How to derive a shared key with ECDH and HKDF in PHP

1<?php
2// ext-openssl reads EC and (since PHP 8.4) Curve25519 keys; the key type comes from the PEM
3function agree(string $privateKeyFile, string $peerPublicKeyFile): string
4{
5    $privateKey = openssl_pkey_get_private(file_get_contents($privateKeyFile));
6    $peerKey = openssl_pkey_get_public(file_get_contents($peerPublicKeyFile));
7    $secret = openssl_pkey_derive($peerKey, $privateKey); // peer's public key comes first
8    if ($secret === false) {
9        throw new RuntimeException(openssl_error_string() ?: 'key agreement failed');
10    }
11    return $secret;
12}
13 
14foreach (['X25519' => 'x25519', 'P-256' => 'p256'] as $curve => $name) {
15    $secret = agree("alice-$name.pem", "bob-$name.pub.pem");
16    // hash_hkdf(algo, key material, length, info, salt) — info comes before salt
17    $key = hash_hkdf('sha256', $secret, 32, 'AES-256-GCM key v1', '');
18    echo "$curve shared secret: ", bin2hex($secret), PHP_EOL;
19    echo "$curve derived key: ", bin2hex($key), PHP_EOL;
20}

1<?php
2// ext-sodium works on raw 32-byte X25519 keys
3$alice = openssl_pkey_get_details(openssl_pkey_get_private(file_get_contents('alice-x25519.pem')));
4$bob = openssl_pkey_get_details(openssl_pkey_get_public(file_get_contents('bob-x25519.pub.pem')));
5 
6$secret = sodium_crypto_scalarmult($alice['x25519']['priv_key'], $bob['x25519']['pub_key']);
7echo 'sodium X25519 shared secret: ', bin2hex($secret), PHP_EOL;