The invention relates to a group of public-key cryptography schemas that use the discrete logarithm problem with the purpose of reducing the cost of developing, producing and maintaining a cryptographic unit. One of the entities (10) performs a calculation comprising at most a small number of additions, subtractions and multiplications of integers, said calculation being common to all of the schemas of the group. The aforementioned calculation is preferably the main calculation to be performed by the entity in question while most of the other calculations can be performed in advance. In particular, said calculation is of the y=ar+bs type, wherein r is a random number and s is a secret key that is specific to the entity (10). The calculation is common to a group of schemas for entity authentication, message authentication, digital signatures and key exchange.