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Date: Sun, 27 Jun 2021 00:52:04 -0700
From: David Sontheimer <david.sontheimer@...il.com>
To: john-users@...ts.openwall.com
Subject: Cracking nested hashes with unknown salts
Hello Alexander,
I'm curious if John can crack this second password generation heuristic as
well (beyond incremental mode):
A password that is the truncated hash digest of a hashing algorithm, with a
known input (or wordlist of possible inputs), along with an unknown salt of
known length and of a known character set.
For simplicity's sake, let's call this a "hashword."
A hashword is now salted and hashed as usual - for our experiments, using
sha1_crypt - and written to file.
For additional specifics, let's say the following for generating hashwords:
1. The hashing algorithm is sha256_crypt - but again, only the digest is
known - the initial salt remains unknown, removed from the output.
2. The digest is truncated to a known length between 4-8.
3. The salt is of length 8, of the custom alphabet [lowercase letters] +
the five digits [0-4] + the three specials [!@$].
I'm comfortable writing an external script for generating these candidates,
and using John's --stdin option, but I'm curious if John can generate these
hashword candidates internally with a wordlist and appropriate rules.
My goal is similar to the work done cracking pwdhash passwords using
hashcat, as described here:
https://www.flypig.co.uk/papers/dlj-gr-passwords16.pdf
One difference is that pwdhash uses a website TLD as additional input,
assumed to be known to the attacker; our hashword uses a client-side salt
unknown to the attacker.
I recognize incremental mode would eventually find the hashword. Yet I'd
like to find the word and client-side salt used for initial input.
I hope hashword generation makes sense. Please let me know if I've left out
important details.
Regards,
-David
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