83Plus:BCALLs:80A5
Synopsis
Official Name: TransformHash
BCALL Address: 80A5
Applies one of the four f-transformations as appropriate for the application to be validated.
Inputs
- 8291: The MD5 hash as a big integer
- 83E6: The parameter f as a big integer
- 8000: The modulus
Outputs
- MD5Buffer: The MD5 hash appropriately transformed.
Destroys
- 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
Comments
Given p, q prime, p â‰¡ 3 mod 8, q â‰¡ 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.