package main

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

type rot13Reader struct {
    r io.Reader
}

func (reader *rot13Reader) Read(b []byte) (int, error) {
    rot13 := func(r rune) rune {
        switch {
        case r >= 'A' && r <= 'Z':
            return 'A' + (r-'A'+13)%26
        case r >= 'a' && r <= 'z':
            return 'a' + (r-'a'+13)%26
        }
        return r
    }
    n, err := reader.r.Read(b)
    result := []byte(strings.Map(rot13, string(b)))
    b = []byte(result)
    fmt.Println(string(b))
    return n, err
}

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

You cracked the code!
Lbh penpxrq gur pbqr!You cracked the code!

Lbh penpxrq gur pbqr!