RSA

RSA is one of the most common modern cryptosystems, known for it's public and private key encryption. If you see public and private keys, there is a strong chance that it is an RSA problem, or at least RSA derived.

Read this: https://ctf101.org/cryptography/what-is-rsa/

Follow this writeup and submit your solution: https://qy.sg/x-ctf-fact0r!z3-and-fact0r!z3_aga!n-crypto/

Some RSA problems will not be solved by default RSAtool runs. For those, you will often need an extension named Sage Math. I keep a cloud server setup with a few crypto tools and Sage installed, and a ton of cores and vmem so that if I need to, I can turn it on, download the ciphers, brrrrrrrrrt and solve with zero effort. Definitely a CTF power move, and it is free as long as you remember to turn it off once you are done, because it costs about $1.20 a minute. If you forget to turn it off, you'll burn through your free credits instantly and then have an $1000 dollar bill at the end of the month.

High risk, high reward?