Anonymity and authenticity are apparently conflicting goals. Anonymity means hiding a party’s identity whereas authenticity means proving a party’s identity. So how can a set of senders authenticate their messages without revealing their identity? Despite the paradoxical nature of this problem, there exist many cryptographic schemes designed to achieve both goals simultaneously, in some form. This paper provides a composable treatment of communication channels that achieve different forms of anonymity and authenticity. More specifically, three channel functionalities for many senders and one receiver are introduced which provide some trade-off between authenticity and anonymity (of the senders). For each of them, composably realizing it is proved to corresponds to the use of a certain type of cryptographic scheme, namely (1) a new type of scheme which we call bilateral signatures (syntactically related to designated verifier signatures), (2) partial signatures , and (3) ring signatures . This treatment hence provides composable semantics for (game-based) security definitions for these types of schemes. The results of this paper can be interpreted as the dual of the work by Kohlweiss et al. (PETS 2013), where composable notions for anonymous confidential communication were introduced and related to the security definitions of certain types of public-key encryption schemes, and where the treatment of anonymous authenticated communication was stated as an open problem.
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