verifyKeyFile = privAccessFile(sf,Attribute.CRYPTO_EXTERNAL_KEY_VERIFY_FILE,"r");
// then read the checksum length
int checksumLen = verifyKeyFile.readInt();
byte[] originalChecksum = new byte[checksumLen];
verifyKeyFile.readFully(originalChecksum);
verifyKeyFile.readFully(data);
// decrypt data with key
CipherProvider tmpCipherProvider = createNewCipher(DECRYPT,mainSecretKey,mainIV);