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Versions: 00 01 02 03 04 RFC 3602

Internet Draft                                       IPsec Working Group
June 2002                                               S. Frankel, NIST
Expiration Date: December 2002            S. Kelly, Black Storm Networks
                                                          R. Glenn, NIST


            The AES Cipher Algorithm and Its Use With IPsec
                 <draft-ietf-ipsec-ciph-aes-cbc-04.txt>




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
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   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-Drafts 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. Comments are solicited and should be addressed
   to the working group mailing list (ipsec@lists.tislabs.com) or to the
   editors.

   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).













Frankel,Glenn,Kelly                                             [Page 1]

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                             Table of Contents


 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .   3
    1.1 Specification of Requirements  . . . . . . . . . . . . . . .   3
 2. The AES Cipher Algorithm . . . . . . . . . . . . . . . . . . . .   4
    2.1 Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
    2.2 Key Size . . . . . . . . . . . . . . . . . . . . . . . . . .   4
    2.3 Weak Keys  . . . . . . . . . . . . . . . . . . . . . . . . .   4
    2.4 Block Size and Padding . . . . . . . . . . . . . . . . . . .   5
    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. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . . .   6
 5. IKE Interactions . . . . . . . . . . . . . . . . . . . . . . . .  10
    5.1 Phase 1 Identifier . . . . . . . . . . . . . . . . . . . . .  10
    5.2 Phase 2 Identifier . . . . . . . . . . . . . . . . . . . . .  10
    5.3 Key Length Attribute . . . . . . . . . . . . . . . . . . . .  10
    5.4 Diffie-Hellman Groups  . . . . . . . . . . . . . . . . . . .  10
        5.4.1 Relative Strength  . . . . . . . . . . . . . . . . . .  11
    5.5 Hash Algorithm Considerations  . . . . . . . . . . . . . . .  12
 6. Security Considerations  . . . . . . . . . . . . . . . . . . . .  12
 7. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . .  12
 8. Intellectual Property Rights Statement . . . . . . . . . . . . .  12
 9. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  15
12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  16


























Frankel,Glenn,Kelly                                             [Page 2]

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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 [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 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].





Frankel,Glenn,Kelly                                             [Page 3]

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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

   NIST has defined 5 modes of operation for AES and other FIPS-approved
   ciphers [MODES]: CBC (Cipher Block Chaining), ECB (Electronic Code-
   Book), CFB (Cipher FeedBack), OFB (Output FeedBack) and CTR
   (Counter).  The CBC mode is well-defined and well-understood for sym-
   metric 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].

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].





Frankel,Glenn,Kelly                                             [Page 4]

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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
   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 [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 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.

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:



Frankel,Glenn,Kelly                                             [Page 5]

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    +---------------+---------------+---------------+---------------+
    |                                                               |
    +               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 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. Test Vectors

   The first 4 test cases test AES-CBC encryption.  Each test case in-
   cludes 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 char-
   acter ('\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 pro-
   vided by the authors of the Rijndael algorithm



Frankel,Glenn,Kelly                                             [Page 6]

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   (http://csrc.nist.gov/encryption/aes/rijndael/
                          rijndael-unix-refc.tar).

   Case #1: Encrypting 16 bytes (1 blocks) using AES-CBC with 128-bit key
   Key       : 0x06a9214036b8a15b512e03d534120006
   IV        : 0x3dafba429d9eb430b422da802c9fac41
   Plaintext : "Single block msg"
   Ciphertext: 0xe353779c1079aeb82708942dbe77181a

   Case #2: Encrypting 32 bytes (2 blocks) using AES-CBC with 128-bit key
   Key       : 0xc286696d887c9aa0611bbb3e2025a45a
   IV        : 0x562e17996d093d28ddb3ba695a2e6f58
   Plaintext : 0x000102030405060708090a0b0c0d0e0f
                 101112131415161718191a1b1c1d1e1f
   Ciphertext: 0xd296cd94c2cccf8a3a863028b5e1dc0a
                 7586602d253cfff91b8266bea6d61ab1

   Case #3: Encrypting 48 bytes (3 blocks) using AES-CBC with 128-bit key
   Key       : 0x6c3ea0477630ce21a2ce334aa746c2cd
   IV        : 0xc782dc4c098c66cbd9cd27d825682c81
   Plaintext : "This is a 48-byte message (exactly 3 AES blocks)"
   Ciphertext: 0xd0a02b3836451753d493665d33f0e886
                 2dea54cdb293abc7506939276772f8d5
                 021c19216bad525c8579695d83ba2684

   Case #4: Encrypting 64 bytes (4 blocks) using AES-CBC with 128-bit key
   Key       : 0x56e47a38c5598974bc46903dba290349
   IV        : 0x8ce82eefbea0da3c44699ed7db51b7d9
   Plaintext : 0xa0a1a2a3a4a5a6a7a8a9aaabacadaeaf
                 b0b1b2b3b4b5b6b7b8b9babbbcbdbebf
                 c0c1c2c3c4c5c6c7c8c9cacbcccdcecf
                 d0d1d2d3d4d5d6d7d8d9dadbdcdddedf
   Ciphertext: 0xc30e32ffedc0774e6aff6af0869f71aa
                 0f3af07a9a31a9c684db207eb0ef8e4e
                 35907aa632c3ffdf868bb7b29d3d46ad
                 83ce9f9a102ee99d49a53e87f4c3da55

