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Versions: (draft-lebovitz-ietf-tcpm-tcp-ao-crypto) 00 01 02 03 RFC 5926

TCPM                                                         G. Lebovitz
Internet-Draft                                                   Juniper
Intended status: Standards Track                             E. Rescorla
Expires: May 1, 2010                                                RTFM
                                                        October 28, 2009


    Cryptographic Algorithms for TCP's Authentication Option, TCP-AO
                    draft-ietf-tcpm-tcp-ao-crypto-01

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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.

Abstract

   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.  Concrete KDFs  . . . . . . . . . . . . . . . . . . . .  6
     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)  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17












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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 modern 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, message authentication codes (MACs) are used, which then
   rely on shared keys.  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 defines the general requirements for MACs used in TCP-AO,
   both for currently specified MACs and for any future specified MACs.
   It then specifies two MAC algorithms required in all TCP-AO
   implementations.  It also specifies two key derivations functions
   (KDFs) used to create the traffic keys used by the MACs.  These KDFs
   are also required by all TCP-AO implementations.


2.  Requirements

2.1.  Requirements Language

   The key words "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].

   When used in lower case, these words convey their typical use in
   common language, and are not to be interpreted as described in
   [RFC2119].

2.2.  Algorithm Requirements

   This is the initial specification of required cryptography for
   TCP-AO, and indicates two MAC algorithms and two KDFs.  All four
   components MUST be implemented in order for the implementation to be
   fully compliant with this RFC.

   The following table indicates the required MAC algorithms and KDFs
   for TCP-AO:







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

           Requirement      Key Derivation Function (KDF)
           -------------    ------------------------
           MUST             KDF_HMAC_SHA1
           MUST             KDF_AES_128_CMAC

   NOTE EXPLAINING WHY TWO MAC ALGORITHMS WERE MANDATED:

   Two MAC algorithms and two corresponding KDFs are mandated as a
   result of discussion in the TCPM WG, and in consultation with the
   Security Area Directors.  SHA-1 was selected because it is widely
   deployed and currently has sufficient strength and reasonable
   computational cost, so it is a "MUST" for TCP-AO today.  The security
   strength of SHA-1 HMACs should be sufficient for the foreseeable
   future, especially given that the tags are truncated to 96 bits.

   Recently exposed vulnerabilities in other MACs (e.g., MD5 or HMAC
   MD5) aren't practical on SHA-1, but these types of analyses are
   mounting and could potentially pose a concern for HMAC forgery if
   they were significantly improved, over time.  The security issues
   driving the migration from SHA-1 to SHA-256 for digital signatures
   [HMAC-ATTACK] do not immediately render SHA-1 weak for this
   application of SHA-1 in HMAC mode.

   AES-128 CMAC is considered to be a stronger algorithm than SHA-1, but
   may not yet have very wide implementation.  AES-128 CMAC is also a
   "MUST" to implement, in order to drive vendors toward its use, and to
   allow for another MAC option, if SHA-1 were to be compromised.

2.3.  Requirements for Future MAC Algorithms

   TCP-AO is intended to support cryptographic agility.  As a result,
   this document includes recommendations in various places for future
   MAC and KDF algorithms when used for TCP-AO.  For future MAC
   algorithms specifically, they SHOULD protect at least 2**48 messages
   with a collision probability of less than one in 10**9.

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


3.  Algorithms Specified

   TCP-AO requires two classes of cryptographic algorithms used on a



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   particular connection, and refers to this document to define them
   both:


       (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, as described in TCP-AO.
       (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 and authenticity of
            those messages.

   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:

       Traffic_Key = KDF_alg(Master_Key, Context, Output_Length)

   where:


      - KDF_alg:     the specific pseudorandom function (PRF) that is
                     the basic building block used in constructing the
                     given Traffic_Key.

      - Master_Key:  In TCP-AO's manual key mode, this is a key shared
                     by both peers, entered via some interface to their
                     respective configurations.  The Master_Key is used
                     as the seed for the KDF.  We assume that this is a
                     human-readable pre-shared key (PSK), thus we assume
                     it is of variable length.  Master_Keys SHOULD be
                     random, but might not be (e.g., badly chosen by the
                     user).

