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Date: Mon, 11 Jan 2016 10:03:57 -0500
From: Rich Rumble <richrumble@...il.com>
To: john-users@...ts.openwall.com
Subject: Re: format-all-details -> Max. password length in bytes

On Mon, Jan 11, 2016 at 9:52 AM, Frank Dittrich
<frank.dittrich@...lbox.org> wrote:
> 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
>>> ../..
To add to what Frank said, some password lengths are limited by the
type more often than not (LM=7, DES=8) and that's why incremental was
limited by default for so long, but in 1.8 the length of incremental
was changed to 24 bytes.
-rich

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