public static function loadSignatureKeyPair(string $filePath): SignatureKeyPair
    {
        if (!\is_readable($filePath)) {
            throw new CannotPerformOperation(
                'Cannot read keyfile: '. $filePath
            );
        }
        return new SignatureKeyPair(
            new SignatureSecretKey(
                self::loadKeyFile($filePath)
            )
        );
    }