john-users - Dynamic format & truncate

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Message-ID: <001601d574a7$097460c0$1c5d2240$@dexlab.nl>
Date: Thu, 26 Sep 2019 22:14:44 +0200
From: "Vincent" <spam@...lab.nl>
To: <john-users@...ts.openwall.com>
Subject: Dynamic format & truncate

I'd like to create a new dynamic format using truncation but I'm a bit
confused after checking some examples. Simple example from dynamic.conf:

[List.Generic:dynamic_1023]
Expression=sha1($p) (hash truncated to length 32)
Flag=MGF_KEYS_INPUT
Flag=MGF_FLAT_BUFFERS
Flag=MGF_POOR_OMP
MaxInputLen=110
MaxInputLenX86=110
Func=DynamicFunc__SHA1_crypt_input1_to_output1_FINAL
Test=$dynamic_1023$5baa61e4c9b93f3f0682250b6cf8331b:password
# sha1(password) = 5baa61e4c9b93f3f0682250b6cf8331b7ee68fd8, match on [0:31]

Two questions:

- How does this code know that it only needs to compare [0:31] instead of
[0:39]?
- Is it possible to truncate leading instead of trailing nibbles, e.g. match
[8:39]?

Thanks!

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