Secure protocols for password-based user authentication are well-studied in the cryptographic
literature but have failed to see wide-spread adoption on the Internet; most proposals to date
require extensive modifications to the Transport Layer Security (TLS) protocol, making deployment challenging. Recently, a few modular designs have been proposed in which a cryptographically secure password-based mutual authentication protocol is run inside a confidential (but not necessarily authenticated) channel such as TLS; the password protocol is bound to the established channel to prevent active attacks. Such protocols are useful in practice for a variety of reasons: security no longer relies on users’ ability to validate server certificates and can potentially be implemented with no modifications to the secure channel protocol library.
We provide a systematic study of such authentication protocols. Building on recent advances
in modelling TLS, we give a formal definition of the intended security goal, which we call password-authenticated and confidential channel establishment
(PACCE). We show generically that combining a secure channel protocol, such as TLS, with a password authentication or password authenticated key exchange protocol, where the two protocols are bound together
using the transcript of the secure channel’s handshake, the server’s certificate, or the server’s domain name, results in a secure PACCE protocol. Our prototypes based on TLS are available as a cross-platform client-side Firefox browser extension as well as an Android application and a server-side web application that can easily be installed on servers.