Correct and clarify the documentation of GCM whole-message functions
Clarify the roles of the buffer parameter and their sizes.
Remove a statement about input size restrictions that only applies to
mbedtls_gcm_update, not to the whole-message functions.
Document the possible error codes.
Warn that mbedtls_gcm_crypt_and_tag in decrypt mode does not
authenticate the data and recommend using mbedtls_gcm_auth_decrypt
instead.
diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index 1e5a507..4147385 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -106,20 +106,35 @@
* If the buffers overlap, the output buffer must trail at least 8 Bytes
* behind the input buffer.
*
+ * \warning When this function performs a decryption, it outputs the
+ * authentication tag and does not verify that the data is
+ * authentic. You should use this function to perform encryption
+ * only. For decryption, use mbedtls_gcm_auth_decrypt() instead.
+ *
* \param ctx The GCM context to use for encryption or decryption.
* \param mode The operation to perform: #MBEDTLS_GCM_ENCRYPT or
- * #MBEDTLS_GCM_DECRYPT.
- * \param length The length of the input data. This must be a multiple of 16 except in the last call before mbedtls_gcm_finish().
+ * #MBEDTLS_GCM_DECRYPT. Note that during decryption, the
+ * tag is not verified. You should use this function only
+ * to encrypt data, and use mbedtls_gcm_auth_decrypt()
+ * to decrypt.
+ * \param length The length of the input data, which is equal to the length
+ * of the output data.
* \param iv The initialization vector.
* \param iv_len The length of the IV.
* \param add The buffer holding the additional data.
* \param add_len The length of the additional data.
- * \param input The buffer holding the input data.
- * \param output The buffer for holding the output data.
+ * \param input The buffer holding the input data. Its size is \b length.
+ * \param output The buffer for holding the output data. It must have room
+ * for \b length bytes.
* \param tag_len The length of the tag to generate.
* \param tag The buffer for holding the tag.
*
- * \return \c 0 on success.
+ * \return \c 0 if the encryption or decryption was performed
+ * successfully. Note that in #MBEDTLS_GCM_DECRYPT mode,
+ * this does not indicate that the data is authentic.
+ * \return #MBEDTLS_ERR_GCM_BAD_INPUT if the lengths are not valid.
+ * \return #MBEDTLS_ERR_GCM_HW_ACCEL_FAILED or a cipher-specific
+ * error code if the encryption or decryption failed.
*/
int mbedtls_gcm_crypt_and_tag( mbedtls_gcm_context *ctx,
int mode,
@@ -142,18 +157,23 @@
* behind the input buffer.
*
* \param ctx The GCM context.
- * \param length The length of the input data. This must be a multiple of 16 except in the last call before mbedtls_gcm_finish().
+ * \param length The length of the ciphertext to decrypt, which is also
+ * the length of the decrypted plaintext.
* \param iv The initialization vector.
* \param iv_len The length of the IV.
* \param add The buffer holding the additional data.
* \param add_len The length of the additional data.
- * \param tag The buffer holding the tag.
- * \param tag_len The length of the tag.
- * \param input The buffer holding the input data.
- * \param output The buffer for holding the output data.
+ * \param tag The buffer holding the tag to verify.
+ * \param tag_len The length of the tag to verify.
+ * \param input The buffer holding the ciphertext. Its size is \b length.
+ * \param output The buffer for holding the decrypted plaintext. It must
+ * have room for \b length bytes.
*
- * \return 0 if successful and authenticated, or
- * #MBEDTLS_ERR_GCM_AUTH_FAILED if tag does not match.
+ * \return \c 0 if successful and authenticated.
+ * \return #MBEDTLS_ERR_GCM_AUTH_FAILED if the tag does not match.
+ * \return #MBEDTLS_ERR_GCM_BAD_INPUT if the lengths are not valid.
+ * \return #MBEDTLS_ERR_GCM_HW_ACCEL_FAILED or a cipher-specific
+ * error code if the decryption failed.
*/
int mbedtls_gcm_auth_decrypt( mbedtls_gcm_context *ctx,
size_t length,