Good resources

https://linuxconfig.org/using-openssl-to-encrypt-messages-and-files-on-linux

Public key encrypt / private key decrypt – RSA (small files)

Note RSA (rsault) is only suitable for very small files and is typically used to encrypt a randomly chosen private key that a larger file gets encrypted with.  The reason it won't allow use with large files (say over around 512 bytes) is performance.

Generating a key pair

Generate private key

openssl genrsa -out private_key.pem 1024

Then use it to generate the public key

openssl rsa -in private_key.pem -out public_key.pem -outform PEM -pubout

Encrypt file

This will encrypt using RSA and your 1024 bit key.

openssl rsautl -encrypt -inkey public_key.pem -pubin -in encrypt.txt -out encrypt.dat

Decrypt File

openssl rsautl -decrypt -inkey private_key.pem -in encrypt.dat -out new_encrypt.txt

Public key encrypt / private key decrypt – SMIME AES (Large files)

(Good up to around 500MB, dependant on platform and resources)

Generating a key pair

openssl req -x509 -nodes -days 100000 -newkey rsa:2048  -keyout privatekey.pem  -out publickey.pem  -subj '/'

Encrypt file

This will encrypt using RSA and your 1024 bit key.

openssl  smime  -encrypt -aes256  -in  my_large_file.bin  -binary  -outform DEM  -out  my_large_file_encrypted.bin  publickey.pem

Decrypt File

openssl  smime -decrypt  -in  my_large_file.bin  -binary -inform DEM -inkey privatekey.pem  -out  my_large_file.bin