Function decrypt

ciphers/caesar_cipher.py:91–160

decrypt ======= Decodes a given string of cipher-text and returns the decoded plain-text Parameters: ----------- * `input_string`: the cipher-text that needs to be decoded * `key`: the number of letters to shift the message backwards by to decode Optional:

(input_string: str, key: int, alphabet: str | None = None)

def decrypt(input_string: str, key: int, alphabet: str | None = None) -> str:
    """
    decrypt
    =======

    Decodes a given string of cipher-text and returns the decoded plain-text

    Parameters:
    -----------

    *   `input_string`: the cipher-text that needs to be decoded
    *   `key`: the number of letters to shift the message backwards by to decode

    Optional:

    *   `alphabet` (``None``): the alphabet used to decode the cipher, if not
        specified, the standard english alphabet with upper and lowercase
        letters is used

    Returns:

    *   A string containing the decoded plain-text

    More on the caesar cipher
    =========================

    The caesar cipher is named after Julius Caesar who used it when sending
    secret military messages to his troops. This is a simple substitution cipher
    where very character in the plain-text is shifted by a certain number known
    as the "key" or "shift". Please keep in mind, here we will be focused on
    decryption.

    Example:
    Say we have the following cipher-text:
    ``Jgnnq, ecrvckp``

    And our alphabet is made up of lower and uppercase letters:
    ``abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ``

    And our shift is ``2``

    To decode the message, we would do the same thing as encoding, but in
    reverse. The first letter, ``J`` would become ``H`` (remember: we are decoding)
    because ``H`` is two letters in reverse (to the left) of ``J``. We would
    continue doing this. A letter like ``a`` would shift back to the end of
    the alphabet, and would become ``Z`` or ``Y`` and so on.

    Our final message would be ``Hello, captain``

    Further reading
    ===============

    *   https://en.m.wikipedia.org/wiki/Caesar_cipher

    Doctests
    ========

    >>> decrypt('bpm yCqks jzwEv nwF rCuxA wDmz Bpm tiHG lwo', 8)