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Versions: 00 01 02 draft-ietf-tcpm-tcp-ao-crypto

TCPM                                                         G. Lebovitz
Internet-Draft                                                   Juniper
Intended status: Standards Track                           July 27, 2009
Expires: January 28, 2010

  Cryptographic Algorithms, Use, & Implementation Requirments for TCP
                         Authentication Option

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   The TCP Authentication Option, TCP-AO, relies on security algorithms
   to provide authentication between two end-points.  There are many
   such algorithms available, and two TCP-AO systems cannot interoperate
   unless they are using the same algorithm(s).  This document specifies
   the algorithms and attributes that can be used in TCP-AO's current
   manual keying mechanism.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
     2.2.  Algorithm Requirements . . . . . . . . . . . . . . . . . .  3
     2.3.  Requirements for Future MAC Algorithms . . . . . . . . . .  4
   3.  Algorithms Specified . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Key Derivation Functions (KDFs)  . . . . . . . . . . . . .  5
       3.1.1.  The Use of KDF_HMAC_SHA1 . . . . . . . . . . . . . . .  7
       3.1.2.  The Use of KDF_AES_128_CMAC  . . . . . . . . . . . . .  8
       3.1.3.  Tips for User Interfaces regarding KDFs  . . . . . . .  9
     3.2.  MAC Algorithms . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.1.  The Use of HMAC-SHA-1-96 . . . . . . . . . . . . . . . 11
       3.2.2.  The Use of AES-128-CMAC-96 . . . . . . . . . . . . . . 11
   4.  Change History (RFC Editor: Delete before publishing)  . . . . 12
   5.  Needs Work in Next Draft (RFC Editor: Delete Before
       Publishing)  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   This document is a companion to TCP-AO [TCP-AO]
   [I-D.ietf-tcpm-tcp-auth-opt].  Like most security protocols, TCP-AO
   allows users to chose which cryptographic algorithm(s) they want to
   use to meet their security needs.

   TCP-AO provides cryptographic authentication and message integrity
   verification between to end-points.  In order to accomplish this
   function, one employs message authentication codes (MACs).  There are
   various ways to create MACs.  The use of hashed-based MACs (HMAC) in
   Internet protocols is defined in [RFC2104].  The use of cipher-based
   MACs (CMAC) in Internet protocols is defined in [RFC4493].

   This RFC discusses the requirements for implementations to support
   two MACs used in TCP-AO, both now and in the future, and includes the
   rationale behind the present and future requirements.  The document
   then specifies the use of those two MACs with TCP-AO.

2.  Requirements

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.2.  Algorithm Requirements

   In this the first RFC specifying cryptography for TCP-AO, we specify
   two MAC algorithms.  Both MUST be implemented in order for the
   implementation to be fully compliant with this RFC.

   This table lists authentication algorithms for the TCP-AO protocol.

           Requirement      Authentication Algorithm
           ------------     ------------------------
           MUST             HMAC-SHA-1-96 [RFC2404]
           MUST             AES-128-CMAC-96 [RFC4493]

           Requirement      Key Derivation Function (KDF)
           -------------    ------------------------

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           MUST             KDF_HMAC_SHA1
           MUST             KDF_AES_128_CMAC


   The security issues driving the migration from SHA-1 to SHA-256 for
   digital signatures [HMAC-ATTACK] [HMAC-ATTACK]do not immediately
   render SHA-1 weak for this application of SHA-1 in HMAC mode.  The
   security strength of SHA-1 HMACs should be sufficient for the
   foreseeable future, especially given that the tags are truncated to
   96 bits.  However, while it's clear that the attacks aren't practical
   on SHA-1, these types of analysis are mounting and could potentially
   pose a concern for HMAC forgery if they were significantly improved,
   over time.  In anticipation of SHA-1's growing less dependable over
   time, but given its wide deployment and current strength, it is a
   "MUST" for TCP-AO today.  AES-128 CMAC is considered to be far
   stronger algorithm, but may not yet have very wide implementation.
   It is also a "MUST" to implement, in order to drive vendors toward
   its use.

