ESP8266 Arduino request content encoding

Question

I'm trying to send JSON to an Arduino module with a ESP8266. I have a simple web server, which waiting for JSON with SSID and password where device must connect.

ESP8266WebServer server(80);
server.on("/config", HTTP_POST, configHandle);
server.begin();

void handleConfig() {
    String payload = server.arg("plain");
    //convert JSON to char[]
    //parse using jsmn lib
}

What if password contains non ASCII characters? How can I handle request content to put this arguments to method:

WiFi.begin(ssid, pass);

Edit:

Example: If I send JSON like:

{"pass": "test+test"}

Then, when I print this payload I don't get a + sign (but this is ASCII sign)

Request (wireshark):

Char array payload from board:

Answer 1

The ESP8266WebServer library is decoding + into a space character.

You need to URL encode the JSON string, before sending it.

In vanilla JavaScript you need to use encodeURIComponent.
Don't use encodeURI, because it doesn't encode +.
Whatever you use, make sure the + character is encoded into %2b.

This will also save you from potential problems, involving ?, & and = inside your JSON.