A method for creating a group signature of a message to be implemented by a member of a group in a system, the system including a trust authority, the group including at least the member provided with a secure portable electronic entity including storage elements and computing elements wherein are implanted a cryptographic algorithm. The method includes the following steps: generating via the computing elements a signature of the message using a private key common to the members of the group and integrating a data identifying the group member and a temporal data representing a temporal information of the member's membership to the group and of the date of the signature of the message, the private key common to the members of the group, the identifying data and the temporal data being stored in the storage elements.