cipher.codes

Vigenère

About

Vigenère is a simple polyalphabetic substitution cipher which uses a tableau composed of each of the 26 options of the Caesar Cipher.

Supports

✅ Perfect Decoding
✅ Uppercase letters (A-Z)
✅ Lowercase letters (a-z)
❌ Numbers (0-9)
❌ Symbols (!@#$)
❌ Emojis (😍🤬👩🏾‍💻)

Numbers, Symbols, and Emojis
Numbers, symbols, and emoji are outputted as-is by this cipher.

What is “Perfect Decoding”?
Perfect Decoding is when the decoded text exactly matches the text that was encoded.

Source

View Vigenère on Github

Method

Cipher Key

Given a keyPart, create a string of the length as the string to encode or decode. For example, with a key part of hidden and a string to encode hello world, the full cipher key would be: hiddenhidde

key = makeKey('hidden', 'hello world'.length)

function makeKey (keyPart, targetKeyLength) {
  if (typeof keyPart !== 'string' || keyPart.length <= 0) {
    return ''
  }

  const keyPartLen = keyPart.length
  let key = keyPart.repeat(Math.ceil(targetKeyLength / keyPartLen))

  if (key.length > targetKeyLength) {
    key = key.substring(0, targetKeyLength)
  }

  return key
}

Encoding

(letterPosition + keyPosition) mod 26

Decoding Formula

(letterPosition - keyPosition) mod 26

Live Example

Variables

Based on the current settings for Vigenère:

keyPart = "hide"
cipherKey = "hidehidehidehidehidehidehidehidehidehidehidehidehidehidehidehidehidehidehidehidehidehide"

Sample String

Hello World!
ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz
0123456789
!@#$
✨🦄✨

When, Encoded

Phpsw Avzoh!
DFJLHJNPLNRTPRVXTVZBXZDFBD
iegkmikoqmosuqswyuwacyaegc
0123456789
!@#$
✨🦄✨

Then, Decoded

Hello World!
ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz
0123456789
!@#$
✨🦄✨

How it works

letterPosition is the 0-index of the letter in the standard alphabet
keyPosition given the letter at same letterPosition in the key, get the 0-index of that letter in the alphabet

Key

Repeat of the keyPart that matches the length of the string to encode, this key is used to find the keyPosition.

`letterPosition`

Index of the letter in the English alphabet (0-indexed). For example,
A=0, B=1, C=2, and so on until Z=25.

`keyPosition`

Find the letter at letterPosition in the key, then get the 0-index of that letter in the standard English alphabet.

“Magic” Number 26

You’ll see a lot of references to 26, it’s not a magic number, just the
number of letters in the English alphabet.

`mod 26`

Both formulas for encoding and decoding have mod 26 which performs the modulo (%) operation on the output.

This is a safe-keeping action to guarantee the character is one of the 26 letters in the alphabet. Javascript doesn’t have proper support for mod so this formula is used:

export function mod (a, b) {
  return ((a % b) + b) % b
}