GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3686

Network Working Group R. Housley Request for Comments: 3686 Vigil Security Category: Standards Track January 2004

       Using Advanced Encryption Standard (AES) Counter Mode
          With IPsec Encapsulating Security Payload (ESP)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

 This document describes the use of Advanced Encryption Standard (AES)
 Counter Mode, with an explicit initialization vector, as an IPsec
 Encapsulating Security Payload (ESP) confidentiality mechanism.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Conventions Used In This Document. . . . . . . . . . . .  2
 2.  AES Block Cipher . . . . . . . . . . . . . . . . . . . . . . .  2
     2.1.  Counter Mode . . . . . . . . . . . . . . . . . . . . . .  2
     2.2.  Key Size and Rounds. . . . . . . . . . . . . . . . . . .  5
     2.3.  Block Size . . . . . . . . . . . . . . . . . . . . . . .  5
 3.  ESP Payload. . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Initialization Vector. . . . . . . . . . . . . . . . . .  6
     3.2.  Encrypted Payload. . . . . . . . . . . . . . . . . . . .  6
     3.3.  Authentication Data. . . . . . . . . . . . . . . . . . .  6
 4.  Counter Block Format . . . . . . . . . . . . . . . . . . . . .  7
 5.  IKE Conventions. . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Keying Material and Nonces . . . . . . . . . . . . . . .  8
     5.2.  Phase 1 Identifier . . . . . . . . . . . . . . . . . . .  9
     5.3.  Phase 2 Identifier . . . . . . . . . . . . . . . . . . .  9
     5.4.  Key Length Attribute . . . . . . . . . . . . . . . . . .  9
 6.  Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . .  9
 7.  Security Considerations. . . . . . . . . . . . . . . . . . . . 12
 8.  Design Rationale . . . . . . . . . . . . . . . . . . . . . . . 14
 9.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 16

Housley Standards Track [Page 1] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 10. Intellectual Property Statement. . . . . . . . . . . . . . . . 16
 11. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 16
 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     12.1. Normative References . . . . . . . . . . . . . . . . . . 17
     12.2. Informative References . . . . . . . . . . . . . . . . . 17
 13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 18
 14. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 19

1. Introduction

 The National Institute of Standards and Technology (NIST) recently
 selected the Advanced Encryption Standard (AES) [AES], also known as
 Rijndael.  The AES is a block cipher, and it can be used in many
 different modes.  This document describes the use of AES Counter Mode
 (AES-CTR), with an explicit initialization vector (IV), as an IPsec
 Encapsulating Security Payload (ESP) [ESP] confidentiality mechanism.
 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 [ROADMAP].

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 Block Cipher

 This section contains a brief description of the relevant
 characteristics of the AES block cipher.  Implementation requirements
 are also discussed.

2.1. Counter Mode

 NIST has defined five modes of operation for AES and other FIPS-
 approved block ciphers [MODES].  Each of these modes has different
 characteristics.  The five modes are: ECB (Electronic Code Book), CBC
 (Cipher Block Chaining), CFB (Cipher FeedBack), OFB (Output
 FeedBack), and CTR (Counter).
 Only AES Counter mode (AES-CTR) is discussed in this specification.
 AES-CTR requires the encryptor to generate a unique per-packet value,
 and communicate this value to the decryptor.  This specification
 calls this per-packet value an initialization vector (IV).  The same
 IV and key combination MUST NOT be used more than once.  The

