2.3 Trust on first use
Advocates of the trust on first use method (TOFU) reject the idea of having to rely on a third-party-issuing certifi- cates that have to be renewed periodically. Instead users are supposed to make a leap of faith, trusting and pinning the certificate that has been presented by a remote host during the very first connection attempt. This approach is familiar from its use in SSH and it also has been proposed for encrypted e-mails [27]. The application of the TOFU approach for SSL certificate validation has been proposed in [28]. An implementation of the concept is the certificate patrol browser extension [29].
However, certificate renewal is handled poorly in the TOFU trust model. It may be difficult for users to dis- tinguish between the case of a certificate having been changed deliberately and legitimately by the operator of a remote host and the case of an MITM attack. Therefore, Laribus does not rely on the TOFU principle.
2.4 Incorporating out-of-band information
The following approaches try to establish secure connec- tions without a set of fixed, potentially untrusted third parties.
Direct Validation of Certificates (DVCert) [30] allows a client to validate the authenticity of a server certifi- cate without having to rely on third parties at all. DVCert leverages the fact that many websites can be personalized, i. e., users have to log in with application-level creden- tials (username and password). In DVcert, the fact that client and server know the user’s credentials is exploited by the web server to prove to a connecting client that it is in fact talking to the desired web server and not to a man-in-the-middle.