Home Python and the Vigenere Cipher

Python and the Vigenere Cipher

The Vigenere cipher is a polyalphabetic substitution cipher system designed by Giovan Battista Bellaso and improved upon by Blaise de Vigenere. It functions very similarly to a Caesar shift cipher where a shift of lettering occurs. Unlike the Caesar shift cipher the Vigenere cipher performs different shift per character. For example the first letter may have a shift of 4 and the second letter may have a shift of 8 and so on. A key is used to define the shift value for each letter. In this script they key is a letter of the alphabet.

The program works by retrieving the index values of the characters from the key and the plain text in turn. These values are then added together and the resulting number is equal to the index value corresponding to the cipher text. For example:

We have an alphabet with each letter assigned a value a = 0 b = 1 c = 2 and so on. If we were to have the key R and the plain text letter P we would add the values 17 for R and 15 for P. 17 + 15 = 32 If the value is greater than 26 we keep subtracting 26 until we get a number less than 26. 32 – 26 = 6. We now look up the value 6 in the alphabet index. the value at index 6 is G. The program simply loops through this process for each letter in the plain text till the cipher text is complete. Feel free to use the code however you want. Let me know if you do as I would be interested in its implementation.

#Creates the base Alphabet which is used for finding preceeding characters from the ciphertext.
baseAlphabet = ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z')
print ("Welcome to a Vigenere Cipher encrypter. You will first be asked to enter the plain text to be encrypted and then the key you would like to use in the encryption process. The resulting text will be the cipher text.")
plainText = raw_input("Please enter the plain text")
key = raw_input("Please enter the key")
keyList = []
keyLength = 0
while keyLength < len(plainText):
    for char in key:#Adds the users entered key into a list character by character. Also makes the key the same length as plainText
        if keyLength < len(plainText):
            keyLength = keyLength + 1
completeCipherText = [] #The variable each processed letter is appended to
cipherCharIndexValue = 0#This is the value used to temporaily store the ciphertext character during the iteration
keyIncrement = 0
for plainTextChar in plainText:#iterates through the plain text
        cipherCharIndexValue = baseAlphabet.index(keyList[keyIncrement]) + baseAlphabet.index(plainTextChar)#Adds the base alphabets index value of the key and the plain text char
        while cipherCharIndexValue > 25:
            cipherCharIndexValue = cipherCharIndexValue - 26#makes the addition value under 26 as to not go out of range of base alphabet tuple
        completeCipherText.append(baseAlphabet[cipherCharIndexValue])#appends the ciphertext character to the completeCipherText variable. The character is the index of the key + index of the plainTextChar from baseAlphabet
        keyIncrement = keyIncrement + 1#Moves onto the next key
print ''.join(completeCipherText)#Makes the result a strings for printing to the console.
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