The invention relates to a method for signing a message (m), implemented by processing means of a user device of a member (Mi) belonging to a group of members (G), said user device having a secret signature key (ski), said method including a step of generating (E301) a group signature (σ) for the message (m), enabling said member (Mi) to prove his membership in the group (G), and a step of generating (E302) a pseudonym (nymij) identifying the member (Mi) within a domain (Dj) of a service provider (SPj), said domain including a set of terminals in communication with a server of said service provider, said signature (σ) being designed such that said member (Mi) can prove, by signing the message (m), his knowledge of said secret signature key without disclosing it, said group signature (σ) being designed such that the membership of the member (Mi) in the group is verifiable independently from the pseudonym (nymij), said pseudonym and said signature being a function of a portion (xi) of said secret signature key of the member (Mi) and being designed so as to prove that the member identified by the pseudonym is the signatory of the message (m) and said pseudonym (nymij) of said member (Mi) being specific to the domain (Dj).