KCDSA (Korean Certificate-based Digital Signature Algorithm) is a digital signature algorithm created by a team led by the Korea Internet & Security Agency (KISA). It is an ElGamal variant, similar to the Digital Signature Algorithm and GOST R 34.10-94. The standard algorithm is implemented over , but an elliptic curve variant (EC-KCDSA) is also specified. KCDSA requires a collision-resistant cryptographic hash function that can produce a variable-sized output (from 128 to 256 bits, in 32-bit increments). HAS-160, another Korean standard, is the suggested choice. Domain parameters : a large prime such that for . : a prime factor of such that for . : a base element of order in . The revised version of the spec additional requires either that be prime or that all of its prime factors are greater than . User parameters : signer's private signature key such that . : signer's public verification key computed by where . : a hash-value of Cert Data, i.e., . The 1998 spec is unclear about the exact format of the "Cert Data". In the revised spec, z is defined as being the bottom B bits of the public key y, where B is the block size of the hash function in bits (typically 512 or 1024). The effect is that the first input block corresponds to y mod 2^B. : the lower B bits of y. Hash Function : a collision resistant hash function with |q|-bit digests. Signing To sign a message : Signer randomly picks an integer and computes Then computes the first part: Then computes the second part: If , the process must be repeated from the start. The signature is The specification is vague about how the integer be reinterpreted as a byte string input to hash function. In the example in section C.1 the interpretation is consistent with using the definition of I2OSP from PKCS#1/RFC3447. Verifying To verify a signature on a message : Verifier checks that and and rejects the signature as invalid if not. Verifier computes Verifier checks if . If so then the signature is valid; otherwise it is not valid. External links KCDSA specification and analysis Digital signature schemes Elliptic curve cryptography Public-key cryptography Standards of South Korea