Host Keys
Each server host must have a host key. Hosts may have multiple host
keys using multiple different algorithms. Multiple hosts may share
the same host key. Every host must have at least one key using each
required public key algorithm.
The server host key is used during key exchange to verify that the
client is really communicating with the correct server. For this to be possible,
the client must have prior knowledge of the server's public host key.
Two different trust models can be used:
- The client has a local database that associates each host name (as typed by the user) with the corresponding public host key. This method requires no centrally administered infrastructure, and no
third-party coordination. The downside is that the database of
name-key associations may become burdensome to maintain.
- The host name - key association is certified by a
trusted certification authority. The client knows
only the CA root key, and can verify the validity of all host
keys certified by accepted CAs.
The second alternative eases the maintenance problem, since ideally only a
single CA key needs to be securely stored on the client. On the other
hand, each host key must be appropriately certified by a central
authority before authorization is possible. Also, a lot of trust is
placed on the central infrastructure.
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