Easy RSA

No maths!

We're given a pem public/private key pair and an encrypted message so let's take the obvious route and try to decrypt the message using the private key. I used CyberChef to do this:

Phew, no maths for this one, we get the plaintext flag ictf{Rivest–Shamir–Adleman-encryption-means-nothing-if-you-have-the-private-key} straight away.

Another option (if I knew the right arguments to use at the time) would be to use openssl directly with the command openssl rsautl -decrypt -in enc.txt -inkey private.pem.