A one way public key cryptosystem for the linkage of nominal files in epidemiological studies.

An encryption method is proposed for nominal files to allow their linkage while respecting confidentiality rules. In contrast to most encryption algorithms, this encryption method must be non-reversible. The method is based on congruence, like the SAN MARCO algorithm, and satisfies the two following conditions. 1) Non-reversible enciphering, which implies the possibility of collisions. 2) The collision rate must be minimized, to allow file linkage. The security of this method is, in addition, guaranteed by a key, which is destroyed after enciphering and is not given to the recipient of the coded file.