The Secure Shell (SSH) Transport Layer Protocol
RFC 4253

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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.