How to encrypt and decrypt files with public and private keys

The aim of this post is to show case who to do encryption and decryption using a paire of private and public RSA keys (asymmetric cryptography).

Generate paire private and public keys using openssl

$ ssh-keygen -t rsa -C "your.email@example.com" -b 4096

The private and public keys will be stored respectively in ~/.ssh/ida_rsa and ~/.ssh/id_rsa.pub

Get the public key in pkcs8 format from private key or ssh-rsa public key

$ ssh-keygen -f ~/.ssh/id_rsa.pub -e -m pkcs8 > ~/.ssh/id_rsa.pub.pkcs8

Or get the public key from private key

$ openssl rsa -in ~/.ssh/id_rsa -pubout > ~/.ssh/id_rsa.pub.pkcs8

Encrypt files with public keys

First we generate a test file by simply run following command

$ echo "This is my program." > input

To encrypt with public key, run this command

$ openssl pkeyutl -encrypt -pubin -inkey ~/.ssh/id_rsa.pub.pkcs8 -in input -out output.enc

Decrypt files with private key

$ openssl pkeyutl -decrypt -inkey ~/.ssh/id_rsa -in output.enc -out output.dec