IPSECME S. Shen
Internet-Draft Huawei
Updates: RFC4307 Y. Mao
(if approved) H3C
Expires: March 19, 2010 NSS. Murthy
Freescale Semiconductor
September 15, 2009
Using Advanced Encryption Standard (AES) Counter Mode with IKEv2
draft-ietf-ipsecme-aes-ctr-ikev2-02
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Abstract
This document describes the usage of Advanced Encryption Standard
Counter Mode (AES_CTR), with an explicit initialization vector, by
IKEv2 for encrypting the IKEv2 exchanges that follow the IKE_SA_INIT
exchange.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used In This Document . . . . . . . . . . . . 3
2. AES Counter Mode . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Counter Mode . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Key Sizes and Rounds . . . . . . . . . . . . . . . . . . . 6
2.3. Block Size . . . . . . . . . . . . . . . . . . . . . . . . 7
3. IKEv2 Encrypted Payload . . . . . . . . . . . . . . . . . . . 8
3.1. Initialization Vector(IV) . . . . . . . . . . . . . . . . 8
3.2. Integrity Checksum Data . . . . . . . . . . . . . . . . . 8
3.3. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Counter Block Format . . . . . . . . . . . . . . . . . . . . . 9
5. IKEv2 Conventions . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Keying Material and Nonces . . . . . . . . . . . . . . . . 11
5.2. Encryption identifier . . . . . . . . . . . . . . . . . . 12
5.3. Key Length Attribute . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
IKEv2 [RFC4306] is a component of IPsec used for performing mutual
authentication and establishing and maintaining security associations
(SAs). [RFC4307] defines the set of algorithms that are mandatory to
implement as part of IKEv2, as well as algorithms that should be
implemented because they may be promoted to mandatory at some future
time. [RFC4307] requires that an implementation "SHOULD" support
Advanced Encryption Standard [AES] in Counter Mode [MODES] (AES_CTR)
as a Transform Type 1 Algorithm (encryption).
Although the [RFC4307] specifies that the AES_CTR encryption
algorithm feature SHOULD be supported by IKEv2, no existing document
specifies how IKEv2 can support the feature. This document provides
the specification and usage of AES-CTR counter mode by IKEv2.
All the IKEv2 messages that follow the initial exchange(IKE_SA_INIT)
are cryptographically protected using the cryptographic algorithms
and keys negotiated in the first two messages of the IKEv2 exchange.
These subsequent messages use the syntax of the IKEv2 Encrypted
Payload as explained in [RFC4306].
This document explains how IKEv2 makes use of AES_CTR algorithm for
encrypting IKE messages that follow initial exchange: The second pair
of messages (IKE_AUTH) in initial exchange, messages in
CREATE_CHILD_SA exchange, messages in INFORMATIONAL exchange.
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 [RFC2119].
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2. AES Counter Mode
AES [AES] is a symmetric block cipher that can process data blocks of
128 bits, using cipher keys with lengths of 128, 192, and 256 bits.
The use of AES algorithm operations in IKEv2 is the same as what
defined in [AES]. The use of Counter Mode is defined the same as how
AES_CTR is used to encrypt ESP payload [RFC3686]. The choices of Key
Size, Rounds and Block Size are defined as following which are
compatible with [RFC3686].
2.1. Counter Mode
This section gives description for AES Counter Mode algorithm and
cites algorithm description part in section 2.1 of [RFC3686]
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
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
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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
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 [RFC4306] 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 [RFC2404].
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
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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.
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 Sizes and Rounds
AES supports three key sizes: 128 bits, 192 bits, and 256 bits. All
IKEv2 implementations that implement AES-CTR MUST support the 128 key
size. An IKEv2 implementation MAY support key sizes of 192 and 256
bits.
AES MUST use different rounds for each of the 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.