2.3.  Requirements for Future MAC Algorithms

   Since this document provides cryptographic agility, it is also
   important to establish requirements for future MAC algorithms.  The
   TCPM WG should restrict any future MAC algorithms for this
   specification to ones that can protect at least 2**48 messages with a
   probability that a collision will occur of less than one in a

   [Reviewers: Are there any other requirements we want/need to place in
   here?  RFC EDITOR: Please delete this text before publishing as RFC]

3.  Algorithms Specified

   TCP-AO refers to this document saying that the MAC mechanism employed
   for a connection is listed in the TAPD entry, and is chosen from a
   list of MACs both named and described in this document.

   TCP-AO requires two classes of cryptographic algorithms:

       (1)  Key Derivation Functions (KDFs) which name a pseudorandom
            function (PRF) and use a Master_Key and some connection-
            specific Input with that PRF to produce Traffic_Keys, the
            keys suitable for authenticating and integrity checking
            individual TCP segments.

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       (2)  Message Authentication Code (MAC) algorithms which take a
            key and a message and produce an authentication tag which
            can be used to verify the integrity of the messages sent
            over the wire.

   In TCP-AO, these algorithms are always used in pairs.  Each MAC
   algorithm MUST specify the KDF to be used with that MAC algorithm.
   However, a KDF MAY be used with more than one MAC algorithm.

3.1.  Key Derivation Functions (KDFs)

   TCP-AO's Traffic_Keys are derived using KDFs.  The KDFs used in TCP-
   AO's current manual keying have the following interface:

       Derived_Key = KDF(Master_Key, Input, Output_Length)


      - KDF:         the specific pseudorandom function that is the
                     basic building block used in constructing the given

      - Master_Key:  The Master_Key as will be stored into the
                     associated TCP-AO TAPD entry.  In TPC-AO's manual
                     key mode, this is a shared key that both peers
                     enter via some user interface into their respective
                     configurations.  The Master_Key is the seed for the
                     KDF.  We assume that, in manual key mode, this is a
                     human readable pre-shared key (PSK), thus we assume
                     that it is of variable length.  Users SHOULD chose
                     random strings for the Master_Key. However, we
                     assume that some users may not.

      - Input:       the input data for the KDF, in conformance with
                     [NIST-SP800-108], is a concatonation of:

           ( i || Label || Context || Output_Length)


         - "||":      Represents a concatonation operation, between two
                      values X || Y.

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         - i:         A counter, a binary string that is an input to
                      each iteration of a PRF in counter mode and
                      (optionally) in feedback mode.  This will depend
                      on the specific size of the Output_Length desired
                      for an given MAC.

         - Label:     A binary string that clearly identifies the
                      purpose of this KDF's derived keying material.
                      For TCP-AO we use the ASCII string "TCP-AO", where
                      the last character is the capital letter "O", not
                      to be confused with a zero.  While this may seem
                      like overkill in this specification since TCP-AO
                      only describes one call to the KDF, it is included
                      in order to comply with FIPS 140 certifications.

         - Context :  A binary string containing information related to
                      the specific connection for this derived keying
                      material.  In TCP-AO, this is the Data_Block, as
                      defined in [I-D.ietf-tcpm-tcp-auth-opt], Section

         - Output_Length:  The length in bits of the key that the KDF
                      will produce.  This length must be the size
                      required for the MAC algorithm that will use the
                      PRF result as a seed.

   NOTE: The cited NIST document on KDFs calls for an input: (i || Label
   || 0x00 || Context || Output_Length).  That document states that the
   "0x00" is an all zero octet and is "an optional data field used to
   indicate a separation of different variable length data fields".  In
   our case, the "Label" is specified and fixed, thus its data field is
   fixed, not variable, so there is no need for the 0x00 separator.
   Thus, we have dropped it.

   When invoked, a KDF runs a certain PRF, using the Master_Key as the
   seed, and Input as the message input and produces a result of
   Output_Length bits.  This result may then be used as a cryptographic
   key for any algorithm which takes an Output_Length length key as its
   seed.  A KDF MAY specify a maximum Output_Length parameter.

