I find three methods to encrypt/decrypt a single file. Those ways use different utilities respectively, which are vim, gpg and openssl. I prefer to use vim, because vim is really convenient. I will cover two of them.

vim

For example, I have a file named info including many important information in my home directory, so I don’t want others to read it. To encrypt this file, we could do this:

billie@piggy:~$ vim -x info

After run this command, you will get prompt at the bottom of your screen.

Enter encryption key: 
Enter same key again:

Enter your passward according to the prompt, then the job is done. If you want to see the content of this file next time, you need to enter passward, otherwise you will see bullshit.

In addition, you may change your password. To do this, execute the following command in your terminal.

billie@piggy:~$ vim +X info

The command above will prompt some messages.

"info" [crypted] 1L, 3C
Enter encryption key: 
Enter same key again:

Note once you encrypted your file, you will not get its cleartext back unless you remember the encryption key.

gpg

gpg is another tool to encrypt files. Check gpg man page for more details. You can use it like this:

billie@piggy:~$ gpg -c info
Enter passphrase: 
Repeat passphrase:

gpg will create a new file called info.pgp, and the info file still exist, so you need to remove the original file manually. 

billie@piggy:~$ gpg -d info.pgp -o info
Enter passphrase:

The “-o” option redirect the output to the file info.