Fixing memory-access bug.

PiperOrigin-RevId: 243044973
GitOrigin-RevId: 535dc711b71e5f75571755aa0ae85a2fa14ae222

cc/aead/kms_envelope_aead.cc

@@ -108,14 +108,14 @@ util::StatusOr<std::string> KmsEnvelopeAead::Encrypt(
 util::StatusOr<std::string> KmsEnvelopeAead::Decrypt(
     absl::string_view ciphertext, absl::string_view associated_data) const {
   // Parse the ciphertext.
-  auto enc_dek_size = BigEndianLoad32(
-      reinterpret_cast<const uint8_t*>(ciphertext.data()));
-  if (ciphertext.size() < kEncryptedDekPrefixSize ||
-      enc_dek_size > ciphertext.size() - kEncryptedDekPrefixSize) {
+  if (ciphertext.size() < kEncryptedDekPrefixSize) {
     return util::Status(util::error::INVALID_ARGUMENT,
                         "ciphertext too short");
   }
-  if (enc_dek_size < 0) {
+  auto enc_dek_size = BigEndianLoad32(
+      reinterpret_cast<const uint8_t*>(ciphertext.data()));
+  if (enc_dek_size > ciphertext.size() - kEncryptedDekPrefixSize ||
+      enc_dek_size < 0) {
     return util::Status(util::error::INVALID_ARGUMENT,
                         "invalid ciphertext");
   }

cc/aead/kms_envelope_aead_test.cc

@@ -107,14 +107,14 @@ TEST(KmsEnvelopeAeadTest, DecryptionErrors) {
                                                 HasSubstr("too short")));
 
   // Short ciphertext.
-  decrypt_result = aead->Decrypt("short", aad);
+  decrypt_result = aead->Decrypt("sh", aad);
   EXPECT_THAT(decrypt_result.status(), StatusIs(util::error::INVALID_ARGUMENT,
                                                 HasSubstr("too short")));
 
   // Truncated ciphertext.
   decrypt_result = aead->Decrypt(ct.substr(2), aad);
   EXPECT_THAT(decrypt_result.status(), StatusIs(util::error::INVALID_ARGUMENT,
-                                                HasSubstr("too short")));
+                                                HasSubstr("invalid")));
 
   // Corrupted ciphertext.
   auto ct_copy = ct;