The writeups are organized by category, as on the website, then by number. Each writeup is a single PDF file, encrypted with the SHA256 sum of the MD5 sum of the flag as the password.

For example, if the flag is FLAG-SUPERSECRETFLAG, then the password can be found by

echo -n "FLAG-SUPERSECRETFLAG" | md5sum | cut -b-32 -z | sha256sum | cut -b-64 -z

(Note the -n for the echo command, this is to prevent a newline at the end. If the newline is printed it will change the value of the hash. The same for cut, except the flag is -z)

This would output 78d178916fbd806043a43e04081fcc12, so to unzip the file, use that as the password.

Alternatively, use the open.sh script provided. Specify the challenge number and flag (in plaintext) and if it is correct, it will be extracted for you. For example,

./open.sh 999 "FLAG-SUPERSECRETFLAG"