapsulation
+
+ SATP does not depend on the lower layer protocol. This section only
+ gives an example of how packets could look like.
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+ Examples of SATP used with different lower layer and payload
+ protocols
+
+ +------+-----+-------------------------------+
+ | | | +----------------+-----+ |
+ | IPv6 | UDP | SATP | Ethernet 802.3 | ... | |
+ | | | +----------------+-----+ |
+ +------+-----+-------------------------------+
+
+ Tunneling of Ethernet over UDP/IPv6
+
+ +------+-----+---------------------------+
+ | | | +------+-----+-----+ |
+ | IPv4 | UDP | SATP | IPv6 | UDP | RTP | |
+ | | | +------+-----+-----+ |
+ +------+-----+---------------------------+
+
+ Tunneling of IPv6 over UDP/IPv4 with RTP payload
+
+ +------+-------------------------------+
+ | | +----------------+-----+ |
+ | IPv6 | SATP | Ethernet 802.3 | ... | |
+ | | +----------------+-----+ |
+ +------+-------------------------------+
+
+ Tunneling of Ethernet over IPv6
+
+ +------+---------------------------+
+ | | +------+-----+-----+ |
+ | IPv4 | SATP | IPv6 | UDP | RTP | |
+ | | +------+-----+-----+ |
+ +------+---------------------------+
+
+ Tunneling of IPv6 over IPv4 with RTP payload
+
+ Figure 4
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+3. Using SATP on top of IP
+
+3.1. Fragmentation
+
+ The only way of fully supporting fragmentation would be to
+ synchronise fragments between all anycast servers. This is
+ considered to be too much overhead, so there are two non-perfect
+ solutions for these problems. Either fragmentation HAS TO be
+ disabled or if not all fragments arrive at the same server the IP
+ datagramm HAS TO be discarded. As routing changes are not expected
+ to occur very frequently, the encapsulated protocol can do a
+ retransmission and all fragments will arrive at the new server.
+
+ If the payload type is IP and the IP headers' Don't Fragment (DF) bit
+ is set, then the DF bit of the outer IP header HAS TO be set as well.
+
+3.2. ICMP messages
+
+ ICMP messages MUST be relayed according to rfc2003 section 4
+ [RFC2003]. This is needed for path MTU detection.
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+4. Protocol specification
+
+4.1. Header format
+
+ Protocol Format
+
+ 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
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | sequence number | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
+ | sender ID | MUX | |
+ +#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+ |
+ | | payload type | | |
+ | +-------------------------------+ | |
+ | | .... payload ... | |
+ | | +-------------------------------+ |
+ | | | padding (OPT) | pad count(OPT)| |
+ +#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+#+-+
+ | : authentication tag (RECOMMENDED) : |
+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
+ | |
+ +- Encrypted Portion Authenticated Portion ---+
+
+ Figure 5
+
+4.2. sequence number
+
+ The sequence number is a 32 bit unsigned integer in network byte
+ order. The starting point is signaled by the key exchange mechanism
+ and then value is then increased by 1 for every packet sent. After
+ the maximum value it starts over from 0.
+
+4.3. sender ID
+
+ The sender ID is a 16 bit unsigned integer. It HAS TO be unique for
+ every sender sharing the same anycast address.
+
+4.4. MUX
+
+ The MUX (multiplex) field is a 16 bit unsigned integer. It is used
+ to distinguish multiple tunnel connections.
+
+4.5. payload type
+
+ The payload type field defines the payload protocol. ETHER TYPE
+ protocol numbers are used. See IANA assigned ethernet numbers [1] .
+ The values 0000-05DC are reserverd and MUST NOT be used.
+
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+Gsenger & Pointner Expires October 3, 2009 [Page 10]
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+ Some examples for protocol numbers
+
+ HEX
+ 0000 Reserved
+ .... Reserved
+ 05DC Reserved
+ 0800 Internet IP (IPv4)
+ 6558 transparent ethernet bridging
+ 86DD IPv6
+
+ Figure 6
+
+4.6. payload
+
+ A packet of type payload type (e.g. an IP packet).
+
+4.7. padding (OPTIONAL)
+
+ Padding of max 255 octets. None of the pre-defined encryption
+ transforms uses any padding; for these, the plaintext and encrypted
+ payload sizes match exactly. Transforms which may be added in future
+ (see Section 5.3) MUST define wheter they need padding or not and if
+ they need it they MUST define a proper padding format. If the
+ padding count field is present, the padding count field MUST be set
+ to the padding length.
