Encrypted SMS and voice app Signal has passed a security audit with flying colours.

As explained in a paper titled A Formal Security Analysis of the Signal Messaging Protocol [PDF], published by the International Association for Cryptologic Research, Signal has no discernible flaws and offers a well-designed and compromise-resistant architecture.

Signal uses a double rachet algorithm that employs ephemeral key exchanges continually during each session, minimising the amount of text that can be decrypted at any point should a key be compromised.

Signal was examined by a team of five researchers from the UK, Australia, and Canada, namely Oxford University information security Professor Cas Cremers and his PhDs Katriel Cohn-Gordon and Luke Garratt, Queensland University of Technology PhD Benjamin Dowling, and McMaster University Assistant Professor Douglas Stebila.

The team examines Signal threat models in the context of a fully adversarially-controlled network to examine how it stands up, proving that the cryptographic core of Signal is secure. As the authors write, Signal's security is such that even testing it is hard:

Providing a security analysis for the Signal protocol is challenging for several reasons. First, Signal employs a novel and unstudied design, involving over ten different types of keys and a complex update process which leads to various chains of related keys. It therefore does not directly fit into existing analysis models. Second, some of its claimed properties have only recently been formalised. Finally, as a more mundane obstacle, the protocol is not substantially documented beyond its source code.

They conclude that it is impossible to say if Signal meets its goals, as there are none stated, but say their analysis proves it satisfies security standards adding "we have found no major flaws in its design, which is very encouraging".

The team finds some room for improvement which they passed on to the app's developers, namely that the protocol can be further strengthened with negligible cost by using "constructions in the spirit of the NAXOS (authenticated key exchange) protocol" [PDF] by or including a static-static Diffie-Hellman shared secret in the key derivation. This would solve the risk of attackers compromising communications should the random number generator become fully predictable.

The paper does, however, cover only a subsection of Signal's efforts, as it ignores non-Signal library components, plus application and implementation variations. It should therefore be considered a substantial starting point for future analysis, the authors say, rather than the final world on Signal. ®