Difference between revisions of "83Plus:BCALLs:8018"

Latest revision as of 14:12, 24 July 2007

Synopsis

Official Name: MD5Final

Other Name: FinishMD5

BCALL Address: 8018

Finishes an MD5 hash by adding padding and length bits.

Inputs

• 8269: (8 bytes) Holds the length in bits of the data hashed so far.
• 8292: (16 bytes) Current MD5 hash

Outputs

• 8292: (16 bytes) Completed MD5 hash

Destroys

• 8259: (16 bytes) Alternate registers (roughly equivalent to the AA, BB, CC, and DD registers specified in the MD5 standard.)
• 83A5: (64 bytes) Buffer holding data to be hashed.

Comments

This B_CALL "completes" a hash started by MD5Init and MD5Update.

Warning: Early versions of this B_CALL contain a bug, such that computing the hash of a string with length congruent to 55 mod 64 gives an incorrect result. (Rather than adding only one byte of padding, it adds a full 65 bytes, the last of which is E5 rather than zero.)

This is most likely not a security hole if you are simply using MD5 as a black-box hash function, and you don't assume the result on one calculator will match that on another.

If you need the actual, RFC-compliant MD5 hash, you can add the padding and length bytes yourself using MD5Update. (Leaving off the padding and length altogether would be a significant security hole; and in any case, MD5Update only updates the hash every 64 bytes.) See below for a simple implementation of this.

Boot code versions known to contain this bug:

• TI-73 1.3005
• TI-83 Plus BE 1.00, 1.01

Boot code versions known not to contain this bug:

• TI-83 Plus SE 1.00
• TI-84 Plus BE/SE 1.00, 1.02

Replacement code

(uses OP1)

MD5Final:
       ld hl,MD5Length
       rst 20h

       ld hl,MD5Final_Padding
       ld bc,1
       B_CALL MD5Update
MD5Final_PadLoop:
       ld a,(MD5Length)
       cp 0c0h
       jr z,MD5Final_24mod32
MD5Final_ContinuePadding:
       ld hl,MD5Final_Padding+1
       ld bc,1
       B_CALL MD5Update
       jr MD5Final_PadLoop
MD5Final_24mod32:
       ld a,(MD5Length+1)
       and 1
       jr z,MD5Final_ContinuePadding

       ld hl,OP1
       ld bc,8
       B_CALL MD5Update
       ret

MD5Final_Padding:  db 80h, 00h

Example

B_CALL MD5Init
ld bc,14
ld hl,String
B_CALL MD5Update
B_CALL MD5Final
; should give: f96b697d7cb7938d525a2f31aaf161d0

String: .db "message digest"