   This document defines two KDFs:

       *  KDF_HMAC_SHA1  based on PRF-HMAC-SHA1 [RFC2404]

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       *  KDF_AES_128_CMAC  based on AES-CMAC-PRF-128 [RFC4615]

   Other KDFs may be defined in future revisions of this document, and
   SHOULD follow this same format as described above.  When doing so,

   1.   The underlying PRFs specified in this document have fixed sized
        output lengths, 128 bits in the case of the AES-CMAC, and 160
        bits in the case of HMAC-SHA1.
   2.   It is possible to generate an arbitrary number of output bits
        with some given PRF by operating it in a feedback or counter
        mode.  The KDFs described in [NIST-SP800-108] incorporate this
        feature, hence the counter "i", which creates leading "0".
   3.   Each MAC needs a key of a specific length.
   4.   Not totally uncoincidentally, the KDFs we have chosen to use
        with each MAC happen to generate the right key size for use with
        the MAC, thus avoiding the need for the procedure in (2).
   5.   If one wanted to use these KDFs with a MAC requiring a longer
        key (e.g., HMAC-SHA-256) one would need to use the procedure:
        KDF_X = PRF_X(Master_Key, Input).

3.1.1.  The Use of KDF_HMAC_SHA1


        PRF(Master_Key, Input, Output_Length)

   KDF_HMAC_SHA1 for TCP-AO has the following values:

      - PRF:      HMAC-SHA1 [RFC2404]
      - Master_Key:   As provided in the MKT
      - Input:

         - i:             "0"
         - Label:         "TCP-AO"
         - Context:       Data_Block
         - Output_Length  160
      - Result:   Traffic_Key

   The result is computed by performing HMAC-SHA1(Master_Key, Input) and
   then taking the first (high order) Output_Length, 160 here, bits.
   This result is the TCP-AO Traffic_Key. The Traffic_Key is then used
   as the seed for the MAC function on each segment of the connection.

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3.1.2.  The Use of KDF_AES_128_CMAC


        PRF(Master_Key, Input, Output_Length)

   KDF_AES_128_CMAC for TCP-AO has the following values:

      - PRF:      AES-CMAC-PRF-128 [RFC4615]
      - Master_Key:   As provided in the MKT
      - Input:

         - i:             "0"
         - Label:         "TCP-AO"
         - Context:       Data_Block
         - Output_Length  128
      - Result:   Traffic_Key

   Whereas the KDF_SHA1 used only one round to produce the Traffic_Key,
   the KDF_AES will take two steps.  The reasoning follows:

   The Master_Key in TCP-AO's current manual keying mechanism is a
   shared secret, entered by an administrator.  It is passed via an out-
   of-band mechanism between two devices, and often between two
   organizations.  The shared secret does not have to be 16 octets, and
   the length may vary.  However, AES_128_CMAC requires a key of 16
   octets (128 bits) in length.  We could mandate that implementations
   force administrators to input Master_Keys of exactly 128 bit length,
   and with sufficient randomness, but this places undue burdon on the
   deployers.  This specification RECOMMENDS that deployers use a
   randomly generated 128-bit string as the Master_Key, but acknowledges
   that deployers may not.

   To handle variable length Master_Keys we use a similar mechanism to
   the AES-CMAC-PRF-128 mechanism represented in [RFC4615], Sect 3.  We
   do a two step process.

   First we use AES_128_CMAC with a fixed key as a "randomness
   extractor", while using the shared secret Master_Key, MK, as the
   message input to produce a 128 bit key K.

   Second, we run AES_128_CMAC again, this time using K as the key and
   the normal Input I (as described above) as the message input to
   produce Traffic_Key, TK.

