This question already has an answer here:
Hopefully this pattern isn't a repeat of any puzzle posted previously...
Dawn, Dec 17 XXXX
"Our future is in your hands. I wish you the best of luck."
With that, the hooded man lays before you a scrap of dirty paper and exits the room. Peering at the sheet under dim lamplight, you notice the imperial seal of Country Y and the word 'CONFIDENTIAL' printed in block letters. Your stomach tightens in anticipation--the agents had succeeded.
Somehow, they had obtained the communications cipher for the enemy's upcoming attack, and now they require the assistance of the nations greatest code breaker to usher in victory!
The paper reads:
Dec 13 XXXX -> 223
Dec 14 XXXX -> 2213
Dec 15 XXXX -> 221113
Dec 16 XXXX -> 223113
What is the communications password for Dec 17 XXXX?