IPsec Working Group R. Housley Internet Draft Vigil Security expires in six months January 2003 Using AES CCM Mode With IPsec ESP Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This document is a submission to the IETF Internet Protocol Security (IPsec) Working Group. Please send comments on this document to the working group mailing list (ipsec@lists.tislabs.com). Distribution of this memo is unlimited. Abstract This document describes the use of AES CCM Mode, with an explicit initialization vector, as an IPsec Encapsulating Security Payload (ESP) mechanism to provide confidentiality, data origin authentication, connectionless integrity. Housley [Page 1] INTERNET DRAFT January 2003 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-CMM), with an explicit initialization vector (IV), as an IPsec Encapsulating Security Payload (ESP) [ESP] mechanism to provide confidentiality, data origin authentication, 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. 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 Housley [Page 2] INTERNET DRAFT January 2003 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 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 IV same 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 can be used to establish fresh keys. 3. ESP Payload The ESP payload is comprised of the IV followed by the ciphertext. Housley [Page 3] INTERNET DRAFT January 2003 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]. 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. Housley [Page 4] INTERNET DRAFT January 2003 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 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 ee 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Truncated SPI | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initialization Vector | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2. Nonce Format The components of the nonce are as follows: Truncated SPI The truncated SPI field is 24 bits. As the name implies, it contains the least significant 24 bits of the ESP SPI. 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 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. Housley [Page 5] INTERNET DRAFT January 2003 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 4. Packet Expansion The initialization vector (IV) and the integrity check value (ICV) is 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. 5. Test Vectors To be supplied. 6. 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 Housley [Page 6] INTERNET DRAFT January 2003 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, since: (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 Internet Key Exchange (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 SPI values to the security association (SA). Further, since the counter block only contains the least significant 24 bits of the SPI, such implementations MUST ensure that the two SPI values differ in the least significant 24 bits. AES with a 128-bit key is vulnerable to the birthday attack after 2^64 blocks are encrypted with a single key, regardless of the mode used. Since ESP with Extended Sequence Numbers allows for up to 2^64 packets in a single security association (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 Housley [Page 7] INTERNET DRAFT January 2003 will not exceed 2^64 blocks even if all of the packets are maximum- length Jumbograms. 7. 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. LSFRs offer an alternative that executes in constant time. 3. Complexity is in control of the implementer. Further, 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. Housley [Page 8] INTERNET DRAFT January 2003 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-CMM confidentiality processing with an explicit IV has about one-third of the overhead as AES-CBC, and the overhead is constant for each packet. 8. IANA Considerations IANA has assigned nine ESP transform numbers for use with AES-CCM with an explicit IV: for AES-CCM with a 128 bit AES key and an 8 octet ICV; for AES-CCM with a 192 bit AES key and a 12 octet ICV; for AES-CCM with a 256 bit AES key and a 16 octet ICV; for AES-CCM with a 128 bit AES key and an 8 octet ICV; for AES-CCM with a 192 bit AES key and a 12 octet ICV; for AES-CCM with a 256 bit AES key and a 16 octet ICV; for AES-CCM with a 128 bit AES key and an 8 octet ICV; for AES-CCM with a 192 bit AES key and a 12 octet ICV; and for AES-CCM with a 256 bit AES key and a 16 octet ICV. 9. 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 10. References This section provides normative and informative references. 10.1. Normative References [AES] NIST, FIPS PUB 197, "Advanced Encryption Standard (AES)," November 2001. Housley [Page 9] INTERNET DRAFT January 2003 [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)," Work In Progress. . [CMM] D. Whiting, D., Housley, R., and N. Ferguson, "Counter with CBC-MAC (CCM)," Work In Progress. . [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels," RFC 2119, March 1997. 10.2. Informative References [ARCH] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol," RFC 2401, November 1998. [IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)," RFC 2409, November 1998. [ROAD] Thayer, R., N. Doraswamy and R. Glenn, "IP Security Document Roadmap," RFC 2411, November 1998. 11. Author's Address Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA housley@vigilsec.com Housley [Page 10] INTERNET DRAFT January 2003 12. Full Copyright Statement Copyright (C) The Internet Society 2003. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Housley [Page 11]