   Therefore this KDF is always a 2 step function, as follows (borrowing
   the format from [RFC4615]):

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   +                        KDF-AES-128-CMAC                           +
   +                                                                   +
   + Input  : MK (Master_Key, the variable-length shared secret)       +
   +        : I (Input, i.e., the input data of the PRF)               +
   +        : MKlen (length of MK in octets)                           +
   +        : len (length of I in octets)                              +
   + Output : TK (Traffic_Key, 128-bit Pseudo-Random Variable)         +
   +                                                                   +
   + Variable: K (128-bit key for AES-CMAC)                            +
   +                                                                   +
   + Step 1.     K := AES-CMAC(0^128, MK, MKlen);                      +
   + Step 2.    TK := AES-CMAC(K, I, len);                             +
   +           return TK;                                              +
   +                                                                   +

            Figure 1: The AES-CMAC-PRF-128 Algorithm for TCP-AO

      o In Step 1, the 128-bit key, K, for AES-CMAC is derived by
      applying the AES-CMAC algorithm using the 128-bit all-zero string
      as the key and MK as the input message.

      o In Step 2, we apply the AES-CMAC algorithm again, this time
      using the output of Step 1, K, as the key.  The input message is
      now I, just as it is described at the beginning of this section.
      The output of this algorithm returns TK, the Traffic_Key, which is
      128 bits suitable for the seed into the AES-CMAC operation over
      the actual TCP segment.

3.1.3.  Tips for User Interfaces regarding KDFs

   This section provides suggested representations for the KDFs in
   implementation user interfaces.  Following these guidelines across
   common implementations will make interoperability easier and simpler
   for deployers.

   UIs SHOULD refer to the choice of KDF_HMAC_SHA1 as simply "SHA1".

   UIs SHOULD refer to the choice of KDF_AES_128_CMAC as simply

   UIs SHOULD use KDF_HMAC_SHA1 as the default selection in TCP-AO
   settings.  KDF_HMAC_SHA1 is preferred at this time solely because it
   has wide support, being present in most implementations in the
   marketplace.  When such a time arrives as KDF_AES_128_CMAC becomes

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   widely deployed, this document should be updated so that it becomes
   the default KDF on implementations.

3.2.  MAC Algorithms

   MACs for TCP-AO have the following interface:

       MAC (Traffic_Key(KDF), Message, Truncation)


      - MAC-algo:    MAC Algorithm used
      - Traffic_Key: Variable; Result of KDF.

         - KDF:      Name of the TCP-AO KDF used

      - Key_Length:  Length in bits required for the Traffic_Key used in
                     this MAC
      - Truncation:  Length in bits to which the final MAC result is
                     truncated before being placed into TCP-AO header

   This document specifies two MAC algorithm options for generating the
   MAC for TCP-AO's option header:

       *  HMAC-SHA-1-96  based on [RFC2404]

       *  AES-128-CMAC-96  based on [RFC4493]

   Both provide a high level of security and efficiency.  The AES-128-
   CMAC-96 is potentially more efficient, particularly in hardware, but
   HMAC-SHA-1-96 is more widely used in Internet protocols and in most
   cases could be supported with little or no additional code in today's
   deployed software and devices.

   An important aspect to note about these algorithms' definitions for
   use in TCP-AO is the fact that the MAC outputs are truncated to 96
   bits.  AES-128-CMAC-96 produces a 128 bit MAC, and HMAC SHA-1
   produces a 160 bit result.  The MAC output are then truncated to 96
   bits to provide a reasonable tradeoff between security and message
   size, for fitting into the TCP-AO header.

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3.2.1.  The Use of HMAC-SHA-1-96

   By definition, HMAC [RFC2104] requires a cryptographic hash function.
   SHA1 will be that has function used for authenticating and providing
   integrity validation on TCP segments with HMAC.


        MAC (Traffic_Key(KDF), Message, Truncation)

   HMAC-SHA-1-96 for TCP-AO has the following values:

      - MAC-algo:    MAC Algorithm used
      - Traffic_Key: Variable; Result of KDF.