      - Context :    A binary string containing information related to
                     the specific connection for this derived keying
                     material, as defined in
                     [I-D.ietf-tcpm-tcp-auth-opt], Section 7.2]






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      - 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.

   When invoked, a KDF generates a string of length Output_Length bits
   based on Master_Key and context value.  This result may then be used
   as a cryptographic key for any algorithm which takes an Output_Length
   length key.  A KDF MAY specify a maximum Output_Length parameter.

3.1.1.  Concrete KDFs

   This document defines two KDF algorithms, each paired with a
   corresponding PRF algorithm as explained below:


       *  KDF_HMAC_SHA1  based on PRF-HMAC-SHA1 [RFC2404]
       *  KDF_AES_128_CMAC  based on AES-CMAC-PRF-128 [RFC4615]


   Each

   Both of these KDFs are based on the iteration mode KDFs specified in
   [NIST-SP800-108].  This means that they use an underlying
   pseudorandom function (PRF) with a fixed-length output lengths, 128
   bits in the case of the AES-CMAC, and 160 bits in the case of HMAC-
   SHA1.  The KDF generates an arbitrary number of output bits by
   operating the PRF in a "counter mode", where each invocation of the
   PRF uses a different input block differentiated by a block counter.

   Each input block is constructed as follows:

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

      Where

      - "||":      For any X || Y, "||" represents a concatonation
                   operation of the binary strings X and Y.

      - i:         A counter, a binary string that is an input to each
                   iteration of the PRF in counter mode.  The number of
                   iterations will depend on the specific size of the
                   Output_Length desired for a given MAC.







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      - 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 :  The context argument provided to the KDF interface,
                   as described above in Section 3.1 .

      - 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.

   The ouput of mutiple PRF invocations is simply concatenated.  For the
   Traffic_Key, values of multiple PRF invocations are concatenated and
   truncated as needed to create a Traffic_Key of the desired length.
   For instance, if one were using KDF_HMAC_SHA1, which uses a 160-bit
   internal PRF to generate 320 bits of data, one would execute the PRF
   twice, once with i=0 and once with i=1.  The result would be the
   entire output of the first invocation concatenated with the second
   invocation.  E.g.,

          Traffic_Key =
            KDF_alg(Master_Key, 0 || Label || Context || Output_length) ||
            KDF_alg(Master_Key, 1 || Label || Context || Output_length)

   If the number of bits required is not an even multiple of the output
   size of the PRF, then the output of the final invocation of the PRF
   is truncated as necessary.

3.1.1.1.  KDF_HMAC_SHA1

   For KDF_HMAC_SHA1:

   - PRF for KDF_alg:  HMAC-SHA1 [RFC2404].

   - Use:       HMAC-SHA1(Key, Input).

   - Key:       Master_Key, configured by user, and passed to the KDF.







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   - Input:     ( i || Label || Context || Output_Length)

   - Output_Length:  160 bits.

   - Result:    Traffic_Key, used in the MAC function by TCP-AO.

3.1.1.2.  KDF_AES_128_CMAC

   For KDF_AES_128_CMAC:

   - PRF for KDF_alg:  AES-CMAC-PRF-128 [RFC4615].

   - Use:       AES-CMAC(Key, Input).

   - Key:       Master_Key (see usage below)

   - Input:     ( i || Label || Context || Output_Length)

   - Output_Length:  128 bits.