   Case #5: Sample transport-mode ESP packet (ping 192.168.123.100)
   Key: 90d382b4 10eeba7a d938c46c ec1a82bf
   SPI: 4321
   Source address: 192.168.123.3
   Destination address: 192.168.123.100
   Sequence number: 1
   IV: e96e8c08 ab465763 fd098d45 dd3ff893

   Original packet:
   IP header (20 bytes): 45000054 08f20000 4001f9fe c0a87b03 c0a87b64
   Data (64 bytes):
   08000ebd a70a0000 8e9c083d b95b0700 08090a0b 0c0d0e0f 10111213 14151617
   18191a1b 1c1d1e1f 20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637

   Augment data with:
   Padding: 01020304 05060708 090a0b0c 0d0e
   Pad length: 0e



Frankel,Glenn,Kelly                                             [Page 7]

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   Next header: 01 (ICMP)

   Pre-encryption Data with padding, pad length and next header (80 bytes):
   08000ebd a70a0000 8e9c083d b95b0700 08090a0b 0c0d0e0f 10111213 14151617
   18191a1b 1c1d1e1f 20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637
   01020304 05060708 090a0b0c 0d0e0e01

   Post-encryption packet with SPI, Sequence number, IV:
   IP header: 4500007c 08f20000 4032f9a5 c0a87b03 c0a87b64
   SPI/Seq #: 00004321 00000001
   IV: e96e8c08 ab465763 fd098d45 dd3ff893
   Encrypted Data (80 bytes):
   f663c25d 325c18c6 a9453e19 4e120849 a4870b66 cc6b9965 330013b4 898dc856
   a4699e52 3a55db08 0b59ec3a 8e4b7e52 775b07d1 db34ed9c 538ab50c 551b874a
   a269add0 47ad2d59 13ac19b7 cfbad4a6

   Case #6: Sample transport-mode ESP packet (ping -p 77 -s 20 192.168.123.100)
   Key: 90d382b4 10eeba7a d938c46c ec1a82bf
   SPI: 4321
   Source address: 192.168.123.3
   Destination address: 192.168.123.100
   Sequence number: 8
   IV: 69d08df7 d203329d b093fc49 24e5bd80

   Original packet:
   IP header (20 bytes): 45000030 08fe0000 4001fa16 c0a87b03 c0a87b64
   Data (28 bytes):
   0800b5e8 a80a0500 a69c083d 0b660e00 77777777 77777777 77777777

   Augment data with:
   Padding: 0102
   Pad length: 02
   Next header: 01 (ICMP)

   Pre-encryption Data with padding, pad length and next header (32 bytes):
   0800b5e8 a80a0500 a69c083d 0b660e00 77777777 77777777 77777777 01020201

   Post-encryption packet with SPI, Sequence number, IV:
   IP header: 4500004c 08fe0000 4032f9c9 c0a87b03 c0a87b64
   SPI/Seq #: 00004321 00000008
   IV: 69d08df7 d203329d b093fc49 24e5bd80
   Encrypted Data (32 bytes):
   f5199588 1ec4e0c4 488987ce 742e8109 689bb379 d2d750c0 d915dca3 46a89f75

   Case #7: Sample tunnel-mode ESP packet (ping 192.168.123.200)
   Key: 01234567 89abcdef 01234567 89abcdef
   SPI: 8765
   Source address: 192.168.123.3
   Destination address: 192.168.123.200
   Sequence number: 2
   IV: f4e76524 4f6407ad f13dc138 0f673f37

   Original packet:
   IP header (20 bytes): 45000054 09040000 4001f988 c0a87b03 c0a87bc8



Frankel,Glenn,Kelly                                             [Page 8]

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   Data (64 bytes):
   08009f76 a90a0100 b49c083d 02a20400 08090a0b 0c0d0e0f 10111213 14151617
   18191a1b 1c1d1e1f 20212223 24252627 28292a2b 2c2d2e2f 30313233 34353637

   Augment data with:
   Padding: 01020304 05060708 090a
   Pad length: 0a
   Next header: 04 (IP-in-IP)

   Pre-encryption Data with original IP header, padding, pad length and
                            next header (96 bytes):
   45000054 09040000 4001f988 c0a87b03 c0a87bc8 08009f76 a90a0100 b49c083d
   02a20400 08090a0b 0c0d0e0f 10111213 14151617 18191a1b 1c1d1e1f 20212223
   24252627 28292a2b 2c2d2e2f 30313233 34353637 01020304 05060708 090a0a04

