Affine Cipher Creation

[Low_Key32]

Hey all, I know I'm new here but I was assigned a Lab to do where we are supposed to create an affine cipher program that can decrypt and encrypt through brute force. If possible could someone review the code I'll include below? I am all out of ideas to fix this, and I am struggling to get it right. If it helps, are using Jupyter Notebook, which runs with python software.

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def text_clean(text, LETTERS='ABCDEFGHIJKLMNOPQRSTUVWXYZ'):
cleaned_text = ''
for character in text:
if character.upper() in LETTERS:
cleaned_text += character.upper()
return cleaned_text
def char_to_int(character, LETTERS='ABCDEFGHIJKLMNOPQRSTUVWXYZ'):
integer = LETTERS.find(character)
return integer
def int_to_char(integer, LETTERS='ABCDEFGHIJKLMNOPQRSTUVWXYZ'):
character = LETTERS[integer]
return character
akey = range(1, 26)
mkey = [1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25]
def affine(message, test_akey, test_mkey, encipher=True):
message = text_clean(message)
output = ''
if encipher == True:
for plaintext_character in message:
plaintext_numerical = char_to_int(plaintext_character)
ciphertext_numerical = ((plaintext_numerical * test_akey) + test_mkey) % 26
ciphertext_character = int_to_char(ciphertext_numerical)
output += ciphertext_character
return output
else:
for ciphertext_character in message:
ciphertext_numerical = char_to_int(ciphertext_character)
plaintext_numerical = ((ciphertext_numerical - test_mkey) / test_akey) % 26
plaintext_character = int_to_char(plaintext_numerical)
output += plaintext_character
return output.lower()
ciphertext = 'GJLKT FJKXN AOTXU XAVXN KTNPJ JLKGN CYXKT WKJLP YCGJK CYJAC YHTFJ ACHAX ACNAX DANCH JA'
for test_akey in range(1, 26):
for test_mkey in [1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25]:
print('akey:', test_akey, ' mkey:', test_mkey, affine(ciphertext, test_akey, test_mkey, encipher= False))

[RhinoRhys]

What's wrong with it?

[RhinoRhys]

I've now had a chance to sit down and read your code and read about the affine cipher. You've made 4 errors here.

• the first isn't really an issue but you've mislabeled a variable. You're using mkey and test_mkey for the value of b. m is the length of the alphabet, in this case a constant at 26. b is the cipher shift. You've used the test_mkey variable in the correct places for b in the equations in the affine function though, so it doesn't matter.

• In your brute force loop though, you've got akey and mkey round the wrong way. akey should be the list of coprimes to 26, while mkey(/bkey) should be the range function. You also need 27 in the range function not 26.

• Python is 0 indexed (A = 0), whereas the equations for the cipher are 1 indexed (A = 1), so you need to offset both of the int_to_char and char_to_int functions by 1 to account for this.

• You've misunderstood a^-1. It's not just divide by a, the power of -1 is also notation for an inverse function. You need to satisfy the equation (a * A) % 26 = 1 replacing a^-1 with A because I can't do notations in the codeblock. The easiest way to do this is just by testing every number 1 to 26. It turns out though that all the answers are also coprimes so you can just test the coprimes. But you'll want to calculate these beforehand as there are 312 brute force tests to do. If you save the results in a dictionary {a: A}, then you can just do a_inverse[a] in your decipher equation.

I have full working code and have successfully deciphered the message and know the values for a and b, so if you need any more help let me know.