AES-CCM for ESP: Using Advanced Encryption Standard (AES) CCM Mode with IPsec Encapsulating Security Payload (ESP)

Network Working Group                                         R. Housley
Request for Comments: 4309                                Vigil Security
Category: Standards Track                                  December 2005


           Using Advanced Encryption Standard (AES) CCM Mode
            with IPsec Encapsulating Security Payload (ESP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes the use of Advanced Encryption Standard (AES)
   in Counter with CBC-MAC (CCM) Mode, with an explicit initialization
   vector (IV), as an IPsec Encapsulating Security Payload (ESP)
   mechanism to provide confidentiality, data origin authentication, and
   connectionless integrity.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. AES CCM Mode ....................................................2
   3. ESP Payload .....................................................4
      3.1. Initialization Vector (IV) .................................4
      3.2. Encrypted Payload ..........................................4
      3.3. Authentication Data ........................................5
   4. Nonce Format ....................................................5
   5. AAD Construction ................................................6
   6. Packet Expansion ................................................7
   7. IKE Conventions .................................................7
      7.1. Keying Material and Salt Values ............................7
      7.2. Phase 1 Identifier .........................................8
      7.3. Phase 2 Identifier .........................................8
      7.4. Key Length Attribute .......................................8
   8. Test Vectors ....................................................8
   9. Security Considerations .........................................8
   10. Design Rationale ...............................................9
   11. IANA Considerations ...........................................11
   12. Acknowledgements ..............................................11
   13. References ....................................................11
      13.1. Normative References .....................................11
      13.2. Informative References ...................................12

1.  Introduction

   The Advanced Encryption Standard (AES) [AES] is a block cipher, and
   it can be used in many different modes.  This document describes the
   use of AES in CCM (Counter with CBC-MAC) mode (AES CCM), with an
   explicit initialization vector (IV), as an IPsec Encapsulating
   Security Payload (ESP) [ESP] mechanism to provide confidentiality,
   data origin authentication, and connectionless integrity.

   This document does not provide an overview of IPsec.  However,
   information about how the various components of IPsec and the way in
   which they collectively provide security services is available in
   [ARCH] and [ROAD].

1.1.  Conventions Used in This Document

   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 [STDWORDS].

2.  AES CCM Mode

   CCM is a generic authenticate-and-encrypt block cipher mode [CCM].
   In this specification, CCM is used with the AES [AES] block cipher.

   AES CCM has two parameters:

      M  M indicates the size of the integrity check value (ICV).  CCM
         defines values of 4, 6, 8, 10, 12, 14, and 16 octets; However,
         to maintain alignment and provide adequate security, only the
         values that are a multiple of four and are at least eight are
         permitted.  Implementations MUST support M values of 8 octets
         and 16 octets, and implementations MAY support an M value of 12
         octets.

      L  L indicates the size of the length field in octets.  CCM
         defines values of L between 2 octets and 8 octets.  This
         specification only supports L = 4.  Implementations MUST
         support an L value of 4 octets, which accommodates a full
         Jumbogram [JUMBO]; however, the length includes all of the
         encrypted data, which also includes the ESP Padding, Pad
         Length, and Next Header fields.

   There are four inputs to CCM originator processing:

      key
         A single key is used to calculate the ICV using CBC-MAC and to
         perform payload encryption using counter mode.  AES supports
         key sizes of 128 bits, 192 bits, and 256 bits.  The default key

         size is 128 bits, and implementations MUST support this key
         size.  Implementations MAY also support key sizes of 192 bits
         and 256 bits.

      nonce
         The size of the nonce depends on the value selected for the
         parameter L.  It is 15-L octets.  Implementations MUST support
         a nonce of 11 octets.  The construction of the nonce is
         described in Section 4.

      payload
         The payload of the ESP packet.  The payload MUST NOT be longer
         than 4,294,967,295 octets, which is the maximum size of a
         Jumbogram [JUMBO]; however, the ESP Padding, Pad Length, and
         Next Header fields are also part of the payload.

      AAD
         CCM provides data integrity and data origin authentication for
         some data outside the payload.  CCM does not allow additional
         authenticated data (AAD) to be longer than
         18,446,744,073,709,551,615 octets.  The ICV is computed from
         the ESP header, Payload, and ESP trailer fields, which is
         significantly smaller than the CCM-imposed limit.  The
         construction of the AAD described in Section 5.

   AES CCM requires the encryptor to generate a unique per-packet value
   and to communicate this value to the decryptor.  This per-packet
   value is one of the component parts of the nonce, and it is referred
   to as the initialization vector (IV).  The same IV and key
   combination MUST NOT be used more than once.  The encryptor can
   generate the IV in any manner that ensures uniqueness.  Common
   approaches to IV generation include incrementing a counter for each
   packet and linear feedback shift registers (LFSRs).

