path: root/papers/draft-gsenger-pointner-secure-anycast-tunneling-protocol-01.txt

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

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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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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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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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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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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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                                 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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   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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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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