Please note that openssl_seal() cannot be used for EC encryption.
Took me literally two hours to find out because the OpenSSL documentation is so bad.
(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)
openssl_seal — Seal (encrypt) data
$data,&$sealed_data,&$encrypted_keys,$public_key,$cipher_algo,&$iv = null
openssl_seal() seals (encrypts) data using the
specified cipher_algo with a randomly generated secret key. The key is
then encrypted with each of the public keys in public_key array,
and each encrypted envelope key is returned in encrypted_keys. This allows
sealed data to be sent to multiple recipients (provided their public keys are available). Each
recipient must receive both the sealed data and the envelope key that was encrypted with the
recipient's public key. The IV (Initialization Vector) is generated, and its value is returned in
iv.
dataThe data to seal.
sealed_dataThe sealed data.
encrypted_keysArray of encrypted keys.
public_keyArray of OpenSSLAsymmetricKey instances containing public keys.
cipher_algoThe cipher method.
The default value for PHP versions prior to 8.0 is ('RC4') which is
considered insecure. It is strongly recommended to explicitly specify a secure cipher
method.
iv
The initialization vector for decryption of data. It is required if
the cipher method requires IV. This can be found out by calling
openssl_cipher_iv_length() with cipher_algo.
The IV cannot be set explicitly. Any value set in it is overwritten by randomly generated value.
Returns the length of the sealed data on success, or false on error.
If successful the sealed data is returned in
sealed_data, and the envelope keys in
encrypted_keys.
| Version | Description |
|---|---|
| 8.0.0 |
public_key accepts an array of
OpenSSLAsymmetricKey instances now;
previously, an array of resources of type OpenSSL key
was accepted.
|
| 8.0.0 |
cipher_algo is no longer an optional parameter.
|
| 8.0.0 |
iv is nullable now.
|
Example #1 openssl_seal() example
<?php
// $data is assumed to contain the data to be sealed
$data = "test";
// fetch public keys
$pk1 = openssl_get_publickey("file://cert1.pem");
$pk2 = openssl_get_publickey("file://cert2.pem");
// seal message, only owners of $pk1 and $pk2 can decrypt $sealed with keys
// $ekeys[0] and $ekeys[1] respectively.
if (openssl_seal($data, $sealed, $ekeys, array($pk1, $pk2), 'AES256', $iv) > 0) {
// possibly store the $sealed and $iv values and use later in openssl_open
echo "success\n";
}
?>Please note that openssl_seal() cannot be used for EC encryption.
Took me literally two hours to find out because the OpenSSL documentation is so bad.while the default is using RC4, it is possible to use other more secure algorithms. These are specified as the fifth parameter. Also, one needs to add an initialization vector (random bytes). Eg.
<?php
$data = "This is top secret.";
// fetch public keys for our recipients, and ready them
$cert = file_get_contents('./cert.pem');
$pk1 = openssl_get_publickey($cert);
$iv = openssl_random_pseudo_bytes(32);
openssl_seal($data, $sealed, $ekeys, array($pk1), "AES256", $iv);
// free the keys from memory
openssl_free_key($pk1);
echo base64_encode($sealed);
?>Some critical details that are not in the docs, nor widely written about elsewhere.
- The envelope key is a 128-bit RSA key, randomly generated.
- The data is encrypted with (A)RC4 using the envelope key.
- The envelope key is encrypted for transmission with PKCS1 v1.5. It is NOT the OAEP padding variant. PKCS1 v1.5 is even older, and not widely supported anymore.
At least this was true for openssl_seal in PHP 7.2 that we are using.
(Note: In Python you can decrypt this envelope key with the Cryptography package, using padding.PKCS1v15())
The combination of RC4 and PKCS1 v1.5 make this function actually semi-obsolete for security use in my opinion."seals (encrypts) data by using RC4 with a randomly generated secret key"
It should be noted that the randomly generated secret key is 128 bits long (openssl: EVP_rc4(void): RC4 stream cipher. This is a variable key length cipher with default key length 128 bits.)According to several sources (e.g. crypto101.io or Wikipedia) RC4 is not safe and not supposed to be used anymore.
So, shouldn't openssl_seal use another stream cipher in place of RC4?openssl_seal() can work well when you need to pass data securely to other platforms / languages. What openssl_seal() does is;
1. Generate a random key
2. Encrypt the data symmetrically with RC4 using the random key
3. Encrypt the random key itself with RSA using the public key / certificate
4. Returns the encrypted data and the encrypted key
So to decrypt the steps are simply;
1. Decrypt the key using RSA and your private key
2. Decrypt the data using RC4 and the decrypted key
The trickiest part may be figuring out how handle the private key - BouncyCastle ( http://www.bouncycastle.org/ ) provides a PEMReader for Java and C# while Not Yet commons-ssl ( http://juliusdavies.ca/commons-ssl/ ) has a KeyStoreBuilder to build Java keystores out of a PEM certificate.
A complete example in Java is described at http://blog.local.ch/archive/2007/10/29/openssl-php-to-java.html