openssl_seal()/_open() is not able to handle gcm cipers, e.g. aes-256-gcm

[rotdrop]

Description

The following code:

<?php

const NUM_KEYS = 2;

for ($i = 0; $i < NUM_KEYS; $i++) {
  $keys[] = $key = openssl_pkey_new();
  $details = openssl_pkey_get_details($key);
  $pubKeys[] = $details['key'];
  $privKeys[] = openssl_pkey_get_private($key);
}

$data = 'Test Data String';

$ciphers = [
  'aes-256-ctr',
  'aes-256-gcm',
];

foreach ($ciphers as $cipherAlgo) {

  echo "*** TESTING $cipherAlgo ***" . PHP_EOL . PHP_EOL;
  $iv = \random_bytes(openssl_cipher_iv_length($cipherAlgo));
  $result = openssl_seal($data, $sealedData, $sealedKeys, $pubKeys, $cipherAlgo, $iv);

  echo "DATA  :   " . strlen($data) . ' ' . $data . PHP_EOL;
  echo "IV-LEN:   " . openssl_cipher_iv_length($cipherAlgo) . PHP_EOL;
  echo "IV    :   " . bin2hex($iv) . PHP_EOL;
  echo "ENC DATA: " . strlen($sealedData) . ' ' . bin2hex($sealedData) . PHP_EOL;
  echo "RESULT:   " . ($result ? 'true' : 'false') . PHP_EOL;
  echo PHP_EOL;

  // Try decrypt
  foreach ($keys as $i => $key) {
    $decrypted = null; // ;)
    $result = openssl_open($sealedData, $decrypted, $sealedKeys[$i], $key, $cipherAlgo, $iv);
    echo "OPEN:    " . $decrypted . PHP_EOL;
    echo "RESULT:  " . ($result ? 'true' : 'false') . PHP_EOL;

    $result = openssl_private_decrypt($sealedKeys[$i], $unsealedKey, $key);
    echo "UNSEAL:  " . bin2hex($unsealedKey) . PHP_EOL;
    echo "RESULT:  " . ($result ? 'true' : 'false') . PHP_EOL;
    echo "DECRYPT: " . openssl_decrypt($sealedData, $cipherAlgo, $unsealedKey, OPENSSL_RAW_DATA, $iv) . PHP_EOL;
  }

  echo PHP_EOL;
}

Resulted in this output:

$ ./openssl-seal-test.php
*** TESTING aes-256-ctr ***

DATA  :   16 Test Data String
IV-LEN:   16
IV    :   37ff67041864b987b8ec2dcd0879f55e
ENC DATA: 16 2a748aa6ae176caf59cf22868304d65a
RESULT:   true

OPEN:    Test Data String
RESULT:  true
UNSEAL:  8745e00bdf3d88bd1caa8ec0150a9b369eade92e57fc2d26ae283f224c66af14
RESULT:  true
DECRYPT: Test Data String
OPEN:    Test Data String
RESULT:  true
UNSEAL:  8745e00bdf3d88bd1caa8ec0150a9b369eade92e57fc2d26ae283f224c66af14
RESULT:  true
DECRYPT: Test Data String

*** TESTING aes-256-gcm ***

DATA  :   16 Test Data String
IV-LEN:   12
IV    :   033b1589f62bebbfffa3adda
ENC DATA: 16 a531f88fc51e373235c928cae1eb7c29
RESULT:   true

OPEN:    
RESULT:  false
UNSEAL:  600cf6e76cf4ae9d48a881e0649af4682aa9065e63bec515a6b8c9678e398d36
RESULT:  true
DECRYPT: 
OPEN:    
RESULT:  false
UNSEAL:  600cf6e76cf4ae9d48a881e0649af4682aa9065e63bec515a6b8c9678e398d36
RESULT:  true
DECRYPT: 

But I expected this output instead:

[...]
*** TESTING aes-256-gcm ***

DATA  :   16 Test Data String
IV-LEN:   12
IV    :   033b1589f62bebbfffa3adda
ENC DATA: 16 a531f88fc51e373235c928cae1eb7c29
RESULT:   true

OPEN:    Test Data String
RESULT:  true
UNSEAL:  600cf6e76cf4ae9d48a881e0649af4682aa9065e63bec515a6b8c9678e398d36
RESULT:  false
DECRYPT: 
OPEN:    Test Data String
RESULT:  true
UNSEAL:  600cf6e76cf4ae9d48a881e0649af4682aa9065e63bec515a6b8c9678e398d36
RESULT:  false
DECRYPT: 

The point is that the gcm-cipher need the authentication tag for decrypting the data. This however does not seem to be saved by the openssl_seal() function. I have also placed a question in the OpenSSL issue tracker: openssl/openssl#17235

PHP Version

PHP 8.0.13 (cli) (built: Nov 27 2021 17:17:19) ( ZTS )

Operating System

Gentoo Linux 5.15.6, but should not matter

[cmb69]

@bukka, could you please clarify whether this is a bug, or a feature request, or a documentation issue?

[rotdrop]

Well, AFAIK PHP's openssl_seal() is not able to use the gcm-ciphers. So probably

• it should be documented
• one might regard this as a missing feature
• as openssl_seal() does not complain (return value is true) this is also a bug. Arguably, the encoding does succeed. Its just that there is no chance to decode the sealed data without the authentication tag.

There is also an attempt to use openssl_seal() with aes-256-gcm within the Nextcloud-project: nextcloud/server#25551

[bukka]

I don't think there's any issue in OpenSSL so you can probably close that linked ticket in OpenSSL. This is just a missing functionality in PHP and it's really a feature request. Basically EVP_Seal* are just wrappers around EVP_Encrypt* that allows encrypting the key with the supplied pkey. What PHP doesn't do is to provide a way to return a tag. Although you could probably write your version of sealing in PHP to give you the same functionality ( you might get an idea when you look to https://github.com/openssl/openssl/blob/defe51c178e3dc9f07514c179121021fd78691b4/crypto/evp/p_seal.c ). That said we could possibly add extra parameters to openssl_seal to return tag and also extend openssl_open accordingly. So that's why this should be a feature request.

[rotdrop]

So then. Kind of a missing feature.

You are probably right that an extra parameter string &$tag or so is more in the spirit of the current interface than simply appending the tag to the encrypted keys.

I am still tempted to interpret the current state as a documentation bug. It is not so clear from reading the manual that the the ...-gcm ciphes to not work out of the box. And it still feels a little bit weird that openssl_seal() very happily accepts the ...-gcm cipher just to produce something which cannot be decrypted again.

[bukka]

I agree that it should be noted in docs but doc bugs should be reported to https://github.com/php/doc-en . So for this repo, it's just a feature request... :)

[rotdrop]

Agreed. I have just filed an issue which, well, is just a link to this issue.