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2.3. Block Size
The AES algorithm has a block size of 128 bits (16 octets), i.e., AES
generate 128 bits of key stream. For encryption or decryption, a
user XOR the key stream with 128 bits of plaintext or ciphertext
blocks. 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.
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3. IKEv2 Encrypted Payload
Section 3.14 of IKEv2 [RFC4306] explains the IKEv2 Encrypted Payload.
The encrypted Payload, denoted SK{...} contains other IKEv2 payloads
in encrypted form.
The payload includes an Initialization Vector(IV) whose length is
defined by the encryption algorithm negotiated. It also includes
Integrity Checksum data. These two fields are not encrypted.
3.1. Initialization Vector(IV)
The IV field MUST be eight octets when AES_CTR algorithm is used for
encryption. The IV MUST be chosen by the encryptor in a manner that
ensures that the same IV value is NOT used more than once with a
given encryption 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).
3.2. Integrity Checksum Data
Since it is trivial to construct a forgery AES_CTR ciphertext from a
valid AES_CTR ciphertext, an integrity algorithm must be used when
using AES_CTR. IKEv2 does require Integrity Checksum Data for
Encrypted Payload as described in section 3.14 of [RFC4306]. The
choice of integrity algorithms in IKEv2 is defined in [RFC4307] or
its future update documents.
3.3. Padding
AES-CTR does not require the plaintext to be padded to a multiple of
the block size. For the Padding field in the Encrypted Payload, as
required in [RFC4306]: the sender SHOULD set the Pad Length to the
minimum value that makes the combination of the Payloads, the
Padding, and the Pad Length a multiple of the block size, but the
recipient MUST accept any length that results in proper alignment.
In this case when AES-CTR is used in IKEv2, the Padding field of the
Encrypted Payload SHOULD be empty, and the Pad Length field SHOULD be
zero.
It should be noticed that ESP [RFC4303] Encrypted Payload requires
alignment on a 4-byte boundary while IKEv2 [RFC4306] Encrypted
Payload does not have such a requirement.
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4. Counter Block Format
All the IKEv2 messages following the initial exchange are
cryptographically protected using the cryptographic algorithms and
keys negotiated in the first two messages of the IKEv2 exchange.
These subsequent messages use the syntax of the IKEv2 Encrypted
Payload.
The Encrypted Payload is the XOR of the plaintext and key stream.
The key stream is generated by inputing Counter Blocks into AES
algorithm. The AES counter block cipher block is 128 bits. Counter
Blocks are defined as in Figure 1.
All messages carry the IV that is necessary to construct the sequence
of counter blocks used to generate the key stream necessary to
decrypt the payload.
Figure 1 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 1: 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 (IV)
The IV field is 64 bits. 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 encryption key. The encryptor includes the IV in the
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IKEv2 message containing encrypted payloads.
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.
Section 2 provides references to other documents for implementing
AES_CTR encryption/decryption process.
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5. IKEv2 Conventions
This section describes the conventions used by IKEv2 protocol to
generate encryption keys and nonces for use with AES-CTR algorithm in
IKE-SA negotiation. The identifiers and attributes related to AES-
CTR required during IKE-SA and Child-SA negotiation are also defined.
5.1. Keying Material and Nonces
IKEv2 can be used to establish fresh keys and nonces, as the same
combination of IV and encryption key values MUST not be reused when
AES_CTR algorithm is used for encryption. This section describes the
conventions for generating an unpredictable and secret Nonce and an
encryption key of required lengths using IKEv2.
IKEv2 negotiates four cryptographic algorithms with its peer using
IKE_SA_INIT exchange. They include an encryption algorithm and a
pseudo-random function(PRF). All the payloads of IKEv2 messages that
follow the IKE_SA_INIT exchange are encrypted using the negotiated
encryption algorithm. The pseudo-random function(PRF)is used to
generate the keying material required for the encryption algorithm.
AES_CTR encryption algorithm needs an encryption key and a nonce.