   Post-encryption packet with SPI, Sequence number, IV:
   IP header: 4500008c 09050000 4032f91e c0a87b03 c0a87bc8
   SPI/Seq #: 00008765 00000002
   IV: f4e76524 4f6407ad f13dc138 0f673f37
   Encrypted Data (96 bytes):
   773b5241 a4c44922 5e4f3ce5 ed611b0c 237ca96c f74a9301 3c1b0ea1 a0cf70f8
   e4ecaec7 8ac53aad 7a0f022b 859243c6 47752e94 a859352b 8a4d4d2d ecd136e5
   c177f132 ad3fbfb2 201ac990 4c74ee0a 109e0ca1 e4dfe9d5 a100b842 f1c22f0d

   Case #8: Sample tunnel-mode ESP packet (ping -p ff -s 40 192.168.123.200)
   Key: 01234567 89abcdef 01234567 89abcdef
   SPI: 8765
   Source address: 192.168.123.3
   Destination address: 192.168.123.200
   Sequence number: 5
   IV: 85d47224 b5f3dd5d 2101d4ea 8dffab22

   Original packet:
   IP header (20 bytes): 45000044 090c0000 4001f990 c0a87b03 c0a87bc8
   Data (48 bytes):
   0800d63c aa0a0200 c69c083d a3de0300 ffffffff ffffffff ffffffff ffffffff
   ffffffff ffffffff ffffffff ffffffff

   Augment data with:
   Padding: 01020304 05060708 090a
   Pad length: 0a
   Next header: 04 (IP-in-IP)

   Pre-encryption Data with original IP header, padding, pad length and
                            next header (80 bytes):
   45000044 090c0000 4001f990 c0a87b03 c0a87bc8 0800d63c aa0a0200 c69c083d
   a3de0300 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
   ffffffff 01020304 05060708 090a0a04

   Post-encryption packet with SPI, Sequence number, IV:
   IP header: 4500007c 090d0000 4032f926 c0a87b03 c0a87bc8
   SPI/Seq #: 00008765 00000005
   IV: 85d47224 b5f3dd5d 2101d4ea 8dffab22
   Encrypted Data (80 bytes):



Frankel,Glenn,Kelly                                             [Page 9]

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   15b92683 819596a8 047232cc 00f7048f e45318e1 1f8a0f62 ede3c3fc 61203bb5
   0f980a08 c9843fd3 a1b06d5c 07ff9639 b7eb7dfb 3512e5de 435e7207 ed971ef3
   d2726d9b 5ef6affc 6d17a0de cbb13892

5. IKE Interactions

5.1 Phase 1 Identifier

   For Phase 1 negotiations, IANA has assigned an Encryption Algorithm
   ID of 7 for AES-CBC.

5.2 Phase 2 Identifier

   For Phase 2 negotiations, IANA has assigned an ESP Transform Identi-
   fier of 12 for ESP_AES.

5.3 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].

5.4 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
   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.




Frankel,Glenn,Kelly                                            [Page 10]

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   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.

5.4.1 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].

   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.


   +===========+=================+================+===============+
   | 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]

   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 2002. 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.




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   [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].

5.5 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, 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).  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 Trans-
   form Identifiers 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
   128-bit keys.

6. 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.

   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].

7. IANA Considerations

   IANA has assigned Encryption Algorithm ID 7 to AES-CBC.
   IANA has assigned ESP Transform Identifier 12 to ESP_AES.

8. 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



Frankel,Glenn,Kelly                                            [Page 12]

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   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.

9. 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.  We also thank Scott Fluhrer for his
   helpful comments and recommendations.

10. References


     [AES]       NIST, FIPS PUB 197, "Advanced Encryption Standard
                 (AES)," November 2001.
        http://csrc.nist.gov/publications/fips/fips197/fips-197.{ps,pdf}

     [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/papers/probtxt.{ps, pdf}

     [CRYPTO-S]  B. Schneier, "Applied Cryptography Second Edition",
                 John Wiley & Sons, New York, NY, 1995, ISBN
                 0-471-12845-7.





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     [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.
                 http://www.counterpane.com/ipsec.{pdf,ps.zip}

     [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.

     [KEYLEN]    Orman, H. and P. Hoffman, "Determining Strengths For
                 Public Keys Used For Exchanging Symmetric Keys," draft-
                 orman-public-key-lengths-01.txt, August 2000.

     [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."



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                 http://www.counterpane.com/aes-performance.{pdf,ps.zip}

     [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.

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

     [SHA2-1]    NIST, Draft FIPS PUB 180-2 "Specifications for the
                 Secure Hash Standard," May 2001.
                 http://csrc.nist.gov/encryption/shs/dfips-180-2.pdf

     [SHA2-2]    "Descriptions of SHA-256, SHA-384, and SHA-512."
                 http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf


11. Authors' Addresses


        Sheila Frankel
        NIST
        820 West Diamond Ave.
        Room 680
        Gaithersburg, MD 20899
        Phone: +1 (301) 975-3297
        Email: sheila.frankel@nist.gov

        Scott Kelly
        Black Storm Networks
        250 Cambridge Ave
        Palo Alto CA 94304
        Phone: +1 (650) 617-2934
        Email: scott@bstormnetworks.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 Ts'o
        Massachusetts Institute of Technology
        Email: tytso@mit.edu



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12. 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
   "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 HERE-
   IN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
   CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.































Frankel,Glenn,Kelly                                            [Page 16]


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