diff --git a/sha3sum/main.go b/sha3sum/main.go
index 63127a6..c5255b5 100644
--- a/sha3sum/main.go
+++ b/sha3sum/main.go
@@ -1,33 +1,68 @@
 package main
 
 import (
-	"flag"
-	"fmt"
 	"io"
 	"os"
 
 	"github.com/magical/keccak"
 )
 
+const usage = "usage: sha3sum [-h] files..."
+
+var (
+	stdout = os.Stdout
+	stderr = os.Stderr
+)
+
 func main() {
-	flag.Parse()
+	// Parse flags
+	if len(os.Args) <= 1 {
+		stderr.WriteString(usage + "\n")
+		return
+	}
+	args := os.Args[1:]
+	for _, a := range args {
+		switch a {
+		case "-h", "--help":
+			stdout.WriteString(usage + "\n")
+			return
+		case "", "-":
+		default:
+			if a[0] == '-' {
+				stderr.WriteString("sha3sum: unrecognized flag " + a + "\n")
+				os.Exit(1)
+			}
+		}
+	}
+
+	// compute file hashes
 	var sum [keccak.Size]byte
 	h := keccak.New256()
-	files := flag.Args()
+	files := args
 	for _, filename := range files {
 		f, err := os.Open(filename)
 		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
+			stderr.WriteString("sha3sum: " + err.Error() + "\n")
 			continue
 		}
 		h.Reset()
 		_, err = io.Copy(h, f)
 		f.Close()
 		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
+			stderr.WriteString("sha3sum: " + err.Error() + "\n")
 			continue
 		}
 		h.Sum(sum[:0])
-		fmt.Printf("%x %s\n", sum, filename)
+		stdout.WriteString(toHex(sum[:]) + " " + filename + "\n")
 	}
 }
+
+func toHex(data []byte) string {
+	const digits = "0123456789abcdef"
+	out := make([]byte, len(data)*2)
+	for i, v := range data {
+		out[i*2] = digits[v>>4]
+		out[i*2+1] = digits[v&15]
+	}
+	return string(out)
+}