picoCTF{decrypted_message}
H98A9W_H6UM8W_6A_9_D6C_5ZCI9C8I_D9FF6IFD
, then set the alphabet to abcdefghijklmnopqrstuvwxyz0123456789
, then increment the shift until the message is clear. The offset I found was 5 (a --> f
)picoCTF{C4354R_C1PH3R_15_4_817_0U7D473D_84AA1DA8}