Let's imagine we want to have custom implementation of EncryptionInterface, e.g. because we want to add custom header info. It's the instruction directly in the code BTW:

    /**
     * Override to create a custom header
     */
    protected function generateJwtHeader($payload, $algorithm)
    {
        return array(
            'typ' => 'JWT',
            'alg' => $algorithm,
        );
    }

So, we create a custom implementation, but there is no way to pass it to instantiation of IdToken class, because it doesn't accept anything as last parameter here: return new IdToken($this->storages['user_claims'], $this->storages['public_key'], $config); (in createDefaultIdTokenResponseType()).
So to solve that we need to create a custom implementation of createDefaultIdTokenResponseType() or whole response type