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 original SI110 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).

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

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