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Network Working Group Request for Comments: 3602 Category: Standards Track |
S. Frankel R. Glenn NIST S. Kelly Airespace September 2003 |
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 © The Internet Society (2003). All Rights Reserved.
This document describes the use of the Advanced Encryption Standard (AES) Cipher Algorithm in Cipher Block Chaining (CBC) Mode, with an explicit Initialization Vector (IV), as a confidentiality mechanism within the context of the IPsec Encapsulating Security Payload (ESP).
1. Introduction
1.1. Specification of Requirements
2. The AES Cipher Algorithm
2.1. Mode
2.2. Key Size and Number of Rounds
2.3. Weak Keys
2.4. Block Size and Padding
2.5. Additional Information
2.6. Performance
3. ESP Payload
3.1. ESP Algorithmic Interactions
3.2. Keying Material
4. Test Vectors
5. IKE Interactions
5.1. Phase 1 Identifier
5.2. Phase 2 Identifier
5.3. Key Length Attribute
5.4. Hash Algorithm Considerations
6. Security Considerations
7. IANA Considerations
8. Intellectual Property Rights Statement
9. References
9.1. Normative References
9.2. Informative References
10. Acknowledgments
11. Authors' Addresses
12. Full Copyright Statement
As the culmination of a four-year competitive process, NIST (the National Institute of Standards and Technology) has selected the AES (Advanced Encryption Standard), the successor to the venerable DES (Data Encryption Standard). The competition was an open one, with public participation and comment solicited at each step of the process. The AES [AES], formerly known as Rijndael, was chosen from a field of five finalists.
The AES selection was made on the basis of several characteristics:
+ security
+ unclassified
+ publicly disclosed
+ available royalty-free, worldwide
+ capable of handling a block size of at least 128 bits
+ at a minimum, capable of handling key sizes of 128, 192, and
256 bits
+ computational efficiency and memory requirements on a variety
of software and hardware, including smart cards
+ flexibility, simplicity and ease of implementation
The AES will be the government's designated encryption cipher. The expectation is that the AES will suffice to protect sensitive (unclassified) government information until at least the next century. It is also expected to be widely adopted by businesses and financial institutions.
It is the intention of the IETF IPsec Working Group that AES will eventually be adopted as the default IPsec ESP cipher and will obtain the status of MUST be included in compliant IPsec implementations.
The remainder of this document specifies the use of the AES within the context of IPsec ESP. For further information on how the various pieces of ESP fit together to provide security services, refer to [ARCH], [ESP], and [ROAD].
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" that appear in this document are to be interpreted as described in [RFC-2119].
All symmetric block cipher algorithms share common characteristics and variables, including mode, key size, weak keys, block size, and rounds. The following sections contain descriptions of the relevant characteristics of the AES cipher.
NIST has defined 5 modes of operation for AES and other FIPS-approved ciphers [MODES]: CBC (Cipher Block Chaining), ECB (Electronic CodeBook), CFB (Cipher FeedBack), OFB (Output FeedBack) and CTR (Counter). The CBC mode is well-defined and well-understood for symmetric ciphers, and is currently required for all other ESP ciphers. This document specifies the use of the AES cipher in CBC mode within ESP. This mode requires an Initialization Vector (IV) that is the same size as the block size. Use of a randomly generated IV prevents generation of identical ciphertext from packets which have identical data that spans the first block of the cipher algorithm's block size.
The IV is XOR'd with the first plaintext block before it is encrypted. Then for successive blocks, the previous ciphertext block is XOR'd with the current plaintext, before it is encrypted.
More information on CBC mode can be obtained in [MODES, CRYPTO-S]. For the use of CBC mode in ESP with 64-bit ciphers, see [CBC].
AES supports three key sizes: 128 bits, 192 bits, and 256 bits. The default key size is 128 bits, and all implementations MUST support this key size. Implementations MAY also support key sizes of 192 bits and 256 bits.
AES uses a different number of rounds for each of the defined key sizes. When a 128-bit key is used, implementations MUST use 10 rounds. When a 192-bit key is used, implementations MUST use 12 rounds. When a 256-bit key is used, implementations MUST use 14 rounds.
At the time of writing this document there are no known weak keys for the AES.