   - Result:    Traffic_Key, used in the MAC function by TCP-AO

   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 exactly
   16 octets (128 bits) in length.  We could mandate that
   implementations force administrators to input Master_Keys of exactly
   128 bit length when using AES_128_CMAC, and with sufficient
   randomness, but this places undue burden on the implementors and
   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 the same mechanism as
   described in [RFC4615], Sect 3.  First we use AES_128_CMAC with a
   fixed key of all zeros as a "randomness extractor", while using the
   shared secret Master_Key, MK, as the message input, to produce a 128
   bit key Derived_Master_Key (K).  Second, we use the result as a key,
   and run AES-128_CMAC again, this time using the result K as the Key,
   and the true input block as the Input to yield the Traffic_Key (TK)
   used in the MAC over the message.  The description follows:







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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 M in octets)                              +
   + Output : TK (Traffic_Key, 128-bit Pseudo-Random Variable)+
   +                                                                   +
   +-------------------------------------------------------------------+
   + Variable: K (128-bit key for AES-CMAC)                            +
   +                                                                   +
   + Step 1.   If MKlen is equal to 16                                 +
   + Step 1a.  then                                                    +
   +               K := MK;                                            +
   + Step 1b.  else                                                    +
   +               K := AES-CMAC(0^128, MK, MKlen);                    +
   + Step 2.   TK := AES-CMAC(K, I, len);                             +
   +           return TK;                                             +
   +                                                                   +
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


                                 Figure 1

   In step 1, the 128-bit key, K, for AES-CMAC is derived as follows:

   o If the Master_Key, MK, provided by the administrator is exactly 128
   bits, then we use it as-is.

   o If it is longer or shorter than 128 bits, then we derive the key K
   by applying the AES-CMAC algorithm using the 128-bit all-zero string
   as the key and MK as the input message.  This step is described in
   step 1b.

   In step 2, we apply the AES-CMAC algorithm again, this time using K
   as the key and I as the input message.

   The output of this algorithm returns TK, the Traffic_Key, which is
   128 bits suitable for use in the MAC function on each TCP segment of
   the connection.

3.1.1.3.  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



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   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
   "AES128".

   The initial IANA registry values will reflect these two entries.

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

3.2.  MAC Algorithms

   Each MAC_alg defined for TCP-AO has three fixed elements as part of
   its definition:

   - KDF_Alg:     Name of the TCP-AO KDF algorithm used to generate the
                  Traffic_Key
   - Key_Length:  Length in bits required for the Traffic_Key used in
                  this MAC
   - Output:      The final length of the bits used in the TCP-AO MAC
                  field.  This value may be a truncation of the MAC
                  function's original output length.

   MACs computed for TCP-AO have the following interface:

       MAC = MAC_alg(Traffic_Key, Message)

   where:


      - MAC_alg:     MAC Algorithm used
      - Traffic_Key: Variable; the result of KDF.
      -  Message     The message to be authenticated, as specified in
                     [I-D.ietf-tcpm-tcp-auth-opt] Section 7.1.

   This document specifies two MAC algorithm options for generating the
   MAC as used by TCP-AO:


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







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       *  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.

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.

   The three fixed elements for HMAC-SHA-1-96 are:

   - KDF_Alg:     KDF_HMAC_SHA1.
   - Key_Length:  160 bits.
   - Output:      96 bits.

   For:

        MAC = MAC_alg (Traffic_Key, Message)


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


      - MAC_alg:     HMAC-SHA1
      - Traffic_Key: Variable; the result of the KDF.
      - Message:     The message to be authenticated, as specified in
                     [I-D.ietf-tcpm-tcp-auth-opt] Section 7.1.



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
   [NIST-SP800-38B].



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   The three fixed elements for AES-128-CMAC-96 are:

   - KDF_Alg:     KDF_AES_128_CMAC.
   - Key_Length:  128 bits.
   - Output:      96 bits.

   For:

        MAC = MAC_alg (Traffic_Key, Message)


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


      - MAC_alg:     AES-128-CMAC-96 [RFC4493]
      - Traffic_Key: Variable; the result of the KDF.
      - Message:     The message to be authenticated, as specified in
                     [I-D.ietf-tcpm-tcp-auth-opt] Section 7.1.




