Repeated Vignère Cipher I was looking into the Vignère 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?
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)

If this is the case, this is exactly like doing a Caesar cipher with 1 shift for all of the letters.