Housley Standards Track [Page 2] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 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).
 This specification calls for the use of a nonce for additional
 protection against precomputation attacks.  The nonce value need not
 be secret.  However, the nonce MUST be unpredictable prior to the
 establishment of the IPsec security association that is making use of
 AES-CTR.
 AES-CTR has many properties that make it an attractive encryption
 algorithm for in high-speed networking.  AES-CTR uses the AES block
 cipher to create a stream cipher.  Data is encrypted and decrypted by
 XORing with the key stream produced by AES encrypting sequential
 counter block values.  AES-CTR is easy to implement, and AES-CTR can
 be pipelined and parallelized.  AES-CTR also supports key stream
 precomputation.
 Pipelining is possible because AES has multiple rounds (see section
 2.2).  A hardware implementation (and some software implementations)
 can create a pipeline by unwinding the loop implied by this round
 structure.  For example, after a 16-octet block has been input, one
 round later another 16-octet block can be input, and so on.  In AES-
 CTR, these inputs are the sequential counter block values used to
 generate the key stream.
 Multiple independent AES encrypt implementations can also be used to
 improve performance.  For example, one could use two AES encrypt
 implementations in parallel, to process a sequence of counter block
 values, doubling the effective throughput.
 The sender can precompute the key stream.  Since the key stream does
 not depend on any data in the packet, the key stream can be
 precomputed once the nonce and IV are assigned.  This precomputation
 can reduce packet latency.  The receiver cannot perform similar
 precomputation because the IV will not be known before the packet
 arrives.
 AES-CTR uses the only AES encrypt operation (for both encryption and
 decryption), making AES-CTR implementations smaller than
 implementations of many other AES modes.
 When used correctly, AES-CTR provides a high level of
 confidentiality.  Unfortunately, AES-CTR is easy to use incorrectly.
 Being a stream cipher, any reuse of the per-packet value, called the
 IV, with the same nonce and key is catastrophic.  An IV collision
 immediately leaks information about the plaintext in both packets.
 For this reason, it is inappropriate to use this mode of operation

Housley Standards Track [Page 3] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 with static 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-CTR.  The Internet
 Key Exchange (IKE) [IKE] protocol can be used to establish fresh
 keys.  IKE can also provide the nonce value.
 With AES-CTR, it is trivial to use a valid ciphertext to forge other
 (valid to the decryptor) ciphertexts.  Thus, it is equally
 catastrophic to use AES-CTR without a companion authentication
 function.  Implementations MUST use AES-CTR in conjunction with an
 authentication function, such as HMAC-SHA-1-96 [HMAC-SHA].
 To encrypt a payload with AES-CTR, the encryptor partitions the
 plaintext, PT, into 128-bit blocks.  The final block need not be 128
 bits; it can be less.
    PT = PT[1] PT[2] ... PT[n]
 Each PT block is XORed with a block of the key stream to generate the
 ciphertext, CT.  The AES encryption of each counter block results in
 128 bits of key stream.  The most significant 96 bits of the counter
 block are set to the nonce value, which is 32 bits, followed by the
 per-packet IV value, which is 64 bits.  The least significant 32 bits
 of the counter block are initially set to one.  This counter value is
 incremented by one to generate subsequent counter blocks, each
 resulting in another 128 bits of key stream.  The encryption of n
 plaintext blocks can be summarized as:
    CTRBLK := NONCE || IV || ONE
    FOR i := 1 to n-1 DO
      CT[i] := PT[i] XOR AES(CTRBLK)
      CTRBLK := CTRBLK + 1
    END
    CT[n] := PT[n] XOR TRUNC(AES(CTRBLK))
 The AES() function performs AES encryption with the fresh key.
 The TRUNC() function truncates the output of the AES encrypt
 operation to the same length as the final plaintext block, returning
 the most significant bits.

Housley Standards Track [Page 4] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 Decryption is similar.  The decryption of n ciphertext blocks can be
 summarized as:
    CTRBLK := NONCE || IV || ONE
    FOR i := 1 to n-1 DO
      PT[i] := CT[i] XOR AES(CTRBLK)
      CTRBLK := CTRBLK + 1
    END
    PT[n] := CT[n] XOR TRUNC(AES(CTRBLK))

2.2. Key Size and Rounds

 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.

2.3. Block Size

 The AES has a block size of 128 bits (16 octets).  As such, when
 using AES-CTR, each AES encrypt operation generates 128 bits of key
 stream.  AES-CTR encryption is the XOR of the key stream with the
 plaintext.  AES-CTR decryption is the XOR of the key stream with the
 ciphertext.  If the generated key stream is longer than the plaintext
 or ciphertext, the extra key stream bits are simply discarded.  For
 this reason, AES-CTR does not require the plaintext to be padded to a
 multiple of the block size.  However, to provide limited traffic flow
 confidentiality, padding MAY be included, as specified in [ESP].

