In python NOT using fernet function please

W. A "symmetric-key" cryptosystem is a system for two people, Alice and Bob, who have agreed on a single secret key before-hand, to encrypt and decrypt messages sent over an insecure channel. The shift cipher, Vigenere cipher, and one-time pad are all examples of symmetric-key cryptosystems. There are various notions of security, but essentially what we desire is that an eavesdropper Eve who sees the ciphertexts cannot gain much useful information about the plaintexts. Your task is to design, implement, and describe a symmetric-key cryptosystem with the following specifications: A/ message to be encrypted consists of a string containing only the characters A,B,... ,z. A message of any length that is a multiple of 12 can be encrypted. The encrypted message must be a string of characters (but not necessarily just capital letters). Page 1 of 5 . The key can have a variety of forms, but you must explicitly specify which keys are allowable. There can be at most 261-95428956661682176 valid choices of key. The key must be describable with a string (but not necesarily just capital letters). So, for instance, one example of the set of valid posesible cboices of key is (s : s is a string consisting of capital letters of length 12). Another example is (s s is a binary (0,1) string of length 56). (the last example satisfies the condition on number of possible keys since 238