Comparison of SSH v2 vs. SSH v1

Glossary

Secure Shell (SSH) is a cryptographic protocol that securely transports data over an unsecured network. Usually it operates on TCP port 22.

In 1995 Tatu Ylonen developed the first version of the SSH protocol and released the source code to the public.

In 2006, a revised version of the protocol, called SSH-2, was adopted as a new standard. Compared to SSH-1 it has improved security and new features like new key exchange mechanisms and has an improved protocol structure in which the protocol is divided into separate layers. Other improvements are negotiable hashing mechanisms, new encryption methods, etc.

Table of Contents

• Glossary
• SSH2 Introduction
• Comparison of SSH v2 vs SSH v1
• SSH2 vs SSH1 Feature Comparison Table

SSH2 Introduction

In 2006 the SSH-2 protocol became a proposed industry standard by having been submitted as an RFC (Request For Comment) with the Internet Engineering Task Force (IETF). This version is not compatible with SSH-1, but it has improved security and new features compared to SSH-1. For example, it introduces an improved and layered protocol structure with three layers building on top of each other, a negotiable hashing mechanism (where SSH v1 always uses CRC-32), new negotiable key exchange and encryption methods, more flexible naming conventions that allow implementing extensions in a way that they will not collide with names that are required to be registered with the IANA.

Other improvements are the possibility to periodically replace the session key ("rekeying") and a mechanism to provide certification for public keys. Another new aspect is the implementation of a new file transfer protocol named SFTP (Secure File Transfer Protocol) in addition to the existing SCP (Secure Copy Protocol).

Comparison of SSH v2 vs SSH v1

Architecture

While SSH-1 is monolithic, SSH-2 separates itself into three layers: SSH Transport Layer Protocol (SSH-TRANS), SSH Authentication Protocol (SSH-AUTH) and SSH Connection Protocol (SSH-CONN)

Integrity Checking

SSH v1 uses CRC-32 to check the integrity of data packets. In SSH v2 the two parties (client and server) can negotiate a hashing mechanism of their choice, e.g. there are several flavors of HMAC or SHA.

Algorithm Choice

While SSH1 lets the client and server negotiate the encryption algorithm that is used to encrypt the session data, other aspects involving encryption were hard coded. SSH2 also allows the negotiation of algorithms for key exchange, hashing and compression.

Algorithm Namespace

SSH-1 depended on names for the algorithms that were registered with the IANA. SSH-2 has a naming scheme that divides the algorithms into two classes: one where new mechanisms can be added that are guaranteed not to collide with IANA names, and one that needs to be registered by the IANA.

Authentication

SSH1 has a predetermined sequence of authentication mechanisms. In SSH2 the server is more flexible and can change the authentication methods, e.g. depending on what happened with previous authentication attempts.

Cryptographic Algorithms

SSH1 did support a range of cryptographic algorithms, e.g. 3-des, arcfour, blowfish, DSA, RSA or AES, most of which are now thoroughly outdated because they can be brute-forced even by non-state actors with moderate financial or computational means. SSH2 did deprecate some of these and introduced a range of new algorithms like aes-ctr, aes-cbc, aes-gcm, chacha-poly, etc.

The same is true for cryptographic keys, where SSH-2 offers algorithms like Diffie-Hellman, ECDSA, and Curve25519.

Session Rekeying

SSH2 has a method where the encryption key can be changed periodically during a session, while SSH1 used the same key throughout the duration of the whole session.

Hostbased Authentication

SSH-1 has limited ways to authenticate the client based on its host (only via IP address), while SSH-2 can also use the host name via DNS lookup.

Certification

Similar to the SSL protocol, SSH2 introduces a mechanism where a server can certify a key (public/private key pair) that is used for authentication, thus proving that the key was issued by a trusted authority and subsequently allowing such keys to be used for authentication without them being added to the authorized_keys file.

File Transfer

While SSH1 supports the SCP file transfer method, SSH2 adds SFTP which implements features that are similar to FTP, e.g. the retrieval of the list of file names from the remote server, so that applications can be built that allow browsing the remote file system without using the shell.

SSH2 vs SSH1 Feature Comparison Table

The following table indicates the main differences between SSH v1 and SSH v2.