3. ESP Payload

 The ESP payload is comprised of the IV followed by the ciphertext.
 The payload field, as defined in [ESP], is structured as shown in
 Figure 1.

Housley Standards Track [Page 5] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

  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-CTR

3.1. Initialization Vector

 The AES-CTR 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 packets
 are lost or reordered.

3.2. Encrypted Payload

 The encrypted payload contains the ciphertext.
 AES-CTR mode does not require plaintext padding.  However, ESP does
 require padding to 32-bit word-align the authentication data.  The
 padding, Pad Length, and the Next Header MUST be concatenated with
 the plaintext before performing encryption, as described in [ESP].

3.3. Authentication Data

 Since it is trivial to construct a forgery AES-CTR ciphertext from a
 valid AES-CTR ciphertext, AES-CTR implementations MUST employ a non-
 NULL ESP authentication method.  HMAC-SHA-1-96 [HMAC-SHA] is a likely
 choice.

Housley Standards Track [Page 6] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

4. Counter Block Format

 Each packet conveys the IV that is necessary to construct the
 sequence of counter blocks used to generate the key stream necessary
 to decrypt the payload.  The AES counter block cipher block is 128
 bits.  Figure 2 shows the format of the counter block.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            Nonce                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Initialization Vector (IV)                   |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Block Counter                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 2.  Counter Block Format
 The components of the counter block are as follows:
 Nonce
    The Nonce field is 32 bits.  As the name implies, the nonce is a
    single use value.  That is, a fresh nonce value MUST be assigned
    for each security association.  It MUST be assigned at the
    beginning of the security association.  The nonce value need not
    be secret, but it MUST be unpredictable prior to the beginning of
    the security association.
 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.
 Block Counter
    The block counter field is the least significant 32 bits of the
    counter block.  The block counter begins with the value of one,
    and it is incremented to generate subsequent portions of the key
    stream.  The block counter is a 32-bit big-endian integer value.
 Using the encryption process described in section 2.1, this
 construction permits each packet to consist of up to:
    (2^32)-1 blocks  =  4,294,967,295 blocks
                     = 68,719,476,720 octets

Housley Standards Track [Page 7] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 This construction can produce enough key stream for each packet
 sufficient to handle any IPv6 jumbogram [JUMBO].

5. IKE Conventions

 This section describes the conventions used to generate keying
 material and nonces for use with AES-CTR using the Internet Key
 Exchange (IKE) [IKE] protocol.  The identifiers and attributes needed
 to negotiate a security association which uses AES-CTR are also
 defined.

5.1. Keying Material and Nonces

 As described in section 2.1, implementations MUST use fresh keys with
 AES-CTR.  IKE can be used to establish fresh keys.  This section
 describes the conventions for obtaining the unpredictable nonce value
 from IKE.  Note that this convention provides a nonce value that is
 secret as well as unpredictable.
 IKE makes use of a pseudo-random function (PRF) to derive keying
 material.  The PRF is used iteratively to derive keying material of
 arbitrary size, called KEYMAT.  Keying material is extracted from the
 output string without regard to boundaries.
 The size of the requested KEYMAT MUST be four octets longer than is
 needed for the associated AES key.  The keying material is used as
 follows:
 AES-CTR with a 128 bit key
    The KEYMAT requested for each AES-CTR key is 20 octets.  The first
    16 octets are the 128-bit AES key, and the remaining four octets
    are used as the nonce value in the counter block.
 AES-CTR with a 192 bit key
    The KEYMAT requested for each AES-CTR key is 28 octets.  The first
    24 octets are the 192-bit AES key, and the remaining four octets
    are used as the nonce value in the counter block.
 AES-CTR with a 256 bit key
    The KEYMAT requested for each AES-CTR key is 36 octets.  The first
    32 octets are the 256-bit AES key, and the remaining four octets
    are used as the nonce value in the counter block.

Housley Standards Track [Page 8] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

5.2. Phase 1 Identifier

 This document does not specify the conventions for using AES-CTR for
 IKE Phase 1 negotiations.  For AES-CTR to be used in this manner, a
 separate specification is needed, and an Encryption Algorithm
 Identifier needs to be assigned.

