  Double Blind Comparison is a new cryptographic primitive that allows a user
who is in possession of a ciphertext to determine if the corresponding
plaintext is identical to the plaintext for a different ciphertext held by a
different user, but only if both users co-operate. Neither user knows anything
about the plaintexts corresponding to either ciphertext, and neither user
learns anything about the plaintexts as a result of the comparison, other than
whether the two plaintexts are identical. Neither user can determine whether
the plaintexts are equal without the other user's co-operation. Double Blind
Comparisons have potential application in Anonymous Credentials and the
Database Aggregation Problem. This paper shows how Double Blind Comparisons can
be implemented using a Strong Associative One-Way Function (SAOWF). Proof of
security is given, making an additional assumption that the SAOWF is
implemented on a Group with Infeasible Inversion (GII), whose existence was
postulated by Hohenberger and Molnar.
