Difference between revisions of "83Plus:BCALLs:80A2"
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=== Inputs === | === Inputs === | ||
| − | * HL | + | * HL points to ciphertext |
| − | * | + | * [[83Plus:RAM:8000|8000]] holds the modulus n of the public key |
=== Outputs === | === Outputs === | ||
Revision as of 08:14, 29 March 2005
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
- 8100h = 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:
