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Date: Mon, 11 Jan 2016 15:52:07 +0100
From: Frank Dittrich <frank.dittrich@...lbox.org>
To: john-users@...ts.openwall.com
Subject: Re: format-all-details -> Max. password length in bytes
On 01/11/2016 02:35 PM, patpro@...pro.net wrote:
> I wonder how those limits are set: how are they implemented,
The max. password length issue is quite complex.
This link to an old john-dev discussion might provide some background
information:
http://thread.gmane.org/gmane.comp.security.openwall.john.devel/12718/focus=12776
> are they tunable at run time, and why 39 (arbitrary decision?) ?
It is not really tunable by an end user.
For some formats, a developer could look into the problem and see what
performance impact increasing the max. password length would have.
Salted SHA1 currently supports a max. salt size of 16 bytes:
salted_sha1_common.h:13:#define MAX_SALT_LEN 16
The max. password length is defined so that just a single SHA1 block
needs to be computed per candidate:
salted_sha1_fmt_plug.c:58:#define PLAINTEXT_LENGTH (55-MAX_SALT_LEN)
So, if all your hashes do have a salt length < 16 bytes, you could use a
larger max.password length for salted-sha1 and salted-sha1-opencl by
decreasing MAX_SALT_LEN.
OTOH, dynamic_24 and dynamic_25 are quite similar to salted-sha1.
One of these dynamic formats computes SHA1($p.$s), the other SHA1($s.$p).
Both of them do have max. password length of 110 and a salt size of 64,
but you can see that the speed of these formats is considerably slower.
(Since the dynamic formats use hex encoding and salted-sha1 uses base64,
the hashes would need to be converted to be used by dynamic).
> However, I do remember that incremental is limited to 8 char. at compile time. But I'm interested in the limit set for formats.
>
>> $ ./john --list=format-all-details | grep "Max. password length"
>> Max. password length in bytes 8
>> Max. password length in bytes 64
>> Max. password length in bytes 15
>> Max. password length in bytes 72
>> Max. password length in bytes 125
>> Max. password length in bytes 7
>> ../..
$ ./john --list=format-details 2>/dev/null |cut -f 1,2 |head -n 20
descrypt 8
bsdicrypt 64
md5crypt 15
bcrypt 72
scrypt 125
LM 7
AFS 63
tripcode 8
dynamic_0 55
dynamic_1 23
dynamic_2 55
dynamic_3 55
dynamic_4 31
dynamic_5 31
dynamic_6 55
dynamic_8 23
dynamic_9 55
dynamic_10 32
dynamic_11 32
dynamic_12 110
For descrypt and tripcode, length 8 is fine, since these algorithms just
ignore additional bytes.
The same applies to LM's max. length of 7.
For md5crypt, max. length 15 might be too short.
As a workaround, you could use --format=crypt with a max. password
length of 72 bytes.
But make sure to reject all candidates shorter than 16 bytes, because
--format=crypt is a lot slower than --format=md5crypt when cracking
md5crypt hashes.
Frank
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