International Association
for Cryptologic Research

Digital signatures are one of the most extensively used cryptographic primitives today. It is well-understood that they guarantee practical security only if the corresponding verification keys are distributed authentically; however, arguably, satisfying solutions for the latter haven\'t been found yet, or at least aren\'t in large-scale deployment. This paper introduces a novel approach for cryptographic message authentication where this problem does not arise: A linkable message tagging scheme (LMT) identifies pairs of messages and accompanying authentication tags as related if and only if these tags were created using the same secret key. In other words, in contrast to signature schemes, our primitive does not aim at detecting whether individually considered messages originate from an explicitly specified entity, but instead decides whether all messages from a given collection originate from the same (possibly anonymous) source. The appealing consequence is that our primitive does not involve public keys at all, and hence elegantly sidesteps the key distribution problem of signature schemes.

As an interesting application of LMT we envision an email authentication system with minimal user interaction. Email clients could routinely generate a secret LMT key upon their first invocation, and then equip all outgoing messages with corresponding tags. On the receiver\'s side, client software could automatically verify whether incoming messages originate from the same entity as previously or subsequently received messages with an (allegedly) identical sender address. Although this form of message authentication does not provide as strong guarantees of sender\'s origin as signature schemes would do, we do believe that trading the apparently discouraging obstacles implied by the authentic distribution of signature verification keys for the assumption that an attacker does not forge every message exchanged between parties is quite attractive.

On the technical side, we formalize the notions of LMT and its (more efficient) variant CMT (classifiable message tagging), including corresponding notions of unforgeability. For both variants we propose a range of provably secure constructions, basing on different hardness assumptions, with and without requiring random oracles.