Bug in a custom encryption/decryption program - encryption

Question

I'm creating an encryption software in python 3.5. It should go along a key, using key[0] to shift raw[0], then key[1] to shift raw[1] etc, going back to key[0] when raw[i] is greater then key[i%len(key)].
# Converts the key into a numerical list.
def convert(alph, key):
for i in range(0, len(key)):
rem = alph.index(key[i])
numkey.append(rem)
print(numkey)
return numkey
#shifts the text dependant on the key
def encrypt (numkey, raw, alph):
encr = ""
emi = ()
emi = list(emi)
for i in range (0, len(raw)):
rem = raw[i]
rem = alph.index(rem)
suba = i%len(numkey)
ram = numkey[suba]
shift = (rem + ram) % 28 #ensures that shift is an index of alph
shift = alph[shift]
emi.append(shift)
for i in range(0, len(emi)):
encr = encr + str(emi[i])
print (encr)
letters = [
' ', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k',
'l', 'm', 'n', 'o', 'p', 'q', 'r', 't', 's', 'u', 'v', 'w',
'x', 'y', 'z', '.', ',', '!', '?']
raw_key = input("Please enter the key:\n")
raw_text = input("Please enter the text you would like to encrypt (no numbers or capitals):")
numkey = convert(letters, raw_key)
encrypt(numkey, raw_text, letters)
My problem is with the decryption program(below).
# Converts the key into a numerical list.
def convert(alph, key):
numkey = ()
numkey = list(numkey) # parse numkey as list
for i in range(0, len(key)):
rem = alph.index(key[i])
numkey.append(rem)
return numkey
# shifts the text dependant on the key
def encrypt (numkey,raw,alph):
encr = ""
emi = ()
emi = list(emi)
for i in range (0, len(raw)):
rem = raw[i]
rem = alph.index(rem)
suba = i%len(numkey)
ram = numkey[suba]
shift = (rem - ram)
if shift < 0:
shift = shift + 28
else:
pass
shift = alph[shift]
emi.append(shift)
for i in range(0, len(emi)):
encr = encr + str(emi[i])
print (encr)
letters = [
' ', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k',
'l', 'm', 'n', 'o', 'p', 'q', 'r', 't', 's', 'u', 'v', 'w',
'x', 'y', 'z', '.', ',' ,'!' ,'?']
raw_key = input("Please enter the key:\n")
raw_text = input("Please enter the text you would like to decrypt:\n")
numkey = convert(letters, raw_key)
encrypt(numkey, raw_text, letters)
For some reason, after encrypting the characters ",", "?" & "!", if I pass them beck through the decryption they always returns as " ", "a" and "b" respectively. This isn't a problem with any other element in the characters list.
If anyone can spot the problem I would be extremely grateful.

Answer 1

The problem is here in the encryption program:
shift = (rem + ram) % 28
The length of letters is 31 not 28. This is where you're looping back to the beginning of the array prematurely.
The problem is mirrored here in the decryption program:
shift = shift + 28
There are other problems as well. Just a few examples:
In the encryption program numkey is not initialized in convert()
no need to use range(), just use for char in key:
no need for the lst = () followed by lst = list(lst) pattern, just use a list in the first place, lst = []
no checking for invalid characters
function is still named encrypt() in the decryption program
Here's a quick first pass at cleaning both up.
Encryption:
import sys
LETTERS = (
' ', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k',
'l', 'm', 'n', 'o', 'p', 'q', 'r', 't', 's', 'u', 'v', 'w',
'x', 'y', 'z', '.', ',', '!', '?')
# Converts the key into a numerical list.
def convert(alph, key):
numkey = []
for char in key:
if char not in alph:
sys.exit("Invalid character")
numkey.append(alph.index(char))
print(numkey)
return numkey
# Shifts the text dependant on the key.
def encrypt (numkey, raw, alph):
encr = ""
for i, char in enumerate(raw):
if char not in alph:
sys.exit("Invalid character")
rem = alph.index(char)
ram = numkey[i % len(numkey)]
# Ensure that shift is an index of alph
shift = (rem + ram) % len(alph)
encr = encr + alph[shift]
print(encr)
raw_key = input("Please enter the key: ")
raw_text = input("Please enter the text you would like to encrypt (no numbers or capitals):\n")
numkey = convert(LETTERS, raw_key)
encrypt(numkey, raw_text, LETTERS)
Decryption:
import sys
LETTERS = (
' ', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k',
'l', 'm', 'n', 'o', 'p', 'q', 'r', 't', 's', 'u', 'v', 'w',
'x', 'y', 'z', '.', ',' ,'!' ,'?')
# Converts the key into a numerical list.
def convert(alph, key):
numkey = []
for char in key:
if char not in alph:
sys.exit("Invalid character")
numkey.append(alph.index(char))
return numkey
# Shifts the text dependant on the key.
def decrypt(numkey, raw, alph):
decr = ""
for i, char in enumerate(raw):
if char not in alph:
sys.exit("Invalid character")
rem = alph.index(char)
ram = numkey[i % len(numkey)]
shift = rem - ram
if shift < 0:
shift = shift + len(alph)
decr = decr + alph[shift]
print(decr)
raw_key = input("Please enter the key: ")
raw_text = input("Please enter the text you would like to decrypt:\n")
numkey = convert(LETTERS, raw_key)
decrypt(numkey, raw_text, LETTERS)