   AES CCM employs counter mode for encryption.  As with any stream
   cipher, reuse of the same IV value with the same key is catastrophic.
   An IV collision immediately leaks information about the plaintext in
   both packets.  For this reason, it is inappropriate to use this CCM
   with statically configured keys.  Extraordinary measures would be
   needed to prevent reuse of an IV value with the static key across
   power cycles.  To be safe, implementations MUST use fresh keys with
   AES CCM.  The Internet Key Exchange (IKE) [IKE] protocol or IKEv2
   [IKEv2] can be used to establish fresh keys.

3.  ESP Payload

   The ESP payload is composed of the IV followed by the ciphertext.
   The payload field, as defined in [ESP], is structured as shown in
   Figure 1.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Initialization Vector                     |
      |                            (8 octets)                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                  Encrypted Payload (variable)                 ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                 Authentication Data (variable)                ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 1.  ESP Payload Encrypted with AES CCM

3.1.  Initialization Vector (IV)

   The AES CCM IV field MUST be eight octets.  The IV MUST be chosen by
   the encryptor in a manner that ensures that the same IV value is used
   only once for a given key.  The encryptor can generate the IV in any
   manner that ensures uniqueness.  Common approaches to IV generation
   include incrementing a counter for each packet and linear feedback
   shift registers (LFSRs).

   Including the IV in each packet ensures that the decryptor can
   generate the key stream needed for decryption, even when some
   datagrams are lost or reordered.

3.2.  Encrypted Payload

    The encrypted payload contains the ciphertext.

    AES CCM mode does not require plaintext padding.  However, ESP does
    require padding to 32-bit word-align the authentication data.  The
    Padding, Pad Length, and Next Header fields MUST be concatenated
    with the plaintext before performing encryption, as described in
    [ESP].  When padding is required, it MUST be generated and checked
    in accordance with the conventions specified in [ESP].

3.3.  Authentication Data

   AES CCM provides an encrypted ICV.  The ICV provided by CCM is
   carried in the Authentication Data fields without further encryption.
   Implementations MUST support ICV sizes of 8 octets and 16 octets.
   Implementations MAY also support ICV 12 octets.

4.  Nonce Format

   Each packet conveys the IV that is necessary to construct the
   sequence of counter blocks used by counter mode to generate the key
   stream.  The AES counter block is 16 octets.  One octet is used for
   the CCM Flags, and 4 octets are used for the block counter, as
   specified by the CCM L parameter.  The remaining octets are the
   nonce.  These octets occupy the second through the twelfth octets in
   the counter block.  Figure 2 shows the format of the nonce.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |                  Salt                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Initialization Vector                     |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 2.  Nonce Format

   The components of the nonce are as follows:

      Salt The salt field is 24 bits.  As the name implies, it contains
         an unpredictable value.  It MUST be assigned at the beginning
         of the security association.  The salt value need not be
         secret, but it MUST NOT be predictable prior to the beginning
         of the security association.

      Initialization Vector The IV field is 64 bits.  As described in
         Section 3.1, the IV MUST be chosen by the encryptor in a manner
         that ensures that the same IV value is used only once for a
         given key.

   This construction permits each packet to consist of up to:

         2^32 blocks  =  4,294,967,296 blocks
                      = 68,719,476,736 octets

   This construction provides more key stream for each packet than is
   needed to handle any IPv6 Jumbogram [JUMBO].

5.  AAD Construction

   The data integrity and data origin authentication for the Security
   Parameters Index (SPI) and (Extended) Sequence Number fields is
   provided without encrypting them.  Two formats are defined: one for
   32-bit sequence numbers and one for 64-bit extended sequence numbers.
   The format with 32-bit sequence numbers is shown in Figure 3, and the
   format with 64-bit extended sequence numbers is shown in Figure 4.

   Sequence Numbers are conveyed canonical network byte order.  Extended
   Sequence Numbers are conveyed canonical network byte order, placing
   the high-order 32 bits first and the low-order 32 bits second.
   Canonical network byte order is fully described in RFC 791, Appendix
   B.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               SPI                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     32-bit Sequence Number                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 3.  AAD Format with 32-bit Sequence Number

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               SPI                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 64-bit Extended Sequence Number               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 4.  AAD Format with 64-bit Extended Sequence Number

6.  Packet Expansion

   The initialization vector (IV) and the integrity check value (ICV)
   are the only sources of packet expansion.  The IV always adds 8
   octets to the front of the payload.  The ICV is added at the end of
   the payload, and the CCM parameter M determines the size of the ICV.
   Implementations MUST support M values of 8 octets and 16 octets, and
   implementations MAY also support an M value of 12 octets.

