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?

  • $\begingroup$ The concept is that you shift each separate character in the plaintext with every character in the key (in essence a for loop) $\endgroup$
    – patricab
    Commented Feb 14, 2019 at 12:55
  • $\begingroup$ 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' $\endgroup$
    – patricab
    Commented Feb 14, 2019 at 13:27
  • $\begingroup$ I've edited your question, please check it. If it is correct then the answer is correct. $\endgroup$
    – kelalaka
    Commented Feb 14, 2019 at 13:52
  • $\begingroup$ Yeah. It's correct. Guess it's just an ordinary Caesar cipher then as abdul says. Thanks for the help though :) $\endgroup$
    – patricab
    Commented Feb 14, 2019 at 14:17

1 Answer 1


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.


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