# Arup Guha
# Substitution Cipher Encrypter

# Precondition: message is all lowercase letters, key is 26 letters long
#               and a permutation of 'a' - 'z', so also all lowercase.
# Postcondition: returns the encryption of message with key (substitution)
def encrypt(message,key):

    # Store result here.
    res = ""

    # Go through each character.
    for i in range(len(message)):

        # Subtract ascii values and use that as the index into key.

        res = res + (key[ord(message[i])-ord('a')])

    return res

# Returns the inverse function stored in key.
def makedecryptkey(key):

    # List of 26 characters.
    newkey = [' ']*26

    # Do the inverse mapping.
    for i in range(len(key)):
        temp = ord(key[i])-ord('a')
        newkey[temp] = chr(i+ord('a'))

    # There must be a better way to do this!
    res = ""
    for i in range(len(newkey)):
        res = res + newkey[i]
    return res

# My hard-coded test case.
myKey = "qwertyuiopasdfghjklzxcvbnm"
plain = "thequickbrownfoxjumpsoverthelazydog"

# Encrypt and print ciphertext.
cipher = encrypt(plain, myKey)
print("The ciphertext is",cipher)

# Make the decryption key and show it for testing purposes.
decryptKey = makedecryptkey(myKey)
print("Key to decrypt is", decryptKey)

# We can reuse the encrypt function here!
plainBack = encrypt(cipher, decryptKey)
print("The decrypted message is", plainBack)