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.]

   draft-ietf...-02 - 6th submission

   o  3.1.1.1 & 3.1.1.2, s/PRF:/PRF for KDF_alg: for clarification
   o

   draft-ietf...-01 - 5th submission

   o  simplified title of document - Touch
   o  structure of sect 3 changed: 3.1 becomes "Concrete KDF's" and the
      description of the two KDF's became 3.1.1.1 & 3.1.1.2. used a
      template (of sorts) in both sections that can be easily re-used by
      any future KDF definition.
   o  in 3.1, switched "Derived_Key" back to "Traffic_Key" in order to
      sync with -auth-opt spec.
   o  in 3.1, for Traffic_Key definition, changed the name of the
      element "Input" to "Context", and removed the hard binding to
      NIST-SP800-108 at the generic interface level.  The NIST inclusion
      is still present in the concrete definitions of the two presented
      KDF's, but this way, if NIST changes, or if the community wants to
      define some other, non-NIST-like KDFs, the interface is flexible
      enough to handle that.  Changed "KDF" to "KDF-alg".



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   o  in 3.2, clarified that the output of the function is called "MAC
      =", to have a similar look and feel to sect 3.1's function. also
      changed MAC(foo) to "MAC_alg", and dropped "Output_Length" from
      the function parameters, as it is specified as a fixed element for
      each MAC defined, but not passed as a parameter of the function.
   o  sect 3.2 - removed "truncation" from interface definition, and
      placed it as a fixed element of the MAC definition. sect 3.2.1 and
      3.2.2 - changed "truncation:" field to "Output:".  Removed text
      about RFC4493 from end of 3.2.2.
   o  sect 3.1.1.2, changed back to follow 4615 exactly (agreement with
      polk, mcgrew, lebovitz, ekr)
   o  sect 3.1.1.3, deleted the following: "When such a time arrives as
      KDF_AES_128_CMAC becomes widely deployed, this document should be
      updated so that it becomes the default KDF on implementations."
   o  added the two IANA values SHA1 and AES128 to IANA section
   o  Minor text change in section 3.1 and 3.2, to the definition of
      "Context" in both - Joe Touch
   o  minor text change in 3.1.1 clarifying that the concatenation is of
      binary strings - Joe Touch
   o  copy edits for read-ability throughout - Touch

   draft-ietf...-00 - 4th submission

   o  Submitting draft-lebovitz...-02 as a WG document.  Added EKR as
      co- author, and gave him credit for rewrite of sect 3.1.x .

   draft-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
      "context"
   o  changed "conn_key" to "traffic_key" throughout, to follow auth-
      opt-05
   o  changed "tsad" to "mkt" throughout, to follow auth-opt-05
   o  changed "conn_block" to "data_block" throughout, to follow auth-
      opt-05

   draft-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.





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   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.
   o  cleaned up all references
   o  copy editing

   draft-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  should be ready for WG LC.  Any changes will come from final WG
      review or IESG review.


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



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   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.

   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
   implementing.


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.

   The registry has the following initial values:


      SHA1           [This RFC when published]
      AES128         [This RFC when published]



8.  Acknowledgements

   Eric "EKR" Rescorla, who provided a ton of input and feedback,
   including a somewhat heavy re-write of section 3.1.x, earning him and
   author slot on the document.

   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.




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   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
   document.


9.  References

9.1.  Normative References

   [I-D.ietf-tcpm-tcp-auth-opt]
              Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", draft-ietf-tcpm-tcp-auth-opt-07
              (work in progress), October 2009.

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

9.2.  Informative References

   [HMAC-ATTACK]
              "On the Security of HMAC and NMAC Based on HAVAL, MD4,
              MD5, SHA-0 and SHA-1"", 2006,
              <http://eprint.iacr.org/2006/187
              http://www.springerlink.com/content/00w4v62651001303>.

   [I-D.narten-iana-considerations-rfc2434bis]
              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.

   [NIST-SP800-108]
              National Institute of Standards and Technology,
              "Recommendation for Key Derivation Using Pseudorandom
              Functions", SP 800-108, April 2008.




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   [NIST-SP800-38B]
              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.

   [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.











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Authors' Addresses

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

   Phone:
   Email: gregory.ietf@gmail.com


   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303
   US

   Phone: 650-678-2350
   Email: ekr@rtfm.com































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