Red Hat ENTERPRISE LINUX 3 Reference Manual page 295

Chapter 19. SSH Protocol
19.3.1. Transport Layer
The primary role of the transport layer is to facilitate safe and secure communication between the two
hosts at the time of authentication and during subsequent communication. The transport layer accom-
plishes this by handling the encryption and decryption of data, and by providing integrity protection
of data packets as they are sent and received. The transport layer also 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:
Keys are exchanged
•
The public key encryption algorithm is determined
•
The symmetric encryption algorithm is determined
•
The message authentication algorithm is determined
•
The hash algorithm is determined
•
During the key exchange, the server identifies itself to the client with a unique host key. If the client has
never communicated with this particular server before, the server's host 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 the acceptance of 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 an 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, verify the integrity of a new SSH server by contacting the server
administrator before connecting for the first time or in the event of a host key mismatch.
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, generating another set of hash
values and a new shared secret value. Even if an attacker is able to determine the hash and shared
secret value, this information is only useful for a limited period of time.
19.3.2. 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 then tries to authenticate itself to the server using
one of these supported methods.
SSH servers and clients can be configured to allow different types of authentication, which gives each
side the optimal amount of control. The server can decide which encryption methods it supports based
on its security model, and the client can choose the order of authentication methods to attempt from
the available options. Thanks to the secure nature of the SSH transport layer, even seemingly insecure
authentication methods, such as a host and password-based authentication, are safe to use.
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