Proof is established by means of the following parameters: m pairs of private values Q.sub.1 and public values G.sub.1 m>1, a public module n made of the product of f first factors p.sub.j, f>2, a public exponent v, linked to each other by relations of the type: G.sub.1.Q.sub.i.sup.v=1 mod n or G.sub.1=Q.sub.1.sup.v mod n. Said exponent v is such that v=2.sup.k where k>1 is a security parameter. Public value G.sub.1 is the square g.sub.1.sup.2 of a base number g.sub.i that is lower than f first factors p.sub.j, so that the two equations: x2=g.sub.i mod n and x.sup.2=-g.sub.1 mod n do not have a solution in x in the ring of the modulo n integers and such that the equation x.sup.v=g.sub.1.sup.2 mod n has solutions in x in the ring of the modulus n integers.