Strict Avalanche Criterion and Avalanche criterion? What is the difference between Strict Avalanche Criterion(SAC) and Avalanche criterion?
The definition of avalanche effect is given in the paper of Webster, A. F. "On the design of S-boxes". Advances in Cryptology - Crypto '85 as :


  For a given transformation to exhibit the avalanche effect, an average of one half of the output bits should change whenever a single input bit is complemented.


It is also seen as each bit should have 50% chances to change if you change 1 bit of the input. (strict avalanche)



In the avalanche criterion you look at the output as a whole (average 50% of the bits changes).

In the strict avalanche criterion, you look at each bit one by one and you verify that what ever the other bits will change, it will have a 50% probability to switch.

Related questions:


Hash functions and the Avalanche effect
Calculation of the avalanche effect coefficient