Network Working Group M. Bellare
Request for Comments: 4344 T. Kohno
Category: Standards Track UC San Diego
C. Namprempre
Thammasat University
January 2006
The Secure Shell (SSH) Transport Layer Encryption Modes
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 (2006).
Abstract
Researchers have discovered that the authenticated encryption portion
of the current SSH Transport Protocol is vulnerable to several
attacks.
This document describes new symmetric encryption methods for the
Secure Shell (SSH) Transport Protocol and gives specific
recommendations on how frequently SSH implementations should rekey.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Rekeying ........................................................2
3.1. First Rekeying Recommendation ..............................3
3.2. Second Rekeying Recommendation .............................3
4. Encryption Modes ................................................3
5. IANA Considerations .............................................6
6. Security Considerations .........................................6
6.1. Rekeying Considerations ....................................7
6.2. Encryption Method Considerations ...........................8
Normative References ...............................................9
Informative References ............................................10
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1. Introduction
The symmetric portion of the SSH Transport Protocol was designed to
provide both privacy and integrity of encapsulated data. Researchers
([DAI,BKN1,BKN2]) have, however, identified several security problems
with the symmetric portion of the SSH Transport Protocol, as
described in [RFC4253]. For example, the encryption mode specified
in [RFC4253] is vulnerable to a chosen-plaintext privacy attack.
Additionally, if not rekeyed frequently enough, the SSH Transport
Protocol may leak information about payload data. This latter
property is true regardless of what encryption mode is used.
In [BKN1,BKN2], Bellare, Kohno, and Namprempre show how to modify the
symmetric portion of the SSH Transport Protocol so that it provably
preserves privacy and integrity against chosen-plaintext, chosen-
ciphertext, and reaction attacks. This document instantiates the
recommendations described in [BKN1,BKN2].
2. 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].
The used data types and terminology are specified in the architecture
document [RFC4251].
The SSH Transport Protocol is specified in the transport document
[RFC4253].
3. Rekeying
Section 9 of [RFC4253] suggests that SSH implementations rekey after
every gigabyte of transmitted data. [RFC4253] does not, however,
discuss all the problems that could arise if an SSH implementation
does not rekey frequently enough. This section serves to strengthen
the suggestion in [RFC4253] by giving firm upper bounds on the
tolerable number of encryptions between rekeying operations. In
Section 6, we discuss the motivation for these rekeying
recommendations in more detail.
This section makes two recommendations. Informally, the first
recommendation is intended to protect against possible information
leakage through the MAC tag, and the second recommendation is
intended to protect against possible information leakage through the
block cipher. Note that, depending on the block length of the
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underlying block cipher and the length of the encrypted packets, the
first recommendation may supersede the second recommendation, or vice
versa.
3.1. First Rekeying Recommendation
Because of possible information leakage through the MAC tag, SSH
implementations SHOULD rekey at least once every 2**32 outgoing
packets. More explicitly, after a key exchange, an SSH
implementation SHOULD NOT send more than 2**32 packets before
rekeying again.
SSH implementations SHOULD also attempt to rekey before receiving
more than 2**32 packets since the last rekey operation. The
preferred way to do this is to rekey after receiving more than 2**31
packets since the last rekey operation.
3.2. Second Rekeying Recommendation
Because of a birthday property of block ciphers and some modes of
operation, implementations must be careful not to encrypt too many
blocks with the same encryption key.
Let L be the block length (in bits) of an SSH encryption method's
block cipher (e.g., 128 for AES). If L is at least 128, then, after
rekeying, an SSH implementation SHOULD NOT encrypt more than 2**(L/4)
blocks before rekeying again. If L is at least 128, then SSH
implementations should also attempt to force a rekey before receiving
more than 2**(L/4) blocks. If L is less than 128 (which is the case
for older ciphers such as 3DES, Blowfish, CAST-128, and IDEA), then,
although it may be too expensive to rekey every 2**(L/4) blocks, it
is still advisable for SSH implementations to follow the original
recommendation in [RFC4253]: rekey at least once for every gigabyte
of transmitted data.
Note that if L is less than or equal to 128, then the recommendation
in this subsection supersedes the recommendation in Section 3.1. If
an SSH implementation uses a block cipher with a larger block size
(e.g., Rijndael with 256-bit blocks), then the recommendations in
Section 3.1 may supersede the recommendations in this subsection
(depending on the lengths of the packets).
4. Encryption Modes
This document describes new encryption methods for use with the SSH
Transport Protocol. These encryption methods are in addition to the
encryption methods described in Section 6.3 of [RFC4253].