         - KDF:      KDF_HMAC_SHA1

      - Key_Length:  160 bits
      - Truncation:  96 bits

3.2.2.  The Use of AES-128-CMAC-96

   In the context of TCP-AO, when we say "AES-128-CMAC-96" we actually
   define a usage of AES-128 as a cipher-based MAC according to


        MAC (Traffic_Key(KDF), Message, Truncation)

   AES-128-CMAC-96 for TCP-AO has the following values:

      - MAC-algo:    AES-128-CMAC-96 [RFC4493]
      - Traffic_Key: Variable; Result of KDF.

         - KDF:      KDF_AES_128_CMAC

      - Key_Length:  128 bits

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      - Truncation:  96 bits

   According to [RFC4493], by default, "the length of the output of AES-
   128-CMAC is 128 bits.  It is possible to truncate the MAC.  The
   result of the truncation is then taken in most significant bits first
   order.  The MAC length must be specified before the communication
   starts, and it must not be changed during the lifetime of the key."
   Therefore, we explicitly specify the employed MAC length for TCP-AO
   to be 96 bits.

4.  Change History (RFC Editor: Delete before publishing)

   [NOTE TO RFC EDITOR: this section for use during I-D stage only.
   Please remove before publishing as RFC.]

   lebovitz...-02 - 3rd submission

   o  cleaned up explanation in 3.1.2
   o  in 3.1.2, changed the key extractor mechanism back, from using an
      alphanumeric string for the fixed key C to use 0^128 as the key
      (as it was in -00) (Polk & Ekr)
   o  deleted cut-and-paste error text from sect 3.1 between "label" and
   o  changed "conn_key" to "traffic_key" throughout, to follow auth-
   o  changed "tsad" to "mkt" throughout, to follow auth-opt-05
   o  changed "conn_block" to "data_block" throughout, to follow auth-

   lebovitz...-01- 2nd submission

   o  removed the whole section on labels (previously section 4), per WG
      consensus at IETF74.  Added 3.1.3 to specify that implementations
      SHOULD make HMAC-SHA1 the default choice for the time being, and
      to suggest common names for the KDF's universally in UI's.
   o  changed KDF = PRF... to Derived_Key = KDF...  (EKR)
   o  added the text on how to deal with future KDF to end of s3.1 (EKR)
   o  removed references to TCP-AO "manual key mode".  Changed to TCP-
      AO's "current mode of manual keying".  (Touch)
   o  removed the whole MUST- / SHOULD+ thing.  Both KDF's are MUST now,
      per wg consensus at ietf74.
   o  in 3.1.2, changed the mechanism to force the K to be 128bits from
      using 0^128, to using a fixed 128-bit string of random characters
      (Dave McGrew)
   o  sect 3.1, in Input description, dropped "0x00".  Added "NOTE"
      explaining why right after the output_length description.

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   o  cleaned up all references
   o  copy editing

   lebovitz...-00 - original submission

5.  Needs Work in Next Draft (RFC Editor: Delete Before Publishing)

   [NOTE TO RFC EDITOR: this section for use during I-D stage only.
   Please remove before publishing as RFC.]

   List of stuff that still needs work
   o  fix the iana registry section.  Need registry entries for the KDFs
      and all the other values?
   o  this was supposed to be named
      draft-ietf-tcpm-tcp-ao-crypto-00.txt, but I forgot that since we
      were moving from a personal submission to a wg sub, it had to go
      back to a -00, thus needed to be done a week earlier.  Oops.  Will
      fix as soon as the window opens for submitting -00's again.

6.  Security Considerations

   This document inherits all of the security considerations of the
   TCP-AO, the AES-CMAC, and the HMAC-SHA-1 documents.

   The security of cryptographic-based systems depends on both the
   strength of the cryptographic algorithms chosen and the strength of
   the keys used with those algorithms.  The security also depends on
   the engineering of the protocol used by the system to ensure that
   there are no non-cryptographic ways to bypass the security of the
   overall system.

   Care should also be taken to ensure that the selected key is
   unpredictable, avoiding any keys known to be weak for the algorithm
   in use. ][RFC4086] contains helpful information on both key
   generation techniques and cryptographic randomness.

