
MessageID: <20150903003454.GA22733@openwall.com>
Date: Thu, 3 Sep 2015 03:34:54 +0300
From: Aleksey Cherepanov <lyosha@...nwall.com>
To: johndev@...ts.openwall.com
Subject: reverse of full sha1 and sha256 limb when hash and block are known
tl;dr: if W and hash are known, initial state can be computed. ... PROFIT!
Looking at sha1 round, it is obvious that each round can be computed
backwards (i.e. starting from hash and going to initial state):
consider a, b, c, d, e after round known, then there are only 2
unknown variables: w[i] and e at the beginning of round. So knowing
w[i], it is trivial to right a formula to compute e. This way we go
reverse.
sha256 is a bit more complex, but it has only 1 unknown variable too.
So it is possible to write reverse algo straightforward. PoC is
attached.
So sha1 and sha256 behave like block ciphers: you "encrypt" initial
state ("block of data") with W, that is expanded block of message
("key"). And it is possible to "decrypt" hash to get initial state.
There are several applications. The following 2 were pointed out to me
by Alexander Cherepanov, there are my ideas then.
1) To check 1 password against 1 hash, it is possible to meet in the
middle computing halves in parallel. First of all, it is
interleaving, but it is possible to vectorize shared part of
regular and reverse algorithms. It is possible to speed up
defensive usage this way.
2) To check 1 checksum against 1 big file, it is possible to compute
checksum using 2 threads. (But usually(really?) checksums are
checked for multiple files at the same time, thus multithreading is
already there.)
3) Passwords longer than 1 block with repeating tails and same length
have trade off: 1 computation for each hash for each tail or 1 full
computation for each passwords. Lengths 5664 (inclusive) need 2
limbs but second block does not change at all (1 value for each
length).
4) It is possible to hash beginning and tail independently if memory
permits: compute initial states for each tail for each hash, put
all of them into bitmask (lookup table), compute beginnings and
check against the bitmask. (Is not it called "meet in the middle"
attack?)
Easy practical application
Consider a hash sha256(sha256(...).sha256(...)), for instance
sha256(sha256($p).sha256($s))
sha256($p).sha256($s) produces exactly 1 block of message, so the
second block is 0x80, padding and constant length always. So we can
reverse the second block and check intermediate state computing only 1
limb instead of 2. That's up to 50% higher speed (considering that we
already have other optimizations including caching for sha256($p) and
sha256($s)). With all optimizations and many salts, it should have
same c/s as single block rawsha256.
I only checked sha1 and sha256. But (many?) other hash functions can
have this behaviour. Also other crypto schemes can give constant
blocks. It is needed to investigate it more.
Thanks!

Regards,
Aleksey Cherepanov
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