5.3. Phase 2 Identifier

 For IKE Phase 2 negotiations, IANA has assigned an ESP Transform
 Identifier of 13 for AES-CTR with an explicit IV.

5.4. Key Length Attribute

 Since the AES supports three key lengths, the Key Length attribute
 MUST be specified in the IKE Phase 2 exchange [DOI].  The Key Length
 attribute MUST have a value of 128, 192, or 256.

6. Test Vectors

 This section contains nine test vectors, which can be used to confirm
 that an implementation has correctly implemented AES-CTR.  The first
 three test vectors use AES with a 128 bit key; the next three test
 vectors use AES with a 192 bit key; and the last three test vectors
 use AES with a 256 bit key.
 Test Vector #1: Encrypting 16 octets using AES-CTR with 128-bit key
 AES Key          : AE 68 52 F8 12 10 67 CC 4B F7 A5 76 55 77 F3 9E
 AES-CTR IV       : 00 00 00 00 00 00 00 00
 Nonce            : 00 00 00 30
 Plaintext String : 'Single block msg'
 Plaintext        : 53 69 6E 67 6C 65 20 62 6C 6F 63 6B 20 6D 73 67
 Counter Block (1): 00 00 00 30 00 00 00 00 00 00 00 00 00 00 00 01
 Key Stream    (1): B7 60 33 28 DB C2 93 1B 41 0E 16 C8 06 7E 62 DF
 Ciphertext       : E4 09 5D 4F B7 A7 B3 79 2D 61 75 A3 26 13 11 B8
 Test Vector #2: Encrypting 32 octets using AES-CTR with 128-bit key
 AES Key          : 7E 24 06 78 17 FA E0 D7 43 D6 CE 1F 32 53 91 63
 AES-CTR IV       : C0 54 3B 59 DA 48 D9 0B
 Nonce            : 00 6C B6 DB
 Plaintext        : 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
                  : 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
 Counter Block (1): 00 6C B6 DB C0 54 3B 59 DA 48 D9 0B 00 00 00 01
 Key Stream    (1): 51 05 A3 05 12 8F 74 DE 71 04 4B E5 82 D7 DD 87
 Counter Block (2): 00 6C B6 DB C0 54 3B 59 DA 48 D9 0B 00 00 00 02
 Key Stream    (2): FB 3F 0C EF 52 CF 41 DF E4 FF 2A C4 8D 5C A0 37
 Ciphertext       : 51 04 A1 06 16 8A 72 D9 79 0D 41 EE 8E DA D3 88
                  : EB 2E 1E FC 46 DA 57 C8 FC E6 30 DF 91 41 BE 28