Some cipher algorithms have weak keys or keys that MUST not be used due to their interaction with some aspect of the cipher's definition. If weak keys are discovered for the AES, then weak keys SHOULD be checked for and discarded when using manual key management. When using dynamic key management, such as [IKE], weak key checks SHOULD NOT be performed as they are seen as an unnecessary added code complexity that could weaken the intended security [EVALUATION].
The AES uses a block size of sixteen octets (128 bits).
Padding is required by the AES to maintain a 16-octet (128-bit) blocksize. Padding MUST be added, as specified in [ESP], such that the data to be encrypted (which includes the ESP Pad Length and Next Header fields) has a length that is a multiple of 16 octets.
Because of the algorithm specific padding requirement, no additional padding is required to ensure that the ciphertext terminates on a 4- octet boundary (i.e., maintaining a 16-octet blocksize guarantees that the ESP Pad Length and Next Header fields will be right aligned within a 4-octet word). Additional padding MAY be included, as specified in [ESP], as long as the 16-octet blocksize is maintained.
AES was invented by Joan Daemen from Banksys/PWI and Vincent Rijmen from ESAT-COSIC, both in Belgium, and is available world-wide on a royalty-free basis. It is not covered by any patents, and the Rijndael homepage contains the following statement: "Rijndael is
available for free. You can use it for whatever purposes you want, irrespective of whether it is accepted as AES or not." AES's description can be found in [AES]. The Rijndael homepage is: http://www.esat.kuleuven.ac.be/~rijmen/rijndael/.
The AES homepage, http://www.nist.gov/aes, contains a wealth of information about the AES, including a definitive description of the AES algorithm, performance statistics, test vectors and intellectual property information. This site also contains information on how to obtain an AES reference implementation from NIST.
For a comparison table of the estimated speeds of AES and other cipher algorithms, please see [PERF-1], [PERF-2], [PERF-3], or [PERF-4]. The AES homepage has pointers to other analyses.
The ESP payload is made up of the IV followed by raw cipher-text. Thus the payload field, as defined in [ESP], is broken down according to the following diagram:
+---------------+---------------+---------------+---------------+ | | + Initialization Vector (16 octets) + | | +---------------+---------------+---------------+---------------+ | | ~ Encrypted Payload (variable length, a multiple of 16 octets) ~ | | +---------------------------------------------------------------+
The IV field MUST be the same size as the block size of the cipher algorithm being used. The IV MUST be chosen at random, and MUST be unpredictable.
Including the IV in each datagram ensures that decryption of each received datagram can be performed, even when some datagrams are dropped, or datagrams are re-ordered in transit.
To avoid CBC encryption of very similar plaintext blocks in different packets, implementations MUST NOT use a counter or other low-Hamming distance source for IVs.
Currently, there are no known issues regarding interactions between
the AES and other aspects of ESP, such as use of certain
authentication schemes.
The minimum number of bits sent from the key exchange protocol to the ESP algorithm must be greater than or equal to the key size.
The cipher's encryption and decryption key is taken from the first
<x> bits of the keying material, where <x> represents the required key size.
The first 4 test cases test AES-CBC encryption. Each test case includes the key, the plaintext, and the resulting ciphertext. The values of keys and data are either hexadecimal numbers (prefixed by "0x") or ASCII character strings (surrounded by double quotes). If a value is an ASCII character string, then the AES-CBC computation for the corresponding test case DOES NOT include the trailing null character ('\0') of the string. The computed cyphertext values are all hexadecimal numbers.
The last 4 test cases illustrate sample ESP packets using AES-CBC for encryption. All data are hexadecimal numbers (not prefixed by "0x").
These test cases were verified using 2 independent implementations:
the NIST AES-CBC reference implementation and an implementation
provided by the authors of the Rijndael algorithm
(http://csrc.nist.gov/encryption/aes/rijndael/
rijndael-unix-refc.tar).
For Phase 1 negotiations, IANA has assigned an Encryption Algorithm ID of 7 for AES-CBC.
For Phase 2 negotiations, IANA has assigned an ESP Transform Identifier of 12 for ESP_AES.
Since the AES allows variable key lengths, the Key Length attribute MUST be specified in both a Phase 1 exchange [IKE] and a Phase 2 exchange [DOI].
A companion competition, to select the successor to SHA-1, the widely-used hash algorithm, recently concluded. The resulting hashes, called SHA-256, SHA-384 and SHA-512 [SHA2-1, SHA2-2] are capable of producing output of three different lengths (256, 384 and 512 bits), sufficient for the generation (within IKE) and authentication (within ESP) of the three AES key sizes (128, 192 and 256 bits).
