How to Setup SDP Secure Connection for WhatsApp Cloud API Call?

Question

I'm building on Python the "User-initiated Calls" handling by WhatsApp Cloud API:

1. Accept SDP offer in the WhatsApp user call and send answer in "200 OK".
2. Send the incoming call acceptance request (skipping the call pre-acceptance).

I successfully completed the Prerequisites steps to enable the voice calling in my WhatsApp app: "Cloud API Calling | Get Started | Step 1: Prerequisites".

The WebRTC / SDP connection setup is implemented using aiortc:

async def handle_call_offer(self, sdp_req, wa_call_id):        

    ''' accept incoming call request '''
    
    player = MediaPlayer(os.path.join(os.path.dirname(__file__), 'alstand/sample.mp3'))
    pc = RTCPeerConnection() # create WebRTC peer connection
    pc.addTrack(player.audio)
    offer = RTCSessionDescription(sdp=sdp_req['sdp'], type=sdp_req['sdp_type'])
    await pc.setRemoteDescription(offer)
    answer = await pc.createAnswer() # Create an answer
    await pc.setLocalDescription(answer)

    ''' send accept WhatsApp call request '''
    
    payload = json.dumps({
        'messaging_product': 'whatsapp',
        'call_id': wa_call_id,
        'action': 'accept',
        'session': {
            'sdp_type': 'answer',
            'sdp': pc.localDescription.sdp # re-use respone's sdp
        }
    })

    response = requests.request('POST', f'{self.API_URL}/calls', headers=self.headers, data=payload)

    return pc.localDescription.sdp

Eventually, the incoming call request is successfully accepted, the response is sent, but an error is returned to the incoming call acceptance request:

// 400 Bad Request
{
    "error": {
        "message": "Invalid parameter",
        "type": "OAuthException",
        "code": 100,
        "error_data": {
            "messaging_product": "whatsapp",
            "details": "Fingerprint algo is not SHA256"
        },
        "error_subcode": 2494010,
        "is_transient": false,
        "error_user_title": "Parameter Invalid",
        "error_user_msg": "Check input parameters and retry the request",
        "fbtrace_id": "ApaWkAViZR1ZYOg3XpILtzj"
    }
}

I checked the request's SDP format and see that it does contain the SHA256 fingerprint:

v=0
...
a=fingerprint:sha-256 AB:27:4E:3F:...
a=fingerprint:sha-384 CE:BD:D0:63:...
a=fingerprint:sha-512 94:98:EF:02:...
...

What could I miss in my implementation or initial config so the SDP offer is not accepted?

Answer 1

I was facing the exact same problem today. So for unknown reason WhatsApp is pretty nerdy about its requests formatting. WhatsApp expects capital letters in SDP:

a=fingerprint:SHA-256 AB:27:4E:3F:...

Moreover, replacing sha-512 and sha-384 with capital letters also doesn't work. Consider deleting these lines for good, as it is the only option that worked for me.