CFHTTPMessageAddAuthentication fails to add authentication data to request

I'm trying extend SocketRocket library which is using CFNetwor freamwork with authentication feature.

I need Digest authentication for web socket.

I've wrote sucjh code which creates a request afrer response with authentication chellenge has been recived:

- (CFHTTPMessageRef)createAuthenticationHandShakeRequest: (CFHTTPMessageRef)chalengeMessage {
    CFHTTPMessageRef request = [self createHandshakeRequest];

    BOOL result = CFHTTPMessageAddAuthentication(request,
                                                 chalengeMessage,
                                                 (__bridge CFStringRef)self.credentials.user,
                                                 (__bridge CFStringRef)self.credentials.password,
                                                 NULL, / use strongest supplied authentication */
                                                 NO / for proxy */);
    if (!result) {
        NSString *chalengeDescription = [[NSString alloc] initWithData: CFBridgingRelease(CFHTTPMessageCopySerializedMessage(chalengeMessage))
                                                              encoding: NSUTF8StringEncoding];
        NSString  *requestDescription = [[NSString alloc] initWithData: CFBridgingRelease(CFHTTPMessageCopySerializedMessage(request))
                                                              encoding: NSUTF8StringEncoding];
        SRFastLog(@"Failed to add authentication data `%@` to a request:\n%@After a chalenge:\n%@",
                  self.credentials, requestDescription, chalengeDescription);
    }
    return request;
}

Now problem is that CFHTTPMessageAddAuthentication returns NO.

I see that requestDescription content is (it reamins unchanged):

GET /digest-auth/auth/user/passwd HTTP/1.1
Host: httpbin.org
Sec-WebSocket-Version: 13
Upgrade: websocket
Sec-WebSocket-Key: 3P5YiQDt+g/wgxHe71Af5Q==
Connection: Upgrade
Origin: http://httpbin.org/

and chalengeDescription contains:

HTTP/1.1 401 UNAUTHORIZED
Server: nginx
Content-Type: text/html; charset=utf-8
Set-Cookie: fake=fake_value
Access-Control-Allow-Origin: http://httpbin.org/
Access-Control-Allow-Credentials: true
Date: Mon, 29 Jun 2015 12:21:33 GMT
Proxy-Support: Session-Based-Authentication
Www-Authenticate: Digest nonce="0c7479b412e665b8685bea67580cf391", opaque="4ac236a2cec0fc3b07ef4d628a4aa679", realm="me@kennethreitz.com", qop=auth
Content-Length: 0
Connection: keep-alive

user

password

Why

CFHTTPMessageAddAuthentication

NO

I've used

http://httpbin.org/