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→Building Example
Where the CW303 is the XMEGA target on the ChipWhisperer-Lite / UFO target. Note an existing binary is present in the repository, if using a release you can simply use that existing .hex-file.
=== Firmware Description ===
The example firmware file (<code>simpleserial-rsa.c</code>) pulls in a RSA implementation from avr-crypto-lib.
The main firmware file defines two functions:
simpleserial_addcmd('t', 0, real_dec);
simpleserial_addcmd('p', 16, get_pt);
The <code>real_dec</code> function performs a real RSA decryption. The input plaintext and key are fixed, and loaded internally in the software.
<syntaxhighlight lang="c">
/* Perform a real RSA decryption, be aware this is VERY SLOW on AVR/XMEGA. At 7.37MHz using the default
1024 byte key it takes about 687 seconds (over 10 mins). */
uint8_t real_dec(uint8_t * pt)
{
/* Load encrypted message */
load_bigint_from_os(&cp, ENCRYPTED, sizeof(ENCRYPTED));
//Do an encryption on constant data
trigger_high();
if (rsa_dec(&cp, &priv_key)){
putch('F');
}
trigger_low();
return 0;
}
</syntaxhighlight>
The VERY slow encryption can be seen by simply sending a <code>t</code> command. You will see a series of dots printed to the console as the RSA algorithm is running. Instead we will use the second function which performs a much smaller operation on a 16-byte key. The exact same code is used, only it runs on a MUCH smaller key that can be easily captured. The end of the tutorial will discuss how you could apply this to the full algorithm.
== Finding SPA Leakage ==