From WikiTI
Jump to: navigation, search


Official Name: TransformHash

BCALL Address: 80A5

Applies one of the four f-transformations as appropriate for the application to be validated.



  • MD5Buffer: The MD5 hash appropriately transformed.


  • 8100: 130-byte area which the multiplication routine uses to store its result
  • 8182: 65-byte area used as the first argument to the multiplication routine
  • 81C3: 65-byte area used as the second argument to the multiplication routine


Given p, q prime, p ≡ 3 mod 8, q ≡ 7 mod 8, and m relatively prime to both, exactly one of the following is a quadratic residue modulo pq:

  • pq-2m
  • pq-m
  • m
  • 2m

Which of the transformations is used is specified by the parameter f. In order to ensure that the message is nonzero, or perhaps just to make life difficult, m is defined to be the MD5 of the application, multiplied by 256, plus 1. Therefore:

  • If f=0, the result is n-2*(MD5*256+1)
  • If f=1, the result is n-(MD5*256+1)
  • If f=2, the result is MD5*256+1
  • If f=3, the result is 2*(MD5*256+1)

If the signature is valid, that number is a quadratic residue mod n, and more to the point, it is the square of the Rabin signature. That square is computed by the Rabin B_CALL, and only if the two numbers match is the application considered valid.