[C++] Vigenère Cipher (Alphabet)

          Hello, Reader ^^! Back again with source code ^^! This time the source code is Vigenère Cipher for Alphabet only. The source code written in C++. With this type of encryption, the plaintext is encrypted based on key which consists of a keyword. Each letter will be ciphered differently based on the sequence of its letter on plaintext and the key. Let's take an example. 

Just say I have plaintext "Hello World!". I want to encrypt it using key "KEY". If encrypt it using Vigenère Cipher, here will be happens:

Plaintext

H

E

L

L

O

W

O

R

L

D

!

Ciphertext

K

E

Y

K

E

Y

K

E

Y

K

The key becomes "KEYKE YKEYK". Yup, it is repeated based on the length of the plaintext.

Using the C_i = (P_i + K_i ) mod 26 (C is Ciphertext, P is Plaintext, K is Key, i is Index), it will give result :
"RIJVS GSPVH!"

With that type of encryption, cracking the message using frequency analysis can be prevented.

Now, I also made another variant. It is Auto-Key. Using the example above, the key will be "KEYHE LLOWO", taking some part of the plaintext as key. The result will be "AIDSS HZFHR!". Okay, that's all I can say ^^. Thank you for reading ^^!

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