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Recall from [RFC4253] that the encryption methods in each direction
of an SSH connection MUST run independently of each other and that,
when encryption is in effect, the packet length, padding length,
payload, and padding fields of each packet MUST be encrypted with the
chosen method. Further recall that the total length of the
concatenation of the packet length, padding length, payload, and
padding MUST be a multiple of the cipher's block size when the
cipher's block size is greater than or equal to 8 bytes (which is the
case for all of the following methods).
This document describes the following new methods:
aes128-ctr RECOMMENDED AES (Rijndael) in SDCTR mode,
with 128-bit key
aes192-ctr RECOMMENDED AES with 192-bit key
aes256-ctr RECOMMENDED AES with 256-bit key
3des-ctr RECOMMENDED Three-key 3DES in SDCTR mode
blowfish-ctr OPTIONAL Blowfish in SDCTR mode
twofish128-ctr OPTIONAL Twofish in SDCTR mode,
with 128-bit key
twofish192-ctr OPTIONAL Twofish with 192-bit key
twofish256-ctr OPTIONAL Twofish with 256-bit key
serpent128-ctr OPTIONAL Serpent in SDCTR mode, with
128-bit key
serpent192-ctr OPTIONAL Serpent with 192-bit key
serpent256-ctr OPTIONAL Serpent with 256-bit key
idea-ctr OPTIONAL IDEA in SDCTR mode
cast128-ctr OPTIONAL CAST-128 in SDCTR mode,
with 128-bit key
The label <cipher>-ctr indicates that the block cipher <cipher> is to
be used in "stateful-decryption counter" (SDCTR) mode. Let L be the
block length of <cipher> in bits. In stateful-decryption counter
mode, both the sender and the receiver maintain an internal L-bit
counter X. The initial value of X should be the initial IV (as
computed in Section 7.2 of [RFC4253]) interpreted as an L-bit
unsigned integer in network-byte-order. If X=(2**L)-1, then
"increment X" has the traditional semantics of "set X to 0." We use
the notation <X> to mean "convert X to an L-bit string in network-
byte-order." Naturally, implementations may differ in how the
internal value X is stored. For example, implementations may store X
as multiple unsigned 32-bit counters.
To encrypt a packet P=P1||P2||...||Pn (where P1, P2, ..., Pn are each
blocks of length L), the encryptor first encrypts <X> with <cipher>
to obtain a block B1. The block B1 is then XORed with P1 to generate
the ciphertext block C1. The counter X is then incremented, and the
process is repeated for each subsequent block in order to generate
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the entire ciphertext C=C1||C2||...||Cn corresponding to the packet
P. Note that the counter X is not included in the ciphertext. Also
note that the keystream can be pre-computed and that encryption is
parallelizable.
To decrypt a ciphertext C=C1||C2||...||Cn, the decryptor (who also
maintains its own copy of X) first encrypts its copy of <X> with
<cipher> to generate a block B1 and then XORs B1 to C1 to get P1.
The decryptor then increments its copy of the counter X and repeats
the above process for each block to obtain the plaintext packet
P=P1||P2||...||Pn. As before, the keystream can be pre-computed, and
decryption is parallelizable.
The "aes128-ctr" method uses AES (the Advanced Encryption Standard,
formerly Rijndael) with 128-bit keys [AES]. The block size is 16
bytes.
At this time, it appears likely that a future specification will
promote aes128-ctr to be REQUIRED; implementation of this
algorithm is very strongly encouraged.
The "aes192-ctr" method uses AES with 192-bit keys.
The "aes256-ctr" method uses AES with 256-bit keys.
The "3des-ctr" method uses three-key triple-DES (encrypt-decrypt-
encrypt), where the first 8 bytes of the key are used for the first
encryption, the next 8 bytes for the decryption, and the following 8
bytes for the final encryption. This requires 24 bytes of key data
(of which 168 bits are actually used). The block size is 8 bytes.
This algorithm is defined in [DES].
The "blowfish-ctr" method uses Blowfish with 256-bit keys [SCHNEIER].
The block size is 8 bytes. (Note that "blowfish-cbc" from [RFC4253]
uses 128-bit keys.)
The "twofish128-ctr" method uses Twofish with 128-bit keys [TWOFISH].
The block size is 16 bytes.
The "twofish192-ctr" method uses Twofish with 192-bit keys.
The "twofish256-ctr" method uses Twofish with 256-bit keys.
The "serpent128-ctr" method uses the Serpent block cipher [SERPENT]
with 128-bit keys. The block size is 16 bytes.
The "serpent192-ctr" method uses Serpent with 192-bit keys.
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The "serpent256-ctr" method uses Serpent with 256-bit keys.
The "idea-ctr" method uses the IDEA cipher [SCHNEIER]. The block
size is 8 bytes.
