What's Wrong With My Vigenere Cypher Encrypt Function?

I get the error string index out of range inside of the encrypt function I don't know how to get rot to repeat over text. the code only works when both inputs are the same length.

Solution 1:

You are going out-of-bounds by using the iterator in rot[i] in line 36 of your program - i will go up to the length of your plaintext, which can be larger than the key.

Try doing a module-div by length of the key, that way you should wrap around the key nicely:

lis.append(rotate_character(text[i], alphabet_position(rot[i % len(rot)])))

Edit You were still unhappy with the result your script returned, so I dug a bit deeper. The underlying problem is that you are trying to implement what some online-tools for Vigenere call "enhanced mode": neither plain- nor ciphertext are guarenteed to be from [a-zA-Z], but may "special characters", e.g. elements from [0-9] or characters like <space>, <, etc.

In case your script encounters a special character, it will not rotate it, but rather copy it over as-is, as done in the else-branch of rotate_character; this is correct. In encrypt(), however, you consume what we shall call a "keysymbol" for every symbol of the plaintext you encounter. This means that effectively you are "wasting" a keysymbol on a plaintext symbol that will not be encrypted (vulgo rotated) at all.

Once you realize this, the fix becomes evident: when we encounter a special character, copy it over into the output list, but don't advance the keystream. The keystream should only be advanced if we actually need to consume a keysymbol.

Translated into code:

def encrypt(text, rot):
    lis = []
    keystream = 0
    for i in range(len(text)):
        keychar = keystream % len(rot)
        if text[i].isalpha():
            lis.append(rotate_character(text[i], alphabet_position(rot[keychar])))
            keystream += 1
        else:
            lis.append(text[i])

    return (''.join(lis))