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 Mar2 awarded Critic Mar2 comment Vigenere ciphers : Need help for math analysis @EkoJunaidiSalam in math you don't use 'looping'. This is done with recursion. If you take a close look at my formula for $M_i$, you see that its second case ($i\ge|K|$) is recursive: if you want to apply it, you'll first need to compute $M_{i-|K|}$. If $i-|K| < |K|$ you can apply the first rule, otherwise you'll first have to compute $M_{i-|K|-|K|}$, et cetera... The formula for decryption relies on itself. Mar2 comment Vigenere ciphers : Need help for math analysis @EkoJunaidiSalam please clarify. $i$ is a number, not a character. About which formula are you talking exactly? Mar2 comment Vigenere ciphers : Need help for math analysis @EkoJunaidiSalam why do you think so? For $i<|K|$ we have $M_i=C_i-K_i=M_i+K_i-K_i=M_i$ and for $i\ge|K|$ we have $M_i=C_i-M_{i-|K|} = M_i + M_{i-|K|} - M_{i-|K|} = M_i$. Mar2 awarded Commentator Mar2 comment Vigenere ciphers : Need help for math analysis @EkoJunaidiSalam you're right, I had forgotten your question a bit. $C_i$ should've been $M_i$ in two places; I edited the answer. Mar2 revised Vigenere ciphers : Need help for math analysis deleted 136 characters in body Mar2 awarded Editor Mar2 revised Vigenere ciphers : Need help for math analysis added 802 characters in body Mar1 comment How does $g$ being a generator imply Diffie-Hellman's correctness? @fgrieu absolutely. I did not answer the question the OP asked, only addressed the problem he turned out to have. Mar1 awarded Teacher Mar1 answered How does $g$ being a generator imply Diffie-Hellman's correctness? Feb26 comment Vigenere ciphers : Need help for math analysis Then your question is unclear. Try editing it to make the problem clear. Feb26 answered Vigenere ciphers : Need help for math analysis Feb17 comment What prime lengths are used for RSA? Good luck implementing RSA with $p =1$... Feb8 comment Simple multiplication as an encryption method @Melab your numbers will be too big for small texts already, is my point. Breaking into smaller blocks will make a difference, this will allow frequency analysis. Feb8 comment Simple multiplication as an encryption method @Melab ah, you're not dividing the plaintext in blocks at all? Then, you would have extremely large numbers. Suppose I have a document of 10kB. Do you have an idea of how large a number that would be? To prevent brute force attacks, you would need a large key as well, making this scheme impossible to handle. Feb8 answered Simple multiplication as an encryption method Jan17 awarded Citizen Patrol Jan9 awarded Supporter