Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I started to implement some MAC since last week with the specifications given here. I'm currently testing the OMAC (one-key CBC) with test vectors. In the OMAC specifications at page 4, they explain how the padding works. Just below that, they said :

[...] Where the empty string counts as one block.

This is strange, because the block is empty and that cause me a problem. How could I use M[i] and Xor it with Y[i-1] if M[i] is empty ? (see the algorithm definition at page 8)

Following this question, should I pad the message to get a full block before working with M even if the padding is applied after the for-loop ? I think this would be logic but am I right with this logic ?

A clear explanation of that will be very appreciated. Thanks for all help I'll got.

share|improve this question
up vote 2 down vote accepted

What the specification is saying is that prior to processing, the message is padded to a full block length, with the empty message padded to a single block.

The spec on page 4 describes the input into the algorithm as:

  • Define $||a||_n = max\{1, \lceil|a|/n\rceil \}$, where the empty string counts as one block
  • Let $m = ||M||_n$
  • Partition $M$ into $M[1] ... M[m]$, where $|M[i]| = n$ for $ 1 \le i < m$

i.e. The number of blocks of size $n$ in a message $M$ is rounded up, and must be at least 1 before it can be passed to the $CBC{-}MAC$ portion of the algorithm.

The padding applied to the last block (which is an empty block for the empty message) is defined in formula $(1)$. In byte terms it translates to:

  • Set the first byte to $0x80$ (i.e. set the high bit)
  • Set all remaining bytes to $0$

In implementation terms, this means that you would buffer up to (and including) a complete block, and only process that block when either more data arrives, or the MAC is finalised - an empty message is thus treated as a 0 byte block that needs padding, and a full last block is not padded.

If you're not trying to implement from the spec as an exercise, the Bouncy Castle CMAC implementation should make the buffer/padding scheme clear (CMAC being the NIST standarised name for OMAC1).

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.