http://go-tour-jp.appspot.com/#61

gistfile1.txt

package main

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

type rot13Reader struct {
    r io.Reader
}

func (self *rot13Reader)Read(b []byte)(n int, err error) {
	n, err = self.r.Read(b)
	
    for i, c := range b {
        switch {
        case c < 'A':
        case c < 'N':
            b[i] = c+13
        case c <= 'Z':
            b[i] = c-13
        case c < 'a':
        case c < 'n':
            b[i] = c+13
        case c <= 'z':
            b[i] = c-13
        default:
        }
    }
    
	return n, err
}

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