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Mon, 06 Aug 2007

Breaking Enigma code or cracking SHA1 hash for fun.

Do you recall my intention to crack this hash. Well, it is first 20 rounds of the most widely used cryptographic digest called SHA1. SHA1 contains 80 rounds.

I found a simple way to form a workspace, which, after being processed, results in the given hash value, i.e. algorithm takes needed hash value as parameter and creates something used as input, hash of which is exactly the same as requested.

This is not a complete crack of the reduced SHA1 algorithm yet, since workspace (80 bytes for the first 20 rounds) must be turned into 64 bytes of input, but it is not that complex task.
I do not know if this algorithm will work with full sized SHA1 (80 rounds instead of 20), but right now I do not see any problems with it.

Here is an example workspace data:

Input data:
5e ca 9b e6 38 cf cd 33 41 cf 61 b3 fb cd 39 df 65 87 61 b8 2c 1e 56 ac 69 d7 d0 18 7f 9b 0f a3 
9c 13 99 4c c0 08 c2 de 2d ed c2 d5 99 f8 94 57 d7 a1 e2 35 93 73 0c 11 5a 80 5e 80 ff a8 54 fe

digest: 136be2b1 e949ef99 b85caa61 c97e39cc 7c53ccc5

Cracked data:

workspace (substitute W in sha_transform() with this data):
a57d8667 a57d8667 a57d8667 a57d8667 a57d8667 a57d8667 a57d8667 a57d8667
a57d8667 a57d8667 a57d8667 a57d8667 a57d8667 a57d8667 a57d8667 96ccb97c 
a1900adc c7d34989 3c218123 b2380816

digest: 136be2b1 e949ef99 b85caa61 c97e39cc 7c53ccc5

W[i] = rol32(W[i-3] ^ W[i-8] ^ W[i-14] ^ W[i-16], 1);

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