Housley Standards Track [Page 9] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 Test Vector #3: Encrypting 36 octets using AES-CTR with 128-bit key
 AES Key          : 76 91 BE 03 5E 50 20 A8 AC 6E 61 85 29 F9 A0 DC
 AES-CTR IV       : 27 77 7F 3F  4A 17 86 F0
 Nonce            : 00 E0 01 7B
 Plaintext        : 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
                  : 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
                  : 20 21 22 23
 Counter Block (1): 00 E0 01 7B 27 77 7F 3F 4A 17 86 F0 00 00 00 01
 Key Stream    (1): C1 CE 4A AB 9B 2A FB DE C7 4F 58 E2 E3 D6 7C D8
 Counter Block (2): 00 E0 01 7B 27 77 7F 3F 4A 17 86 F0 00 00 00 02
 Key Stream    (2): 55 51 B6 38 CA 78 6E 21 CD 83 46 F1 B2 EE 0E 4C
 Counter Block (3): 00 E0 01 7B 27 77 7F 3F 4A 17 86 F0 00 00 00 03
 Key Stream    (3): 05 93 25 0C 17 55 36 00 A6 3D FE CF 56 23 87 E9
 Ciphertext       : C1 CF 48 A8 9F 2F FD D9 CF 46 52 E9 EF DB 72 D7
                  : 45 40 A4 2B DE 6D 78 36 D5 9A 5C EA AE F3 10 53
                  : 25 B2 07 2F
 Test Vector #4: Encrypting 16 octets using AES-CTR with 192-bit key
 AES Key          : 16 AF 5B 14 5F C9 F5 79 C1 75 F9 3E 3B FB 0E ED
                  : 86 3D 06 CC FD B7 85 15
 AES-CTR IV       : 36 73 3C 14 7D 6D 93 CB
 Nonce            : 00 00 00 48
 Plaintext String : 'Single block msg'
 Plaintext        : 53 69 6E 67 6C 65 20 62 6C 6F 63 6B 20 6D 73 67
 Counter Block (1): 00 00 00 48 36 73 3C 14 7D 6D 93 CB 00 00 00 01
 Key Stream    (1): 18 3C 56 28 8E 3C E9 AA 22 16 56 CB 23 A6 9A 4F
 Ciphertext       : 4B 55 38 4F E2 59 C9 C8 4E 79 35 A0 03 CB E9 28
 Test Vector #5: Encrypting 32 octets using AES-CTR with 192-bit key
 AES Key          : 7C 5C B2 40 1B 3D C3 3C 19 E7 34 08 19 E0 F6 9C
                  : 67 8C 3D B8 E6 F6 A9 1A
 AES-CTR IV       : 02 0C 6E AD C2 CB 50 0D
 Nonce            : 00 96 B0 3B
 Plaintext        : 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
                  : 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
 Counter Block (1): 00 96 B0 3B 02 0C 6E AD C2 CB 50 0D 00 00 00 01
 Key Stream    (1): 45 33 41 FF 64 9E 25 35 76 D6 A0 F1 7D 3C C3 90
 Counter Block (2): 00 96 B0 3B 02 0C 6E AD C2 CB 50 0D 00 00 00 02
 Key Stream    (2): 94 81 62 0F 4E C1 B1 8B E4 06 FA E4 5E E9 E5 1F
 Ciphertext       : 45 32 43 FC 60 9B 23 32 7E DF AA FA 71 31 CD 9F
                  : 84 90 70 1C 5A D4 A7 9C FC 1F E0 FF 42 F4 FB 00

Housley Standards Track [Page 10] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 Test Vector #6: Encrypting 36 octets using AES-CTR with 192-bit key
 AES Key          : 02 BF 39 1E E8 EC B1 59 B9 59 61 7B 09 65 27 9B
                  : F5 9B 60 A7 86 D3 E0 FE
 AES-CTR IV       : 5C BD 60 27 8D CC 09 12
 Nonce            : 00 07 BD FD
 Plaintext        : 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
                  : 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
                  : 20 21 22 23
 Counter Block (1): 00 07 BD FD 5C BD 60 27 8D CC 09 12 00 00 00 01
 Key Stream    (1): 96 88 3D C6 5A 59 74 28 5C 02 77 DA D1 FA E9 57
 Counter Block (2): 00 07 BD FD 5C BD 60 27 8D CC 09 12 00 00 00 02
 Key Stream    (2): C2 99 AE 86 D2 84 73 9F 5D 2F D2 0A 7A 32 3F 97
 Counter Block (3): 00 07 BD FD 5C BD 60 27 8D CC 09 12 00 00 00 03
 Key Stream    (3): 8B CF 2B 16 39 99 B2 26 15 B4 9C D4 FE 57 39 98
 Ciphertext       : 96 89 3F C5 5E 5C 72 2F 54 0B 7D D1 DD F7 E7 58
                  : D2 88 BC 95 C6 91 65 88 45 36 C8 11 66 2F 21 88
                  : AB EE 09 35
 Test Vector #7: Encrypting 16 octets using AES-CTR with 256-bit key
 AES Key          : 77 6B EF F2 85 1D B0 6F 4C 8A 05 42 C8 69 6F 6C
                  : 6A 81 AF 1E EC 96 B4 D3 7F C1 D6 89 E6 C1 C1 04
 AES-CTR IV       : DB 56 72 C9 7A A8 F0 B2
 Nonce            : 00 00 00 60
 Plaintext String : 'Single block msg'
 Plaintext        : 53 69 6E 67 6C 65 20 62 6C 6F 63 6B 20 6D 73 67
 Counter Block (1): 00 00 00 60 DB 56 72 C9 7A A8 F0 B2 00 00 00 01
 Key Stream    (1): 47 33 BE 7A D3 E7 6E A5 3A 67 00 B7 51 8E 93 A7
 Ciphertext       : 14 5A D0 1D BF 82 4E C7 56 08 63 DC 71 E3 E0 C0
 Test Vector #8: Encrypting 32 octets using AES-CTR with 256-bit key
 AES Key          : F6 D6 6D 6B D5 2D 59 BB 07 96 36 58 79 EF F8 86
                  : C6 6D D5 1A 5B 6A 99 74 4B 50 59 0C 87 A2 38 84
 AES-CTR IV       : C1 58 5E F1 5A 43 D8 75
 Nonce            : 00 FA AC 24
 Plaintext        : 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
                  : 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
 Counter block (1): 00 FA AC 24 C1 58 5E F1 5A 43 D8 75 00 00 00 01
 Key stream    (1): F0 5F 21 18 3C 91 67 2B 41 E7 0A 00 8C 43 BC A6
 Counter block (2): 00 FA AC 24 C1 58 5E F1 5A 43 D8 75 00 00 00 02
 Key stream    (2): A8 21 79 43 9B 96 8B 7D 4D 29 99 06 8F 59 B1 03
 Ciphertext       : F0 5E 23 1B 38 94 61 2C 49 EE 00 0B 80 4E B2 A9
                  : B8 30 6B 50 8F 83 9D 6A 55 30 83 1D 93 44 AF 1C

