Ccrypt

apt install ccrypt

To encrypt a file called myfile.txt:

ccrypt -e myfile.txt  OR
ccencrypt myfile.txt

You will be asked for your password twice. The file myfile.txt will be replaced with a file named myfile.txt.cpt.

To decrypt myfile.txt.cpt:

ccrypt -d myfile.txt.cpt  OR
ccdecrypt myfile.txt.cpt

Enter the password at the prompt. The encrypted file will be replaced with a decrypted myfile.txt.