I would like to encrypt a file using a symmetric key algorithm. AES is a common algorithm, but I don't quite trust it, and I heard that Serpent is a very strong alternative. How can I encrypt/decrypt a file with Serpent, preferably through a command line?
if you are really concerned about the security of your computer's files and directories then you should try encrypting with triple encryption algorithm AES-Twofish-Serpent.
There is a great tutorial detailing the steps needed to encrypt both individual files and also directories. It is too detailed to post here and also I do not have permission to copy the content.
The steps outlined by the blogger on his post seem to suggest that it is better. Follow this link to see the steps he outlined. Hopefully this helps you with your situation.