How to encrypt a file non-interactively (using a stored private key)

Question

I do not want to run something like gpg -c as it prompts me for a passphrase. I will have cron job that backs-up and encypts some files. I want to pass in a location to a private key (via arguements) to an application that can encrypt a stream (ie: instead of encrypting just a file, it can use the pipe character, |, as input).

Symmetric encryption like AES or the like is preferred.

One more thing, I want it to be a private key that I store somewhere on the file system. Not a private key that is managed by a key server or some kind of application specific key container.

Answer 1

Then use gpg -c --passphrase-file=... or gpg -c --passphrase-fd=.... For example:

dmesg | gpg -c --passphrase-fd=3 --batch > dmesg.txt.gpg 3< zombies.key

To decrypt:

gpg --batch --passphrase-fd=3 3< zombies.key --output decrypted.txt --decrypt dmesg.txt.gpg

(I think gpg does default to CAST, though, so also add --cipher-algo AES256.)

Alternatively, openssl cipher -aes256.