Ever security minded, Santa is. So switched to a prime encoding system he has, after contemplating for long.
At first I tried to search for different prime encodings. This proved to be the wrong approach and I took a step back. While googling around I came across the Cistercian numerals. I then used this decoder to get the actual numbers:
2314
6344
6333
4675
2268
3533
763
5940
1707
7377
4022
4870
7382
6109
385
4221
At this point I tried different encodings to get the flag but was unsuccessful. The largest numer (7382) needs 13 bits to be encoded. Therefore, I padded all the numbers to 13 bits, then looked at them in group of 8 bits and interpreteted that as ASCII encoding:
from textwrap import wrap
i = [2314, 6344, 6333, 4675, 2268, 3533, 763, 5940, 1707, 7377, 4022, 4870, 7382, 6109, 385, 4221]
all = ""
for e in i:
all += bin(e)[2:].zfill(13)
for group in wrap(all, 8):
print(chr(int(group, 2)), end='')
print()This gave the flag HV21{$4n74_w45_4_m0nk_t00}.