WordClockESP/src/ArduinoJsonJWT.cpp

#include "ArduinoJsonJWT.h"

ArduinoJsonJWT::ArduinoJsonJWT(String secret) : _secret(secret) { }

void ArduinoJsonJWT::setSecret(String secret){
    _secret = secret;
}

String ArduinoJsonJWT::sign(String &value) {
  // create signature
  Sha256.initHmac((uint8_t*) _secret.c_str(), _secret.length());
  Sha256.print(value);

  // trim and return
  return encode(Sha256.resultHmac(), 32);
}

String ArduinoJsonJWT::decode(unsigned char * value) {
  // create buffer of approperate length
  size_t decodedLength = decode_base64_length(value) + 1;
  char decoded[decodedLength];

  // decode
  decode_base64(value, (unsigned char *) decoded);
  decoded[decodedLength-1] = 0;

  // return as arduino string
  return String(decoded);
}

String ArduinoJsonJWT::encode(unsigned char * value , int length) {
  int encodedIndex = encode_base64_length(length);

  // encode
  char encoded[encodedIndex];
  encode_base64(value, length, (unsigned char *) encoded);

  // trim padding
  while (encoded[--encodedIndex] == '=') {
    encoded[encodedIndex] = 0;
  }  

  // return as string
  return String(encoded);
}

String ArduinoJsonJWT::buildJWT(JsonObject &payload) {
  // serialize, then encode payload
  String jwt;
  serializeJson(payload, jwt);
  jwt = encode((unsigned char *) jwt.c_str(), jwt.length());

  // add the header to payload
  jwt = JWT_HEADER + '.' + jwt;
 
  // add signature
  jwt  += '.' + sign(jwt);

  return jwt;
}

void ArduinoJsonJWT::parseJWT(String jwt, JsonDocument &jsonDocument) {
  // clear json document before we begin, jsonDocument wil be null on failure
  jsonDocument.clear();

  // must be of minimum length or greater
  if (jwt.length() <= JWT_SIG_SIZE + JWT_HEADER_SIZE + 2) {
    return;
  }
  // must have the correct header and delimiter
  if (!jwt.startsWith(JWT_HEADER) || jwt.indexOf('.') != JWT_HEADER_SIZE) {
    return;
  }
  // must have signature of correct length
  int signatureDelimieterIndex = jwt.length() - JWT_SIG_SIZE - 1;
  if (jwt.lastIndexOf('.') != signatureDelimieterIndex) {
    return;
  }

  // signature must be correct
  String signature = jwt.substring(signatureDelimieterIndex + 1);
  jwt = jwt.substring(0, signatureDelimieterIndex);
  if (sign(jwt) != signature){
    return;
  }

  // decode payload
  jwt = jwt.substring(JWT_HEADER_SIZE + 1); 
  jwt = decode((unsigned char *) jwt.c_str());
  
  // parse payload, clearing json document after failure
  DeserializationError error = deserializeJson(jsonDocument, jwt);
  if (error != DeserializationError::Ok || !jsonDocument.is<JsonObject>()){
    jsonDocument.clear();
  }
}