john-users - skip or try passwords with repeated characters

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Message-ID: <20191113154010.GA6379@openwall.com>
Date: Wed, 13 Nov 2019 16:40:11 +0100
From: Solar Designer <solar@...nwall.com>
To: john-users@...ts.openwall.com
Subject: skip or try passwords with repeated characters

Hi,

I've just written the following two external modes, which I think others
might find useful as well.  We'll likely add them to default john.conf.

--external=Filter_NoRepeats can be used to skip candidate passwords that
contain the same character more than once.  --external=Filter_Repeats is
the opposite, and can be used e.g. to search the remainder of the
initial set of passwords after a run with --external=Filter_NoRepeats.

My immediate use case is for trying to recover a presumably
random-looking yet user-chosen password.  I split the initial candidate
password list in two: first those without repeats, and then the rest.
In my case, the first sub-list ended up being 15+ times smaller than the
second one, although this varies (primarily) by the character set size.
There are not a lot of different characters in total in my case, which
is why seeing a character used more than once in a password is so common
in the full list of candidate passwords.

My guess is this will improve average time until success if people tend
to avoid using a character more than once when asked to produce a string
of random characters.  (Someone might want to test this hypothesis.)

[List.External:Filter_NoRepeats]
int seen[0x100], now;

void init()
{
	now = 0;
}

void filter()
{
	int i, c;

	if (!now--) {
		i = 0;
		while (i < 0x100)
			seen[i++] = 0;
		now = 1000000000;
	}

	i = 0;
	while (c = word[i++]) {
		if (seen[c] == now) {
			word = 0; return;
		}
		seen[c] = now;
	}
}

[List.External:Filter_Repeats]
int seen[0x100], now;

void init()
{
	now = 0;
}

void filter()
{
	int i, c;

	if (!now--) {
		i = 0;
		while (i < 0x100)
			seen[i++] = 0;
		now = 1000000000;
	}

	i = 0;
	while (c = word[i++]) {
		if (seen[c] == now)
			return;
		seen[c] = now;
	}

	word = 0;
}

On a related note, we might want to add a way to invert the filter from
the command-line, which would eliminate the need for defining both
modes.  Alternatively, we can share most code of the two modes above by
including a common section into them, like we do for some other external
modes already.

Alexander

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