Internet DRAFT - draft-ietf-tls-ctr
draft-ietf-tls-ctr
Network Working Group N. Modadugu
Internet-Draft Stanford University
Expires: December 15, 2006 E. Rescorla
Network Resonance
June 13, 2006
AES Counter Mode Cipher Suites for TLS and DTLS
draft-ietf-tls-ctr-01.txt
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Copyright (C) The Internet Society (2006).
Abstract
This document describes the use of the Advanced Encryption Standard
(AES) Counter Mode for use as a Transport Layer Security (TLS) and
Datagram Transport Layer Security (DTLS) confidentiality mechanism.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used In This Document . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Encrypting Records with AES Counter Mode . . . . . . . . . . . 4
3.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Encryption . . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. Decryption . . . . . . . . . . . . . . . . . . . . . . 5
3.1.3. Counter Block Construction . . . . . . . . . . . . . . 5
3.2. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Session Resumption . . . . . . . . . . . . . . . . . . . . 7
4. Design Rationale . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Maximum Key Lifetime . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
Transport Layer Security [3] provides channel-oriented security for
application layer protocols. In TLS, cryptographic algorithms are
specified in "Cipher Suites, which consist of a group of algorithms
to be used together."
Cipher suites supported by TLS are divided into stream and block
ciphers. Counter mode ciphers behave like stream ciphers, but are
constructed based on a block cipher primitive (that is, counter mode
operation of a block cipher results in a stream cipher.) This
specification is limited to discussion of the operation of AES in
counter mode (AES-CTR.)
Counter mode ciphers (CTR) offer a number of attractive features over
other block cipher modes and stream ciphers such as RC4:
Low Bandwidth: AES-CTR provides a saving of 17-32 bytes per record
compared to AES-CBC as used in TLS 1.1 and DTLS. 16 bytes are
saved from not having to transmit an explicit IV, and another 1-16
bytes are saved from the absence of the padding block.
Random Access: AES-CTR is capable of random access within the key
stream. For DTLS, this implies that records can be processed out
of order without dependency on packet arrival order, and also
without keystream buffering.
Parallelizable: As a consequence of AES-CTR supporting random access
within the key stream, making the cipher amenable to parallelizing
and pipelining in hardware.
Multiple mode support: AES-CTR support in TLS/DTLS allows for
implementator to support both a stream (CTR) and block (CBC)
cipher through the implementation of a single symmetric algorithm.
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 [1].
2. Terminology
This document reuses some terminology introduced in [2] and [3]. The
term 'counter block' has the same meaning as used in [2]. However,
the term 'IV' in this document, holds the meaning defined in [3].
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3. Encrypting Records with AES Counter Mode
AES-CTR is functionally equivalent to a stream cipher; it generates a
pseudo-random cipher stream that is XORed into the plaintext to form
ciphertext.
The cipher stream is generated by applying the AES encrypt operation
on a sequence of 128-bit counter blocks. Counter blocks, in turn,
are generated based on record sequence numbers (in the case of TLS),
or a combination of record sequence and epoch numbers (in the case of
DTLS.)
It should be noted that although the client and server use the same
sequence number space, they use different write keys and counter
blocks.
There is one important constraint on the use of counter mode ciphers:
for a given key, a counter block value MUST never be used more than
once.
This constraint is required because a given key and counter block
value completely specify a portion of the cipher stream. Hence, a
particular counter block value when used (with a given key) to
generate more than one ciphertext leaks information about the
corresponding plaintexts. For a detailed explanation, see Section 7
of [2].
Given this constraint, the challenge then is in the design of the
counter block. We describe the construction of the counter block in
the following sections.
TLS/DTLS records encrypted with AES-CTR mode use a
CipherSpec.cipher_type of GenericStreamCipher (Section 6.2.3 of [3]).
3.1. TLS
AES counter mode requires the encryptor and decryptor to share a per-
record unique counter block. As previously stated, a given counter
block MUST never be used more than once with the same key. The
following description of AES-CTR mode has been adapted from [2].
3.1.1. Encryption
To encrypt a payload with AES-CTR, the encryptor sequentially
partitions the plaintext (PT) into 128-bit blocks. The final PT
block MAY be less than 128-bits. This partitioning is denoted as:
PT = PT[1] PT[2] ... PT[n]
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In order to encrypt, each PT block is XORed with a block of the key
stream to generate the ciphertext (CT.) The keystream is generated
via the AES encryption of each counter block value, with each
encryption operation producing 128-bits of key stream.
The encryption operation is performed as follows:
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 leftmost bits.
3.1.2. Decryption
Decryption is similar to encryption. The decryption of n ciphertext
blocks is performed as follows:
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))
The AES() and TRUNC() operate identically as in the case of
encryption.
3.1.3. Counter Block Construction
To construct the counter block, the leftmost 48-bits of the counter
block are set to the rightmost 48-bits of the client_write_IV (for
the half-duplex stream originated by the client) or the rightmost 48-
bits of the server_write_IV (for the half-duplex stream originated by
the server.) The following 64-bits of the counter block are set to
record sequence number, and the remaining 16-bits function as the
block counter. The block counter is a 16-bit unsigned integer in
network byte order (i.e. big-endien). The block counter is initially
set to one, and is incremented by one to generate subsequent counter
blocks, each resulting in another 128-bits of key stream.
