ccrypt

# Install
sudo apt install ccrypt
# Encrypt
ccencrypt .ssh/id_rsa
# Decrypt
ccdecdcrypt .ssh/id_rsa.cpt

icekimo@ip-172-31-10-86:~$ sudo apt show ccrypt
Package: ccrypt
---
Description: secure encryption and decryption of files and streams
 ccrypt is a utility for encrypting and decrypting files and streams. It was
 designed as a replacement for the standard unix crypt utility, which is
 notorious for using a very weak encryption algorithm. ccrypt is based on the
 Rijndael cipher, which is the U.S. government's chosen candidate for the
 Advanced Encryption Standard (AES, see http://www.nist.gov/aes). This cipher is
 believed to provide very strong security.