Alternatives to a Vigenere Cipher

Patoshi パトシ

3/8/24, 5:04 PM

A vigenere cipher is just a ceasar cipher (ROT13) based on a SECRET word from my understanding. Are there variations of a vigenere cipher that are similar? I want to create a new type of vigenere cipher where the SECRET is used 2-3 times. Once forward for encrypting it once, then a second time encrypting it twice with the SECRET word backwards, and last encrypting it again with just the first letter of the SECRET. Does such a thing exist already or something similar?

107

2 + 3

Maarten Bodewes

3/8/24, 6:52 PM

The Vigenere cipher is more complex than the Caesar cipher. You could actually argue that the Caesar cipher is a Vigenere cipher with just a single character key. ROT13 is not a cipher as it doesn't use a key; the shift is part of the algorithm description. As such you could call it an obfuscation scheme or even an encoding scheme rather than an encryption scheme.

Patoshi パトシ

3/8/24, 8:23 PM

what if I removed the letters O and E from the source then encode it, would that be considered an encryption scheme? weak, but reasoning is the letters O and E by itself don't represent any words compared to the other vowels and one can still read the source text if needed.

tylo

3/10/24, 3:24 PM

I'd there is no key, it's not a cipher. Any fixed encoding is just an encoding.

Score:5

Crypto

name

3/8/24, 6:43 PM

Your scheme does not address the main weakness of Vigenere cipher - repetition of key after [length of secret - n] characters.

{scheme 1} You effectively take n character word (SECRET), add it (mod 26) with another n character word (TERCES) and add it (mod 26) with another n character word (SSSSSS). Result of this calculation is different SECRET word with same length (that means it is exactly as (un)safe against frequency analysis).

{scheme 2} What you COULD do is take 2 different secrets (SECRET1 and SECRET2), but this two secrets MUST be different length... In that case, key will repeat itself after [Length of SECRET1 × Length of SECRET2] characters.

So, when you use secret SECRET in scheme 1, length of KEY is 6 characters; in scheme 2, you can split word to two secrets SE - CRET, so length of KEY is 8 Characters (so it is actually better).

That being said, Vigenere cipher is still really weak (unless your SECRET word is generated by CSPRNG AND it's length is at least same as length of open text AND you do not reuse KEY for more but 1 message - but in that case it is closer to One-time pad then to Vigenere's cipher).

+ 0

Score:4

Crypto

kodlu

3/8/24, 5:51 PM

Repeatedly adding symbols is equivalent to adding a single symbol. So this is trivially equivalent to Vigenere encrypting once with DAWAA since

SECRET+TECRES+SSSSS=DAWAAD

via addition modulo 26 letterwise, if I haven't made a mistake in the addition.

+ 1

bmm6o

3/9/24, 1:03 AM

`DAWAA` is too short by 1 char

Elon Musk

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EN: Alternatives to a Vigenere Cipher

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