AbstractBrief history of SSHWhat is SSH?CS265 Term Paper Spring 2003SSH: The Secure ShellCS265 Term PaperRachana MaheswariApril 14 2003Submitted to Dr. Mark Stamp as fulfillment of the Term Paper Requirement for CS265, Spring 2003SSH: The Secure Shell 1 Rachana MaheswariCS265 Term Paper Spring 2003AbstractMost widely used utilities to login through a network in Unix environment are telnet and rlogin.In both of the above cases user's login name and password are transmitted as clear text. Inaddition, data transmission after login is also in clear text. An intruder can easily see such data byusing freeware programs. Secure shell provides a secure channel for data transmission. SSH, theSecure Shell is a popular, powerful, software-based approach to network security.This paper explains what are secure shell (SSH), brief history, its features and its securitymechanisms. For the most part, this paper assumes SSH is implemented on Unix platform.Secure shell is a protocol as well as a product. The protocol is indicated as SSH-1 for version 1and SSH-2 for version 2.Finally, the differences of SSH-1 and SSH-2 features are covered. Brief history of SSHTatu Ylonen developed SSH (secure shell) in 1995 while he was working as a researcher at theHelsinki University of Technology, Finland. The product SSH was developed as an answer to anetwork attack on the university computer. In the same year SSH was released as free softwarefor public use. Later in the year, Ylonen published the protocol SSH-1 as an IETF (InternetEngineering Task Force) draft. Term SSH is widely used to refer to the protocol as well as thesoftware.To commercialize and further popularize the use of SSH, Ylonen founded SSH communicationssecurity Ltd., in late 1995 (http://www.ssh.com). SSH-2 protocol was released in 1996, which inessence fixed the bugs in SSH-1.But SSH-2 did not have backward compatibility with SSH-1.The product SSH2 was released in 1998. Unlike original version, SSH-2 was more for paidcommercial use. Even after the release of SSH-2, people continue to use SSH (or known asSSH1) because of its useful features. A freeware version of SSH known as openssh(http://www.openssh.com, mainly developed under openBSD project), is available. Now SSH isported freely on Linux and is available in major flavors of Unix. What is SSH?SSH is a powerful, convenient approach to protecting communications on a computer network.Through secure encryption technologies and authentication, SSH supports secure remote logins,secure remote command execution, secure file transfers, and other secure network services.Secure shell provides a secure channel for data transmission. To some extent the same secureshell is a misnomer; it is not a shell like born shell or korn shell. SSH is not a commandinterpreter. SSH does not give the user a prompt to execute commands like born shell or kornshell. It is only a secure pipe to open up a command interpreter. SSH encrypts any data sentthrough this pipe. SSH automatically decrypts incoming encrypted data. SSH uses modern,SSH: The Secure Shell 2 Rachana MaheswariCS265 Term Paper Spring 2003secure encryption algorithms and is effective enough to be found within mission criticalapplications.SSH has a client server architecture. An SSH server program, typically installed by a systemadministrator accepts or rejects incoming connections to its host computer. In most Unix SSHimplementations, the server is sshd. Users then run SSH client programs to make requests to theSSH server. In SSH1, SSH2 and OpenSSH, the major clients are ssh and scp.Features of SSHThe major features of the Secure Shell protocol are Privacy, Integrity, Authentication,Authorization, and Forwarding.- Privacy: of data is obtained via strong end-to-end encryption that is based on randomkeys which are securely negotiated for that session, and then destroyed when the sessionis over. SSH supports encryption algorithms such as DES, IDEA, Blowfish, ARC-FOUR, and triple-DES. The next section goes into the details of the security mechanism.- Integrity: .SSH1 uses a weak method which is a 32 bit Cyclic Redundancy Check(CRC-32) on the unencrypted data in each packet.SSH2 uses keyed hash algorithmsbased on MD5 and SHA1.- Authentication: involves server authentication which is done using the servers hostkey, and client authentication is usually done by password authentication or public keyauthentication. - Authorization: occurs after authentication, and is controlled at a server wide level or peraccount basis. - SSH supports Forwarding, means encapsulating another TCP based service such asTelnet within an SSH session. For example, by forwarding telnet through SSH, all dataare automatically encrypted, integrity checked, and authenticated using SSH credentials.SSH Security MechanismThis section gives an overall picture of how the features mentioned in the above section areimplemented in Secure Shell to guarantee security of a network connection. Four keys are usedin SSH to establish a secure connection. They are the user key, session key, host key and serverkey. Of these, the session key is private and all the other three keys are public keys.Establishing the Secure ConnectionTypically, the client initiates the connection by sending a request to the TCP port of the SSHserver. At this point all communication is un-encrypted. Server reveals it's SSH protocol versionto the client. If the client and server decide their versions are compatible, the connection processcontinues; other wise either party may decide to terminate the connection. After protocol versionis accepted, client and server switch to a packet-based protocol. SSH server then sends the following to the client - its host key which is used to identify itself,the server key which helps to establish the secure connection, a list of supported encryption,SSH: The Secure Shell 3 Rachana MaheswariCS265 Term Paper Spring 2003compression and authentication methods, and a sequence of eight random bytes. Client checksidentity of server by using the host key, and checking it in the known-hosts database. If Clientrejects host key, connection ends. Client then, has to prove its identity by including the eightrandom bytes called check bytes in its next response. This is a built-in mechanism to
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