In December 2019 I participated in PST (Norwegian Police Security Service) advent capture the flag calendar. I ended up in 32nd place, with one unsolved task. I had to read up a lot to be able to solve the tasks, as this is my first CTF, so was quite happy with that result. Here are some useful tools I ended up using during the challenge.
https://cryptii.com/
Super easy to use tool to decrypt/encode various ciphers.
https://gchq.github.io/CyberChef/
Nice tool with lots of possibilities for decrypting and encoding various ciphers.
https://incoherency.co.uk/chess-steg/
One of the tasks involved a chess game, this online tool decoded it perfectly.
https://www.mygeocachingprofile.com/codebreaker.vigenerecipher.aspx
This Vigenere codebreaker analyzes the encrypted text to determine the most probable key length and then tries to guess the key based on known character frequencies/words in the English language. Helped me a lot in one of the tasks.
https://www.sleuthkit.org/
Very nice tool to analyze a raw dump of a hard drive.
https://github.com/zed-0xff/zsteg
This quickly became my goto tool in all of the steganography tasks. Very nice and easy to run with the -all mode.
https://georgeom.net/StegOnline/upload
Nice online tool for steganography, also a very handy checklist on what to check.
https://29a.ch/photo-forensics/#forensic-magnifier
Also a nice online tool that helped me with the first steganography task in the CTF.