Internet Draft IPsec Working Group
November 2001 S. Frankel, NIST
Expiration Date: May 2002 S. Kelly, SonicWALL
R. Glenn, NIST
The AES Cipher Algorithm and Its Use With IPsec
<draft-ietf-ipsec-ciph-aes-cbc-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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This document is a submission to the IETF Internet Protocol Security
(IPsec) Working Group. Comments are solicited and should be addressed
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Distribution of this memo is unlimited.
Abstract
This document describes the use of the AES Cipher Algorithm in Cipher
Block Chaining Mode, with an explicit IV, as a confidentiality mecha-
nism within the context of the IPsec Encapsulating Security Payload
(ESP).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Specification of Requirements . . . . . . . . . . . . . . . 3
2. The AES Cipher Algorithm . . . . . . . . . . . . . . . . . . . . 3
2.1 Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Key Size . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Weak Keys . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4 Block Size and Padding . . . . . . . . . . . . . . . . . . . 4
2.5 Rounds . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.6 Additional Information . . . . . . . . . . . . . . . . . . . 5
2.7 Performance . . . . . . . . . . . . . . . . . . . . . . . . 5
3. ESP Payload . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 ESP Algorithmic Interactions . . . . . . . . . . . . . . . . 6
3.2 Keying Material . . . . . . . . . . . . . . . . . . . . . . 6
4. IKE Interactions . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 Phase 1 Identifier . . . . . . . . . . . . . . . . . . . . . 6
4.2 Phase 2 Identifier . . . . . . . . . . . . . . . . . . . . . 6
4.3 Key Length Attribute . . . . . . . . . . . . . . . . . . . . 6
4.4 Diffie-Hellman Groups . . . . . . . . . . . . . . . . . . . 6
4.4.1 Relative Strength . . . . . . . . . . . . . . . . . . 7
4.5 Hash Algorithm Considerations . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
7. Intellectual Property Rights Statement . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
As the culmination of a four-year competitive process, NIST (the Na-
tional Institute of Standards and Technology) has selected the AES
(Advanced Encryption Standard), the successor to the venerable DES.
The competition was an open one, with public participation and com-
ment solicited at each step of the process. The AES, formerly known
as Rijndael, was chosen from a field of five finalists.
The final AES selection was made on the basis of several additional
characteristics:
+ 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, and
will be definitively described in a FIPS (Federal Information Pro-
cessing Standard), expected to be completed by summer 2001. The
expectation is that the AES will suffice to protect sensitive
(unclassified) government information at least until the next cen-
tury. 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].
1.1 Specification of Requirements
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].
2. The AES Cipher Algorithm
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.
2.1 Mode
No operational modes are currently defined for the AES cipher. NIST
is in the process of developing a modes of operation FIPS for AES
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[MODES]. However, the Cipher Block Chaining (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 Ini-
tialization Vector (IV) that is the same size as the block size. Use
of a randomly generated IV prevents generation of identical cipher-
text 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 [CRYPTO-S]. For the
use of CBC mode in ESP with 64-bit ciphers, see [CBC].
2.2 Key Size
Some cipher algorithms allow for variable sized keys, while others
only allow specific, pre-defined key sizes. The length of the key
typically correlates with the strength of the algorithm; thus larger
keys are usually harder to break than shorter ones.
This document specifies the default (i.e. MUST be supported) key size
for the AES cipher algorithm. The default key size that implementa-
tions MUST support for IPsec is 128 bits. In addition, implementa-
tions MAY support key sizes of 192 and 256 bits.
2.3 Weak Keys
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 com-
plexity that could weaken the intended security [EVALUATION].
2.4 Block Size and Padding
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
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within a 4-octet word). Additional padding MAY be included, as
specifed in [ESP], as long as the 16-octet blocksize is maintained.
2.5 Rounds
This variable determines how many times a block is encrypted. While
this variable MAY be negotiated, a default value MUST always exist
when it is not negotiated. Within IPsec, the AES MUST support 10
rounds, corresponding to the mandatory 128-bit keysize.
The AES's default number of rounds is 12 for a 192-bit keysize and 14
for a 256-bit keysize.
2.6 Additional Information
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 Rijn-
dael homepage contains the following statement: "Rijndael is avail-
able for free. You can use it for whatever purposes you want, irre-
spective of whether it is accepted as AES or not." AES's description
can be found in [RIJNDAEL]. The Rijndael homepage is:
http://www.esat.kuleuven.ac.be/~rijmen/rijndael/.
The AES homepage, http://www.nist.gov/aes, contains a wealth of in-
formation 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.
2.7 Performance
For a comparison table of the estimated speeds of AES and other ci-
pher algorithms, please see [PERF-1], [PERF-2], [PERF-3], or
[PERF-4]. The AES homepage has pointers to other analyses. The AES
cypher document [RIJNDAEL] also contains performance statistics.
