def alphabet_of_cipher(symbols="ABCDEFGHIJKLMNOPQRSTUVWXYZ"):
    """
    Returns the list of characters in the string input defining the alphabet.

    Notes
    =====

    First, some basic definitions.

    A *substitution cipher* is a method of encryption by which
    "units" (not necessarily characters) of plaintext are replaced with
    ciphertext according to a regular system. The "units" may be
    characters (ie, words of length `1`), words of length `2`, and so forth.

    A *transposition cipher* is a method of encryption by which
    the positions held by "units" of plaintext are replaced by a
    permutation of the plaintext. That is, the order of the units is
    changed using a bijective function on the characters' positions
    to perform the encryption.

    A *monoalphabetic cipher* uses fixed substitution over the entire
    message, whereas a *polyalphabetic cipher* uses a number of substitutions
    at different times in the message.

    Each of these ciphers require an alphabet for the messages to be
    constructed from.

    Examples
    ========

    >>> from sympy.crypto.crypto import alphabet_of_cipher
    >>> alphabet_of_cipher()
    ['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']
    >>> L = [str(i) for i in range(10)] + ['a', 'b', 'c']; L
    ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c']
    >>> A = "".join(L); A
    '0123456789abc'
    >>> alphabet_of_cipher(A)
    ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c']
    >>> alphabet_of_cipher()
    ['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']

    """
    symbols = "".join(symbols)
    return list(symbols)