Cifrado Vigenere0.1

The Vigenere cipher is a symmetric cryptosystem, ie uses the same key to encrypt and decrypt. The Vigenère cipher closely resembles the Caesar cipher, but the difference is that the former uses a longer to counteract the great problem of the Caesar cipher key: the fact that a letter can only be coded in a way. To solve this problem, a keyword instead of a single character is used.
First, each letter is associated with a corresponding number.

ABCDEFGHIJKLMNOPQRSTU VWXYZ
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

A text is encoded with a word by adding the letters of another word (called keyword) to each of its letters. The keyword is added indefinitely in the text to be encrypted, then the ASCII of each of the letters of the keyword in the text to be encrypted code is added. For example, the text "rendezvousamidi" your keyword "bonjour" will be encoded as follows:

Original text:
r e n d e z v o u s a m i d i
114 101 110 100 101 122 118 111 117 115 97 109 105 100 105

Keyword:
b o n j o u r
98 111 110 106 111 117 114

Ciphertext
\ R + B E + O \ n + \ n -d + J E + O u + \ r O + O + B u s -a + J M + O i d + \ ri
114 + 98 101 + 111 110 + 110 100 + 106 101 + 111 122 + 117 118 + 114 111 + 98 117 + 111 115 + 110 97 + 106 109 + 111 105 + 117 100 + 114 105 + 98

To decrypt this message, the secret key is needed and reverse decryption is performed using subtraction.

Although encryption is much stronger than the Caesar cipher, yet it can easily break. When messages are much longer than the keyword, you can identify the length of the keyword and use, for each keyword sequence, the method of calculating the frequency of occurrence of the letters, and determine characters Key words one at a time ...

To avoid this problem, one solution is to use a keyword that is almost as long as the text, to avoid a statistical study of the ciphertext. This type of encryption system is called one-time pad system. The problem with this type of method is the length of the encryption key (the longer the text to be encrypted, the larger will be the key) which prevents its memorization and implies a much larger probability of errors in the key (one single error makes the text almost impossible to read ...).