The "cast128-ctr" method uses the CAST-128 cipher with 128-bit keys
[RFC2144]. The block size is 8 bytes.
5. IANA Considerations
The thirteen encryption algorithm names defined in Section 4 have
been added to the Secure Shell Encryption Algorithm Name registry
established by Section 4.11.1 of [RFC4250].
6. Security Considerations
This document describes additional encryption methods and
recommendations for the SSH Transport Protocol [RFC4253].
[BKN1,BKN2] prove that if an SSH application incorporates the methods
and recommendations described in this document, then the symmetric
cryptographic portion of that application will resist a large class
of privacy and integrity attacks.
This section is designed to help implementors understand the
security-related motivations for, as well as possible consequences of
deviating from, the methods and recommendations described in this
document. Additional motivation and discussion, as well as proofs of
security, appear in the research papers [BKN1,BKN2].
Please note that the notion of "prove" in the context of [BKN1,BKN2]
is that of practice-oriented reductionist security: if an attacker is
able to break the symmetric portion of the SSH Transport Protocol
using a certain type of attack (e.g., a chosen-ciphertext attack),
then the attacker will also be able to break one of the transport
protocol's underlying components (e.g., the underlying block cipher
or MAC). If we make the reasonable assumption that the underlying
components (such as AES and HMAC-SHA1) are secure, then the attacker
against the symmetric portion of the SSH protocol cannot be very
successful (since otherwise there would be a contradiction). Please
see [BKN1,BKN2] for details. In particular, attacks are not
impossible, just extremely improbable (unless the building blocks,
like AES, are insecure).
Note also that cryptography often plays only a small (but critical)
role in an application's overall security. In the case of the SSH
Transport Protocol, even though an application might implement the
symmetric portion of the SSH protocol exactly as described in this
document, the application may still be vulnerable to non-protocol-
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based attacks (as an egregious example, an application might save
cryptographic keys in cleartext to an unprotected file).
Consequently, even though the methods described herein come with
proofs of security, developers must still execute caution when
developing applications that implement these methods.
6.1. Rekeying Considerations
Section 3 of this document makes two rekeying recommendations: (1)
rekey at least once every 2**32 packets, and (2) rekey after a
certain number of encrypted blocks (e.g., 2**(L/4) blocks if the
block cipher's block length L is at least 128 bits). The motivations
for recommendations (1) and (2) are different, and we consider each
recommendation in turn. Briefly, (1) is designed to protect against
information leakage through the SSH protocol's underlying MAC, and
(2) is designed to protect against information leakage through the
SSH protocol's underlying encryption scheme. Please note that,
depending on the encryption method's block length L and the number of
blocks encrypted per packet, recommendation (1) may supersede
recommendation (2) or vice versa.
Recommendation (1) states that SSH implementations should rekey at
least once every 2**32 packets. If more than 2**32 packets are
encrypted and MACed by the SSH Transport Protocol between rekeyings,
then the SSH Transport Protocol may become vulnerable to replay and
re-ordering attacks. This means that an adversary may be able to
convince the receiver to accept the same message more than once or to
accept messages out of order. Additionally, the underlying MAC may
begin to leak information about the protocol's payload data. In more
detail, an adversary looks for a collision between the MACs
associated to two packets that were MACed with the same 32-bit
sequence number (see Section 4.4 of [RFC4253]). If a collision is
found, then the payload data associated with those two ciphertexts is
probably identical. Note that this problem occurs regardless of how
secure the underlying encryption method is. Also note that although
compressing payload data before encrypting and MACing and the use of
random padding may reduce the risk of information leakage through the
underlying MAC, compression and the use of random padding will not
prevent information leakage. Implementors who decide not to rekey at
least once every 2**32 packets should understand these issues. These
issues are discussed further in [BKN1,BKN2].
One alternative to recommendation (1) would be to make the SSH
Transport Protocol's sequence number more than 32 bits long. This
document does not suggest increasing the length of the sequence
number because doing so could hinder interoperability with older
versions of the SSH protocol. Another alternative to recommendation
(1) would be to switch from basic HMAC to a another MAC, such as a
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MAC that has its own internal counter. Because of the 32-bit counter
already present in the protocol, such a counter would only need to be
incremented once every 2**32 packets.
Recommendation (2) states that SSH implementations should rekey
before encrypting more than 2**(L/4) blocks with the same key
(assuming L is at least 128). This recommendation is designed to
minimize the risk of birthday attacks against the encryption method's
underlying block cipher. For example, there is a theoretical privacy
attack against stateful-decryption counter mode if an adversary is
allowed to encrypt approximately 2**(L/2) messages with the same key.
