# decodes messages from a file enciphered by substitution clipher # see http://norvig.com/ngrams/ for explanation and additional code # the hillclimb in decode_subst() is run in parallel

cliphertexts.txt

NKDIF SERLJ MIBFK FKDLV NQIBR HLCJU KFTFL KSTEN YQNDQ NTTEB TTENM QLJFS
NOSUM MLQTL CTENC QNKRE BTTBR HKLQT ELCBQ QBSFS KLTML SSFAI NLKBR RLUKT LCJUK
FTFLK FKSUC CFRFN KRYXB

CNLGV QVELH WTTAI LEHOT WEQVP CEBTQ FJNPP EDMFM LFCYF SQFSP NDHQF OEUTN
PPTPP CTDQN IFSQD TWHTN HHLFJ OLFSD HQFED HEGNQ TWVNQ HTNHH LFJWE BBITS PTHDT
XQQFO EUTYF SLFJE DEFDN IFSQG NLNGN PCTTQ EDOED FGQFI TLXNI

declipher.py

# decodes messages from a file enciphered by substitution clipher
# see http://norvig.com/ngrams/ for explanation and additional code
# the hillclimb in decode_subst() is run in parallel

from multiprocessing import Pool
import sys

import ngrams # written by Peter Norvig for his chapter in the book Beautiful Data

sys.setrecursionlimit(10000)

def func(msg):
  steps=4000
  return ngrams.hillclimb(ngrams.encode(msg, key=ngrams.cat(ngrams.shuffled(ngrams.alphabet))), 
                            ngrams.logP3letters, ngrams.neighboring_msgs, steps)

def decode_subst(msg, restarts=90): 
    "Decode a substitution cipher with random restart hillclimbing." 
    pool = Pool()
    msg = ngrams.cat(ngrams.allwords(msg)) 
    candidates = pool.map(func, [msg]*restarts)
    p, words = max(ngrams.segment2(c) for c in candidates) 
    return ' '.join(words) 

texts = ['']
with open('cliphertexts.txt') as f:
  lines = f.readlines()
  for line in lines:
    if line == '\n':
      texts.append('')
    else:
      texts[-1] += ' ' + line.strip()

for i, text in enumerate(texts):
  print 'CLIPHERTEXT', i
  print '\t', text
  print '\t', decode_subst(text)