# Defining functions in Vigenere cipher

I'm trying to learn the process of defining/calling a (somewhat) complex function.

I understand the main idea, and can do it on a small (simple) scale.

A simple example:

def multiply(x, y):
return x*y

numb1 = 2
numb2 = 3

print(multiply(numb1, numb2))


Now what I'm trying to do is clean up my Vigenere cipher by defining the functions, and what's stumping me is what parameters to use and why. I don't want to change the (possibly poorly-written) code, however I just want to see how it would look after making use of defined functions.

message = input("Enter a message to encrypt:\n").upper().replace(" ", "")
print("Enter your encryption key ("+str(len(message)),"or less letters.): ")
key = input().upper().replace(" ", "")
div_times = int(int(len(message))/int(len(key)))
remainder = int(len(message))%int(len(key))
key_ring = (((key)*(div_times+1))[:-(len(key)-remainder)])
alph = 26

print("-------\n"
"Message:  ",message,"\n"
"Key:      ",key_ring,
"\n-------")
mvalues = [0]*len(message)
kvalues = [0]*len(key_ring)
m_position = 0
k_position = 0

for letter in message:
mvalues[m_position] = ord(letter)
m_position += 1

for key in key_ring:
num = alph - (int(ord("Z")) - int(ord(key)))
kvalues[k_position] = num - 1
k_position += 1

m_position = 0
k_position = 0
print("\nEncrypted message: ", end="")
for character in message:
newletter_v = (mvalues[m_position] + kvalues[k_position])
if newletter_v > ord("Z"):
newletter_v -= 26
elif newletter_v < ord("A"):
newletter_v += 26
print(chr(newletter_v)+"", end="")
m_position += 1
k_position += 1

• How/why is keyRing different from len(key)? Commented Oct 22, 2014 at 2:05
• If I understand your question, key_ring is the encryption key after it has been formatted to repeat so that it is the same length as message. Key is each individual letter in key_ring. So len(key) will always be 1 and len(key_ring)=len(message). Commented Oct 22, 2014 at 2:10

I think you're making this a lot more complicated than it needs to be. Forgive my rusty python, but you should be able to do something like

def viginere_encrypt(message, key):
length = len(message)
mnum = [0]*length
for i in range(0, length):
mnum[i] = letter_to_number(message[i])
knum = [0]*len(key)
for i in range(0, len(key)):
knum[i] = letter_to_number(key[i]
result = [0]*length
for i in range(0, length):
result[i] = number_to_letter(mnum[i] + knum[i % len(key])
return result

def letter_to_num(letter):
return ord(letter) - ord("a")

def number_to_letter(number):
return chr(number%alpha + ord("a"))


Using list comprehensions, this can be shortened to

def viginere_encrypt(message, key):
mnum = [letter_to_number(x) for x in message]
knum = [letter_to_number(x) for x in key]
return [number_to_letter(mnum[i] + knum[i%len(key)])for i in range(0, len(message)]


or even further to

def viginere_cipher(message, key):
return [number_to_letter(letter_to_number(message[i]) + letter_to_number(key[i % len(key)])) for i in range(0, len(message)])


though that's probably too mushed together by now.

Also, I don't know python too well; there's probably a better way to convert between "a-z" and 0-25.

Anyways, functions are a Very Good Idea. Output can then be separated from business logic, so if, say, you need to use the cipher elsewhere in your code, you're not tied to the terminal. It also allows for much easier testing.

• Is it possible to add a char to a char like that? Neither of them have been assigned any type of value at this point. Commented Oct 22, 2014 at 2:32
• @DrakkorNoir thanks for pointing that out! I think it should be good now. Commented Oct 22, 2014 at 2:37