7.  IKE Conventions

   This section describes the conventions used to generate keying
   material and salt values for use with AES CCM using the Internet Key
   Exchange (IKE) [IKE] protocol.  The identifiers and attributes needed
   to negotiate a security association that uses AES CCM are also
   defined.

7.1.  Keying Material and Salt Values

   As previously described, implementations MUST use fresh keys with AES
   CCM.  IKE can be used to establish fresh keys.  This section
   describes the conventions for obtaining the unpredictable salt value
   for use in the nonce from IKE.  Note that this convention provides a
   salt value that is secret as well as unpredictable.

   IKE makes use of a pseudo-random function (PRF) to derive keying
   material.  The PRF is used iteratively to derive keying material of
   arbitrary size, called KEYMAT.  Keying material is extracted from the
   output string without regard to boundaries.

   The size of KEYMAT MUST be three octets longer than is needed for the
   associated AES key.  The keying material is used as follows:

      AES CCM with a 128-bit key
         The KEYMAT requested for each AES CCM key is 19 octets.  The
         first 16 octets are the 128-bit AES key, and the remaining
         three octets are used as the salt value in the counter block.

      AES CCM with a 192-bit key
         The KEYMAT requested for each AES CCM key is 27 octets.  The
         first 24 octets are the 192-bit AES key, and the remaining
         three octets are used as the salt value in the counter block.

      AES CCM with a 256-bit key
         The KEYMAT requested for each AES CCM key is 35 octets.  The
         first 32 octets are the 256-bit AES key, and the remaining
         three octets are used as the salt value in the counter block.

7.2.  Phase 1 Identifier

   This document does not specify the conventions for using AES CCM for
   IKE Phase 1 negotiations.  For AES CCM to be used in this manner, a
   separate specification is needed, and an Encryption Algorithm
   Identifier needs to be assigned.

7.3.  Phase 2 Identifier

   For IKE Phase 2 negotiations, IANA has assigned three ESP Transform
   Identifiers for AES CCM with an explicit IV:

      14 for AES CCM with an 8-octet ICV;
      15 for AES CCM with a 12-octet ICV; and
      16 for AES CCM with a 16-octet ICV.

7.4.  Key Length Attribute

   Because the AES supports three key lengths, the Key Length attribute
   MUST be specified in the IKE Phase 2 exchange [DOI].  The Key Length
   attribute MUST have a value of 128, 192, or 256.

8.  Test Vectors

   Section 8 of [CCM] provides test vectors that will assist
   implementers with AES CCM mode.

9.  Security Considerations

   AES CCM employs counter (CTR) mode for confidentiality.  If a counter
   value is ever used for more that one packet with the same key, then
   the same key stream will be used to encrypt both packets, and the
   confidentiality guarantees are voided.

   What happens if the encryptor XORs the same key stream with two
   different packet plaintexts?  Suppose two packets are defined by two
   plaintext byte sequences P1, P2, P3 and Q1, Q2, Q3, then both are
   encrypted with key stream K1, K2, K3.  The two corresponding
   ciphertexts are:

      (P1 XOR K1), (P2 XOR K2), (P3 XOR K3)

      (Q1 XOR K1), (Q2 XOR K2), (Q3 XOR K3)

   If both of these two ciphertext streams are exposed to an attacker,
   then a catastrophic failure of confidentiality results, because:

      (P1 XOR K1) XOR (Q1 XOR K1) = P1 XOR Q1
      (P2 XOR K2) XOR (Q2 XOR K2) = P2 XOR Q2
      (P3 XOR K3) XOR (Q3 XOR K3) = P3 XOR Q3

   Once the attacker obtains the two plaintexts XORed together, it is
   relatively straightforward to separate them.  Thus, using any stream
   cipher, including AES CTR, to encrypt two plaintexts under the same
   key stream leaks the plaintext.

   Therefore, AES CCM should not be used with statically configured
   keys.  Extraordinary measures would be needed to prevent the reuse of
   a counter block value with the static key across power cycles.  To be
   safe, implementations MUST use fresh keys with AES CCM.  The IKE
   [IKE] protocol can be used to establish fresh keys.

   When IKE is used to establish fresh keys between two peer entities,
   separate keys are established for the two traffic flows.  If a
   different mechanism is used to establish fresh keys, one that

   establishes only a single key to encrypt packets, then there is a
   high probability that the peers will select the same IV values for
   some packets.  Thus, to avoid counter block collisions, ESP
   implementations that permit use of the same key for encrypting and
   decrypting packets with the same peer MUST ensure that the two peers
   assign different salt values to the security association (SA).

