  A GNSS signal authentication concept named semi-assisted authentication is
proposed. It is based on the re-encryption and publication of keystream
sequences of some milliseconds from an already existing encrypted signal. Some
seconds after the keystreams are transmitted in the signal-in-space, the signal
broadcasts the key allowing to decrypt the sequences and the a-posteriori
correlation at the receiver. The concept is particularized as Galileo Assisted
Commercial Authentication Service, or ACAS, for Galileo E1-B, with OSNMA used
for the decryption keys, and E6C, assumed to be encrypted in the near future.
This work proposes the ACAS cryptographic operations and a model for signal
processing and authentication verification. Semi-assisted authentication can be
provided without any modification to the signal plan of an existing GNSS,
without the disclosure of signal encryption keys, and for several days of
receiver autonomy, depending on its storage capabilities.
