Vigenère Cipher exntended to support UTF, implemented in Go.

main.go

import (
	"fmt"
	"strings"
)

func cypher(str string, key string) string {
	s, ix := new(strings.Builder), 0
	for _, cs := range str {
		s.WriteRune(cs + rune(key[ix%len(key)]))
		ix++
	}
	return s.String()
}

func decypher(str string, key string) string {
	s, ix := new(strings.Builder), 0
	for _, cs := range str {
		s.WriteRune(cs - rune(key[ix%len(key)]))
		ix++
	}
	return s.String()
}