edwardbetts / decrypting-polyalphabetic-ciphers

Decrypting polyalphabetic ciphers usign Kasiski's Method and Index of Coincidence

Decrypting-polyalphabetic-ciphers

Decrypting polyalphabetic ciphers using Kasiski's Method and Index of Coincidence

Usage

usage: decrypt_polycipher.py [-h] [-m] [-spa] [-i INPUT_FILE] [-o OUTPUT_FILE]
                             [-nc] [-v]

decrypt a polyalphabetic substitution ciphered text

optional arguments:
  -h, --help            show this help message and exit
  -m, --manual          interacts with the user
  -spa, --spanish       suppose the ciphertext is in Spanish (English by
                        default
  -i INPUT_FILE, --input-file INPUT_FILE
                        the input file with the encrypted text
  -o OUTPUT_FILE, --output-file OUTPUT_FILE
                        the output file with the decrypted text
  -nc, --no-colors      don't color the output
  -v, --verbosity       increase output verbosity

Example

The attack in automatic mode only works with big texts. If you have a short text, you will have to use the option --manual

Automatic attack

Suppose we encrypt the speech of Martin Luther King I have a dream using Vigenère's cipher with the key LUTHER. We have used Sharky's Vigenere Cipher but you can use whatever tool you want as long as it keeps non-alphabet characters and it isn't case sensitive.

Let's try to decrypt:

$ python3 decrypt_polycipher.py -i ihaveadream_encrypted.txt -o ihaveadream_decrypted.txt
INFO: Getting encrypted text from ihaveadream_encrypted.txt
INFO: Periods guessed using Kasiski's Method [2, 3, 6, 4, 12]
INFO: Period with closest IC to English IC: 12
INFO: Period with closest IC to ciphertext IC: 6
INFO: Periods with confidence: [(6, '32.19%'), (12, '23.72%'), (2, '17.69%'), (3, '17.42%'), (4, '8.98%')]
INFO: Key: LUTHER
INFO: Saving decrypted text in ihaveadream_decrypted.txt.

We have obtained the correct key and the decrypted text is in the file ihaveadream_decrypted.txt.

Manual attack

Suppose we encrypt the monologue Tears in Rain from Blade Runner:

I've seen things you people wouldn't believe.
Attack ships on fire off the shoulder of Orion.
I watched C-beams glitter in the dark near the Tannhauser Gate.
All those moments will be lost in time, like tears...in...rain.
Time to die.

We encrypt it using Vigenère's cipher with the key ROY and we obtain:

A'kd ktdf igacfk nnm edgekw lnmacf'i awahwkd.
Sissrj kwhhh nf uhjt nxu szt rzdtdsdj de Gghgc.
H opsuwdv R-awplk vkaiswg hf igw szjz mwpq lwd Lpmfwzmhdj Vzlt.
Zda szdrw bnetmlh vaak tt kghs ac sabd, dxjw idsgr...ac...qsxm.
Lxlw in vxd.

If we try the automatic decryption:

$ python3 decrypt_polycipher.py
Note: if you want to read/write from/to a file, print more information or interact with the decryption, use command-line arguments. For more information, type:

        python3 decrypt_polycipher.py --help

Introduce the ciphertext: A'kd ktdf igacfk nnm edgekw lnmacf'i awahwkd. Sissrj kwhhh nf uhjt nxu szt rzdtdsdj de Gghgc. H opsuwdv R-awplk vkaiswg hf igw szjz mwpq lwd Lpmfwzmhdj Vzlt. Zda szdrw bnetmlh vaak tt kghs ac sabd, dxjw idsgr...ac...qsxm. Lxlw in vxd.

INFO: Periods guessed using Kasiski's Method [2, 3, 4, 5, 6]
INFO: Period with closest IC to English IC: 3
INFO: Period with closest IC to ciphertext IC: 3
INFO: Periods with confidence: [(3, '41.67%'), (2, '23.33%'), (4, '11.67%'), (5, '11.67%'), (6, '11.67%')]
INFO: Key: RDY
INFO: Decrypted text: i'ge spen ehiygs jou aeoale houwdn'e bewiege. aetank ssipd on qirp ofq thp shzuloer zf ocioy. i wltcsed n-belms rlietec in ehe oarv nelr tse tlnnsauder ratp. alw thzse xompntd wiwl bp lodt iy tixe, ltke eeacs...iy...ratn. ttme eo dte.

It failed. However, we can guess that the beginning must be I've, so we have to change the second monoalphabetic cipher (the second letter of the key).

If the letter K from the ciphertext correspond to the letter V in the plaintext, the letter E must be encrypted to T.

We add the argument --manual to interact with the decryption:

$ python3 decrypt_polycipher.py -m
Introduce the ciphertext: A'kd ktdf igacfk nnm edgekw lnmacf'i awahwkd. Sissrj kwhhh nf uhjt nxu szt rzdtdsdj de Gghgc. H opsuwdv R-awplk vkaiswg hf igw szjz mwpq lwd Lpmfwzmhdj Vzlt. Zda szdrw bnetmlh vaak tt kghs ac sabd, dxjw idsgr...ac...qsxm. Lxlw in vxd.

INFO: Periods guessed using Kasiski's Method [2, 3, 4, 5, 6]
INFO: Period with closest IC to English IC: 3
INFO: Period with closest IC to ciphertext IC: 3
INFO: Periods with confidence: [(3, '41.67%'), (2, '23.33%'), (4, '11.67%'), (5, '11.67%'), (6, '11.67%')]

Introduce period: 3

INFO: Subsequence 0. Most common letters: [('W', 10), ('A', 7), ('L', 5), ('F', 5), ('K', 4)]

Encryption of E: W

INFO: Subsequence 0. Key: R <-> 18
INFO: Subsequence 1. Most common letters: [('I', 7), ('T', 6), ('X', 4), ('H', 4), ('C', 4)]

Encryption of E: T

INFO: Subsequence 1. Key: O <-> 15
INFO: Subsequence 2. Most common letters: [('D', 11), ('S', 7), ('N', 6), ('H', 6), ('Z', 4)]

Encryption of E: D

INFO: Subsequence 2. Key: Y <-> 25
INFO: Key: ROY
INFO: Decrypted text: i've seen things you people wouldn't believe. attack ships on fire off the shoulder of orion. i watched c-beams glitter in the dark near the tannhauser gate. all those moments will be lost in time, like tears...in...rain. time to die.

Try another period [y/N]: N
--------------
INFO: Key: ROY
INFO: Decrypted text: i've seen things you people wouldn't believe. attack ships on fire off the shoulder of orion. i watched c-beams glitter in the dark near the tannhauser gate. all those moments will be lost in time, like tears...in...rain. time to die.

We can see now why the automatic mode failed. The letter which E must be encrypted to is usually the most common letter but in the second subsequence the letter E is encrypted to the second most common letter. For that reason the program prints the 5 most common letters.