4.4 Selecting an appropriate authentication method

This section provides some guidance on how to use the available authentication methods in GnuTLS in various scenarios.

4.4.1 Two peers with an out-of-band channel

Let’s consider two peers who need to communicate over an untrusted channel (the Internet), but have an out-of-band channel available. The latter channel is considered safe from eavesdropping and message modification and thus can be used for an initial bootstrapping of the protocol. The options available are:

• Pre-shared keys (see PSK authentication). The server and a client communicate a shared randomly generated key over the trusted channel and use it to negotiate further sessions over the untrusted channel.
• Passwords (see SRP authentication). The client communicates to the server its username and password of choice and uses it to negotiate further sessions over the untrusted channel.
• Public keys (see Certificate authentication). The client and the server exchange their public keys (or fingerprints of them) over the trusted channel. On future sessions over the untrusted channel they verify the key being the same (similar to Verifying a certificate using trust on first use authentication).

Provided that the out-of-band channel is trusted all of the above provide a similar level of protection. An out-of-band channel may be the initial bootstrapping of a user’s PC in a corporate environment, in-person communication, communication over an alternative network (e.g. the phone network), etc.

4.4.2 Two peers without an out-of-band channel

When an out-of-band channel is not available a peer cannot be reliably authenticated. What can be done, however, is to allow some form of registration of users connecting for the first time and ensure that their keys remain the same after that initial connection. This is termed key continuity or trust on first use (TOFU).

The available option is to use public key authentication (see Certificate authentication). The client and the server store each other’s public keys (or fingerprints of them) and associate them with their identity. On future sessions over the untrusted channel they verify the keys being the same (see Verifying a certificate using trust on first use authentication).

To mitigate the uncertainty of the information exchanged in the first connection other channels over the Internet may be used, e.g., DNSSEC (see Verifying a certificate using DANE).

4.4.3 Two peers and a trusted third party

When a trusted third party is available (or a certificate authority) the most suitable option is to use certificate authentication (see Certificate authentication). The client and the server obtain certificates that associate their identity and public keys using a digital signature by the trusted party and use them to on the subsequent communications with each other. Each party verifies the peer’s certificate using the trusted third party’s signature. The parameters of the third party’s signature are present in its certificate which must be available to all communicating parties.

While the above is the typical authentication method for servers in the Internet by using the commercial CAs, the users that act as clients in the protocol rarely possess such certificates. In that case a hybrid method can be used where the server is authenticated by the client using the commercial CAs and the client is authenticated based on some information the client provided over the initial server-authenticated channel. The available options are:

• Passwords (see SRP authentication). The client communicates to the server its username and password of choice on the initial server-authenticated connection and uses it to negotiate further sessions. This is possible because the SRP protocol allows for the server to be authenticated using a certificate and the client using the password.
• Public keys (see Certificate authentication). The client sends its public key to the server (or a fingerprint of it) over the initial server-authenticated connection. On future sessions the client verifies the server using the third party certificate and the server verifies that the client’s public key remained the same (see Verifying a certificate using trust on first use authentication).