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Internet Engineering Task Force (IETF)                       G. LebovitzRequest for Comments: 5926                                       JuniperCategory: Standards Track                                    E. RescorlaISSN: 2070-1721                                                     RTFM                                                               June 2010Cryptographic Algorithms for the TCP Authentication Option (TCP-AO)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 algorithms.  This document specifies   the algorithms and attributes that can be used in TCP-AO's current   manual keying mechanism and provides the interface for future message   authentication codes (MACs).Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5926.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Lebovitz & Rescorla          Standards Track                    [Page 1]

RFC 5926                    Crypto for TCP-AO                  June 2010Table of Contents1. Introduction ....................................................22. Requirements ....................................................32.1. Requirements Language ......................................32.2. Algorithm Requirements .....................................32.3. Requirements for Future MAC Algorithms .....................33. Algorithms Specified ............................................43.1. Key Derivation Functions (KDFs) ............................43.1.1. Concrete KDFs .......................................53.1.1.1. KDF_HMAC_SHA1 ..............................63.1.1.2. KDF_AES_128_CMAC ...........................73.1.1.3. Tips for User Interfaces Regarding KDFs ....93.2. MAC Algorithms .............................................93.2.1. The Use of HMAC-SHA-1-96 ...........................103.2.2. The Use of AES-128-CMAC-96 .........................114. Security Considerations ........................................115. IANA Considerations ............................................136. Acknowledgements ...............................................137. References .....................................................147.1. Normative References ......................................147.2. Informative References ....................................141.  Introduction   This document is a companion to [RFC5925].  Like most modern security   protocols, TCP-AO allows users to choose which cryptographic   algorithm(s) they want to use to meet their security needs.   TCP-AO provides cryptographic authentication and message integrity   verification between two 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 hash-based MACs (HMACs) is defined in [RFC2104].  The use of   cipher-based MACs (CMACs) is defined in [NIST-SP800-38B].   This RFC defines the general requirements for MACs used in TCP-AO,   both for currently specified MACs and for any future specified MACs.   It specifies two MAC algorithms required in all TCP-AO   implementations.  It also specifies two key derivation functions   (KDFs) used to create the traffic keys used by the MACs.  These KDFs   are also required by all TCP-AO implementations.Lebovitz & Rescorla          Standards Track                    [Page 2]

RFC 5926                    Crypto for TCP-AO                  June 20102.  Requirements2.1.  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 lowercase, these words convey their typical use in   common language, and they 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:           Requirement      Authentication Algorithm           ------------     ------------------------           MUST             HMAC-SHA-1-96 [RFC2104][FIPS-180-3]           MUST             AES-128-CMAC-96 [NIST-SP800-38B][FIPS197]           Requirement      Key Derivation Function (KDF)           -------------    ------------------------           MUST             KDF_HMAC_SHA1           MUST             KDF_AES_128_CMAC   For an explanation of why two MAC algorithms were mandated, see theSection 4.2.3.  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.Lebovitz & Rescorla          Standards Track                    [Page 3]

RFC 5926                    Crypto for TCP-AO                  June 20103.  Algorithms Specified   TCP-AO requires two classes of cryptographic algorithms used on a   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 that 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).  For interoperability, the                     management interface by which the PSK is configured                     MUST accept ASCII strings, and SHOULD also allow                     for the configuration of any arbitrary binary                     string in hexadecimal form.  Other configuration                     methods MAY be supported.Lebovitz & Rescorla          Standards Track                    [Page 4]

RFC 5926                    Crypto for TCP-AO                  June 2010      - Context:     A binary string containing information related to                     the specific connection for this derived keying                     material, as defined in[RFC5925], Section 5.2.      - 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 the Master_Key and context value.  This result may then be   used as a cryptographic key for any algorithm that 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 [RFC2104][FIPS-180-3]       *  KDF_AES_128_CMAC  based on AES-CMAC-PRF-128                            [NIST-SP800-38B][FIPS197]   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, 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 concatenation                   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 counter                   "i" is represented in a single octet.  The number of                   iterations will depend on the specific size of the                   Output_Length desired for a given MAC. "i" always                   starts = 1.Lebovitz & Rescorla          Standards Track                    [Page 5]

RFC 5926                    Crypto for TCP-AO                  June 2010      - 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 inSection 3.1 .      - Output_Length:  The length, in bits, of the key that the KDF                        will produce.  The Output_length is represented                        within two octets.  This length must be the size                        required for the MAC algorithm that will use the                        PRF result as a seed.   The output of multiple 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=1 and once with i=2.  The result would be   the entire output of the first invocation concatenated with the   second invocation.  For example,  Traffic_Key =          KDF_alg(Master_Key, 1 || Label || Context || Output_length) ||          KDF_alg(Master_Key, 2 || Label || Context || Output_length)   If the number of bits required is not an exact 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 [RFC2104][FIPS-180-3].   - Use:       HMAC-SHA1(Key, Input).   - Key:       Master_Key, configured by user, and passed to the KDF.   - Input:     ( i || Label || Context || Output_Length)   - Output_Length:  160 bits.Lebovitz & Rescorla          Standards Track                    [Page 6]

