HMAC empty inputs without panicing

re #7

hmac.go

@@ -72,6 +72,9 @@ type hmacImplementation struct {
 	// Cleanup function
 	cleanup func()
 
+	// Count of updates
+	updates uint64
+
 	// Result, or nil if we don't have the answer yet
 	result []byte
 }
@@ -167,6 +170,7 @@ func (hi *hmacImplementation) initialize() (err error) {
 		hi.cleanup()
 		return
 	}
+	hi.updates = 0
 	hi.result = nil
 	return
 }
@@ -181,13 +185,21 @@ func (hi *hmacImplementation) Write(p []byte) (n int, err error) {
 	if err = hi.session.Ctx.SignUpdate(hi.session.Handle, p); err != nil {
 		return
 	}
+	hi.updates++
 	n = len(p)
 	return
 }
 
 func (hi *hmacImplementation) Sum(b []byte) []byte {
 	if hi.result == nil {
 		var err error
+		if hi.updates == 0 {
+			// http://docs.oasis-open.org/pkcs11/pkcs11-base/v2.40/os/pkcs11-base-v2.40-os.html#_Toc322855304
+			// We must ensure that C_SignUpdate is called _at least once_.
+			if err = hi.session.Ctx.SignUpdate(hi.session.Handle, []byte{}); err != nil {
+				panic(err)
+			}
+		}
 		hi.result, err = hi.session.Ctx.SignFinal(hi.session.Handle)
 		hi.cleanup()
 		if err != nil {

hmac_test.go

@@ -98,6 +98,15 @@ func testHmac(t *testing.T, keytype int, mech int, length int, xlength int, full
 		}
 	})
 	if full { // Independent of hash, only do these once
+		t.Run("Empty", func(t *testing.T) {
+			// Must be able to MAC empty inputs without panicing
+			var h1 hash.Hash
+			if h1, err = key.NewHMAC(mech, length); err != nil {
+				t.Errorf("key.NewHMAC: %v", err)
+				return
+			}
+			h1.Sum([]byte{})
+		})
 		t.Run("MultiSum", func(t *testing.T) {
 			input := []byte("a different short string")
 			var h1 hash.Hash
@@ -158,5 +167,19 @@ func testHmac(t *testing.T, keytype int, mech int, length int, xlength int, full
 				return
 			}
 		})
+		t.Run("ResetFast", func(t *testing.T) {
+			// Reset() immediately after creation should be safe
+			var h1 hash.Hash
+			if h1, err = key.NewHMAC(mech, length); err != nil {
+				t.Errorf("key.NewHMAC: %v", err)
+				return
+			}
+			h1.Reset()
+			if n, err := h1.Write([]byte{2}); err != nil || n != 1 {
+				t.Errorf("h1.Write: %v/%d", err, n)
+				return
+			}
+			h1.Sum([]byte{})
+		})
 	}
 }