RFC 4253 - The Secure Shell (SSH) Transport Layer Protocol

Network Working Group: T. Ylonen, SSH Communications Security Corp
Request for Comments: 4253
Category: Standards Track
Editor: C. Lonvick, Cisco Systems, Inc.
Date: January 2006

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.

Contents

• 1. Introduction
• 2. Contributors
• 3. Conventions Used in This Document
• 4. Connection Setup
  • 4.1. Use over TCP/IP
  • 4.2. Protocol Version Exchange
• 5. Compatibility With Old SSH Versions
  • 5.1. Old Client, New Server
  • 5.2. New Client, Old Server
  • 5.3. Packet Size and Overhead
• 6. Binary Packet Protocol
  • 6.1. Maximum Packet Length
  • 6.2. Compression
  • 6.3. Encryption
  • 6.4. Data Integrity
  • 6.5. Key Exchange Methods
  • 6.6. Public Key Algorithms
• 7. Key Exchange
  • 7.1. Algorithm Negotiation
  • 7.2. Output from Key Exchange
  • 7.3. Taking Keys Into Use
• 8. Diffie-Hellman Key Exchange
  • 8.1. diffie-hellman-group1-sha1
  • 8.2. diffie-hellman-group14-sha1
• 9. Key Re-Exchange
• 10. Service Request
• 11. Additional Messages
  • 11.1. Disconnection Message
  • 11.2. Ignored Data Message
  • 11.3. Debug Message
  • 11.4. Reserved Messages
• 12. Summary of Message Numbers
• 13. IANA Considerations
• 14. Security Considerations
• 15. References
  • 15.1. Normative References
  • 15.2. Informative References
• Authors' Addresses
• Trademark Notice