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Network Working Group                                         R. HousleyRequest for Comments: 4309                                Vigil SecurityCategory: Standards Track                                  December 2005Using Advanced Encryption Standard (AES) CCM Modewith 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 Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................22. AES CCM Mode ....................................................23. ESP Payload .....................................................43.1. Initialization Vector (IV) .................................43.2. Encrypted Payload ..........................................43.3. Authentication Data ........................................54. Nonce Format ....................................................55. AAD Construction ................................................66. Packet Expansion ................................................77. IKE Conventions .................................................77.1. Keying Material and Salt Values ............................77.2. Phase 1 Identifier .........................................87.3. Phase 2 Identifier .........................................87.4. Key Length Attribute .......................................88. Test Vectors ....................................................89. Security Considerations .........................................810. Design Rationale ...............................................9Housley                     Standards Track                     [Page 1]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 200511. IANA Considerations ...........................................1112. Acknowledgements ..............................................1113. References ....................................................1113.1. Normative References .....................................1113.2. Informative References ...................................121.  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.Housley                     Standards Track                     [Page 2]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 2005   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 inSection 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 inSection 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 acrossHousley                     Standards Track                     [Page 3]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 2005   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 CCM3.1.  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 concatenatedHousley                     Standards Track                     [Page 4]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 2005    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 inSection 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.Housley                     Standards Track                     [Page 5]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 2005   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 inRFC 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 NumberHousley                     Standards Track                     [Page 6]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 20056.  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.Housley                     Standards Track                     [Page 7]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 20057.2.  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:Housley                     Standards Track                     [Page 8]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 2005      (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.Housley                     Standards Track                     [Page 9]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 2005      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 inSection 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.Housley                     Standards Track                    [Page 10]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 200511.  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)",RFC4303, 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.Housley                     Standards Track                    [Page 11]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 200513.2.  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.comHousley                     Standards Track                    [Page 12]

RFC 4309           Using AEC CCM Mode with IPsec ESP       December 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Housley                     Standards Track                    [Page 13]