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Date: Thu, 8 Sep 2011 16:13:33 -0500
From: "jfoug" <jfoug@....net>
To: <john-dev@...ts.openwall.com>
Subject: Question over validity of concept in JtR's benchmarking code.

I have been re-working the pkzip format.  I have it running quite a bit
faster, with a huge improvement in the 1 byte checksum type zips (which ARE
the most common, outside of the unix infoZip built zip files).

 

On my test machine, I have it running well, in real world tests. However,
benchmark testing was REALLY off, WAY below what I was seeing in real world
(7250k real world, vs .  45k benchmark)

 

Here is the reasoning why, and then a description of what the pkzip format
is doing under the hood.  The reason the timings are SO far off, is that in
the benchmark code, we are always providing the correct password to the
format in the inner testing loop.   Here is the ‘question’ I have from the
title of this message:

 

Is sending correct passwords to the benchmark functions a ‘valid’ way to
benchmark?    In real testing, we assume that a correct password, is a VERY
rare event. There may be billions, or 100’s of billions of passwords tested,
before a correct password is found (if ever).  

 

Would it not make more sense, to provide incorrect passwords when
benchmarking the format?

 

 

 

NOTE, the description below is the current (WIP) pkzip_fmt logic.  It is
very long winded.  Skip it if you are not interested.  It was included to
add a real world example to the above question.

 

 

 

Now, how pkzip is setup.  Note, an incorrect password, is several orders of
magnitude quicker within the crypt function than the correct one (the actual
amount of difference will depend upon the size of the smallest encrypted zip
blob that was in the zip file).

 

In the pkzip format, testing for correct password, requires a full
decryption of the encrypted zip blob of data, and then inflating that
compressed zip blob, and then performing a CRC32 against the decompressed
data, to make sure it checks out.  If the inflation works, and gives the
proper sized file blob, and the CRC32 of the data gives the correct data,
then we have a valid password crack.    This is SLOW, and can be orders of
magnitude slower than even MsCash2, if the zip blob is large.

 

Thus, to make this format work faster, I have used multiple levels of
testing.  I first take the knowledge that the last byte (or 2) of the 12
byte IV, is a known value.  This is used by unzip programs to not have to
query for a password multiple times, and processing multiple encrypted files
in the same zip (some unzip programs even ‘remember’  the password between
files and sessions).  Also it is a quick check within the unzip, so it does
not have to decrypt megabytes (or more) of data, and inflate it, (which can
take a good long while), only to then be able to inform the user that they
have entered the wrong password.  Now, just because a password causes the
decryption of the IV to have the last byte (or 2) to match the crc value,
does not mean this is the right password, but it is good enough for the
unzipper to continue forward.

 

I have taken this ‘knowledge’ as my first level of checking.  What happens,
is I have to convert the password into the key, then run the 12 bytes of IF
through the decrypt and key update functions.  Then the last byte (or 2) is
checked.  This one quick check alone, will remove 255/256 bogus passwords
(or 65535/65536 for 2 byte checksum).

 

I then perform what I am calling a ‘level 2’ checking. In this checking, I
look at the very first inflate ‘code’, and act accordingly. This code is 2
bits (the 2nd and 3rd bit of the very first byte of the decrypted zip
stream).  If the code is a 3 (both bits set), then I know this cannot be the
password, since that is not a valid inflate code (only 0, 1 and 2 are
valid).  This simple 1 byte check will remove 25% of anything getting past
the initial ‘level 1’ checksum testing, while only requiring 1 additional
byte of decryption.  If the zip code is a 0 (a store), then I can decrypt 4
more bytes, (which is a 2 byte length, with a 2 byte checksum), and make
sure the checksum matches.  For the type 0, I also only allow it to proceed,
if the first byte is 0 or 1. Since the deflate/inflate algorithm will
discard the remaining 5 bits of that first byte, I am making an assumption
that all zippers would have initially zero’d out that byte. I strongly
believe that is a valid assumption.  Now, if the code was a 0, only about 1
out of (2^16 * 2^5) will pass. Thus, we have pretty much effectively remove
another ¼ of the possible bad passwords.  

 

I also have code which handles the code type 1 (fixed Huffman table), and
code type 2, (variable Huffman table).  The variable Huffman checking only
requires 10 bytes to be decrypted (in worse case). The fixed Huffman code, I
have to decrypt more bytes, and actually perform the inflate algorithm
(using the fixed tables), until I either arrive at a bad code, a bad reverse
lookup (looking too far back in the data stream), or I run out of data.   I
am currently testing whether 24 bytes, or 36 bytes is the ‘right’ amount.
It is somewhere in that range.

 

So, for level 2, I perform some data validation within a single inflate code
block.  I do not allocate any memory, or do any of the overhead which a
‘real’ inflate handler would do.  The level 2 is checking is a new thing
being done with the new pkzip format I will be releasing soon (still have a
nit or 2 to get worked out).  

 

There used to be (and still is, but is #if  commented out) code that would
perform a small ASCII test, and also was code added which would search the
start of files for ‘magic’ values, if the hash string told the pkzip_fmt to
do this.  The logic on ‘what magic’ to use was put into zip2john.  So a .zip
file inside of a .zip file would have a specific 4 byte ‘magic’ signature to
match.  A .gif file would have 2 signatures (GIF87a or GIF89a). NOTE, this
code has been commented out of the current format, due to level 2 AND level
3 checking, AND due to the magic signature ‘could’ be wrong.  Remember the
file IS encrypted, so just because it is called a .DOC, or .BMP, or .GIF,
does not mean that the file really IS that type, and if we guess wrong, and
compare the starting decrypted/inflated bytes against what we ‘thought’ it
was supposed to be, but in actuality it is NOT, then we will never be able
to crack the password.   So this code is still ‘there’, but simply removed
from the build right now.

 

Level 3 checking, is simply to decrypt a little more of the file (currently
this is 140 bytes, and is ONLY done, if the zip blob is larger than 200
bytes).  This decrypted data is then sent to zlib.  If there is any error at
all (any return other than Z_OK), then we know the zip data was bogus, and
we have the wrong password.  Note, that most of the bogus passwords which
passed the checksum validation (level 1), have already been removed in level
2 (which does much of the same error checking as in the ‘real’ zlib inflate
function).  However we are passing a larger buffer to inflate, and it finds
ALMOST all invalid items.  

 

Finally, a level 4 test is performed. This is the ‘real’ test.  The format
decrypts the entire zip blob, and uses zlib to inflate this properly, and
the inflated data is CRC32 checked.  If the data decrypts with no errors,
it’s length is correct, and the data CRC32’s to the proper value, then we
are assured of having the correct password.

 

 

At this time, ALL 4 levels of checking are being done within the crypt
function.  The prior version of pkzip_fmt (the one that has been released on
the wiki), only perform level 1 in the crypt() function, and perform the
ascii/magic testing, and then the level 4 testing, within the cmp_all()
function.  However, doing this, we lose multi-threading.   The current
crypt() does all work incrementally.  It continues to decrypt more and more
bytes, the deeper into the function we go, and then continues to use the
zlib buffer deeper and deeper, AS LONG AS this password continues to appear
to be the right one.

 

Thus, when bench gives only the right passwords, it causes the format to
always have to perform the worst case scenario (CPU cycle wise and memory
allocation wise), which is a password was found.

 

NOTE, it will be very likely that the level 4 (the full .zip checking), WILL
be pulled out from the crypt function, and put into the cmp_all function,
for several technical reasons.  However, since that is supposed to be a VERY
RARE event, this should make no impact at all on the speed of the format.

 

Sorry for the long winded email.  I wanted to post some information on the
howto/why the format is built the way it is, and also post about the bench
sending correct passwords (and this email killed both birds with one stone).

 

Jim.


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