Code Breaking. PyPool attempts to solve programming challenge: http://pi3.sites.sheffield.ac.uk/mini-challenges/chal6

decypher.py

"""Code Breaking.

PyPool attempts to solve programming challenge:
http://pi3.sites.sheffield.ac.uk/mini-challenges/chal6

"""

import string

# Encoded sentences

sentence1 = "Uijt jt b Dbftbs djqifs. Opx vtf b gsfrvfodz bobmztjt up efdpef uif xfmm-lopxo gjstu qbsbhsbqi, boe mppl gps b tfdsfu nfttbhf bu uif foe"

sentence2 = "Ts tg d swjsu jvtkhwgdmme dfovbpmhnlhn, suds d gtvlmh cdv tv qbgghggtbv ba d lbbn abwsjvh cjgs ih tv pdvs ba d ptah. Ubphkhw mtssmh ovbpv suh ahhmtvlg bw kthpg ba gjfu d cdv cde ih bv utg atwgs hvshwtvl d vhtluibjwubbn, sutg swjsu tg gb phmm atrhn tv suh ctvng ba suh gjwwbjvntvl adctmthg, suds uh tg fbvgtnhwhn dg suh wtlusajm qwbqhwse ba gbch bvh bw bsuhw ba suhtw ndjlushwg. Jgh suh ntltsg ba qt sb gutas suh mhsshwg tv suh vhrs chggdlh abwpdwng."

sentence3 = "qgamznyhntzjzlvauxjzqgfhfxebjzrm"


# Problem 1

def decypher_caeser(sentence, shift):
    "Decrypt Caesar cypher encrypted `sentence` based on `shift`."
    # Create strings of shifted alpha characters
    shifted_upper = string.uppercase[shift:] + string.uppercase[:shift]
    shifted_lower = string.lowercase[shift:] + string.lowercase[:shift]
    # Create translation tables
    # Much more efficient than repeated use of `sentence.replace(a, b)`
    # Also avoids over-writing already decryted characters
    upper_table = string.maketrans(string.uppercase, shifted_upper)
    lower_table = string.maketrans(string.lowercase, shifted_lower)
    # Chain the two translations
    # We can do this because the output of the method is also a string
    decoded = sentence.translate(upper_table).translate(lower_table)
    return decoded


def brute_force(sentence):
    """Attempt to brute-force a simple Caesar cypher in `sentence`.
    """
    for shift in range(1, 26):
        decoded = decypher_caeser(sentence, shift)
        # Check for some really common words
        if ' a ' in decoded or ' the ' in decoded:
            # Print the shift and the decoded line
            print shift, decoded
            # Add an extra newline for output readability
            print '\n'


brute_force(sentence1)


# Problem 2

sentence2 = sentence2.lower()

words = sentence2.split()
wordfreqs = {}
for word in words:
    if len(word) < 5:
        if word in wordfreqs:
            wordfreqs[word] += 1
        else:
            wordfreqs[word] = 1

cfreqs = {}
for c in sentence2:
    if c.isalpha():
        if c in cfreqs:
            cfreqs[c] += 1
        else:
            cfreqs[c] = 1


common = "e t a o i n s r h l d c u m f p g w y b v k x j q z".replace(' ', '')
common2 = "e t a o h n i s r d l u w m c g f y p v k b j x z q".replace(' ', '')
common3 = "e t o r i n h s a d l u w m c g f y p v k x j b z q".replace(' ', '')
common3 = ''.join(['e','t', 'i', 's', 'o', 'n', 'h', 'r', 'a', 'f', 'u', 'l', 'd', 'g', 'm', 'w', 'p', 'y', 'c', 'b', 'v', 'k', 'x', 'j', 'q', 'z'])

# Create a decorated list
sortlist = [(value, key) for key, value in wordfreqs.items()]
# Sort it
sortlist.sort(reverse=True)
# Undecorate (print the keys)
for value, key in sortlist[:5]:
    print key, '=', value

# Create a decorated list
sortlist = [(value, key) for key, value in cfreqs.items()]
# Sort it
sortlist.sort(reverse=True)
# Undecorate (print the keys)
#for value, key in sortlist[:10]:
#    print key, '=', value

ourtextcommon = ''.join([key for value, key in sortlist])
for char in string.lowercase:
    if char not in ourtextcommon:
        ourtextcommon += char

print ourtextcommon
print ""

table = string.maketrans(ourtextcommon, common[:len(ourtextcommon)])
print string.translate(sentence2, table)
print ""
table2 = string.maketrans(ourtextcommon, common2[:len(ourtextcommon)])
print string.translate(sentence2, table2)
print ""
table3 = string.maketrans(ourtextcommon, common3[:len(ourtextcommon)])
print string.translate(sentence2, table3) + '\n'


# Problem 3

# Pi to 100 digits pasted from Wikipedia
pistring = "3.14159 26535 89793 23846 26433 83279 50288 41971 69399 37510 58209 74944 59230 78164 06286 20899 86280 34825 34211 70679".replace('.', '').replace(' ', '')
newchars = []
for i, char in enumerate(sentence3):
    shift = int(pistring[i % len(pistring)])
    if char.isalpha():
        newchars.append(decypher_caeser(char, shift))
    else:
        newchars.append(char)

print ''.join(newchars) + '\n'


 #decodedsentence = []
# for char in sentence1:
#  if (char.isalpha()):
#   # turn char into a number
#   #x = ord(char)
#   # move it by 10
#   x = string.letters.find(char)
#   x = x+i % len(string.letters)#

#   #turn new number back into character
#   newchar = chr(x)

   #put newchar into a new sequence

#   decodedsentence.append(newchar)
#
#  else:
#   decodedsentence.append(char)


# print  "".join(decodedsentence) + '\n'