[xmodem]

Indeed, for a side project I have, I have a problem I want to be able to solve of "encrypt a file with a passphrase in a way that's secure and can be decrypted with standard tools". PGP is the best option for this, but I'm resisting implementing it in the hope I can find something better.

[technion]

You may wish to look at age: https://github.com/FiloSottile/age

[xmodem]

I've looked at 10 different tools like this, but this doesn't fall well enough into the definition i'm using of "standard tools," by which I mean something installable from apt/yum/ports on a wide variety of systems.

The closest I've found is using openssl's aes modes, but that requires the IV to be stored out-of-band somehow which is a do-able but a hassle I was hoping to avoid.