Housley Standards Track [Page 11] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 Test Vector #9: Encrypting 36 octets using AES-CTR with 256-bit key
 AES Key          : FF 7A 61 7C E6 91 48 E4 F1 72 6E 2F 43 58 1D E2
                  : AA 62 D9 F8 05 53 2E DF F1 EE D6 87 FB 54 15 3D
 AES-CTR IV       : 51 A5 1D 70 A1 C1 11 48
 Nonce            : 00 1C C5 B7
 Plaintext        : 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
                  : 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
                  : 20 21 22 23
 Counter block (1): 00 1C C5 B7 51 A5 1D 70 A1 C1 11 48 00 00 00 01
 Key stream    (1): EB 6D 50 81 19 0E BD F0 C6 7C 9E 4D 26 C7 41 A5
 Counter block (2): 00 1C C5 B7 51 A5 1D 70 A1 C1 11 48 00 00 00 02
 Key stream    (2): A4 16 CD 95 71 7C EB 10 EC 95 DA AE 9F CB 19 00
 Counter block (3): 00 1C C5 B7 51 A5 1D 70 A1 C1 11 48 00 00 00 03
 Key stream    (3): 3E E1 C4 9B C6 B9 CA 21 3F 6E E2 71 D0 A9 33 39
 Ciphertext       : EB 6C 52 82 1D 0B BB F7 CE 75 94 46 2A CA 4F AA
                  : B4 07 DF 86 65 69 FD 07 F4 8C C0 B5 83 D6 07 1F
                  : 1E C0 E6 B8

7. Security Considerations

 When used properly, AES-CTR mode provides strong confidentiality.
 Bellare, Desai, Jokipii, Rogaway show in [BDJR] that the privacy
 guarantees provided by counter mode are at least as strong as those
 for CBC mode when using the same block cipher.
 Unfortunately, it is very easy to misuse this counter mode.  If
 counter block values are 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 plaintexts?  Suppose two plaintext byte sequences P1, P2,
 P3 and Q1, Q2, Q3 are both 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