3. ESP Payload
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
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algorithm being used. The IV MUST be chosen at random. Common prac-
tice is to use random data for the first IV and the last block of en-
crypted data from an encryption process as the IV for the next en-
cryption process.
Including the IV in each datagram ensures that decryption of each re-
ceived 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.
3.1 ESP Algorithmic Interactions
Currently, there are no known issues regarding interactions between
the AES and other aspects of ESP, such as use of certain authentica-
tion schemes.
3.2 Keying Material
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.
4. IKE Interactions
4.1 Phase 1 Identifier
For Phase 1 negotiations, IANA has assigned an Encryption Algorithm
ID of 7 for AES-CBC.
4.2 Phase 2 Identifier
For Phase 2 negotiations, IANA has assigned an ESP Transform Identi-
fier of 12 for ESP_AES.
4.3 Key Length Attribute
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 ex-
change [DOI].
4.4 Diffie-Hellman Groups
The Diffie-Hellman algorithm is the basis of cryptographic key ex-
change within IPsec. The algorithm may be implemented using either
"MODP" (modulus-exponent) groups or "EC" (elliptic curve) groups. The
general procedure is as follows: the initiator chooses a random expo-
nent x with K bits of entropy that is 2K bits in length (the K bits
may be hashed to produce 2K bits), and then computes g^x using the
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group operation:
X = g^x
For MODP the group operation is modular multiplication, while for EC
the operation is point addition on the curve. The notation "g^x"
means "iterate the group operation x times". X is then sent to the
responder. The responder chooses a secret number y, and similarly
computes
Y = g^y
which is in turn sent to the initiator. At this point, both the ini-
tiator and responder may compute a shared secret value by combining
their own secret value with the exponential and applying the group
operation:
Z = g^(xy) = Y^x = X^y
From Z, both derive identical cryptographic keys.
This description is simplified in the interest of brevity, and an in-
depth description of this mechanism is beyond the scope of this memo.
For further details, refer to the wealth of published literature on
this topic.
4.4.1 Relative Strength
The relative strength of the encryption keys derived via the Diffie-
Hellman exchange may be characterized in terms the randomness of the
participant's exponents and the strength of the Diffie-Hellman group;
if an exponent has at least 128 completely random bits, it is said
to have 128-bits of "entropy". If the Diffie-Hellman group cannot be
broken in less time than searching a 128-bit key space, then the de-
rived 128-bit key is said to have 128 bits of "strength". For an in-
depth discussion regarding relative strength of values derived from
DH exchanges, see [KEYLEN-1].
In some cases, one may choose to settle for an amount of entropy
which is less than that of a completely random key of the given size.
There are numerous reasons for making such a choice, among which
might include a concern for the computational effort required to com-
plete the key exchange. For example, the following table lists recom-
mended modulus and exponent sizes for various key lengths using ei-
ther MODP or EC groups.
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+===========+=================+================+===============+
| Key Size | Exponent Size | Modulus Size | Group Type |
+===========+=================+================+===============+
| 128 | 256 | 3240 | MODP |
+-----------+-----------------+----------------+---------------+
| 192 | 384 | 7945 | MODP |
+-----------+-----------------+----------------+---------------+
| 256 | 512 | 15430 | MODP |
+-----------+-----------------+----------------+---------------+
| 128 | 248 | 248 | EC2N |
+-----------+-----------------+----------------+---------------+
| 192 | 376 | 376 | EC2N |
+-----------+-----------------+----------------+---------------+
| 256 | 504 | 504 | EC2N |
+-----------+-----------------+----------------+---------------+
NOTE: This table is based on Section 4.5 in [KEYLEN-1] and on email
communications with Hilarie Orman [KEYLEN-2].
Note that the sizes of the moduli and exponents for the MODP groups
in the table above are very large, and the computational effort re-
quired to complete the exponentiation and modulo operations with such
large values is quite significant using hardware commonly available
in the year 2001. If such considerations are deemed important, then
keys larger than 128 bits SHOULD NOT be used. Further, if it is de-
termined that less than 128 bits of strength will suffice for the se-
curity requirements of the given application, then smaller exponents
and moduli may be used.
[GROUPS] defines four additional Diffie-Hellman MODP groups for IKE.
Two of these groups, a 3072-bit MODP group and a 4096-bit MODP group,
could be used to establish 128-bit AES keys. [IKE-ECC] defines four
additional Diffie-Hellman ECC groups for IKE. Two of these groups,
Group 8 and 9, both of which are 283-bit ECC groups, could be used to
establish 128-bit AES keys. Additional information about the rela-
tionship between the group governing a Diffie-Hellman exchange and
the symmetric keys derived from the exchange can be found in
[KEYLEN-1].
