TUCoPS :: Truly Miscellaneous :: ciscopw.txt

Decrypt cisco "encrypted" passwords

#! /bin/sh
## Decrypts cisco "encrypted" passwords.  Feed this confg files as stdin.
## Anything that looks like a "type 7 encrypted" string gets decrypted.
## This should really be a C program, but is presented as a script just to
## piss off a certain group of people.  One beer, please...

while read xx ; do
  case "$xx" in
    *d\ 7\ [01]??* ) ;;
    *) continue ;;
  DEC=`echo "$xx" | sed -e 's/.* //' -e 's/\(^..\).*/\1/'`
  DP1=`expr $DEC + 1`
  HEX=`echo "$xx" | sed -e 's/.* //' -e 's/^..\(..*\)/\1/'`
  echo 'dsfd;kfoA,.iyewrkldJKDHSUB' | cut -c "${DP1}-30" > /tmp/cis$$.pad
  echo '#' > /tmp/cis$$.in
  for xx in 1-2 3-4 5-6 7-8 9-10 11-12 13-14 15-16 17-18 19-20 21-22 ; do
    echo "${HEX}" | cut -c $xx | sed -e '/^$/q' -e 's/^/0x/' >> /tmp/cis$$.in
  echo -n "${DEC}${HEX}: "
  data -g < /tmp/cis$$.in | xor /tmp/cis$$.pad
  echo ''
rm -f /tmp/cis$$.pad /tmp/cis$$.in
exit 0

# Discussion:

# When "service password-encryption" is configured into a cisco router and
# the configuration subsequently viewed, the passwords are no longer printed
# as plaintext but as strings of randomish-looking garbage.  Analysis of
# several samples reveals the scrambling algorithm to be trivially weak.

# Dr. Delete derived and published an analysis and decryption program some
# time ago, but since that didn't seem to be generally available at the time
# I went looking for it, here is an independent explanation.  This was worked
# out on PAPER over a plate of nachos in a hotel bar in downtown LA, but
# still illustrates where a general-purpose "xor" handler can be useful for
# quickly cracking lame "proprietary" algorithms of this genre.

# Passwords can be up to eleven mixed-case characters.  In the "encrypted"
# representation, the first two bytes of the long string are a random decimal
# offset between 0 and 15 into a magic block of characters, and the remaining
# bytes are ascii-hex representations of the password bytes xored against
# the character-block bytes from the given offset on down.  The character
# block is "dsfd;kfoA,.iyewrkldJKDHSUB", which is enough for a maximum-length
# password at the maximum offset.

# Another character block consisting of "sgvca69834ncxv9873254k;fg87" is
# located after the first one in the IOS image, which may be relevant to
# something else and is simply mentioned here for posterity.  It is also
# interesting to note that the strings "%02d" and "%02x" occur immediately
# afterward, which in light of the above is another clue.

# _H* 960315

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