Housley Standards Track [Page 12] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 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, stream ciphers, including AES-CTR, should not be used with
 static keys.  It is inappropriate to use AES-CTR with static keys.
 Extraordinary measures would be needed to prevent reuse of a counter
 block value with the static key across power cycles.  To be safe, ESP
 implementations MUST use fresh keys with AES-CTR.  The Internet Key
 Exchange (IKE) protocol [IKE] can be used to establish fresh keys.
 IKE can also be used to establish the nonce at the beginning of the
 security association.
 When IKE is used to establish fresh keys between two peer entities,
 separate keys are established for the two traffic flows.  When a
 mechanism other than IKE is used to establish fresh keys, and that
 mechanism 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
 outbound traffic and decrypting incoming traffic with the same peer
 MUST ensure that the two peers assign different Nonce values to the
 security association.
 Data forgery is trivial with CTR mode.  The demonstration of this
 attack is similar to the key stream reuse discussion above.  If a
 known plaintext byte sequence P1, P2, P3 is encrypted with key stream
 K1, K2, K3, then the attacker can replace the plaintext with one of
 his own choosing.  The ciphertext is:
    (P1 XOR K1), (P2 XOR K2), (P3 XOR K3)
 The attacker simply XORs a selected sequence Q1, Q2, Q3 with the
 ciphertext to obtain:
    (Q1 XOR (P1 XOR K1)), (Q2 XOR (P2 XOR K2)), (Q3 XOR (P3 XOR K3))
 Which is the same as:
    ((Q1 XOR P1) XOR K1), ((Q2 XOR P2) XOR K2), ((Q3 XOR P3) XOR K3)
 Decryption of the attacker-generated ciphertext will yield exactly
 what the attacker intended:
    (Q1 XOR P1), (Q2 XOR P2), (Q3 XOR P3)

Housley Standards Track [Page 13] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 Accordingly, ESP implementations MUST use of AES-CTR in conjunction
 with ESP authentication.
 Additionally, since AES has a 128-bit block size, regardless of the
 mode employed, the ciphertext generated by AES encryption becomes
 distinguishable from random values after 2^64 blocks are encrypted
 with a single key.  Since ESP with Enhanced Sequence Numbers allows
 for up to 2^64 packets in a single security association, there is
 real potential for more than 2^64 blocks to be encrypted with one
 key.  Therefore, implementations SHOULD generate a fresh key before
 2^64 blocks are encrypted with the same key.  Note that ESP with 32-
 bit Sequence Numbers will not exceed 2^64 blocks even if all of the
 packets are maximum-length IPv6 jumbograms [JUMBO].
 There are fairly generic precomputation attacks against all block
 cipher modes that allow a meet-in-the-middle attack against the key.
 These attacks require the creation and searching of huge tables of
 ciphertext associated with known plaintext and known keys.  Assuming
 that the memory and processor resources are available for a
 precomputation attack, then the theoretical strength of AES-CTR (and
 any other block cipher mode) is limited to 2^(n/2) bits, where n is
 the number of bits in the key.  The use of long keys is the best
 countermeasure to precomputation attacks.  Therefore, implementations
 that employ 128-bit AES keys should take precautions to make the
 precomputation attacks more difficult.  The unpredictable nonce value
 in the counter block significantly increases the size of the table
 that the attacker must compute to mount a successful attack.

8. 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
 selection is not a cryptographic security issue, as either approach
 will prevent counter block collisions.
 In a very conservative model of encryption security, at most 2^64
 blocks ought to be encrypted with AES-CTR under a single key.  Under
 this constraint, no more than 64 bits are needed to identify each
 packet within a security association.  Since the ESP extended
 sequence number is 64 bits, it is an obvious candidate for use as an
 implicit IV.  This would dictate a single method for the assignment
 of per-packet value in the counter block.  The use of an explicit IV
 does not dictate such a method, which is desirable for several
 reasons.

Housley Standards Track [Page 14] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 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. Allows adders, LFSRs, and any other technique that meets the time
    budget of the encryptor, so long as the technique results in a
    unique value for each packet.  Adders are simple and
    straightforward to implement, but due to carries, they do not
    execute in constant time.  LFSRs offer an alternative that
    executes in constant time.
 3. Complexity is in control of the implementer.  Further, the
    decision made by the implementer of the encryptor does not make
    the decryptor more (or less) complex.
 4. When the encryptor has more than one cryptographic hardware
    device, an IV prefix can be assigned to each device, ensuring that
    collisions will not occur.  Yet, since the decryptor does not need
    to examine IV structure, the decryptor is unaffected by the IV
    structure selected by the encryptor.  One cannot make use of the
    same technique with the ESP sequence numbers, because the
    semantics for them require sequential value generation.
 5.  Assurance boundaries are very important to implementations that
    will be evaluated against the FIPS Pub 140-1 or FIPS Pub 140-2
    [SECRQMTS].  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.
 6. Coupling with the sequence number 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.
 7. Decoupling from the sequence number is possible in those
    architectures where the sequence number assignment is performed
    outside the assurance boundary.
 The use of an explicit IV field directly follows from the decoupling
 of the sequence number and the per-packet counter block value.  The
 overhead associated with 64 bits for the IV field is acceptable.
 This overhead is significantly less than the overhead associated with
 Cipher Block Chaining (CBC) mode.  As normally employed, CBC requires

