[OpenTRV-dev] Strange c code behaviour on the arduino

Deniz Erbilgin deniz.erbilgin at gmail.com
Mon Sep 7 19:24:40 BST 2015

Hi all,

I'm currently putting the AESGCM encryption libraries together for the arduino and have been getting some interesting behaviour from my tag checking function:

static uint8_t checkTag(const uint8_t *tag1, const uint8_t *tag2)
    uint8_t result;

    // compare tags. If any byte fails, will set bits in result
    for (uint8_t i = 0; i < 16; i++) {
        result |= *tag1 ^ *tag2;
        //Serial.print(result, HEX);    // when this is uncommented, the function works correctly
    return result;

The function just loops through an array and sets bits in result whenever there is a difference between bytes.

I had all the functions visible externally while I was debugging another problem and it was working fine. However, when I made all the functions static again, the decrypt function started returning a failed tag check.

I included the arduino headers in the library so that I could print from it to serial, and as long as I print 'result' to the console it works fine. I printed both tags from within the function just in case and they're both identical, whether or not the function fails.

All I can think of is that the compiler is trying to optimise it in some strange way.



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