private static function generate_key_pair($surrogate_key) {
    $keys = array();

    // Generate RSA private key
    $gen_private_key = openssl_pkey_new(
        array(
            'private_key_bits' => 2048,
            'private_key_type' => OPENSSL_KEYTYPE_RSA,
        )
    );
    openssl_pkey_export($gen_private_key, $private_key, $surrogate_key);

    // Generate public key from private key
    $public_key_details = openssl_pkey_get_details($gen_private_key);
    $public_key = $public_key_details['key'];

    return array(
        'private' => $private_key,
        'public' => $public_key
    );
} // generate_key_pair()