diff --git a/src/pkg/crypto/sha1/sha1.go b/src/pkg/crypto/sha1/sha1.go
index 7cfde47..8eb3f7a 100644
--- a/src/pkg/crypto/sha1/sha1.go
+++ b/src/pkg/crypto/sha1/sha1.go
@@ -90,9 +90,13 @@
 func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0 so that caller can keep writing and summing.
 	d := *d0
+	hash := d.checkSum()
+	return append(in, hash[:]...)
+}
 
-	// Padding.  Add a 1 bit and 0 bits until 56 bytes mod 64.
+func (d *digest) checkSum() [Size]byte {
 	len := d.len
+	// Padding.  Add a 1 bit and 0 bits until 56 bytes mod 64.
 	var tmp [64]byte
 	tmp[0] = 0x80
 	if len%64 < 56 {
@@ -120,5 +124,13 @@
 		digest[i*4+3] = byte(s)
 	}
 
-	return append(in, digest[:]...)
+	return digest
+}
+
+// Sum returns the SHA1 checksum of the data.
+func Sum(data []byte) [Size]byte {
+	var d digest
+	d.Reset()
+	d.Write(data)
+	return d.checkSum()
 }
diff --git a/src/pkg/crypto/sha1/sha1_test.go b/src/pkg/crypto/sha1/sha1_test.go
index 57cd431..61ece67 100644
--- a/src/pkg/crypto/sha1/sha1_test.go
+++ b/src/pkg/crypto/sha1/sha1_test.go
@@ -54,6 +54,10 @@
 func TestGolden(t *testing.T) {
 	for i := 0; i < len(golden); i++ {
 		g := golden[i]
+		s := fmt.Sprintf("%x", Sum([]byte(g.in)))
+		if s != g.out {
+			t.Fatalf("Sum function: sha1(%s) = %s want %s", g.in, s, g.out)
+		}
 		c := New()
 		for j := 0; j < 3; j++ {
 			if j < 2 {