+
+4.8. padding count (OPTIONAL)
+
+ The number of octets of the padding field. This field is optional.
+ Its presence is signaled by the key management and not by this
+ protocol. If this field isn't present, the padding field MUST NOT be
+ present as well.
+
+4.9. authentication tag (RECOMMENDED)
+
+ The authentication tag is RECOMMENDED and of configurable length. It
+ contains a cryptographic checksum of the sender ID, sequence number
+ and the encrypted portion. On transmitter side encryption HAS TO be
+ done before calculating the authentication tag. A receiver HAS TO
+ calculate the authentication tag before decrypting the encrypted
+ portion.
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+Internet-Draft secure anycast tunneling protocol (SATP) April 2009
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+5. Cryptography
+
+ As mentioned earlier the cryptography of SATP is based on SRTP
+ [RFC3711]. For that reason we recommend to read this document as
+ well (especially chapter 7 Rationale). However some modifications
+ were made in order to fit the changed conditions of SATP. The
+ following section describes the whole cryptography of SATP.
+
+5.1. Basic Concepts
+
+ In order to cope with anycast and packet loss it is important to be
+ able to process one packet on its own without the need for packets
+ from the past as an additional information source. Therefore SATP as
+ well as SRTP [RFC3711] defines a so called cryptographic context.
+ This context consits of all information which is needed to process a
+ single SATP packet and is divided into packet specific parameters and
+ global parameters. The packet specific parameters can be found in
+ the protocol header and global parameters have to be generated by the
+ key exchange mechanism external to SATP (see Section 6). For anycast
+ sender the global parameters have to be synchronized between all
+ hosts which share the same anycast address. The packet specific
+ parameters MUST NOT be synchronized.
+ SATP uses two types of keys: master keys and session keys. A session
+ key is meant to be used for a cryptographic transform (encrytion or
+ message authentication) for one packet. The master keys are used to
+ derive packet-specific session keys in a cryptographical secure way.
+
+5.1.1. Cryptographic Contexts
+
+5.1.1.1. Global Parameters
+
+ As mentioned above global parameters HAVE TO either be provided by
+ the key exchange mechanism or configured manually.
+
+ o a master key(s) which MUST be random and kept secret.
+
+ o a master salt which MUST be random and MAY be public (RECOMMENDED
+ to be kept secret as well).
+
+ o a role specifier used by the key derivation to determine which
+ session keys to generate for outbound or inbound traffic.
+
+ o identifier for the key derivation pseudo random function.
+
+ o identifier for the encryption algorithm (i.e. cipher and its mode
+ of operation).
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+ o if used an identifier for the authentication algorithm.
+
+ o transform specific parameters such as key lengths, see
+ Section 5.2.
+
+ o if used the length of the authentication tag which should be
+ truncated to the packet.
+
+ o an indicator which specifies if padding is needed or not (presence
+ of padding count field).
+
+ o a replay list for each sender (see Section 5.1.1.3), maintained by
+ the receiver which contains the sequence numbers of received and
+ authenticated packets, this lists may be implemented as a sliding
+ window.
+
+ o a [ From , To ] value pair which specifies the lifetime of a
+ master key (including the range endpoints), expressed in terms of
+ a pair of 32-bit sequence numbers.
+
+5.1.1.2. Packet-Specific Parameters
+
+ o the sequence number
+
+ o the sender id
+
+ o the mux value
+
+5.1.1.3. Mapping SATP packets to Cryptographic Contexts
+
+ A cryptographic contexts SHALL be uniquely identifed by the tuple
+ context identifier:
+
+ context id = [ source address , source port ]
+
+ In order to cope with anycast sender and replay protection there HAS
+ TO be more than one replay list per context. Each replay list inside
+ a cryptographic context SHALL be uniquely identified by the sender
+ id.
+
+5.1.2. SATP Packet Processing
+
+ Before any SATP packet can be processed a cryptographic context HAS
+ TO be initialized by the key management mechanism. After that a SATP
+ sender SHALL do the following to create a SATP packet:
+
+ 1. Determine the next sequence number to use.
+
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+ 2. Determine the crypotgraphic context as described in
+ Section 5.1.1.3.
