scratch.go

package main

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

type rot13Reader struct {
	r io.Reader
}

func (r *rot13Reader) Read(p []byte) (n int, err error) {
	n, err = r.r.Read(p)
	for i := 0; i < n; i++ {
		if p[i] >= 65 && p[i] < (97+26) {
			if p[i] < 97 {
				p[i] = 65 + (p[i]-65+13)%26
			} else {
				p[i] = 97 + (p[i]-97+13)%26
			}
		}
	}
	return
}

func main() {
	s := strings.NewReader("Lbh penpxrq gur pbqr!")
	r := rot13Reader{s}
	io.Copy(os.Stdout, &r)
}