There you should read hexadecimal numbers, whether they are encrypted or plaintext.
Maybe you have problems with connections to the DS2431 chip, or you have omitted something in the configuration of your Bus Pirate.
Did you enabled PULL-UPs on your Bus Pirate?
On your Bus Pirate are you using the STANDARD or OVERDRIVE speed?
Which revision of the Bus Pirate are you using, 3 or 4?
What firmware does your Bus Pirate have?
Be seeing you.