KCDSA

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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[edit]

  • : 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[edit]

  • : 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[edit]

  • : a collision resistant hash function with |q|-bit digests.

Signing[edit]

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[edit]

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[edit]