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132 Chapter 10. SSH Protocol

• Interception of communication between two systems — In this scenario, a third party exists some-

where on the network between communicating entities and makes a copy of the information being

passed between them. The intercepting party may intercept and keep the information, or it may alter

the information and send it on to the intended recipient.

• Impersonation of a particular host — Using this strategy, an intercepting system pretends to be the

intended recipient of a message. If the strategy works, the client remains unaware of the deception

and continues to communicate with the interceptor as if its trafﬁc had successfully reached its

destination.

Both techniques cause information to be intercepted, possibly for hostile reasons. The results can be

disastrous, whether that goal is achieved by listening for all packets on a LAN or a hacked DNS server

pointing to a maliciously duplicated host.

If SSH is used for remote shell logins and ﬁle copying, these security threats can be greatly diminished.

A server’s digital signature provides veriﬁcation for its identity. The entire communication between

client and server systems cannot be used if intercepted, because each of the packets is encrypted.

Attempts to spoof the identity of either side of a communication will not work, since each packet is

encrypted using a key known only by the local and remote systems.

10.2. Event Sequence of an SSH Connection

The following series of events helps protect the integrity of SSH communication between two hosts.

First, a secure transport layer is created so that the client knows that it is communicating with the

correct server. Then, the communication is encrypted between the client and server using a symmetric

cipher.

With a secure connection to the server in place, the client authenticates itself to the server without

worrying that the authentication information may be compromised.

Finally, with the client authenticated to the server, several different services can be safely and securely

used through the connection, such as an interactive shell session, X11 applications, and tunneled

TCP/IP ports.

10.3. Layers of SSH Security

The SSH protocol allows any client and server programs built to the protocol’s speciﬁcations to com-

municate securely and to be used interchangeably.

Two varieties of SSH currently exist. SSH version 1 contains several patented encryption algorithms

(however, several of these patents have expired) and a security hole that potentially allows for data to

be inserted into the data stream. The OpenSSH suite under Red Hat Linux 7.3 uses SSH version 2.0 by

default, although it also supports version 1. It is recommended that you use SSH version 2-compatible

servers and clients whenever possible

SSH protocol versions 1 and 2 both add layers of security with each of these layers providing its own

type of protection.

10.3.1. Transport Layer

The primary role of the transport layer is to facilitate safe and secure communication between the

two hosts at the time of and after authentication. Usually running over TCP/IP, the transport layer

accomplishes this by handling the encryption and decryption of data and providing integrity protection

of data packets as they are sent and received. In addition, the transport layer provides compression,

speeding the transfer of information.

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