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 ]]]])
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 = "" ]]]] )
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PHP 7.1
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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.
Voting started on January 20th at 17:00 UTC and ended on January 27th at 17:00 UTC.
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.
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.