Chapter 9. SSH Protocol

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

type of protection.

9.3.1. 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.

Once an SSH client contacts a server, key information is exchanged so that the two systems can

correctly construct the transport layer. The following steps occur during this exchange:

Key exchange

The public key algorithm to be used

The symmetric encryption algorithm to be used

The message authentication algorithm to be used

The hash algorithm to be used

During the key exchange, the server identifies itself to the client with a host key. If the client has never

communicated with this particular server before, the server's key will be unknown to the client and it

will not connect. OpenSSH gets around this problem by accepting the server's host key after the user

is notified and verifies that he will accept the new host key. In subsequent connections, the server's

host key is checked against the saved version on the client, providing confidence that the client is

indeed communicating with the intended server. If, in the future, the host key no longer matches, the

user must remove the client's saved version before a connection can occur.

Caution

It is possible for an attacker to masquerade as the SSH server during the initial contact since the

local system does not know the difference between the intended server and a false one set up by an

attacker. To help prevent this you should verify the integrity of a new SSH server by contacting the

server administrator before connecting for the first time or after a host key has changed.

SSH is designed to work with almost any kind of public key algorithm or encoding format. After

an initial key exchange creates a hash value used for exchanges and a shared secret value, the two

systems immediately begin calculating new keys and algorithms to protect authentication and future

data sent over the connection.

After a certain amount of data has been transmitted using a given key and algorithm (the exact amount

depends on the SSH implementation), another key exchange occurs, which generates another set of

hash values and a new shared secret value. Even if an attack is able to determine the hash and shared

secret value, he would have to determine this information each time a new key exchange is made in

order to monitor the communication.

9.3.2. Authentication

Once the transport layer has constructed a secure tunnel to pass information between the two systems,

the server tells the client the different authentication methods supported, such as using a private key 

encoded signature or typing a password. The client will then try to authenticate itself to the server

using any of the supported methods.