Tectia uses cryptographic authentication for Secure Shell server hosts. Each server has a cryptographic key pair (a public key and a private key) that identifies the server. Whenever a Secure Shell client connects to a Secure Shell server, the server authenticates itself to the client cryptographically. This ensures that encryption and integrity protection are provided end-to-end between the client and the intended server. Server authentication also eliminates the danger of certain cryptographic attacks, especially man-in-the-middle attacks.
For the cryptographic authentication to work, the client must know the server's public key so that it can securely authenticate the server. The public key of the server must be distributed to each client. The private key of the server is never sent anywhere outside the server computer, but it is used by the server to create a digital signature that can then be verified by the client using the public key.
Secure Shell authenticates the Secure Shell server service of the server host. A host may run several Secure Shell server listeners on different ports. On Unix, each server can have a unique identity, if desired. However, typically there is only one Secure Shell server listener. If separate policies are needed or different services need to be offered for multiple use cases on a particular host, they can be defined dynamically in the Tectia Server configuration.
The server is authenticated with a digital signature based on a DSA or RSA public-key algorithm. Each server must have a public-private key pair. In implementations without support for certificates, clients refer to a local database of trusted server public keys.
Secure Shell also supports certificate authentication. Servers can authenticate themselves to the client with X.509 v3 certificates. When certificates are used, the client does not need to have a local database of trusted server public keys or server certificates. Instead, just the few trusted CA (certification authority) certificates are stored on the client, and the client trusts the servers whose certificates are signed by a trusted CA and certificate contents match the server hostname. Certificates provide scalability for authentication.
The Secure Shell server may have multiple identities (one DSA key, one RSA key, one DSA certificate, and one RSA certificate). During the key exchange in the Secure Shell connection, the Secure Shell client and server agree on which identity will be used in server authentication. Tectia Client prefers certificates over keys if trusted CA certificates have been configured, and otherwise DSA keys over RSA keys.