Network Working Group K.M. Igoe
Internet Draft National Security Agency
Intended Status: Informational J.A. Solinas
Expires: December 27, 2008 National Security Agency
June 27, 2008
AES Galois Counter Mode for the Secure Shell Transport Layer Protocol
draft-igoe-secsh-aes-gcm-00
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Copyright (C) The IETF Trust (2008).
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Abstract
Secure Shell (SSH) [RFC4251] is a secure remote-login protocol. SSH
provides for algorithms that provide authentication , key agreement.
confidentiality and data integrity services. This purpose of this
document is to show how the AES Galois/Counter Mode can be used to
provide both confidentiality and data integrity.
Table of Contents
1. Introduction.....................................................1
2. Requirements Terminology.........................................1
3. Applicability Statement..........................................1
4. Two New AEAD Algorithms..........................................1
4.1. aead-aes-128-gcm-ssh........................................2
4.2. aead-aes-256-gcm-ssh........................................2
5. Size of the Message Authentication Code..........................2
6. Maximum Payload Size.............................................3
7. Linkage of Confidentiality and Data Integrity....................3
8. Security Considerations..........................................3
9. IANA Considerations..............................................4
10. References......................................................4
10.1. Normative References.......................................4
1. Introduction
Galois/Counter Mode (GCM) is a block cipher mode of operation that
provides both confidentiality and data integrity services. The
purpose of this document is to show how AES-GCM can be intergrated
into the Secure Shell Transport Layer Protocol.
2. Requirements Terminology
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].
3. Applicability Statement
Using AES-GCM to provide both confidentiality and data integrity is
generally more efficient than using two separate algorithms to
provide these security services.
4. Two New AEAD Algorithms
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4.1. aead-aes-128-gcm-ssh
aead-aes-128-gcm-ssh is a variant of the algorithm AEAD_AES_128_GCM
specified in section 5.1 of [RFC5116]. The only differences between
the two algorithms are in the input and output lengths. Using the
notation defined in [RFC5116], the input and output lengths for
aead-aes-128-gcm-ssh are as follows:
PARAMETER Meaning Value
K_LEN AES key length 16 octets
P_MAX maximum plaintext length 2^32 octets
A_MAX maximum additional 0 octets
authenticated data length
N_MIN minimum nonce (IV) length 12 octets
N_MAX maximum nonce (IV) length 12 octets
C_MAX maximum cipher length 2^32 octets
Test cases are provided in the appendix of [GCM].
The reader is reminded that due to the presence of length fields and
padding in SSH packets, the plaintext length is not the same as the
payload length. See section 6 below.
4.2. aead-aes-256-gcm-ssh
aead-aes-256-gcm-ssh is a variant of the algorithm AEAD_AES_256_GCM
specified in section 5.2 of [RFC5116]. The only differences between
the two algorithms are in the input and output lengths. Using the
notation defined in [RFC5116], the input and output lengths for
aead-aes-256-gcm-ssh are as follows:
PARAMETER Meaning Value
K_LEN AES key length 32 octets
P_MAX maximum plaintext length 2^32 octets
A_MAX maximum additional 0 octets
authenticated data length
N_MIN minimum nonce (IV) length 12 octets
N_MAX maximum nonce (IV) length 12 octets
C_MAX maximum cipher length 2^32 octets
Test cases are provided in the appendix of [GCM].
The reader is reminded that due to the presence of length fields and
padding in SSH packets, the plaintext length is not the same as the
payload length. See section 6 below.
5. Size of the Message Authentication Code
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Both aead-aes-128-gcm-ssh and aead-aes-256-gcm-ssh produce a 16-octet
message authentication code. ([RFC5116] calls this an
"authentication tag" rather than a "message authentication code".)
6. Maximum Payload Size
The value of P_MAX and C_MAX listed above are determined by
constraints on the structure of an SSH packet. Referring to [RFC
4253], one finds that an SSH packet consists of five fields:
uint32 packet_length; // 0 <= packet_length < 2^32
byte padding_length; // 4 <= padding_length < 256
byte[n1] payload; // n1 = packet_length - padding_length - 1
byte[n2] random_padding; // n2 = padding_length
byte[m] mac; // m = mac_length
(= 16 for aes128/256_gcm)
All save the mac field are encrypted, and the total length of the
data to be encrypted (plaintext length) must be a multiple of the
block length. When using either aead-aes-128-gcm-ssh or
aead-aes-256-gcm-ssh (or any other algorithm with either a 16 octet
or 8 octet block size) the largest possible payload is achieved when
payload length = 2^32-9 octets
padding length = 4 octets
packet length = 1 + (2^32-9) + 4 = 2^32 - 4 octets
plaintext length = 4 + 1 + (2^32 - 9) + 4 = 2^32 octets.
7. Linkage of Confidentiality and Data Integrity
When either aead-aes-128-gcm-ssh or aead-aes-256-gcm-ssh is being
employed it SHOULD be used both as the confidentiality mechanism and
as the data integrity mechanism.
8. Security Considerations
The security considerations in [RFC4251] apply.
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9. IANA Considerations
IANA will add the following two entries to the AEAD Registry
described in [RFC5116]:
+----------------------+-------------+--------------------+
| | | Proposed |
| Name | Reference | Numeric Identifier |
+----------------------+-------------+--------------------+
| aead-aes-128-gcm-ssh | Section 4.1 | 5 |
| | | |
| aead-aes-256-gcm-ssh | Section 4.2 | 6 |
+----------------------+-------------+--------------------+
IANA will add the following two entries to the Secure Shell
Encryption Algorithm name Registry described in [RFC4250]:
+----------------------+-------------+
| | |
| Name | Reference |
+----------------------+-------------+
| aead-aes-128-gcm-ssh | Section 4.1 |
| | |
| aead-aes-256-gcm-ssh | Section 4.2 |
+----------------------+-------------+
IANA will add the following two entries to the Secure Shell MAC
Algorithm name Registry described in [RFC4250]:
+----------------------+-------------+
| | |
| Name | Reference |
+----------------------+-------------+
| aead-aes-128-gcm-ssh | Section 4.1 |
| | |
| aead-aes-256-gcm-ssh | Section 4.2 |
+----------------------+-------------+
10. References
10.1. Normative References
[GCM] Dworkin, M, "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", NIST
Special Publication 800-30D, November 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 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.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryptions", RFC 5116, January 2008.
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Author's Addresses
Kevin M. Igoe
NSA/CSS Commercial Solutions Center
National Security Agency
EMail: kmigoe@nsa.gov
Jerome A. Solinas
National Information Assurance Research Laboratory
National Security Agency
EMail: jasolin@orion.ncsc.mil
Trademark Notice
"SSH" is a registered trademark in the United States.
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