  Messaging Layer security (MLS) and its underlying Continuous Group Key
Agreement (CGKA) protocol allows a group of users to share a cryptographic
secret in a dynamic manner, such that the secret is modified in member
insertions and deletions. Although this flexibility makes MLS ideal for
implementations in distributed environments, a number of issues need to be
overcome. Particularly, the use of digital certificates for authentication in a
group goes against the group members' privacy. In this work we provide an
alternative method of authentication in which the solicitors, instead of
revealing their identity, only need to prove possession of certain attributes,
dynamically defined by the group, to become a member. Instead of digital
certificates, we employ Attribute-Based Credentials accompanied with Selective
Disclosure in order to reveal the minimum required amount of information and to
prevent attackers from linking the activity of a user through multiple groups.
We formally define a CGKA variant named Attribute-Authenticated Continuous
Group Key Agreement (AA-CGKA) and provide security proofs for its properties of
Requirement Integrity, Unforgeability and Unlinkability. We also provide
guidelines for an integration of our construction in MLS.