However, HMAC-SHA-1 [HMAC-SHA] and HMAC-MD5 [HMAC-MD5] are currently considered of sufficient strength to serve both as IKE generators of 128-bit AES keys and as ESP authenticators for AES encryption using 128-bit keys.
Implementations are encouraged to use the largest key sizes they can when taking into account performance considerations for their particular hardware and software configuration. Note that encryption necessarily impacts both sides of a secure channel, so such consideration must take into account not only the client side, but the server as well. However, a key size of 128 bits is considered secure for the foreseeable future.
For more information regarding the necessary use of random IV values, see [CRYPTO-B].
For further security considerations, the reader is encouraged to read [AES].
IANA has assigned Encryption Algorithm ID 7 to AES-CBC.
IANA has assigned ESP Transform Identifier 12 to ESP_AES.
The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication 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 Secretariat.
The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director.
[AES] NIST, FIPS PUB 197, "Advanced Encryption Standard
(AES)," November 2001.
http://csrc.nist.gov/publications/fips/fips197/
fips-197.{ps,pdf}
[CBC] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
Algorithms", RFC 2451, November 1998.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[ARCH] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[CRYPTO-B] Bellovin, S., "Probable Plaintext Cryptanalysis of the
IP Security Protocols", Proceedings of the Symposium on
Network and Distributed System Security, San Diego, CA,
pp. 155-160, February 1997.
http://www.research.att.com/~smb/papers/probtxt.pdf
[CRYPTO-S] B. Schneier, "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995, ISBN 0-471-12845-7.
[DOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[EVALUATION] Ferguson, N. and B. Schneier, "A Cryptographic
Evaluation of IPsec," Counterpane Internet Security,
Inc., January 2000.
http://www.counterpane.com/ipsec.pdf
[HMAC-MD5] Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within
ESP and AH", RFC 2403, November 1998.
[HMAC-SHA] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
within ESP and AH", RFC 2404, November 1998.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[MODES] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Methods and Techniques," NIST Special
Publication 800-38A, December 2001.
http://csrc.nist.gov/publications/nistpubs/
800-38a/sp800-38a.pdf
[PERF-1] Bassham, L. III, "Efficiency Testing of ANSI C
Implementations of Round1 Candidate Algorithms for the
Advanced Encryption Standard."
http://csrc.nist.gov/encryption/aes/round1/r1-ansic.pdf
[PERF-2] Lipmaa, Helger, "AES/Rijndael: speed."
http://www.tcs.hut.fi/~helger/aes/rijndael.html
[PERF-3] Nechvetal, J., E. Barker, D. Dodson, M. Dworkin, J.
Foti and E. Roback, "Status Report on the First Round of
the Development of the Advanced Encryption Standard."
http://csrc.nist.gov/encryption/aes/round1/r1report.pdf
[PERF-4] Schneier, B., J. Kelsey, D. Whiting, D. Wagner, C.
Hall, and N. Ferguson, "Performance Comparison of the
AES Submissions."
http://www.counterpane.com/aes-performance.pdf
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[ROAD] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.
[SHA2-1] NIST, FIPS PUB 180-2 "Specifications for the Secure Hash
Standard," August 2002.
http://csrc.nist.gov/publications/fips/fips180-2/
fips180-2.pdf
[SHA2-2] "Descriptions of SHA-256, SHA-384, and SHA-512."
http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf
Portions of this text, as well as its general structure, were unabashedly lifted from [CBC].
The authors want to thank Hilarie Orman for providing expert advice (and a sanity check) on key sizes, requirements for Diffie-Hellman groups, and IKE interactions. We also thank Scott Fluhrer for his helpful comments and recommendations.
Sheila Frankel
NIST
820 West Diamond Ave.
Room 677
Gaithersburg, MD 20899
Phone: +1 (301) 975-3297
EMail: sheila.frankel@nist.gov
Scott Kelly
Airespace
110 Nortech Pkwy
San Jose CA 95134
Phone: +1 408 635 2000
EMail: scott@hyperthought.com
Rob Glenn
NIST
820 West Diamond Ave.
Room 605
Gaithersburg, MD 20899
Phone: +1 (301) 975-3667
EMail: rob.glenn@nist.gov
Copyright © The Internet Society (2003). All Rights Reserved.
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