wow thanks....i missed your post,

The CRC is very weak for encryption, because it is a reversible mathematical operation.
can i ask how to do it? now im reading app note from dallas.



CRC was originally designed for use with a shift register with parallel access and a number of XOR gates. The intention was to be able to compute a very very good checksum with almost no hardware at a time when the world did not have any microprocessors.


your right, its like using checksum :)

For cryptography, you have to look at solutions that are not trivially reversible.

but it should be reversible, on decoding

Anyway - you have still not described your problem. You mention RTC time. What RTC time? To/from a known product or component?


i could set the time on the it, if i have the right sequence


And why didn't you start with telling that you had listened to a communication stream and had two bytes in the packet that you could not find how to compute?


i thought it was not relevant.



There are many different 16-bit CRC alternatives. Several polynomials. With or without pre- or post-conditioning (setting bits initial bits to 1 or 0, inverting or not inverting the answer), and some implementations are intended to be fed the most significant bit first, while some are intended to be fed the most significant bit first.

I THINK THIS IS THE ANSWER IM LOOKING FOR. :(