+
+ 3. Determine the master key and master salt for the packets sequence
+ number.
+
+ 4. Compute all session keys and session salts which are needed by
+ the encryption transform using the key derivation pseudo random
+ function.
+
+ 5. Encrypt the payload type field concatenated with the payload to
+ produce the encrypted portion of the packet using the encryption
+ algorithm defined by the cryptographic context.
+
+ 6. Fill in sender id, mux and sequence number fields.
+
+ 7. If needed compute the session authentication key using the key
+ derivation pseudo random function.
+
+ 8. Generate the authentication tag over the authenticated portion
+ using the authentication algorithm defined by the cryptographic
+ context and append it to the packet.
+
+ On receiver side the packet SHALL be processed as follows:
+
+ 1. Determine the crypotgraphic context as described in
+ Section 5.1.1.3.
+
+ 2. Determine the master key and master salt for the packets
+ sequence number.
+
+ 3. Check if the packet was replayed using the replay list for the
+ packets sender id.
+
+ 4. If needed compute the session authentication key using the key
+ derivation pseudo random function.
+
+ 5. Generate the authentication tag over the authenticated portion
+ using the authentication algorithm defined by the crpyptographic
+ context and compare it with the tag appended to the received
+ packet. If it is equal remove the tag and move on. If it is
+ not equal drop the packet.
+
+ 6. Store the sequence number in the replay list.
+
+ 7. Compute all session keys and session salts which are needed by
+ the encryption transform using the key derivation pseudo random
+ function.
+
+
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+ 8. Decrypt the encrypted portion using the encryption algorithm
+ defined by the cryptographic context.
+
+ 9. Check if the payload type is supported by this tunnel endpoint
+ and discard the packet in case it isn't supported.
+
+ 10. Remove all fields beside the payload itself from the packet.
+
+5.1.3. Key derivation
+
+ Any encryption or message authentication transform which is used
+ (predefined or newly introduced according to Section 5.3) MUST obtain
+ its secret values (keys and salts) using the SATP key derivation.
+ After the key exchange mechanism has signaled all needed parameters
+ (i.e. master key and salt) no additional communiction between sender
+ and receiver is needed until the next rekeying takes place. To
+ achieve this the key derivation uses an pseudo random function seeded
+ by the master key, master salt, the packets sequence number and a
+ label (identifier for the key to compute).
+
+ SATP key derivation
+
+ packet sequence nummber ----+
+ |
+ V
+ +------------+ master +------------+
+ | | key | |--> session encryption key
+ | ext. key |------->| key |
+ | management | | |--> session encryption salt
+ | mechanism |------->| derivation |
+ | | master | |--> session authentication key
+ +------------+ salt +------------+
+
+ Figure 7
+
+ SRTP [RFC3711] defines a pseudo random function as follows:
+ 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.
+
+ For SATP key generation a pseudo random function with at least m =
+ 128 MUST be used. A predefined transform can be found in
+ Section 5.2.3. The input x of the PRF SHOULD be calculated as
+ follows:
+
+ 1. Let key_id = label || sequence_number, with label defined as
+ below.
+
+
+
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+ 2. Let x = key_id XOR master_salt, where key_id and master_salt are
+ aligend so that their least significant bits agree (right-
+ alignment).
+
+ For each key derived by the key derivation there MUST exist a unique
+ label, a 32-bit constant. In order to increase security SATP uses
+ different session keys for inbound and outbound traffic. The role
+ specifier from the cryptographic context is used to determine which
+ session keys to use for inbound and outbound packets. The labels can
+ be computed by calculateing the SHA1 hash over an increasing label-
+ index. The label value are the 32 leftmost bits of this hash value.
+ We currently define 6 labels (label-index from 1 to 6) future
+ extensions may use labels with an index from 7 upwards.
+
+ +--------------------+-------+-------------+------------+
+ | key type | role | label-index | label |
+ +--------------------+-------+-------------+------------+
+ | encryption key | left | 1 | 0x356A192B |
+ | | | | |
+ | encryption key | right | 2 | 0xDA4B9237 |
+ | | | | |
+ | encryption salt | left | 3 | 0x77DE68DA |
+ | | | | |
+ | encryption salt | right | 4 | 0x1B645389 |
+ | | | | |
+ | authentication key | left | 5 | 0xAC3478D6 |
+ | | | | |
+ | authentication key | right | 6 | 0xC1DFD96E |
+ +--------------------+-------+-------------+------------+
+
+ Key Derivation Labels
+
+ The role parameter specifies which label should be used for outbound
+ packets. This means a endpoint with role left MUST use the labels
+ marked with left for outgoing packets and expects inbound packets to
+ be encrypted/authenticated using the labels marked with right.
