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 — 密封 (加密) 数据
$data,&$sealed_data,&$encrypted_keys,$public_key,$cipher_algo,&$iv = null
openssl_seal() 使用随机生成的密钥和给定的 cipher_algo 方法密封（加密）data
数据。密钥用与 public_key 中的标识符相关联的每个公共密钥加密，并且每个加密密钥在 encrypted_keys
中返回。这意味着一个人可以将密封的数据发送给多个接收者（如果一个人已经获得了他们的公钥）。每个接收方都必须同时接收加密的数据和用接收方的公钥加密的信封密钥。
data要密封的数据。
sealed_data被密封后的数据。
encrypted_keys已被加密的密钥数组。
public_key包含公钥的 OpenSSLAsymmetricKey 实例数组。
cipher_algo加密算法。
默认值（'RC4'）认为不安全。强烈建议明确指定安全密码方法。
iv初始化向量。
成功时返回密封后数据的长度，错误为 false。
如果密封后的数据成功地通过 sealed_data 变量返回，那么信封密钥也将会通过 encrypted_keys 变量返回。
| 版本 | 说明 |
|---|---|
| 8.0.0 |
public_key 现在接受 OpenSSLAsymmetricKey
实例 array；之前接受类型 OpenSSL key 的 resource 数组。
|
| 8.0.0 |
cipher_algo 不再是可选参数。
|
| 8.0.0 |
iv 现在可为 null。
|
示例 #1 openssl_seal() 示例
<?php
// $data is assumed to contain the data to be sealed
// fetch public keys for our recipients, and ready them
$fp = fopen("/src/openssl-0.9.6/demos/maurice/cert.pem", "r");
$cert = fread($fp, 8192);
fclose($fp);
$pk1 = openssl_get_publickey($cert);
// Repeat for second recipient
$fp = fopen("/src/openssl-0.9.6/demos/sign/cert.pem", "r");
$cert = fread($fp, 8192);
fclose($fp);
$pk2 = openssl_get_publickey($cert);
// seal message, only owners of $pk1 and $pk2 can decrypt $sealed with keys
// $ekeys[0] and $ekeys[1] respectively.
openssl_seal($data, $sealed, $ekeys, array($pk1, $pk2));
// free the keys from memory
openssl_free_key($pk1);
openssl_free_key($pk2);
?>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);
?>"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?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.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