This challenge gives us the ciphertext: prnea{43clscfif3u}tlr.
After trying several ciphers I found that it was a rail fence cipher with 4 rails.
To decode we write out how the string would be encoded
1 7 13 19
2 6 8 12 14 18 20
3 5 9 11 15 17 21
4 10 16
To encrypt the string, the characters of the string would be written down in the order given above, then read off left to right, to to bottom (1,7,13,19,2,6,...)
So, to decode we just write in the characters of the ciphertext left to right, and read off in the zigzag pattern.
p r n e
a { 4 3 c l s
c f i f 3 u }
t l r
Giving us the flag: pactf{r4ilf3nc3rules}