trouble with the vigenere cipher

Question

print ("")
print ("Welcome to Vigenere cipher!")
print ("")
print ("This program will encrypt then offer you a decryption of your message.")
print ("")
print ("Start this off with inputting your message you would like to encrypt.")
m = input("")
print ("")
print ("Now enter your keyword you would like to use. (it must be (" +str(len(m)),"or less letters.))")
k = input("")

if len(k) > len(m):
    print ("")
    print ("sorry this is an invaild keyword")
    print ("Please re-enter your keyword.")
    k = input("")
else:
    print ("Your keyword is great. Lets keep going")

print ("")

print ("Is this correct?")
print ("")

print("-------\n"
  "Message:  ",m,"\n"
  "Key:      ",k,
  "\n-------")
print ("")

print ("yes or no")
correct = input ("")
if 'yes'.startswith(correct.lower()):
print("Great! Lets keep going.")
else:
    print ("You previously put: " +str(m), " as your message.")
    print ("Please re-enter the keyword you wish to use. (it must be (" +str(len(m)),"or less letters.))")
    k = input("")
encrypted = ("")
print ("")

for a in m:
    print ("The message letters are")
    encrypt2 = ord(a)

    for b in k:
        print ("The keyword letters are")
        encrypt = ord(b)
        encrypt3 = (encrypt) + (encrypt2)
        encrpyted = chr(encrypt3)
        print (chr(encrypt3))

I need it to encrypt with the keyword, any help? p.s it needs to be basic like the code above. it is for GCSE (UK) if it needs to have some commands then can you please tell me what they do :) so i can explain it in the task :)

Answer 1

first, failures in your question:

all the code up for a in m: is unnecessary, all that is not part of your problem, your real problem is to encrypt a word using the vigenere cipher's algorithm, this can be replaced by two lines:

message = "ATTACKATDAWN"
key_adj = "LEMONLEMONLE"

or if you prefer

message = "ATTACKATDAWN"
key = "LEMON"
#key_adj and message equal length
key_adj = key + key*((len(message)/len(key))-1) + key[:len(message)%len(key)]
#key_adj is LEMONLEMONLE

Note: i rename the variables so that the people who help you, better understand,

as the input isn't a problem now, i can solve the problem

second, failures in your algorithm:

The most visible fault is using nested for, more isn't better, this complicates your solution, for this problem you need only one for

for a in m:
    for b in k:
        ....

second failure, is the cipher equation .... for example, first letters:

letter_menssage = "A" -> ord("A") = 65
letter_key = "L"      -> ord("L") = 76
                                  +141 -> chr(141) = "ì"

with all this explanation , now I can show a solution:

#i use zip function, match letter message with letter key
# (A,L) (T,E) (T,M) (A,O) (C,N) (K,L) etc.
encrypt = ""
for letter_msg, letter_key in zip(message, key_adj):
    code_letter_msg = ord(letter_msg)-ord('A')
    code_letter_key = ord(letter_key)-ord('A')
    encrypt += chr((code_letter_msg+code_letter_key) % 26 + ord('A'))

print (encrypt)

you get:

LXFOPVEFRNHR

explanation equation:

letter_menssage = "A" -> ord("A")-65 = 0
letter_key      = "L" -> ord("L")-65 = 11
                                    ((0+11)%26 + 65)-> chr(76) = "L"

letter_menssage = "T" -> ord("A")-65 = 19
letter_key      = "E" -> ord("E")-65 = 4
                                    ((19+4)%26 + 65)-> chr(88) = "X"