Housley Standards Track [Page 15] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 a full block for the IV and, on average, half of a block for padding.
 AES-CTR with an explicit IV has about one-third of the overhead as
 AES-CBC, and the overhead is constant for each packet.
 The inclusion of the nonce provides a weak countermeasure against
 precomputation attacks.  For this countermeasure to be effective, the
 attacker must not be able to predict the value of the nonce well in
 advance of security association establishment.  The use of long keys
 provides a strong countermeasure to precomputation attacks, and AES
 offers key sizes that thwart these attacks for many decades to come.
 A 28-bit block counter value is sufficient for the generation of a
 key stream to encrypt the largest possible IPv6 jumbogram [JUMBO];
 however, a 32-bit field is used.  This size is convenient for both
 hardware and software implementations.

9. IANA Considerations

 IANA has assigned 13 as the ESP transform number for AES-CTR with an
 explicit IV.

10. Intellectual Property Statement

 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 implementors 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.

11. Acknowledgements

 This document is the result of extensive discussions and compromises.
 While not all of the participants are completely satisfied with the
 outcome, the document is better for their contributions.

Housley Standards Track [Page 16] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 The author thanks the members of the IPsec working group for their
 contributions to the design, with special mention of the efforts of
 (in alphabetical order) Steve Bellovin, David Black, Niels Ferguson,
 Charlie Kaufman, Steve Kent, Tero Kivinen, Paul Koning, David McGrew,
 Robert Moskowitz, Jesse Walker, and Doug Whiting.
 The author thanks and Alireza Hodjat, John Viega, and Doug Whiting
 for assistance with the test vectors.

12. References

 This section provides normative and informative references.

12.1. Normative References

 [AES]       NIST, FIPS PUB 197, "Advanced Encryption Standard (AES)",
             November 2001.
 [DOI]       Piper, D., "The Internet IP Security Domain of
             Interpretation for ISAKMP", RFC 2407, November 1998.
 [ESP]       Kent, S. and R. Atkinson, "IP Encapsulating Security
             Payload (ESP)", RFC 2406, November 1998.
 [MODES]     Dworkin, M., "Recommendation for Block Cipher Modes of
             Operation: Methods and Techniques", NIST Special
             Publication 800-38A, December 2001.
 [STDWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

12.2. Informative References

 [ARCH]      Kent, S. and R. Atkinson, "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.
 [BDJR]      Bellare, M, Desai, A., Jokipii, E. and P. Rogaway, "A
             Concrete Security Treatment of Symmetric Encryption:
             Analysis of the DES Modes of Operation", Proceedings 38th
             Annual Symposium on Foundations of Computer Science,
             1997.
 [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.

Housley Standards Track [Page 17] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

 [JUMBO]     Borman, D., Deering, S. and R. Hinden, "IPv6 Jumbograms",
             RFC 2675, August 1999.
 [ROADMAP]   Thayer, R., Doraswamy, N. and R. Glenn, "IP Security
             Document Roadmap", RFC 2411, November 1998.
 [SECRQMTS]  National Institute of Standards and Technology.  FIPS Pub
             140-1: Security Requirements for Cryptographic Modules.
             11 January 1994.
             National Institute of Standards and Technology.  FIPS Pub
             140-2: Security Requirements for Cryptographic Modules.
             25 May 2001. [Supercedes FIPS Pub 140-1]

13. Author's Address

 Russell Housley
 Vigil Security, LLC
 918 Spring Knoll Drive
 Herndon, VA 20170
 USA
 EMail: housley@vigilsec.com

Housley Standards Track [Page 18] RFC 3686 Using AES Counter Mode With IPsec ESP January 2004

14. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  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 assignees.
 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Housley Standards Track [Page 19]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3686.txt · Last modified: 2004/01/28 17:06 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki