I have a key size of 128 bits. My message size is fairly short (e.g., 32 bits - and widely known). Although I am doing a SHA256, the result of the HMAC will be truncated to 128 bits.
So my question is: is there a significant strength difference between the following?
- HMAC256(K,K) 'note that in this case, there is no message) - I just need the result of the hash).
I am trying to prevent someone who knows m and the 128-bit result from figuring out the 128-bit key.