How do I do public-key encryption with openssl?

OpenSSL is a public-key crypto library (plus some other random stuff). Here’s how to do the basics: key generation, encryption and decryption. We’ll use RSA keys, which means the relevant openssl commands are genrsa, rsa, and rsautl.

# Alice generates her private key `priv_key.pem`
openssl genrsa -out priv_key.pem 2048  

# Alice extracts the public key `pub_key.pem` and sends it to Bob
openssl rsa -pubout -in priv_key.pem -out pub_key.pem

# Bob encrypts a message and sends `encrypted_with_pub_key` to Alice
openssl rsautl -encrypt -in cleartext -out encrypted_with_pub_key -inkey pub_key.pem -pubin

# Alice decrypts Bob's message
openssl rsautl -decrypt -in encrypted_with_pub_key -inkey priv_key.pem