PHP RFC: OpenSSL AEAD support

• Version: 0.2
• Date: 2016-01-02
• Author: Jakub Zelenka, bukka@php.net
• Status: Implemented (PHP 7.1)

The PHP OpenSSL extension provides functions for data encryption (openssl_encrypt) and decryption (openssl_decrypt). These function works fine for all cipher algorithms (cipher + mode) except AEAD (Authenticated Encrypt with Associated Data) modes. These modes requires special handling in OpenSSL and a need for supplying resp. retrieving of the authenticated tag and optionally AAD (associated application data).

There are two AEAD modes supported by OpenSSL (up to version 1.0.2) - GCM (Galois Counter Mode) and CCM (Counter with CBC-MAC). Both of these modes currently fails on decryption as there is no way how to supply an authentication tag and internal OpenSSL API doesn't allow it to use it in the same way (e.g. there is no context finalization for CCM mode).

This RFC proposes adding extra parameters to the openssl_encrypt resp. openssl_decrypt for retrieving resp. supplying an authenticated tag and AAD. These parameters are optional and are used only for supported AEAD modes (GCM and CCM). If a tag is used for any modes that doesn't support AEAD, then the warning is triggered.

The parameters differs for each function.

The new prototype for openssl_encrypt is following:

string openssl_encrypt ( string $data , string $method , string $password 
    [, int $options = 0 [, string $iv = "" [, string &$tag = NULL [, string $aad = "" [, int $tag_length = 16 ]]]])

• $tag - The authentication tag will be saved to the variable passed as a reference on successful encryption. If the encryption fails, then the variable is unchanged. The resulted tag length is the same as the length supplied in the $tag_length parameter which default to 16.
• $aad - Additional authentication data.
• $tag_length - The tag length can be set before the encryption and can be between 4 and 16 for GCM mode where it is the same like trimming the tag. On the other side the CCM has no such limits and also the resulted tag is different for each length.

The new prototype for openssl_decrypt is following:

string openssl_decrypt ( string $data , string $method , string $password 
    [, int $options = 0 [, string $iv = "" [, string $tag = "" [, string $aad = "" ]]]] )

• $tag - The authentication tag that will be authenticated. If it's incorrect, then the authentication fails and the function returns FALSE.
• $aad - Additional authentication data.

none

PHP 7.1

none

none

none

none

Tag length and parameters order.

The current encryption and decryption is unaffected. The new parameters are optional.

Adding support for OCB mode once the extension supports OpenSSL 1.1

Required 50%+1 majority for accepting this proposal.

Include AEAD support to OpenSSL extension
Real name	Yes	No
aeoris (aeoris)
ajf (ajf)
bishop (bishop)
bukka (bukka)
bwoebi (bwoebi)
galvao (galvao)
gasolwu (gasolwu)
leigh (leigh)
mrook (mrook)
ocramius (ocramius)
thorstenr (thorstenr)
Final result:	7	4
This poll has been closed.

Voting started on January 20th at 17:00 UTC and ended on January 27th at 17:00 UTC.

https://github.com/php/php-src/pull/1716

OpenSSL AEAD support has been implemented in PHP 7.1 with commit 054b27d3, and is documented in the openssl_encrypt() and openssl_decrypt() man pages.

The PHP language specification is not affected by this RFC.

• https://bugs.php.net/bug.php?id=67304

There has been discussion about introducing an object that would wrap the context and offered functions for setting tag, AAD, key, IV and making partials updates. However such functionality is already implemented in crypto extension and requires much more code (about extra 1000 lines) to address all possible exceptions. The main thing is that this is not contradicting to this proposal as it could easily co-exist as we will still have to keep openssl_ecrypt and openssl_decrypt working. This proposal is just about extending these two function for AEAD mode support.

It's been suggested that new functions openssl_ecrypt_aead and openss_decrytp_aead should be introduced instead of adding new parameters to prevent confusion for users that tries to use a tag with non AEAD modes. That has been rejected in favour of adding warning when tag supplied for non AEAD modes.