The Secure Shell (SSH) Transport Layer Protocol
RFC 4253
Network Working Group T. Ylonen
Request for Comments: 4253 SSH Communications Security Corp
Category: Standards Track C. Lonvick, Ed.
Cisco Systems, Inc.
January 2006
The Secure Shell (SSH) Transport Layer Protocol
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
The Secure Shell (SSH) is a protocol for secure remote login and
other secure network services over an insecure network.
This document describes the SSH transport layer protocol, which
typically runs on top of TCP/IP. The protocol can be used as a basis
for a number of secure network services. It provides strong
encryption, server authentication, and integrity protection. It may
also provide compression.
Key exchange method, public key algorithm, symmetric encryption
algorithm, message authentication algorithm, and hash algorithm are
all negotiated.
This document also describes the Diffie-Hellman key exchange method
and the minimal set of algorithms that are needed to implement the
SSH transport layer protocol.
Ylonen & Lonvick Standards Track [Page 1]
RFC 4253 SSH Transport Layer Protocol January 2006
Table of Contents
1. Introduction ....................................................3
2. Contributors ....................................................3
3. Conventions Used in This Document ...............................3
4. Connection Setup ................................................4
4.1. Use over TCP/IP ............................................4
4.2. Protocol Version Exchange ..................................4
5. Compatibility With Old SSH Versions .............................5
5.1. Old Client, New Server .....................................6
5.2. New Client, Old Server .....................................6
5.3. Packet Size and Overhead ...................................6
6. Binary Packet Protocol ..........................................7
6.1. Maximum Packet Length ......................................8
6.2. Compression ................................................8
6.3. Encryption .................................................9
6.4. Data Integrity ............................................12
6.5. Key Exchange Methods ......................................13
6.6. Public Key Algorithms .....................................13
7. Key Exchange ...................................................15
7.1. Algorithm Negotiation .....................................17
7.2. Output from Key Exchange ..................................20
7.3. Taking Keys Into Use ......................................21
8. Diffie-Hellman Key Exchange ....................................21
8.1. diffie-hellman-group1-sha1 ................................23
8.2. diffie-hellman-group14-sha1 ...............................23
9. Key Re-Exchange ................................................23
10. Service Request ...............................................24
11. Additional Messages ...........................................25
11.1. Disconnection Message ....................................25
11.2. Ignored Data Message .....................................26
11.3. Debug Message ............................................26
11.4. Reserved Messages ........................................27
12. Summary of Message Numbers ....................................27
13. IANA Considerations ...........................................27
14. Security Considerations .......................................28
15. References ....................................................29
15.1. Normative References .....................................29
15.2. Informative References ...................................30
Authors' Addresses ................................................31
Trademark Notice ..................................................31
Ylonen & Lonvick Standards Track [Page 2]
RFC 4253 SSH Transport Layer Protocol January 2006
1. Introduction
The SSH transport layer is a secure, low level transport protocol.
It provides strong encryption, cryptographic host authentication, and
integrity protection.
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