In cryptography, a Schnorr signature is a digital signature produced by the Schnorr signature algorithm. Its security is based on the intractability of certain discrete logarithm problems. It is considered the simplest digital signature scheme to be provably secure in a random oracle model. It is efficient and generates short signatures. It is covered by U.S. Patent 4,995,082, which expired in February 2008.
Choosing parameters 
- All users of the signature scheme agree on a group with generator of prime order in which the discrete log problem is hard. Typically a Schnorr group is used.
- All users agree on a cryptographic hash function .
In the following,
- Exponentiation stands for repeated application of the group operation
- Juxtaposition stands for multiplication on the set of congruence classes or application of the group operation (as applicable)
- Subtraction stands for subtraction on set of equivalence groups
- , the set of finite bit strings
- , the set of congruence classes modulo
- , the multiplicative group of integers modulo (for prime , )
Key generation 
- Choose a private signing key from the allowed set.
- The public verification key is .
To sign a message :
- Choose a random from the allowed set.
- Let .
- Let , where || denotes concatenation and is represented as a bit string.
- Let .
The signature is the pair .
Note that ; if , then the signature representation can fit into 40 bytes.
If then the signature is verified.
Demonstration of correctness 
It is relatively easy to see that if the signed message equals the verified message:
, and hence .
Public elements: , , , , , , . Private elements: , .
Security argument 
No proof of security for the Schnorr signature scheme is known under standard cryptographic assumptions.
The signature scheme was constructed by applying the Fiat-Shamir Transform to Schnorr's identification protocol. Therefore (per Fiat and Shamir's arguments), it is secure if is modeled as a random oracle.
Its security can also be argued in the generic group model, under the assumption that is "random-prefix preimage resistant" and "random-prefix second-preimage resistant". In particular, does not need to be collision resistant.
See also 
- Fiat; Shamir (1986). "How To Prove Yourself: Practical Solutions to Identification and Signature Problems". Proceedings of CRYPTO '86.
- Schnorr (1989). "Efficient Identification and Signatures for Smart Cards". Proceedings of CRYPTO '89.
- Neven, Smart, Warinschi. "Hash Function Requirements for Schnorr Signatures". IBM Research. Retrieved 19 July 2012.
- C.P. Schnorr, Efficient identification and signatures for smart cards, in G. Brassard, ed. Advances in Cryptology—Crypto '89, 239-252, Springer-Verlag, 1990. Lecture Notes in Computer Science, nr 435
- Claus-Peter Schnorr, Efficient Signature Generation by Smart Cards, J. Cryptology 4(3), pp161–174 (1991) (PS).
- Menezes, Alfred J. et al. Handbook of Applied Cryptography CRC Press. 1996.