RFC 5926                    Crypto for TCP-AO                  June 2010   - 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 [NIST-SP800-38B][FIPS197].   - 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], Section 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:Lebovitz & Rescorla          Standards Track                    [Page 7]

RFC 5926                    Crypto for TCP-AO                  June 2010   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++   +                        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   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 and is suitable for use in the MAC function on each TCP   segment of the connection.Lebovitz & Rescorla          Standards Track                    [Page 8]

RFC 5926                    Crypto for TCP-AO                  June 20103.1.1.3.  Tips for User Interfaces Regarding KDFs   This section provides suggested representations for the KDFs in   implementation user interfaces (UIs).  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   "AES128".   The initial IANA registry values 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.   - MAC_Length:  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[RFC5925], Section 5.1.Lebovitz & Rescorla          Standards Track                    [Page 9]

RFC 5926                    Crypto for TCP-AO                  June 2010   This document specifies two MAC algorithm options for generating the   MAC as used by TCP-AO:       *  HMAC-SHA-1-96  based on [RFC2104] and [FIPS-180-3].       *  AES-128-CMAC-96  based on [NIST-SP800-38B][FIPS197]   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 is then truncated to 96   bits to provide a reasonable trade-off between security and message   size, for fitting into the TCP-AO option field.3.2.1.  The Use of HMAC-SHA-1-96   By definition, HMAC [RFC2104] requires a cryptographic hash function.   SHA1 will be that hash 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.   - MAC_Length:  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[RFC5925], Section 5.1.Lebovitz & Rescorla          Standards Track                   [Page 10]

RFC 5926                    Crypto for TCP-AO                  June 20103.2.2.  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].   The three fixed elements for AES-128-CMAC-96 are:   - KDF_Alg:     KDF_AES_128_CMAC.   - Key_Length:  128 bits.   - MAC_Length:  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.  [NIST-SP800-38B]      - Traffic_Key: Variable; the result of the KDF.      - Message:     The message to be authenticated, as specified in[RFC5925], Section 5.1.4.  Security Considerations   This document inherits all of the security considerations of the   TCP-AO [RFC5925], the AES-CMAC [RFC4493], and the HMAC-SHA-1   [RFC2104] documents.   The security of cryptography-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 aLebovitz & Rescorla          Standards Track                   [Page 11]

RFC 5926                    Crypto for TCP-AO                  June 2010   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 it is safe for administrators to use   weak keys.  Administrators are encouraged to follow [RFC4086] as   listed above.  We simply attempted to "put a fence around   foolishness", 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" 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.   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 HMAC-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.Lebovitz & Rescorla          Standards Track                   [Page 12]

RFC 5926                    Crypto for TCP-AO                  June 20105.  IANA Considerations   IANA has created the following registry (http://www.iana.org).   Registry Name: Cryptographic Algorithms for TCP-AO Registration   Procedure: RFC Publication after Expert Review   Initial contents of this registry are:         Algorithm      | Reference        ----------------|-----------         SHA1           | [RFC5926]         AES128         | [RFC5926]6.  Acknowledgements   Eric "EKR" Rescorla, who provided a ton of input and feedback,   including a somewhat heavy re-write ofSection 3.1.x, earning him an   author slot on the document.   Paul Hoffman, from whose [RFC4308] I sometimes copied, to quickly   create a first version 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] &   [RFC4835] text was consulted and sometimes used in the RequirementsSection 2 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.Lebovitz & Rescorla          Standards Track                   [Page 13]

RFC 5926                    Crypto for TCP-AO                  June 20107.  References7.1.  Normative References   [FIPS-180-3]  FIPS Publication 180-3, "Secured Hash Standard",                 FIPS 180-3, October 2008.   [FIPS197]     FIPS Publications 197, "Advanced Encryption Standard                 (AES)", FIPS 197, November 2001.   [NIST-SP800-108]                 National Institute of Standards and Technology,                 "Recommendation for Key Derivation Using Pseudorandom                 Functions, NIST SP800-108", SP 800- 108, October 2009.   [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.   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate                 Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC4493]     Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The                 AES-CMAC Algorithm",RFC 4493, June 2006.   [RFC5925]     Touch, J., Mankin, A., and R. Bonica, "The TCP                 Authentication Option",RFC 5925, June 2010.7.2.  Informative References   [HMAC-ATTACK] "On the Security of HMAC and NMAC Based on HAVAL, MD4,                 MD5, SHA-0 and SHA-1", <http://www.springerlink.com/content/00w4v62651001303> , 2006,                 <http://eprint.iacr.org/2006/187>.   [RFC4086]     Eastlake, D., Schiller, J., and S. Crocker, "Randomness                 Requirements for Security",BCP 106,RFC 4086,                 June 2005.Lebovitz & Rescorla          Standards Track                   [Page 14]

RFC 5926                    Crypto for TCP-AO                  June 2010   [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.   [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.   [RFC4835]     Manral, V., "Cryptographic Algorithm Implementation                 Requirements for Encapsulating Security Payload (ESP)                 and Authentication Header (AH)",RFC 4835, April 2007.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.comLebovitz & Rescorla          Standards Track                   [Page 15]