/**
* Decrypts TTN data_raw payload to data according to the TTN REST API.
*
* @param rawText encrypted message payload from ttn mqtt message
* @param K the TTN application key
* @param IV
* @return decrypted payload
* @throws Exception
*/
public static String decrypt(String rawText, byte[] K, byte[] IV) throws Exception {
byte[] pld = Base64.getDecoder().decode(rawText);
byte[] devAddr = getDevAddr(pld);
byte[] frameCounter = getFrameCounter(pld);
byte[] result = initializeResult(pld);
byte[] Ai = new byte[16];
byte[] Si = null;
for (int i = 0; i < result.length; i += 16) {
int blockSeqCnt = (i >> 4) + 1;
computeAi(Ai, devAddr, frameCounter, blockSeqCnt);
Si = encryptAES(Ai, K, IV);
for (int j = 0; j < 16 && i + j < result.length; j++) {
result[i + j] ^= Si[j];
}
}
return Base64.getEncoder().encodeToString(result);
}
