The following series of events help protect the integrity of SSH
communication between two hosts.
A cryptographic handshake is made so that the client can verify
that it is communicating with the correct server.
The transport layer of the connection between the client and
remote host is encrypted using a symmetric cipher.
The client authenticates itself to the server.
The remote client interacts with the remote host over the
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 accomplishes 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
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 is unknown to the client and it does 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
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
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.
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
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.
After a successful authentication over the SSH transport layer,
multiple channels are opened via a
technique called multiplexing. Each of
these channels handles communication for different terminal
sessions and for forwarded X11 sessions.
Both clients and servers can create a new channel. Each channel
is then assigned a different number on each end of the connection.
When the client attempts to open a new channel, the clients sends
the channel number along with the request. This information is
stored by the server and is used to direct communication to that
channel. This is done so that different types of sessions do not
affect one another and so that when a given session ends, its
channel can be closed without disrupting the primary SSH
Channels also support flow-control,
which allows them to send and receive data in an orderly fashion.
In this way, data is not sent over the channel until the client
receives a message that the channel is open.
The client and server negotiate the characteristics of each
channel automatically, depending on the type of service the client
requests and the way the user is connected to the network. This
allows great flexibility in handling different types of remote
connections without having to change the basic infrastructure of