Applications and the Future - Codes: The Guide to Secrecy from Ancient to Modern Times - page 335

SSH Protocol Architecture
We will assume that Alice is the user on the client computer, and she wishes
to establish secure communications with the (remote) host computer.
Overview of SSH Protocols
1. Transport Layer Protocol : This protocol provides stronghost authenti-
cation, confidentiality via strongencryption, and integrity protection from
the server to the client computer. This layer also thwarts the man-in-the-
middle attack. Moreover, it optionally supports compression. Although
there are other possible data streams over which this transport layer may
run, we assume that it does so over the canonical one, TCP/IP. The other
layers of the SSH protocol run on top of the secure tunnel provided by the
transport layer.
2. User Authentication Protocol : This protocol runs over the transport
layer protocol for the purpose of authenticatingAlice to the server. The
DSA cipher is used for authentication (see page 183). Once this protocol
is completed, there is a mutually authenticated secure channel between
Alice and the host.
3. Connection Protocol : This protocol runs over the encrypted tunnel es-
tablished above. It multiplexes 9.5 that tunnel into numerous logical chan-
nels that may be used for a rich variety of application-support services,
including remote program execution, signal propagation, and connection
forwarding.
SSH Protocols in Detail
SSH Transport Layer : The purpose of this layer is to ensure secure com-
munication between Alice, as the client user, and the remote server, as the host.
Once Alice contacts the server, key data must be exchanged in order to con-
struct the tunnel. With SSH2, it is mandated that DSS be used (see page 183).
The host sends its public key, called the host key e S , as identification. In order
for Alice to be certain that she is communicatingwith the correct server, she
must have prior knowledge of e S , for which two trust models are available.
Trust Models for Host Keys : The first is that Alice has a local database
available to her at the client machine. This database associates each host name,
which Alice enters, with the matchingpublic host key. This requires no PKI
infrastructure, which currently is unavailable to the Internet in any case. How-
ever, it is clear that maintainingsuch a database with matchingkey names may
become onerous.
9.5 Multiplexing means the use of a transmission channel to carry two or more signals at the
same time.