# Repeated Vigenère Cipher

I was looking into the Vigenère cipher when I thought of something.

Would you improve the security of the cipher if you shifted each letter (from the text you want to encrypt) with each letter of the key?

Also, given that the mathematical expression of the cipher is:

$$C_i = E_K(M_i) = (M_i+ \sum_{i=0}^{l-1}K_i) \pmod {26},$$ where $$l$$ is the lenght of the keyword $$K=K_1K_2\ldots K_l$$.

(the encryption part anyway)

How would you express the concept I just explained mathematically?

• The concept is that you shift each separate character in the plaintext with every character in the key (in essence a for loop) Feb 14 '19 at 12:55
• Well, sort of. If you have a key length of 3, the process would be something like this: '(((((M_1 + K_1) mod 26) + K_2) mod 26) + K_3) mod 26' Feb 14 '19 at 13:27
• I've edited your question, please check it. If it is correct then the answer is correct. Feb 14 '19 at 13:52
• Yeah. It's correct. Guess it's just an ordinary Caesar cipher then as abdul says. Thanks for the help though :) Feb 14 '19 at 14:17

Do you mean that if you have a key of length $$l$$ say $$k_1, ..., k_l$$, and a plain $$m_1, ..., m_n$$, that you want to do $$c_i = m_i + \sum_{j=1}^l k_j$$ ? (Each plain letter is added with each key letter)