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The structure of the counter block is depicted below:
struct {
case client:
uint48 client_write_IV; // low order 48-bits
case server:
uint48 server_write_IV; // low order 48-bits
uint64 seq_num;
uint16 blk_ctr;
} CtrBlk;
The seq_num and blk_ctr fields of the counter block are initialized
for each record processed, while the IV is initialized immediately
after a key calculation is made (key calculations are made whenever a
TLS/DTLS handshake, either full or abbreviated, is executed.) seq_num
is set to the sequence number of the record, and blk_ctr is
initialized to 1.
Note that the block counter does not overflow since the maximum size
of input to the record payload protection layer in TLS or DTLS
(TLSCompressed.length) is 2^14 + 1024 octets, and 16 bits of blk_ctr
allow the generation of 2^20 octets (2^16 AES blocks) of keying
material per record.
Note that for TLS, no part of the counter block need be transmitted,
since the client_write_IV and server_write_IV are derived during the
key calculation phase, and the record sequence number is implicit.
3.2. DTLS
The operation of AES-CTR in DTLS is the same as in TLS, with the only
difference being the inclusion of the epoch in the counter block.
The counter block is constructed as follows for DTLS:
struct {
case client:
uint48 client_write_IV; // low order 48-bits
case server:
uint48 server_write_IV; // low order 48-bits
uint16 epoch;
uint48 seq_num;
uint16 blk_ctr;
} CtrBlk;
For decryption, the epoch and seq_num fields are initialized based on
the corresponding values in a received record.
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3.3. Padding
Stream ciphers in TLS and DTLS do not require plaintext padding.
3.4. Session Resumption
TLS supports session resumption via caching of session ID's and
connection parameters on both client and server. While resumed
sessions use the same master secret that was originally negotiated, a
resumed session uses new keys that are derived, in part, using fresh
client_random and server_random parameters. As a result resumed
sessions do not use the same encryption keys or IV's as the original
session.
4. Design Rationale
An alternate design for the construction of the counter block would
be the use of an explicit 'record tag' (as a substitute for the
implicit record sequence number) that could potentially be generated
via an LFSR. Such a design, however, suffers a major drawback when
used in the TLS or DTLS protocol, without offering any significant
benefit: in both TLS and DTLS inclusion of such a tag would incur a
bandwidth cost.
5. Security Considerations
The security considerations for the use of AES-CTR in TLS/DTLS are
specified below. The below text is based heavily on that for AES-CTR
in IPsec [2].
o Counter blocks must not be used more than once with a given key.
Doing so allows a passive attacker to determine the XOR of the
affected plain text blocks. Extracting two plaintexts from their
XOR is a relatively straightforward operation. Because the
counter block is derived from the per-record sequence, this means
that sequence numbers MUST never be re-used with different data.
Note, however, that retransmitting the same record in DTLS is
safe.
o AES-CTR can be used in pre-shared key mode, since session keys and
not pre-shared keys are used for ciphering. Also, since separate
read and write keys are generated, counter blocks generated by
client and server can safely overlap.
o As with other stream ciphers, data forgery is trivial if no
message integrity mechanism is employed. This threat is of no
concern in TLS/DTLS since all ciphersuites that support encryption
also employ message integrity.
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5.1. Maximum Key Lifetime
TLS/DTLS sessions employing AES-CTR MUST be renegotiated before
sequence numbers repeat. In the case of TLS, this implies a maximum
of 2^64 records per session, while for DTLS the maximum is 2^48 (with
the remaining bits reserved for epoch.)
6. IANA Considerations
IANA has assigned the following values for AES-CTR mode ciphers:
CipherSuite TLS_RSA_WITH_AES_128_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DH_DSS_WITH_AES_128_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DH_RSA_WITH_AES_128_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DHE_DSS_WITH_AES_128_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DHE_RSA_WITH_AES_128_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DH_anon_WITH_AES_128_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_RSA_WITH_AES_256_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DH_DSS_WITH_AES_256_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DH_RSA_WITH_AES_256_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DHE_DSS_WITH_AES_256_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DHE_RSA_WITH_AES_256_CTR_SHA = { 0xXX, 0xXX };
CipherSuite TLS_DH_anon_WITH_AES_256_CTR_SHA = { 0xXX, 0xXX };
7. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Housley, R., "Using Advanced Encryption Standard (AES) Counter
Mode With IPsec Encapsulating Security Payload (ESP)", RFC 3686,
January 2004.
[3] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.1", RFC 4346, April 2006.
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Authors' Addresses
Nagendra Modadugu
Stanford University
353 Serra Mall
Stanford, CA 94305
USA
Email: nagendra@cs.stanford.edu
Eric Rescorla
Network Resonance
2483 E. Bayshore Rd., #212
Palo Alto, CA 94303
USA
Email: ekr@networkresonance.com
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