stupidgcm: allow zero-length input data

We used to panic in this case because it is useless.
But Go stdlib supports it, so we should as well.

internal/stupidgcm/common_test.go

@@ -19,6 +19,8 @@ func testCiphers(t *testing.T, our cipher.AEAD, ref cipher.AEAD) {
 	t.Run("testCorruption_c1", func(t *testing.T) { testCorruption(t, our) })
 	t.Run("testCorruption_c2", func(t *testing.T) { testCorruption(t, ref) })
 	t.Run("testConcurrency", func(t *testing.T) { testConcurrency(t, our, ref) })
+	t.Run("testOpenAllZero_our", func(t *testing.T) { testOpenAllZero(t, our) })
+	t.Run("testOpenAllZero_ref", func(t *testing.T) { testOpenAllZero(t, ref) })
 	t.Run("testWipe", func(t *testing.T) { testWipe(t, our) })
 }
 
@@ -37,9 +39,13 @@ func testEncryptDecrypt(t *testing.T, c1 cipher.AEAD, c2 cipher.AEAD) {
 
 	dst := make([]byte, 71) // 71 = arbitrary length
 
-	// Check all block sizes from 1 to 5000
-	for i := 1; i < 5000; i++ {
-		in := make([]byte, i)
+	// Check all block sizes from nil to 0 to 5000
+	for i := -1; i < 5000; i++ {
+		// stays nil at i == -1
+		var in []byte
+		if i >= 0 {
+			in = make([]byte, i)
+		}
 
 		c1out := c1.Seal(dst, iv, in, authData)
 		c2out := c2.Seal(dst, iv, in, authData)
@@ -79,8 +85,8 @@ func testConcurrency(t *testing.T, c1 cipher.AEAD, c2 cipher.AEAD) {
 		for i := 0; i < goroutineCount; i++ {
 			wg.Add(1)
 			go func() {
+				defer wg.Done()
 				testEncryptDecrypt(t, c1, c2)
-				wg.Done()
 			}()
 			wg.Wait()
 		}
@@ -96,8 +102,9 @@ func testInplaceSeal(t *testing.T, c1 cipher.AEAD, c2 cipher.AEAD) {
 	iv := randBytes(c1.NonceSize())
 
 	max := 5016
-	// Check all block sizes from 1 to 5000
-	for i := 1; i < max-16; i++ {
+
+	// Check all block sizes from 0 to 5000
+	for i := 0; i < max-16; i++ {
 		in := make([]byte, i)
 		dst := make([]byte, max-i)
 		dst = dst[:16]
@@ -185,6 +192,27 @@ func testCorruption(t *testing.T, c cipher.AEAD) {
 	}
 }
 
+// testOpenAllZero tests that we do not crash for nil or any block size of zeros
+func testOpenAllZero(t *testing.T, c cipher.AEAD) {
+	authData := make([]byte, 24)
+	iv := make([]byte, c.NonceSize())
+
+	for i := -1; i < 5000; i++ {
+		// stays nil at i == -1
+		var cipher []byte
+		if i >= 0 {
+			cipher = make([]byte, i)
+		}
+		plain, err := c.Open(nil, iv, cipher, authData)
+		if err == nil {
+			t.Error("should have gotten error, but did not")
+		}
+		if len(plain) > 0 {
+			t.Errorf("should not have received data, but got %d bytes", len(plain))
+		}
+	}
+}
+
 func testWipe(t *testing.T, c cipher.AEAD) {
 	switch c2 := c.(type) {
 	case *StupidGCM:

internal/stupidgcm/openssl.go

@@ -20,9 +20,6 @@ func openSSLSeal(a *stupidAEADCommon, dst, iv, in, authData []byte) []byte {
 	if len(iv) != a.NonceSize() {
 		log.Panicf("Only %d-byte IVs are supported, you passed %d bytes", a.NonceSize(), len(iv))
 	}
-	if len(in) == 0 {
-		log.Panic("Zero-length input data is not supported")
-	}
 
 	// If the "dst" slice is large enough we can use it as our output buffer
 	outLen := len(in) + tagLen
@@ -36,7 +33,7 @@ func openSSLSeal(a *stupidAEADCommon, dst, iv, in, authData []byte) []byte {
 	}
 
 	res := int(C.openssl_aead_seal(a.openSSLEVPCipher,
-		(*C.uchar)(&in[0]),
+		slicePointerOrNull(in),
 		C.int(len(in)),
 		(*C.uchar)(&authData[0]),
 		C.int(len(authData)),
@@ -64,7 +61,7 @@ func openSSLOpen(a *stupidAEADCommon, dst, iv, in, authData []byte) ([]byte, err
 	if len(iv) != a.NonceSize() {
 		log.Panicf("Only %d-byte IVs are supported, you passed %d bytes", a.NonceSize(), len(iv))
 	}
-	if len(in) <= tagLen {
+	if len(in) < tagLen {
 		return nil, fmt.Errorf("stupidChacha20poly1305: input data too short (%d bytes)", len(in))
 	}
 
@@ -83,7 +80,7 @@ func openSSLOpen(a *stupidAEADCommon, dst, iv, in, authData []byte) ([]byte, err
 	tag := in[len(in)-tagLen:]
 
 	res := int(C.openssl_aead_open(a.openSSLEVPCipher,
-		(*C.uchar)(&ciphertext[0]),
+		slicePointerOrNull(ciphertext),
 		C.int(len(ciphertext)),
 		(*C.uchar)(&authData[0]),
 		C.int(len(authData)),
@@ -93,7 +90,7 @@ func openSSLOpen(a *stupidAEADCommon, dst, iv, in, authData []byte) ([]byte, err
 		C.int(len(a.key)),
 		(*C.uchar)(&iv[0]),
 		C.int(len(iv)),
-		(*C.uchar)(&buf[0]),
+		slicePointerOrNull(buf),
 		C.int(len(buf))))
 
 	if res < 0 {
@@ -109,6 +106,16 @@ func openSSLOpen(a *stupidAEADCommon, dst, iv, in, authData []byte) ([]byte, err
 	return append(dst, buf...), nil
 }
 
+// slicePointerOrNull returns a C pointer to the beginning of the byte slice,
+// or NULL if the byte slice is empty. This is useful for slices that can be
+// empty, otherwise you can directly use "(*C.uchar)(&s[0])".
+func slicePointerOrNull(s []byte) (ptr *C.uchar) {
+	if len(s) == 0 {
+		return
+	}
+	return (*C.uchar)(&s[0])
+}
+
 // This functions exists to benchmark the C call overhead from Go.
 // See BenchmarkCCall for resuts.
 func noopCFunction() {