It is because of these birthday attacks that implementors are highly
encouraged to use secure block ciphers with large block lengths.
Additionally, recommendation (2) is designed to protect an encryptor
from encrypting more than 2**L blocks with the same key. The
motivation here is that, if an encryptor were to use SDCTR mode to
encrypt more than 2**L blocks with the same key, then the encryptor
would reuse keystream, and the reuse of keystream can lead to serious
privacy attacks [SCHNEIER].
6.2. Encryption Method Considerations
Researchers have shown that the original CBC-based encryption methods
in [RFC4253] are vulnerable to chosen-plaintext privacy attacks
[DAI,BKN1,BKN2]. The new stateful-decryption counter mode encryption
methods described in Section 4 of this document were designed to be
secure replacements to the original encryption methods described in
[RFC4253].
Many people shy away from counter mode-based encryption schemes
because, when used incorrectly (such as when the keystream is allowed
to repeat), counter mode can be very insecure. Fortunately, the
common concerns with counter mode do not apply to SSH because of the
rekeying recommendations and because of the additional protection
provided by the transport protocol's MAC. This discussion is
formalized with proofs of security in [BKN1,BKN2].
As an additional note, when one of the stateful-decryption counter
mode encryption methods (Section 4) is used, then the padding
included in an SSH packet (Section 4 of [RFC4253]) need not be (but
can still be) random. This eliminates the need to generate
cryptographically secure pseudorandom bytes for each packet.
One property of counter mode encryption is that it does not require
that messages be padded to a multiple of the block cipher's block
length. Although not padding messages can reduce the protocol's
network consumption, this document requires that padding be a
multiple of the block cipher's block length in order to (1) not alter
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the packet description in [RFC4253] and (2) not leak precise
information about the length of the packet's payload data. (Although
there may be some network savings from padding to only 8-bytes even
if the block cipher uses 16-byte blocks, because of (1) we do not
make that recommendation here.)
In addition to stateful-decryption counter mode, [BKN1,BKN2] describe
other provably secure encryption methods for use with the SSH
Transport Protocol. The stateful-decryption counter mode methods in
Section 4 are, however, the preferred alternatives to the insecure
methods in [RFC4253] because stateful-decryption counter mode is the
most efficient (in terms of both network consumption and the number
of required cryptographic operations per packet).
Normative References
[AES] National Institute of Standards and Technology, "Advanced
Encryption Standard (AES)", Federal Information
Processing Standards Publication 197, November 2001.
[DES] National Institute of Standards and Technology, "Data
Encryption Standard (DES)", Federal Information
Processing Standards Publication 46-3, October 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
May 1997.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, January 2006.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
Protocols algorithms and source in code in C", Wiley,
1996.
[SERPENT] Anderson, R., Biham, E., and Knudsen, L., "Serpent: A
proposal for the Advanced Encryption Standard", NIST AES
Proposal, 1998.
Bellare, et al. Standards Track [Page 9]
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[TWOFISH] Schneier, B., et al., "The Twofish Encryptions Algorithm:
A 128-bit block cipher, 1st Edition", Wiley, 1999.
Informative References
[BKN1] Bellare, M., Kohno, T., and Namprempre, C.,
"Authenticated Encryption in SSH: Provably Fixing the SSH
Binary Packet Protocol", Ninth ACM Conference on Computer
and Communications Security, 2002.
[BKN2] Bellare, M., Kohno, T., and Namprempre, C., "Breaking and
Provably Repairing the SSH Authenticated Encryption
Scheme: A Case Study of the Encode-then-Encrypt-and-MAC
Paradigm", ACM Transactions on Information and System
Security, 7(2), May 2004.
[DAI] Dai, W., "An Attack Against SSH2 Protocol", Email to the
ietf-ssh@netbsd.org email list, 2002.
Bellare, et al. Standards Track [Page 10]
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Authors' Addresses
Mihir Bellare
Department of Computer Science and Engineering
University of California at San Diego
9500 Gilman Drive, MC 0404
La Jolla, CA 92093-0404
Phone: +1 858-534-8833
EMail: mihir@cs.ucsd.edu
Tadayoshi Kohno
Department of Computer Science and Engineering
University of California at San Diego
9500 Gilman Drive, MC 0404
La Jolla, CA 92093-0404
Phone: +1 858-534-8833
EMail: tkohno@cs.ucsd.edu
Chanathip Namprempre
Thammasat University
Faculty of Engineering
Electrical Engineering Department
Rangsit Campus, Klong Luang
Pathumthani, Thailand 12121
EMail: meaw@alum.mit.edu
Bellare, et al. Standards Track [Page 11]
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