   Regardless of the mode used, AES with a 128-bit key is vulnerable to
   the birthday attack after 2^64 blocks are encrypted with a single
   key.  Since ESP with Extended Sequence Numbers allows for up to 2^64
   packets in a single SA, there is real potential for more than 2^64
   blocks to be encrypted with one key.  Implementations SHOULD generate
   a fresh key before 2^64 blocks are encrypted with the same key, or
   implementations SHOULD make use of the longer AES key sizes.  Note
   that ESP with 32-bit Sequence Numbers will not exceed 2^64 blocks
   even if all of the packets are maximum-length Jumbograms.

10.  Design Rationale

   In the development of this specification, the use of the ESP sequence
   number field instead of an explicit IV field was considered.  This
   section documents the rationale for the selection of an explicit IV.
   This selection is not a cryptographic security issue, as either
   approach will prevent counter block collisions.

   The use of the explicit IV does not dictate the manner that the
   encryptor uses to assign the per-packet value in the counter block.
   This is desirable for several reasons.

      1.  Only the encryptor can ensure that the value is not used for
      more than one packet, so there is no advantage to selecting a
      mechanism that allows the decryptor to determine whether counter
      block values collide.  Damage from the collision is done, whether
      the decryptor detects it or not.

      2.  The use of explicit IVs allows adders, LFSRs, and any other
      technique that meets the time budget of the encryptor, so long as
      the technique results in a unique value for each packet.  Adders
      are simple and straightforward to implement, but due to carries,
      they do not execute in constant time.  LFSRs offer an alternative
      that executes in constant time.

      3.  Complexity is in control of the implementer.  Furthermore, the
      decision made by the implementer of the encryptor does not make
      the decryptor more (or less) complex.

      4.  The assignment of the per-packet counter block value needs to
      be inside the assurance boundary.  Some implementations assign the
      sequence number inside the assurance boundary, but others do not.
      A sequence number collision does not have the dire consequences,
      but, as described in Section 6, a collision in counter block
      values has disastrous consequences.

      5.  Using the sequence number as the IV is possible in those
      architectures where the sequence number assignment is performed
      within the assurance boundary.  In this situation, the sequence
      number and the IV field will contain the same value.

      6.  By decoupling the IV and the sequence number, architectures
      where the sequence number assignment is performed outside the
      assurance boundary are accommodated.

   The use of an explicit IV field directly follows from the decoupling
   of the sequence number and the per-packet counter block value.  The
   additional overhead (64 bits for the IV field) is acceptable.  This
   overhead is significantly less overhead associated with Cipher Block
   Chaining (CBC) mode.  As normally employed, CBC requires a full block
   for the IV and, on average, half of a block for padding.  AES CCM
   confidentiality processing with an explicit IV has about one-third of
   the overhead as AES CBC, and the overhead is constant for each
   packet.

11.  IANA Considerations

   IANA has assigned three ESP transform numbers for use with AES CCM
   with an explicit IV:

      14 for AES CCM with an 8-octet ICV;
      15 for AES CCM with a 12-octet ICV; and
      16 for AES CCM with a 16-octet ICV.

12.  Acknowledgements

   Doug Whiting and Niels Ferguson worked with me to develop CCM mode.
   We developed CCM mode as part of the IEEE 802.11i security effort.
   One of the most attractive aspects of CCM mode is that it is
   unencumbered by patents.  I acknowledge the companies that supported
   the development of an unencumbered authenticated encryption mode (in
   alphabetical order):

      Hifn
      Intersil
      MacFergus
      RSA Security

   Also, I thank Tero Kivinen for his comprehensive review of this
   document.

13.  References

   This section provides normative and informative references.

13.1.  Normative References

   [AES]       NIST, FIPS PUB 197, "Advanced Encryption Standard (AES),"
               November 2001.

   [DOI]       Piper, D., "The Internet IP Security Domain of
               Interpretation for ISAKMP", RFC 2407, November 1998.

   [ESP]       Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
               4303, December 2005.

   [CCM]       Whiting, D., Housley, R., and N. Ferguson, "Counter with
               CBC-MAC (CCM)", RFC 3610, September 2003.

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

13.2.  Informative References

   [ARCH]      Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", RFC 4301, December 2005.

   [IKE]       Harkins, D. and D. Carrel, "The Internet Key Exchange
               (IKE)", RFC 2409, November 1998.

   [IKEv2]     Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
               RFC 4306, December 2005.

   [ROAD]      Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
               Document Roadmap", RFC 2411, November 1998.

   [JUMBO]     Borman, D., Deering, S., and R. Hinden, "IPv6
               Jumbograms", RFC 2675, August 1999.

Author's Address

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   EMail: housley@vigilsec.com

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