Anyone know if MD5, Whirlpool, SHA[n], etc., have any “special” input that might get a hexdigest output to align into:
- All numeric characters
- All alpha characters
- All of the same character/pattern repeated consistently or entirely
Example in python:
>>> from hashlib import sha1
>>> hash = sha1('magic_word').hexdigest()
>>> hash
4040404040404040404040404040404040404040
>>> hash = sha1('^3&#b d *#"').hexdigest()
aedefeebadcdccebefadcedddcbeadaedcbdeadc
Is this even possible? My knowledge of hashing functions is limited to the scope of applying them in databases for storing passwords, which is essentially none.
But sometimes I wonder, when testing for collisions, that these sorts of cases might arise…
A hash function models a random oracle: for each input, if it was not yet queried before, we throw some dice to find an output, then note it to some book. If an input is queried again, simply give back this old value.
By throwing a 16-sided dice 40 times (for each input), we get enough output for an SHA-1 like oracle. (For MD5, we only need 32 times.)
So, we can calculate the probability of “40 times only letters” as (6/16)^40 ≈ 9.15·10^-18, “40 times only digits” has probability (10/16)^40 ≈ 6.8·10^-9.
As “number of tries needed until the first success” is geometrically distributed, we need 1/p tries in average, i.e. around 10^17 tries for “only letters”, and 1.5 ·10^8 tries for “only digits”.
(Now, SHA-1 is not a real random oracle, but there is no weakness known which would say that SHA-1 would have better or worse probabilities for one of these. And for now, brute-force really seems to be the best way to do this.)