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Chapter 10. SSH Protocol 133

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. Of course, if this client

has never communicated with this particular server before, then the server’s key will be unknown to the

client. OpenSSH gets around this problem by allowing the client to accept the server’s host key the ﬁrst

time an SSH connection occurs. Then, in subsequent connections, the server’s host key can be checked

with a saved version on the client, providing conﬁdence that the client is indeed communicating with

the intended server.

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.

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 particular key and algorithm (the exact

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

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

secret values, the attacker would have to determine this information each time a new key exchange is

made to continue to monitor the communication.

10.3.2. 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.

Servers can be conﬁgured to allow different types of authentication, which gives each side the opti-

mal amount of control. The server can decide which encryption methods it will support based on its

security model, and the client can choose the order of authentication methods to attempt from among

the available options. Thanks to the secure nature of the SSH transport layer, even seemingly insecure

authentication methods, such as a host-based authentication, are safe to use.

Most users requiring a secure shell will authenticate using a password. Since the password is encrypted

when moving over the the transport layer, it can be safely sent across any network.

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Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Guida di Installazione Pagina 133