openssl_dh_compute_key returns false - php

Question

I'm trying to compute the shared secret for ECDH (Elliptic Curve Diffie Hellman) using PHP.
Assume I have someone's public key.
$clientPublickey = "BOLcHOg4ajSHR6BjbSBeX_6aXjMu1V5RrUYXqyV_FqtQSd8RzdU1gkMv1DlRPDIUtFK6Nd16Jql0eSzyZh4V2uc";
I generate my keys using OpenSSL.
exec('openssl ecparam -genkey -name prime256v1 -noout -out example-ecc.pem');
$private = openssl_pkey_get_private("file://example-ecc.pem");
Using the following code, I get false as value for $sharedSecret.
$sharedSecret = openssl_dh_compute_key(base64_decode($clientPublickey), $private);
openssl_error_string() doesn't return any error.
I've var_dumped openssl_pkey_get_details($private) and I verified it was created properly.
How do I use openssl_dh_compute_key()?

Answer 1

https://www.openssl.org/docs/manmaster/crypto/DH_compute_key.html describes that function as being for (non-EC) Diffie Hellman. You would need ECDH_compute_key (which I don't know if PHP exposes).
Though https://wiki.openssl.org/index.php/Elliptic_Curve_Diffie_Hellman recommends using the EVP_PKEY wrappers instead of the low level routines. But, again, I can't say what support PHP has.

Answer 2

Quite an old post, but still..
I think your public key is not formatted in Base64, but in URL-safe-Base64.
See: https://en.wikipedia.org/wiki/Base64#URL_applications
Furthermore, you can create the keys also from PHP now. Using openssl_pkey_new().
See: https://www.php.net/manual/en/function.openssl-pkey-new.php

Answer 3

openssl_dh_compute_key() supports ECDH only in PHP 8.1.0, with OpenSSL 3.0.0. Before PHP 8.1.0, openssl_dh_compute_key() always returned false.
Starting from PHP 7.3, there is openssl_pkey_derive() which derives a shared secret from a set of public key and private key. It works either with DH or EC keys. The example code given for this function is simpler than the example given for openssl_dh_compute_key(), which generate a public/private DH keypair using the command line.
$private = openssl_pkey_get_private("-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----
");
$public = openssl_pkey_get_public("-----BEGIN PUBLIC KEY-----
MIICJDCCARcGCSqGSIb3DQEDATCCAQgCggEBAJLxRCaZ933uW+AXmabHFDDyupoj
BIRlbmQLJZfigDaSA1f9YOTsIv+WwVFTX/J1mtCyx9uBcz0Nt2kmVwxWuc2fVtCE
MPsmLsVXX7xRUFLpyX1Y1IYGBVXQOoOvLWYQjpZgnx47Pkh1Ok1+smffztfC0DCN
t4KorWrbsPcmqBejXHN79KvWFjZmXOksRiNu/Bn76RiqvofC4z8Ri3kHXQG2197J
GZzzFXHadGC3xbkg8UxsNbYhVMKbm0iANfafUH7/hoS9UjAVQYtvwe7YNiW/Hnyf
VCrKwcc7sadd8Iphh+3lf5P1AhaQEAMytanrzq9RDXKBxuvpSJifRYasZYsCAQID
ggEFAAKCAQAiCSBpxvGgsTorxAWtcAlSmzAJnJxFgSPef0g7OjhESytnc8G2QYmx
ovMt5KVergcitztWh08hZQUdAYm4rI+zMlAFDdN8LWwBT/mGKSzRkWeprd8E7mvy
ucqC1YXCMqmIwPySvLQUB/Dl8kgau7BLAnIJm8VP+MVrn8g9gghD0qRCgPgtEaDV
vocfgnOU43rhKnIgO0cHOKtw2qybSFB8QuZrYugq4j8Bwkrzh6rdMMeyMl/ej5Aj
c0wamOzuBDtXt0T9+Fx3khHaowjCc7xJZRgZCxg43SbqMWJ9lUg94I7+LTX61Gyv
dtlkbGbtoDOnxeNnN93gwQZngGYZYciu
-----END PUBLIC KEY-----
");
echo bin2hex(openssl_pkey_derive($public, $private));
For other PHP 7 releases, there are pure PHP implementations like phpecc/phpecc, which can generate a shared encryption key using code similar to the following one. (It's one of the library examples.)
use Mdanter\Ecc\EccFactory;
use Mdanter\Ecc\Primitives\GeneratorPoint;
use Mdanter\Ecc\Serializer\PrivateKey\PemPrivateKeySerializer;
use Mdanter\Ecc\Serializer\PrivateKey\DerPrivateKeySerializer;
use Mdanter\Ecc\Serializer\PublicKey\DerPublicKeySerializer;
use Mdanter\Ecc\Serializer\PublicKey\PemPublicKeySerializer;
use Mdanter\Ecc\Util\NumberSize;
// ECDSA domain is defined by curve/generator/hash algorithm,
// which a verifier must be aware of.
$adapter = EccFactory::getAdapter();
$generator = EccFactory::getNistCurves()->generator384();
$useDerandomizedSignatures = true;
$derPub = new DerPublicKeySerializer();
$pemPub = new PemPublicKeySerializer($derPub);
$pemPriv = new PemPrivateKeySerializer(new DerPrivateKeySerializer($adapter, $derPub));
# These .pem and .key are for different keys
$alicePriv = $pemPriv->parse(file_get_contents(__DIR__ . '/../tests/data/openssl-secp256r1.pem'));
$bobPub = $pemPub->parse(file_get_contents(__DIR__ . '/../tests/data/openssl-secp256r1.1.pub.pem'));
$exchange = $alicePriv->createExchange($bobPub);
$shared = $exchange->calculateSharedKey();
echo "Shared secret: " . gmp_strval($shared, 10).PHP_EOL;
# The shared key is never used directly, but used with a key derivation function (KDF)
$kdf = function (GeneratorPoint $G, \GMP $sharedSecret) {
$adapter = $G->getAdapter();
$binary = $adapter->intToFixedSizeString(
$sharedSecret,
NumberSize::bnNumBytes($adapter, $G->getOrder())
);
$hash = hash('sha256', $binary, true);
return $hash;
};
$key = $kdf($generator, $shared);