+
+5.2. Predefined Transforms
+
+ While SATP as well as SRTP allows the use of various encryption and
+ message authentication algorithms interoperable implementations MUST
+ support at least the following transforms. To add additional
+ transforms see Section 5.3.
+
+5.2.1. Encryption
+
+
+
+
+
+
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+5.2.1.1. NULL Encryption
+
+ If confidendtiality of the SATP packet is not an issue the null
+ encryption transform can be used to increase performance. This
+ transform just copies the plaintext input into the ciphertext output
+ wihtout any padding. The identifier for that transfrom SHOULD be
+ NULL and it don't needs any transform specific parameters. It also
+ doesn't need any key or salt values computed by the key derivation.
+
+5.2.1.2. AES in Counter Mode
+
+ The following describes how to use AES in counter mode for SATP
+ encryption. The identifier for that transform SHOULD be AES-CTR-
+ <key_length> or just AES-CTR in which case the key length defaults to
+ 128 bits. Beside the key length there are no additional transfrom
+ specific parameters. This transform needs a key of length
+ <key_length> and a 112 bit salt. These values can be generated using
+ the key derivation pseudo random function as follows:
+
+ session_key = PRF_<key_length>(master_key, x)
+ session_salt = PRF_112(master_key, x)
+ with PRF and x defined as in Section 5.1.3.
+
+ Basically AES in counter mode generates a pseudo random keystream
+ seeded by the session key, session salt as well as the sequence
+ number, sender id and mux value of the packet and encrypts a single
+ SATP packet using this stream. The encryption process consits of the
+ generation of that keystream and then bitwise exclusive-oring it onto
+ the packets payload. If the packet length doesn't fit a multiple of
+ 128 bits the remaining bits (least significant) of the keystream are
+ simple ingored. Therefore this transform does not need any padding.
+ Decryption of the packet can be achieved by generating the same
+ keystream and exclusive-oring it onto the encrypted portion.
+
+5.2.1.2.1. Keystream Generation
+
+ In principle AES in counter mode consists of encrypting an
+ incrementing integer. However the starting point of the integer
+ value has to be randomized to get a good pseudo random key stream. A
+ keystream consits of several keystream segements with a size of 128
+ bits (AES blocksize). Each segement can be computed by applying AES
+ with key k on the block CTR. The whole keystream is a concatination
+ of all its successive segements. Therefore a keystream looks as
+ follows:
+
+ AES(session_key, CTR) || AES(session_key, CTR + 1 mod 2^128) ||
+ AES(session_key, CTR + 2 mod 2^128) ...
+
+
+
+
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+ where the 128 bit value CTR is defined as follows:
+
+ CTR = (session_salt * 2^16) XOR (mux * 2^80) XOR (sender_id * 2^64)
+ XOR (sequence_number * 2^16)
+
+ where each of the four terms are padded with as many leading zeros to
+ form a 128 bit value.
+
+ Mind that the 16 least siginificant bits of CTR are zero. These bits
+ are used for the counter. Therefore the number of blocks generated
+ for one packet MUST NOT exceed 2^16 to avoid keystream reuse. This
+ means that the packet length MUST NOT exceed 2^16 * 128 bits = 2^23
+ bits to ensure the security of the encryption.
+
+5.2.2. Authentication and Integrity
+
+ It is RECOMMENDED to use an authentication tag and if it is used it
+ should be processed as follows. The sender generates the tag over
+ the authenticated portion truncates it to the left-most (most
+ significant) bits to fit the authentication tag length signaled by
+ the key exchange mechanism. After that it simple appends the tag to
+ the packet. The receiver computes the tag in the same way as the
+ sender and compares if with the received tag. If they don't match
+ the packet HAS TO be discarded and the incident SHOULD be logged.
