go tour --Exercise: rot13Reader

package main

import (
	"io"
	"os"
	"strings"
)

type rot13Reader struct {
	r io.Reader
}

func (rot rot13Reader) Read (b []byte) (int, error){
	n, error := rot.r.Read(b)
	if error != nil {
		return n, error
	}
	for i := range b {
		switch {
			case (b[i] >= 'A' && b[i] < 'N'), (b[i] >='a' && b[i] < 'n'): b[i] += 13
			case (b[i] > 'M' && b[i] <= 'Z'), (b[i] > 'm' && b[i] <= 'z'): b[i] -= 13
		}
		
	}
	
	return n, nil
}
func main() {
	s := strings.NewReader("Lbh penpxrq gur pbqr!")
	r := rot13Reader{s}
	io.Copy(os.Stdout, &r)
}

// ===========================================================

You cracked the code!