Toward an operational navigation message authentication service: proposal and justification of additional OSNMA protocol features

Galileo, the European GNSS, has confirmed its intention to provide Navigation Message Authentication for its Open Service already during this decade. A testing specification was made available in November 2016. Being OSNMA a novel service with several potential different users, some details of the protocol are under finalization. In particular, the paper focuses on the authentication of the public keys, renewal and revocation of all keys, and the authentication of the MAC sequence. The authentication of the public keys is proposed through a Merkle tree. The renewal and revocation of keys are handled with the existing flags, in particular NMA status, public key and chain status, and the authentication of the MAC sequence is shared between a MAC lookup table, and a new MAC that replaces previously redundant bits in the protocol. These new features complement the definition of the protocol while they do not affect OSNMA performance or appreciably increase user implementation requirements.