4.5 Hash Algorithm Considerations
A companion competition, to select the successor to SHA-1, the wide-
ly-used hash algorithm, recently concluded. The resulting hashes,
called SHA-256, SHA-384 and SHA-512 [SHA2-1] 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). IANA has already
assigned Phase 1 Hash Algorithm values of 4, 5 and 6 to SHA2-256,
SHA2-384, and SHA2-512. IANA has also assigned AH Transform Identi-
fiers of 5, 6 and 7 to AH_SHA2-256, AH_SHA2-384, and AH_SHA2-512.)
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
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128-bit keys.
5. Security Considerations
Implementations are encouraged to use the largest key sizes they can
when taking into account performance considerations for their partic-
ular hardware and software configuration. Note that encryption nec-
essarily impacts both sides of a secure channel, so such considera-
tion 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.
Because the AES algorithm is relatively new and has only undergone
limited cryptographic analysis, its use in IPsec implementations
should be considered experimental. Once NIST has published the AES
FIPS, and at the recommendation of cryptographic experts, AES should
become a default and mandatory-to-implement cipher algorithm for
IPsec.
For more information regarding the necessary use of random IV values,
see [CRYPTO-B].
For further security considerations, the reader is encouraged to read
[RIJNDAEL].
6. IANA Considerations
IANA has assigned Encryption Algorithm ID 7 to AES-CBC.
IANA has assigned ESP Transform Identifier 12 to ESP_AES.
7. Intellectual Property Rights Statement
Pursuant to the provisions of [RFC-2026], the authors represent that
they have disclosed the existence of any proprietary or intellectual
property rights in the contribution that are reasonably and personal-
ly known to the authors. The authors do not represent that they per-
sonally know of all potentially pertinent proprietary and intellectu-
al property rights owned or claimed by the organizations they repre-
sent or third parties.
The IETF takes no position regarding the validity or scope of any in-
tellectual property or other rights that might be claimed to pertain
to the implementation or use of the technology described in this doc-
ument 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.
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8. Acknowledgments
Portions of this text, as well as its general structure, were un-
abashedly 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.
9. References
[ARCH] Kent, S. and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
[CBC] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
Algorithms," RFC 2451, 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/probtxt.{ps, 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.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[EVALUATION]
Ferguson, N. and B. Schneier, "A Cryptographic
Evaluation of IPsec," Counterpane Internet Security,
Inc., January 2000.
[GROUPS] Kivinen, T. and M. Kojo, "More MODP Diffie-Hellman
groups for IKE," draft-ietf-ipsec-ike-modp-
groups-00.txt, October 2000.
[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.
[IKE-ECC] Panjwani, P. and Y. Poeluev, "Additional ECC Groups For
IKE," draft-ietf-ipsec-ike-ecc-groups-02.txt, May 2000.
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[KEYLEN-1] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys," draft-
orman-public-key-lengths-01.txt, August 2000.
[KEYLEN-2] Orman, H., email communications, February 2000.
[MODES] "Symmetric Key Block Cipher Modes of Operation."
http://www.nist.gov/modes
[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, "Efficiency Testing Table."
http://home.cyber.ee/helger/aes
[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.html
[RFC-2026] Bradner, S., "The Internet Standards Process --
Revision 3", RFC2026, October 1996.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC-2119, March 1997.
[RIJNDAEL] Daemen, J. and V. Rijman, "AES Proposal: Rijndael,"
NIST AES Proposal, Jun 1998.
http://csrc.nist.gov/encryption/aes/round2/AESAlgs/Rijndael/Rijndael.pdf
[ROAD] Thayer, R., N. Doraswamy and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.
[SHA2-1] "Descriptions of SHA-256, SHA-384, and SHA-512."
http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf.
10. Authors' Addresses
Sheila Frankel
NIST
820 West Diamond Ave.
Room 680
Gaithersburg, MD 20899
Phone: +1 (301) 975-3297
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Email: sheila.frankel@nist.gov
Scott Kelly
SonicWALL, Inc.
1160 Bordeaux Dr.
Sunnyvale, CA 94089
Phone: +1 (408) 745-9600
Email: skelly@sonicwall.com
Rob Glenn
NIST
820 West Diamond Ave.
Room 605
Gaithersburg, MD 20899
Phone: +1 (301) 975-3667
Email: rob.glenn@nist.gov
The IPsec working group can be contacted through the chairs:
Barbara Fraser
Cisco Systems Inc.
Email: byfraser@cisco.com
Theodore T'so
Massachusetts Institute of Technology
Email: tytso@mit.edu
11. Full Copyright Statement
Copyright (C) The Internet Society (1998). 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 doc-
ument itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other In-
ternet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights de-
fined in the Internet Standards process must be followed, or as re-
quired 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
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HERE-
IN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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