Cannot correctly set content length after modify the request content

[qiangxinglin]

I need an interceptor to implement the req/rep encryption. For the decrypt part, I've managed it worked, but encryption still struggling.
All the requests and responses are in JSON format, but maybe with data field encryted (depending on the header).

class CryptoInterceptor(httpx.AsyncBaseTransport):
    def __init__(self, cipher, **kwargs):
        self._wrapper = httpx.AsyncHTTPTransport(**kwargs)
        self._cipher = cipher

    async def handle_async_request(self, request):
        # pass public key in header, remote will encrypt the response
        if request.headers.get('response_encrypt', False):
            request.headers['client_public_key'] = local_public_key

        # request public key from remote, sign and encrypt the request body
        if request.headers.get('request_encrypt', False):
            remote_public_key = await self._cipher.get_public_key()
            # sign and encrypt the request body
            data = await request.aread()
            data = {
                'token': token,
                'data': encrypted_data
            }
            request._content = json.dumps(data).encode()
            request.headers['Content-Length'] = str(len(request._content))   # <-- here's the problem

        response = await self._wrapper.handle_async_request(request)
        content = await response.aread()

        if request.headers.get('response_encrypt', False):
            # decrypt by local_private_key...
            response._content = decrypted_content

        return response

If I modify the content of the request in this transport, the system would throw LocalProtocolError: Too little data for declared Content-Length, how to deal with it?