   Note that in the composition of KDF_AES_128_CMAC, the PRF needs a 128
   bit / 16 byte key as the seed.  However, for convenience to the
   administrators/deployers, we did not want to force them to enter a 16
   byte Master_Key. So we specified the sub-key routine that could
   handle a variable length Master_Key, one that might be less than 16
   bytes.  This does NOT mean that administrators are safe to use weak
   keys.  Administrators are encouraged to follow [RFC4086] as listed
   above.  We simply attempted to "put a fence around stupidity", in as
   much as possible.

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   This document concerns itself with the selection of cryptographic
   algorithms for the use of TCP-AO.  The algorithms identified in this
   document as "MUST implement" or "SHOULD implement" are not known to
   be broken at the current time, and cryptographic research so far
   leads us to believe that they will likely remain secure into the
   foreseeable future.  Some of the algorithms may be found in the
   future to have properties significantly weaker than those that were
   believed at the time this document was produced.  Expect that new
   revisions of this document will be issued from time to time.  Be sure
   to search for more recent versions of this document before

7.  IANA Considerations

   IANA has created and will maintain a registry called, "Cryptographic
   Algorithms for TCP-AO".  The registry consists of a text string and
   an RFC number that lists the associated transform(s).  New entries
   can be added to the registry only after RFC publication and approval
   by an expert designated by the IESG.

   [need to finish this section]

8.  Acknowledgements

   Paul Hoffman, from whose [RFC4308] I sometimes copied, to quickly
   create a first draft here.

   Tim Polk, whose email summarizing SAAG's guidance to TCPM on the two
   hash algorithms for TCP-AO is largely cut and pasted into various
   sections of this document.

   Jeff Schiller, Donald Eastlake and the IPsec WG, whose [RFC4307] &
   [RFC4305] text was consulted and sometimes used in the Requirements
   Section 2 section of this document.

   (In other words, I was truly only an editor of others' text in
   creating this document.)

   Eric "EKR" Rescorla and Brian Weis, who brought to clarity the issues
   with the inputs to PRFs for the KDFs.  EKR was also of great
   assistance in how to structure the text, as well as helping to guide
   good cryptographic decisions.

   The TCPM working group, who put up with all us crypto and routing
   folks DoS'ing their WG for 2 years, and who provided reviews of this

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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2.  Informative References

              "On the Security of HMAC and NMAC Based on HAVAL, MD4,
              MD5, SHA-0 and SHA-1"", 2006,

              Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", draft-ietf-tcpm-tcp-auth-opt-05
              (work in progress), July 2009.

              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs",
              draft-narten-iana-considerations-rfc2434bis-09 (work in
              progress), March 2008.

              National Institute of Standards and Technology,
              "Recommendation for Key Derivation Using Pseudorandom
              Functions", SP 800-108, April 2008.

              National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation: The
              CMAC Mode for Authentication", SP 800-38B, May 2005.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC2404]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
              ESP and AH", RFC 2404, November 1998.

   [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

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   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4305]  Eastlake, D., "Cryptographic Algorithm Implementation
              Requirements for Encapsulating Security Payload (ESP) and
              Authentication Header (AH)", RFC 4305, December 2005.

   [RFC4307]  Schiller, J., "Cryptographic Algorithms for Use in the
              Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
              December 2005.

   [RFC4308]  Hoffman, P., "Cryptographic Suites for IPsec", RFC 4308,
              December 2005.

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, June 2006.

   [RFC4615]  Song, J., Poovendran, R., Lee, J., and T. Iwata, "The
              Advanced Encryption Standard-Cipher-based Message
              Authentication Code-Pseudo-Random Function-128 (AES-CMAC-
              PRF-128) Algorithm for the Internet Key Exchange Protocol
              (IKE)", RFC 4615, August 2006.

Author's Address

   Gregory Lebovitz
   Juniper Networks, Inc.
   1194 North Mathilda Ave.
   Sunnyvale, CA  94089-1206

   Email: gregory.ietf@gmail.com

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