The proof is provided by means of the following parameters: a public module n formed by the product of f prime factors p.sub.i, f>2; a public superscript v; m base numbers g.sub.i, m>1. The base numbers g.sub.i are such that the two equations: x.sup.2.ident.g.sub.i mod n and x.sup.2.ident.-g.sub.i mod n cannot de solved in x in the ring of integers modulo n, and such that the equation x.sup.v.ident.g.sub.i.sup.2 mod n can be solved in x in the ring of integers modulo n in the case where the public superscript v is in the form v=2.sup.k, wherein k is a security parameter.