ArduinoJsonJWT.cpp

#include "ArduinoJsonJWT.h"

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

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

String ArduinoJsonJWT::getSecret() {
  return _secret;
}

/*
 * ESP32 uses mbedtls, ESP2866 uses bearssl.
 *
 * Both come with decent HMAC implmentations supporting sha256, as well as others.
 *
 * No need to pull in additional crypto libraries - lets use what we already have.
 */
String ArduinoJsonJWT::sign(String& payload) {
  unsigned char hmacResult[32];
  {
#ifdef ESP32
    mbedtls_md_context_t ctx;
    mbedtls_md_type_t md_type = MBEDTLS_MD_SHA256;
    mbedtls_md_init(&ctx);
    mbedtls_md_setup(&ctx, mbedtls_md_info_from_type(md_type), 1);
    mbedtls_md_hmac_starts(&ctx, (unsigned char*)_secret.c_str(), _secret.length());
    mbedtls_md_hmac_update(&ctx, (unsigned char*)payload.c_str(), payload.length());
    mbedtls_md_hmac_finish(&ctx, hmacResult);
    mbedtls_md_free(&ctx);
#elif defined(ESP8266)
    br_hmac_key_context keyCtx;
    br_hmac_key_init(&keyCtx, &br_sha256_vtable, _secret.c_str(), _secret.length());
    br_hmac_context hmacCtx;
    br_hmac_init(&hmacCtx, &keyCtx, 0);
    br_hmac_update(&hmacCtx, payload.c_str(), payload.length());
    br_hmac_out(&hmacCtx, hmacResult);
#endif
  }
  return encode((char*)hmacResult, 32);
}

String ArduinoJsonJWT::buildJWT(JsonObject& payload) {
  // serialize, then encode payload
  String jwt;
  serializeJson(payload, jwt);
  jwt = encode(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 have the correct header and delimiter
  if (!jwt.startsWith(JWT_HEADER) || jwt.indexOf('.') != JWT_HEADER_SIZE) {
    return;
  }

  // check there is a signature delimieter
  int signatureDelimiterIndex = jwt.lastIndexOf('.');
  if (signatureDelimiterIndex == JWT_HEADER_SIZE) {
    return;
  }

  // check the signature is valid
  String signature = jwt.substring(signatureDelimiterIndex + 1);
  jwt = jwt.substring(0, signatureDelimiterIndex);
  if (sign(jwt) != signature) {
    return;
  }

  // decode payload
  jwt = jwt.substring(JWT_HEADER_SIZE + 1);
  jwt = decode(jwt);

  // parse payload, clearing json document after failure
  DeserializationError error = deserializeJson(jsonDocument, jwt);
  if (error != DeserializationError::Ok || !jsonDocument.is<JsonObject>()) {
    jsonDocument.clear();
  }
}

String ArduinoJsonJWT::encode(const char* cstr, int inputLen) {
  // prepare encoder
  base64_encodestate _state;
#ifdef ESP32
  base64_init_encodestate(&_state);
  size_t encodedLength = base64_encode_expected_len(inputLen) + 1;
#elif defined(ESP8266)
  base64_init_encodestate_nonewlines(&_state);
  size_t encodedLength = base64_encode_expected_len_nonewlines(inputLen) + 1;
#endif
  // prepare buffer of correct length, returning an empty string on failure
  char* buffer = (char*)malloc(encodedLength * sizeof(char));
  if (buffer == nullptr) {
    return "";
  }

  // encode to buffer
  int len = base64_encode_block(cstr, inputLen, &buffer[0], &_state);
  len += base64_encode_blockend(&buffer[len], &_state);
  buffer[len] = 0;

  // convert to arduino string, freeing buffer
  String value = String(buffer);
  free(buffer);
  buffer = nullptr;

  // remove padding and convert to URL safe form
  while (value.length() > 0 && value.charAt(value.length() - 1) == '=') {
    value.remove(value.length() - 1);
  }
  value.replace('+', '-');
  value.replace('/', '_');

  // return as string
  return value;
}

String ArduinoJsonJWT::decode(String value) {
  // convert to standard base64
  value.replace('-', '+');
  value.replace('_', '/');

  // prepare buffer of correct length
  char buffer[base64_decode_expected_len(value.length()) + 1];

  // decode
  int len = base64_decode_chars(value.c_str(), value.length(), &buffer[0]);
  buffer[len] = 0;

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