Let envelope AEAD fail if encrypted DEK is empty.

This shouldn't happen if the AEAD interface is implemented correctly, but it is
better to also cover this case and fail.

PiperOrigin-RevId: 578765381
Change-Id: I320508efa5c181d3cf5094c4442b3e486fd950d2

aead/kms_envelope_aead.go

@@ -91,6 +91,9 @@ func (a *KMSEnvelopeAEAD) Encrypt(pt, aad []byte) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
+	if len(encryptedDEK) == 0 {
+		return nil, errors.New("encrypted dek is empty")
+	}
 	p, err := registry.Primitive(a.dekTemplate.TypeUrl, dek)
 	if err != nil {
 		return nil, err

aead/kms_envelope_aead_test.go

@@ -138,3 +138,23 @@ func TestKMSEnvelopeShortCiphertext(t *testing.T) {
 		t.Error("a.Decrypt([]byte{1}, nil) err = nil, want error")
 	}
 }
+
+type invalidAEAD struct {
+}
+
+func (a *invalidAEAD) Encrypt(plaintext, associatedData []byte) ([]byte, error) {
+	return []byte{}, nil
+}
+
+func (a *invalidAEAD) Decrypt(ciphertext, associatedData []byte) ([]byte, error) {
+	return []byte{}, nil
+}
+
+func TestKMSEnvelopeEncryptWithInvalidAEADFails(t *testing.T) {
+	invalidKEKAEAD := &invalidAEAD{}
+	envAEADWithInvalidKEK := aead.NewKMSEnvelopeAEAD2(aead.AES256GCMKeyTemplate(), invalidKEKAEAD)
+
+	if _, err := envAEADWithInvalidKEK.Encrypt([]byte("plaintext"), []byte("associatedData")); err == nil {
+		t.Error("envAEADWithInvalidKEK.Encrypt(plaintext, associatedData) err = nil, want error")
+	}
+}