Background

The Vigenere Cipher is a bit more complicated that the Caesar Cipher (and a bit more secure). It's still do-able by hand, though with more work. The fundamental operation is, like the Caesar Cipher, to "shift" a letter by some fixed shift-value, but in the Vigenere Cipher each symbol in the key is used as a different shift. If the key is as long as or longer than the plaintext message, each character of the plaintext message gets "caesar shifted" by a different value. Otherwise, we repeat the key over and over until we get something as long as the plaintext message. See the old plebe cyber symmetric encryption notes for more general-purpose information.

For our purposes, all symbols - whether in the key, plaintext or ciphertext - are characters in the ASCII range 42-122 inclusive. We will represent a shift value by a single character in that same range: 42 is a shift of zero, 43 is a shift of 1, and so on. In the following, we will go back and forth between treating a character as an integer (via ASCII) and integer as a character. In code, you must make explicit casts, like (int)c - 42 or (char)(i + 42).

Encrypting a plaintext message based on a key

Given key key $= k_0 k_1 \cdots k_{n-1}$ and plaintext message ptxt $= p_0 p_1 \cdots p_{m-1}$, compute the ciphertext message ctxt $= c_0 c_1 \cdots c_{m-1}$.

We encrypt a character at a time: to encrypt the index i character of the plaintext do the following:

let pc be the index i character of ptxt
let sc be the index (i mod n) character of key
k  = sc - 42
p  = pc - 42
c  = (p + k) mod 81
cc = 42 + c
define the index i character of ctxt to be cc

Note: if any character in the key or the plaintext message is outside of ASCII 42-122 it is an ERROR!

Decrypting a ciphertext message based on a key

Given key key $= k_0 k_1 \cdots k_{n-1}$ and ciphertext message ctxt $= c_0 c_1 \cdots c_{m-1}$, compute the plaintext message ptxt $= p_0 p_1 \cdots p_{m-1}$.

We decrypt a character at a time: to decrypt the index i character of the ciphertext do the following:

let cc be the index i character of ctxt
let sc be the index i mod n character of key
k  = sc - 42
c  = cc - 42
p  = (c + (81 - k)) mod 81
pc = 42 + p
define the index i character of ptxt to be pc

Note: if any character in the key or the ciphertext message is outside of ASCII 42-122 it is an ERROR!