The simplest encryption program - ccrypt

The program is a console program; you work with files in the terminal; you can encrypt several files at once.
For convenience, you can create aliases:
alias enc='ccencrypt'  #encrypt file (with replacement); required package installed ccrypt
alias dec='ccdecrypt'  #decrypt an encrypted file.cpt(with replacement); required package installed ccrypt
alias c-cat='ccat -c'  # View the encrypted text file in the terminal; use: c-cat file.cpt

EXAMPLES:
Encrypt (with replacement):   ccrypt -e file  or ccencrypt file

Decrypt (with replacement):   ccrypt -d file  or ccdecrypt file

Another way to encrypt a file(s) is to first write a command in the terminal, and then simply drag the files into the terminal window, and it does not matter in which folder the terminal is open and where the files were located. It will look like this:

~$ enc  '/home/user/Изображения/картинка2.png'  '/home/user/пример'  '/media/user/KINO/пример-архив.zip'
Enter encryption key:
Enter encryption key: (repeat)
~$

As we see, files were "dragged" from different folders, even from another disk and different in content (picture, text, archive). After encryption is complete, the files in their original locations are replaced with encrypted copies. They can be renamed and moved.
Let's move the encrypted files to the Home folder and decrypt them

~$ dec '/home/user/пример1.cpt' '/home/user/пример-архив1.zip.cpt' '/home/user/картинка2.cpt'
Enter decryption key:
~$
Successfully decrypted

Option -x allows you to change the key of an encrypted file without decrypting it, but for this you still need to know the old key.
ccrypt -x file.cpt   
========================

VeraCrypt - creating an encrypted folder or not a system disk. Documentation and website
https://veracrypt.eu/en/Documentation.html  - here is detailed documentation in English.
https://veracrypt.eu/ru/Beginner's%20Tutorial.html  - instructions for beginners

An easy-to-use program for creating an encrypted folder (disk). The encrypted container can be renamed, moved, with certain settings not visible information about how much space is occupied. Extensive documentation.   
For simple use the approach is a little different. The program needs to allocate a part of the disk space (container), which will be mounted as needed, and where the files selected for encryption will be placed.