SSH can also be run using SCTP rather than TCP as the connection oriented transport layer protocol. ==History and development== ===Version 1.x=== In 1995, Tatu Ylönen, a researcher at Helsinki University of Technology, Finland, designed the first version of the protocol (now called SSH-1) prompted by a password-sniffing attack at his university network.
Ylönen released his implementation as freeware in July 1995, and the tool quickly gained in popularity.
Towards the end of 1995, the SSH user base had grown to 20,000 users in fifty countries. In December 1995, Ylönen founded SSH Communications Security to market and develop SSH.
FISH), released in 1998, which evolved from Unix shell commands over SSH Fast and Secure Protocol (FASP), aka Aspera, uses SSH for control and UDP ports for data transfer. ==Architecture== The SSH-2 protocol has an internal architecture (defined in RFC 4251) with well-separated layers, namely: The transport layer (RFC 4253), which typically runs on top of TCP/IP.
This is not an actual version but a method to identify backward compatibility. ===OpenSSH and OSSH=== In 1999, developers, wanting a free software version to be available, went back to the older 1.2.12 release of the original SSH program, which was the last released under an open source license.
The original version of the SSH software used various pieces of free software, such as GNU libgmp, but later versions released by SSH Communications Security evolved into increasingly proprietary software. It was estimated that by the year 2000 the number of users had grown to 2 million. ===Version 2.x=== "Secsh" was the official Internet Engineering Task Force's (IETF) name for the IETF working group responsible for version 2 of the SSH protocol.
IANA had listed the standard TCP port 22 for SSH servers as one of the well-known ports as early as 2001.
Many of these updated implementations contained a new integer overflow vulnerability that allowed attackers to execute arbitrary code with the privileges of the SSH daemon, typically root. In January 2001 a vulnerability was discovered that allows attackers to modify the last block of an IDEA-encrypted session.
In 2006, a revised version of the protocol, SSH-2, was adopted as a standard.
Due to SSH-2's superiority and popularity over SSH-1, some implementations such as libssh (v0.8.0+), Lsh and Dropbear support only the SSH-2 protocol. ===Version 1.99=== In January 2006, well after version 2.1 was established, RFC 4253 specified that an SSH server which supports both 2.0 and prior versions of SSH should identify its protoversion as 1.99.
Most modern servers and clients support SSH-2. ===CBC plaintext recovery=== In November 2008, a theoretical vulnerability was discovered for all versions of SSH which allowed recovery of up to 32 bits of plaintext from a block of ciphertext that was encrypted using what was then the standard default encryption mode, CBC.
The most straightforward solution is to use CTR, counter mode, instead of CBC mode, since this renders SSH resistant to the attack. ===Possible vulnerabilities=== On December 28, 2014 Der Spiegel published classified information leaked by whistleblower Edward Snowden which suggests that the National Security Agency may be able to decrypt some SSH traffic.
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