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Communication Security: Remote Access and Messaging • Chapter 3 129

SSH

SSH is a cryptographically secure replacement for standard Telnet, Remote Login
(rlogin), Remote Shell (RSH), and RCP commands. SSH consists of both a client
and a server that use public key cryptography to provide session encryption. It also
provides the ability to forward arbitrary ports over an encrypted connection.
SSH has received wide acceptance as the secure mechanism for access to
remote systems interactively. SSH was conceived and developed by Finnish devel-
oper,Tatu Ylonen.When the original version of SSH became a commercial ven-
ture, the license became more restrictive.A public specification was created,
resulting in the development of a number of versions of SSH-compliant client
and server software that do not contain the restrictions (most significantly, those
that restrict commercial use).
SSH deals with the confidentiality and integrity of information being passed
between a client and host. Since programs such as Telnet and rlogin transmit user-
names and passwords in cleartext, sniffing a network is easy. By beginning an
encrypted session before the username and password are transmitted, confidentiality
is guaranteed. SSH protects the integrity of the data being transmitted by the use of
session keys.The client keeps a list of user keys for servers with which it previously
established secure sessions. If the key matches, the secure session is established and
the integrity of the data being transmitted is confirmed. Using SSH helps protect
against different types of attacks including packet sniffing, IP spoofing, and manipu-
lation of data by unauthorized users.

How SSH Works

When a client wants to establish a secure session with a host, the client initiates
communication by requesting an SSH session. Once the server receives the request
from the client, the two perform a handshake, which includes the verification of the
protocol version. Next, session keys are exchanged between the client and the
server. Once session keys have been exchanged and verified against a cache of host
keys, the client can begin the secure session. Figure 3.10 depicts the SSH authenti-
cation process.

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