openssl-encrypt:
encrypt a file using our best settings

Syntax:

openssl-encrypt <input file path> [output file path]

Example:

$ openssl-encrypt example.txt

Output is a new encrypted file:

example.txt.aes

To decrypt the file:

$ openssl-decrypt example.txt.aes

Settings

Symmetric encryption, i.e. we use the same password for encryption and decryption. We choose this because our users can understand symmetric more easily than asymmetic.
Encryption using the aes-256-cbc cipher algorithm. We choose this because it's a good balance of strong, fast, and portable.
Message digest using SHA-256. We choose this because it's the current default of the current openssl tool, and we set it explicity because openssl versions have different defaults.
Salt that is randomly generated.
The output file path defaults to the input file path with suffix ".aes".

The command is:

openssl aes-256-cbc -salt -md sha256
  -in "example.txt"
  -out "example.txt.aes"