Undeniable signatures based on characters: How to sign with one bit

We present a new undeniable signature scheme which is based on the computation of characters. Our signature scheme offers the advantage of having an arbitrarily short signature. Its asymptotic complexity is attractive: the asymptotic complexity of all algorithms (even the key setup) are quadratic in the size of the modulus n in bits when the other parameters are fixed. The practical complexity can be quite low depending on parameter and variant choices. We present also a proof of security of our scheme containing the standard security requirements of an undeniable signature.