func primeBytes(c string) (r int) {
    for _,n := range []byte(c) {
        v := int(n)
        if v > 128 {
            v = 256 - v
        }
        p := 1
        i := 2

        for p > 0 && i*i <= v {
            if v % i < 1 {
                p--
            }
            i++
        }
        r += p
    }
    return
}