# Constant data as input to hash

I want to use hash function on some data, and part of this data, might be constant for a certain period. The output of the hash function is used as authentication data (MAC).

I want to know if there is:

1. Reason to use/not use this data as input to the hash? (In the context of authenticity of data…)
2. Impact on the "strength" of the hash function – can this constant help attacker to find out the source data of the hash function? (Help him do some smart brute force or something.)
• Is the "source data" the constant data or the non-constant data? $\;$ – user991 Aug 3 '14 at 17:18