The two directions of traffic flow use different encryption keys and
nonces. Section 2.14 of [RFC4306] details the process of generating
the keying material. SK_ei and SK_er represent the key material to
be used for encryption purposes in the two directions.
The size of the key material (SK_ei and SK_er) to be generated for
AES_CTR algorithm for different key lengths is as follows:
AES_CTR with a 128 bit key
The key material required 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 key material required 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 key material required 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.
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5.2. Encryption identifier
IKEv2 uses the IANA allocated encryption identifier of 13 for
ENCR_AES_CTR with an explicit IV (ENCR_AES_CTR 13) as the transform
ID during IKE-SA and Child-SA negotiation.
5.3. Key Length Attribute
Since the AES_CTR algorithm supports three key lengths, the Key
Length attribute MUST be specified in both the IKE-SA and Child-SA
negotiations. The Key Length attribute MUST have a value of 128,
192, or 256.
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6. Security Considerations
Security considerations explained in section 7 of [RFC3686] are
entirely relevant for this draft also.
AES_CTR provides high confidentiality when used properly. However,
as a stream mode cipher, the security of will lose when AES-CTR is
misused.
Generally, a stream cipher should not use static keys. This is
because key streams will be easily canceled when two ciphertext use
the same key stream (check detailed description of this attack in
[RFC3686]). Therefore, IKEv2 should avoid an identical key being
used for different IKE SA or a same key stream being used on
different blocks of plaintext. Proper use of Nonce and counter as
defined in Section 4 can successfully avoid the risk.
A stream cipher like AES_CTR is also vulnerable under data forgery
attack (check [RFC3686] for a demonstration of this attack).
However, when integrity protection is provided as Section 3.2
requires, this risk is avoided.
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 IKEv2 are not likely to carry traffics in
such a high quantity, this won't be a big concern here. However,
when large amount of traffic appear in the future or under abnormal
circumstances, implementations SHOULD generate a fresh key before
2^64 blocks are encrypted with the same key.
For generic attacks on AES, such as brute force or precalculations,
the requirement of key size provides reasonable security
[Recommendations].
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7. IANA Considerations
IANA has assigned 13 as the transform ID for ENCR_AES_CTR encryption
with an explicit IV. This ID is to be used during IKE_SA
negotiation.
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8. Acknowledgments
The authors thank Yaron Sheffer, Paul Hoffman and Tero Kivinen for
their direction and comments on this document.
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9. References
9.1. Normative References
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4307] Schiller, J., "Cryptographic Algorithms for Use in the
Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
December 2005.
[AES] National Institute of Standards and Technology, "Advanced
Encryption Standard (AES)", FIPS PUB 197, November 2001, <
http://csrc.nist.gov/publications/fips/fips197/
fips-197.pdf>.
[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>.
9.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2404] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
ESP and AH", RFC 2404, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[draft-ietf-ipsecme-roadmap-02]
Sheila, S. and S. Suresh, "IP Security (IPsec) and
Internet Key Exchange (IKE) Document Roadmap",
draft-ietf-ipsecme-roadmap-02 (work in progress),
July 2009.
[Recommendations]
Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
"Recommendation for Key Management - Part1 -
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General(Revised)", NIST Special Publication 800-57,
March 2007, <http://csrc.nist.gov/publications/nistpubs/
800-57/SP800-57-Part1.pdf>.
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Authors' Addresses
Sean Shen
Huawei
4, South 4th Street, Zhongguancun
Beijing 100190
China
Email: sean.s.shen@gmail.com
Yu Mao
H3C Tech. Co., Ltd
Oriental Electronic Bld.
No.2 Chuangye Road
Shang-Di Information Industry
Hai-Dian District
Beijing 100085
China
Email: maoyu@h3c.com
N S Srinivasa Murthy
Freescale Semiconductor
UMA PLAZA, NAGARJUNA CIRCLE, PUNJAGUTTA
HYDERABAD 500082
INDIA
Email: ssmurthy.nittala@freescale.com
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