Extract keys to decrypt Java TLS stream

extract-pre-master-secret.py

#!/usr/bin/env python3
#
# Extract TLS pre-master secret to decrypt captured TLS stream in Wireshark
#
# Java TLS implementation can be configured to dump information on TLS stream,
# making it possible to extracting TLS key for decrypting the stream for debug
# purposes.
#
# This script is originally by Timothy Basanov
#   https://timothybasanov.com/2016/05/26/java-pre-master-secret.html
#
# Run the application
#   java .... -Djavax.net.debug=ssl,keygen | extract-pre-master-secret.py keys.log
#
# To decode TLS stream in Wireshark
#   1. select packet with TLS layer
#   2. right-click and choose "Protocol preferences / Open Transport Layer Security Preferences"
#   3. select "keys.log" file in (Pre)-Master-Secret log filename
#

import sys
import re

def extract_data_from_line(line):
  m = re.match('\d+:([ 0-9A-F]{51}) .*', line)
  if m:
    return m.group(1).replace(' ', '')
  else:
    raise line


def main(args):

  if len(args) != 2:
    sys.exit('Usage: {} logfile.txt'.format(args[0]))

  logfile = open(args[1], 'w')

  parsing_mastersecret_line = 0
  parsing_clientnonce_line = 0

  for line in sys.stdin:
    print(line, end='')

    if parsing_mastersecret_line:
      parsing_mastersecret_line += 1
    if parsing_clientnonce_line:
      parsing_clientnonce_line += 1

    if line == 'Client Nonce:\n':
      parsing_clientnonce_line = 1
      cn = ''
    if 2 <= parsing_clientnonce_line <= 3:
      cn = cn + extract_data_from_line(line)

    if line == 'Master Secret:\n':
      parsing_mastersecret_line = 1
      ms = ""
    if 2 <= parsing_mastersecret_line <= 4:
      ms = ms + extract_data_from_line(line)

    if 5 == parsing_mastersecret_line:
      print('CLIENT_RANDOM {} {}'.format(cn, ms), file=logfile)
      logfile.flush()


if __name__ == '__main__':
    main(sys.argv)