A method for a provider to provide and prove an association between a first data value A and a second data value B, including: computing evidence E of the form E=H(A∥B)k*(p−1)/q modρ, where H is a secure hash function, E is an evidence of an association between A and B; ensuring that the verifier has A, B and E; and running an interactive proof with the verifier to convince the verifier that the evidence is valid and that the provider knows the value of k without disclosing the value of k to the verifier.