+
+5.2.2.1. HMAC-SHA1
+
+ This transform uses HMAC-SHA1 (as described in [RFC2104]) as message
+ authentication algorithm. The identifier for the transfrom SHOULD be
+ SHA1 and it don't needs any transform specific parameters. The key
+ should be derived using the key derivation pseudo random function:
+
+ session_auth_key = PRF_20(master_key, x)
+ with PRF and x defined as in Section 5.1.3
+
+5.2.3. Key Derivation Pseudo Random Functions
+
+5.2.3.1. AES in Counter Mode
+
+ Section 5.1.3 defines a pseudo random function which SHOULD be used
+ to derive session keys and salts. This describes the use of AES in
+ counter mode as PRF. The identifier for this PRF SHOULD be AES-CTR-
+ <key_length> or just AES-CTR in which case the key length defaults to
+ 128 bits. Beside the key length there are no additional transform
+ specific parameters. This transform needs a master key of length
+ key_length and a 112 bit master salt. The pseudo random string
+ consists of several segements with a size of 128 bits (AES
+ blocksize). The whole string can be computed as follows:
+
+
+
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+ AES(master_key, CTR) || AES(master_key, CTR + 1 mod 2^128) ||
+ AES(master_key, CTR + 2 mod 2^128) ...
+
+ where the 128 bit value CTR is defined as x * 2^16, with x defined as
+ in Section 5.1.3.
+
+ This pseudo random function can produce pseudo random strings up to a
+ length of 2^23 bits. If the requested output length n does not fit
+ multiples of 128 bits the output SHOULD be truncated to the n first
+ (left-most) bits. Therefore there are n/128, rounded up,
+ applications of AES needed to produce the output string.
+
+5.3. Adding SATP Transforms
+
+ If a new transform is to be added to SATP a standard track RFC MUST
+ be written to define the usage of the new transform. Any overlap
+ between the new RFC and this document SHOULD be avoided but it MAY be
+ needed to update some of the information in this document. For
+ example new parameters MAY be added to the cryptographic context or
+ there MAY be additional steps in SATP packet processing.
+
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+6. Key Managment and Anycast Synchronization Considerations
+
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+7. Security Considerations
+
+ As the cryptography of SATP is based on SRTP [RFC3711], it basically
+ shares the same security issues. This section will only discuss some
+ small changes. Please read SRTP RFC3711 section 9 [RFC3711] for
+ details.
+
+7.1. Replay protection
+
+ Replay protection is done by a replay list. Every anycast receiver
+ has its own replay list, which SHOULDN'T be syncronised because of
+ massive overhead. This leads to an additional possible attack. An
+ attacker is able to replay a captured packet once to every anycast
+ receiver. This attack is considered be very unlikely because
+ multiple attack hosts in different locations are needed to reach
+ seperate anycast receivers and the number of replays is limited to
+ count of receivers - 1. Such replays might also happen because of
+ routing problems, so a payload protocol HAS TO be robust against a
+ small number of duplicated packages. The window size and position
+ HAS TO be syncronised between multiple anycast receivers to limit
+ this attack.
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+
+8. IANA Considerations
+
+ The protocol is intended to be used on top of IP or on top of UDP (to
+ be compatible with NAT routers), so UDP and IP protocol numbers have
+ to be assiged by IANA.
+
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+9. References
+
+9.1. Normative References
+
+ [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
+ Norrman, "The Secure Real-time Transport Protocol (SRTP)",
+ RFC 3711, March 2004.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
+ October 1996.
+
+ [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
+ Hashing for Message Authentication", RFC 2104,
+ February 1997.
+
+9.2. Informational References
+
+ [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
+ Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
+ March 2000.
+
+ [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
+ Internet Protocol", RFC 2401, November 1998.
+
+ [RFC1546] Partridge, C., Mendez, T., and W. Milliken, "Host
+ Anycasting Service", RFC 1546, November 1993.
+
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+URIs
+
+ [1] <http://www.iana.org/assignments/ethernet-numbers>
+
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+Authors' Addresses
+
+ Othmar Gsenger
+ Puerstingerstr 32
+ Saalfelden 5760
+ AT
+
+ Phone:
+ Email: satp@gsenger.com
+ URI: http://www.gsenger.com/satp/
+
+
+ Christian Pointner
+ Wielandgasse 19
+ Graz 8010
+ AT
+
+ Phone:
+ Email: equinox@anytun.org
+
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+
+Full Copyright Statement
+
+ Copyright (C) The IETF Trust (2009).
+
+ 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, THE IETF TRUST AND
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+ THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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+
+
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