Cooperative Intelligent Transportation Systems (C-ITS) are gaining ground and are almost part of our everyday life. Within these environments, huge amounts of messages are exchanged. Besides, these messages should be secure in order to ensure users' privacy. Public Key Infrastructures (PKI) represent the most common security solution. Due to the vehicles speed, the communication with the PKI should be fully optimized. The European Telecommunications Standards Institute (ETSI) proposes a PKI architecture for C-ITS environments. However, unlike most of security standards as IEEE 1609.2, there is no Abstract Syntax Notation One (ASN.1) specification for the used certificates. For this reason, in this paper, we propose an ASN.1 definition for the ETSI certificate to help developers in its implementation. In addition, we provide an extensive comparative study of the different encoding schemes, applied to this proposal.