The Vigenère Cipher: A modified version.

cipher.go

package main

import "fmt"

func main() {
	key := "BADboy"
	msg := "This is fun!"
	cm, cmr := initMap()
	fmt.Println(encode(msg, key, cm, cmr))
	fmt.Println(decode(encode(msg, key, cm, cmr), key, cm, cmr))
}

func initMap() (map[rune]int, map[int]rune) {
	str := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 .,!?$&;:"
	cipherMap := make(map[rune]int, len(str))
	cipherReverseMap := make(map[int]rune, len(str))
	for i, ch := range str {
		cipherMap[ch] = i
		cipherReverseMap[i] = ch
	}
	return cipherMap, cipherReverseMap
}

func encode(msg, key string, cm map[rune]int, cmr map[int]rune) string {
	size := len(cm)
	runes := make([]rune, len(msg))
	krunes := []rune(key)
	for i, ch := range msg {
		msgIdx := cm[ch]
		keyIdx := cm[krunes[i%len(key)]]
		idx := (msgIdx + keyIdx) % size
		runes[i] = cmr[idx]
	}

	return string(runes)
}

func abs(i int) int {
	if i < 0 {
		return -i
	}
	return i
}
func decode(msg, key string, cm map[rune]int, cmr map[int]rune) string {
	size := len(cm)
	runes := make([]rune, len(msg))
	krunes := []rune(key)
	for i, ch := range msg {
		msgIdx := cm[ch]
		keyIdx := cm[krunes[i%len(key)]]
		var idx int
		diff := msgIdx - keyIdx
		if diff < 0 {
			idx = (diff + size) % size
		} else {
			idx = diff % size
		}
		runes[i] = cmr[idx]
	}

	return string(runes)
}