83Plus:BCALLs:80A2

Synopsis

Official Name: Rabin

BCALL Address: 80A2

Squares a large integer modulo n.

Inputs

• HL points to ciphertext
• 8000 holds the modulus n of the public key

Outputs

• 8100 holds the plaintext

Destroys

• hardware registers?
• 8100h (large int result register)
• 8182h (large int operand 1)
• 81C3h (large int operand 2)

Comments

This BCALL takes a large integer pointed to by HL and squares it modulo n. The OS uses this to decrypt the Rabin encrypted MD5 hash of an app.

Example

 B_CALL GetFreewareKey  ;loads the modulus into 8000h
 ld     de,tempSwapArea
 push   de
 ld     hl,myInt
 ld     bc,myIntEnd-myInt
 ldir                   ;copy large int to RAM
 pop    hl              ;address of large int to square
 B_CALL Rabin           ;square it
 ;8100h should now contain the number 3412h^2 mod the 0104 key.  
 ;Since the n of the public key is significantly larger, the answer is simply
 ;3412h^2.  So the contents of memory after this BCALL are
 ;8100h: 04 44 51 97 0A
 ;(the integer A975144h)
myInt:
 db  2,12h,34h
myIntEnd: