fcrackzip - a Free/Fast Zip Password Cracker

fcrackzip [-bDBchVvplum2] [--brute-force] [--dictionary] [--benchmark] [--charset characterset] [--help] [--validate] [--verbose] [--init-password string/path] [--length min-max] [--use-unzip] [--method name] [--modulo r/m] file...

fcrackzip searches each zipfile given for encrypted files and tries to guess the password. All files must be encrypted with the same password, the more files you provide, the better.

OPTIONS

  a   include all lowercase characters [a-z]
  A   include all uppercase characters [A-Z]
  1   include the digits [0-9]
  !   include [!:$%&/()=?{[]}+*~#]
  :   the following characters upto the end of the spe-
      cification string are included in the character set.
      This way you can include any character except binary
      null (at least under unix).

Have you ever mis-typed a password for unzip? Unzip reacted pretty fast with 'incorrect password', without decrypting the whole file. While the encryption algorithm used by zip is relatively secure, PK made cracking easy by providing hooks for very fast password-checking, directly in the zip file. Understanding these is crucial to zip password cracking:

For each password that is tried, the first twelve bytes of the file are decrypted. Depending on the version of zip used to encrypt the file (more on that later), the first ten or eleven bytes are random, followed by one or two bytes whose values are stored elsewhere in the zip file, i.e. are known beforehand. If these last bytes don't have the correct (known) value, the password is definitely wrong. If the bytes are correct, the password might be correct, but the only method to find out is to unzip the file and compare the uncompressed length and crc's.

Earlier versions of pkzip (1.xx) (and, incidentally, many zip clones for other operating systems!) stored two known bytes. Thus the error rate was roughly 1/2^16 = 0.01%. PKWARE 'improved' (interesting what industry calls improved) the security of their format by only including one byte, so the possibility of false passwords is now raised to 0.4%. Unfortunately, there is no real way to distinguish one byte from two byte formats, so we have to be conservative.

By default, brute force starts at the given starting password, and successively tries all combinations until they are exhausted, printing all passwords that it detects, together with a rough correctness indicator.

The starting password given by the -p switch determines the length. fcrackzip will not currently increase the password length automatically, unless the -l switch is used.

This mode is similar to brute force mode, but instead of generating passwords using a given set of characters and a length, the passwords will be read from a file that you have to specify using the -p switch.

A CP mask is a method to obscure images or parts of images using a password. These obscured images can be restored even when saved as JPEG files. In most of these files the password is actually hidden and can be decoded easily (using one of the many available viewer and masking programs, e.g. xv). If you convert the image the password, however, is lost. The cpmask crack method can be used to brute-force these images. Instead of a zip file you supply the obscured part (and nothing else) of the image in the PPM-Image Format (xv and other viewers can easily do this).

The cpmask method can only cope with password composed of uppercase letters, so be sure to supply the --charset A or equivalent option, together with a suitable initialization password.

fcrackzip -c a -p aaaaaa sample.zip

checks the encrypted files in sample.zip for all lowercase 6 character passwords (aaaaaa ... abaaba ... ghfgrg ... zzzzzz).

fcrackzip --method cpmask --charset A --init AAAA test.ppm

checks the obscured image test.ppm for all four character passwords. -TP fcrackzip -D -p passwords.txt sample.zip check for every password listed in the file passwords.txt.

fzc, which seems to be widely used as a fast password cracker, claims to make 204570 checks per second on my machine (measured under plain dos w/o memory manager).

fcrackzip, being written in C and not in assembler, naturally is slower. Measured on a slightly loaded unix (same machine), it's 12 percent slower (the compiler used was pgcc, from http://www.gcc.ml.org/).

To remedy this a bit, I converted small parts of the encryption core to x86 assembler (it will still compile on non x86 machines), and now it's about 4-12 percent faster than fzc (again, the fcrackzip performance was measured under a multitasking os, so there are inevitably some meaurement errors), so there shouldn't be a tempting reason to switch to other programs.

Further improvements are definitely possible: fzc took 4 years to get into shape, while fcrackzip was hacked together in under 10 hours. And not to forget you have the source, while other programs (like fzc), even come as an encrypted .exe file (maybe because their programmers are afraid of other people could having a look at their lack of programming skills? nobody knows...)

The reason I wrote fcrackzip was NOT to have the fastest zip cracker available, but to provide a portable, free (thus extensible), but still fast zip password cracker. I was really pissed of with that dumb, nonextendable zipcrackers that were either slow, were too limited, or wouldn't run in the background (say, under unix). (And you can't run them on your superfast 600Mhz Alpha).

No automatic unzip checking.

Stop/resume facility is missing.

Should be able to distinguish between files with 16 bit stored CRC's and 8 bit stored CRC's.

The benchmark does not work on all systems.

It's still early alpha.

Method "cpmask" only accepts ppms.

Could be faster.

fcrackzip was written by Marc Lehmann <pcg@goof.com>. The main fcrackzip page is at http://www.goof.com/pcg/marc/fcrackzip.html)