KCDSA


KCDSA is a digital signature algorithm created by a team led by the Korea Internet & Security Agency. 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 is also specified.
KCDSA requires a collision-resistant cryptographic hash function that can produce a variable-sized output. HAS-160, another Korean standard, is the suggested choice.

Domain parameters

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. 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.

    EC-KCDSA

EC-KCDSA is essentially the same algorithm using Elliptic-curve cryptography instead of discrete log cryptography.
The domain parameters are:
The user parameters and algorithms are essentially the same as for discrete log KCDSA except that modular exponentiation is replaced by point multiplication. The specific differences are:
  • The public key is
  • In signature generation, where
  • In signature verification, the verifier tests whether