A: This depends on if you want symmetric or asymmetric encryption.

For symmetic encryption, you can use the following…

To encrypt:

> openssl aes-256-cbc -salt -a -e -in plaintext.txt -out encrypted.txt

 
To decrypt this, you can use the following.

> openssl aes-256-cbc -salt -a -d -in encrypted.txt -out plaintext.txt

 
Now, for Asymmetric encryption you must first generate your private key and extract the public key.

> openssl genrsa -des3 -out private.key 4096
> openssl -in private.key -pubout -out public.key

 
To encrypt:

> openssl rsautl -encrypt -pubin -inkey public.key \\
                          -in plaintext.txt -out encrypted.txt

 
To decrypt:

> openssl rsautl -decrypt -inkey private.key \\
                          -in encrypted.txt -out plaintext.txt