OpenSSL for files encryption

openssl-encrypt-file

# create a PEM pub key (will be useful later)
openssl rsa -in .ssh/id_rsa -pubout -out id_rsa.pub.pem
# check it
chmod 700 id_rsa.pub.pem
openssl rsa -in id_rsa.pub.pem -pubin -text -noout

# 1) encrypt small file, using only RSA private and pub key 
# create a secret
echo "my secret" > secret.txt
# encrypt the file using your public key 
openssl rsautl -encrypt -inkey id_rsa.pub.pem -pubin -in secret.txt -out secret.enc
# decrypt the file using your private key
openssl rsautl -decrypt -inkey .ssh/id_rsa -in secret.enc

# 2) encrypt large files using RSA 
# generate a random key / password file
openssl rand -base64 128 -out key.bin
# encrypt the large file with the random key
openssl enc -aes-256-cbc -salt -in secret.txt -out secret.enc -pass file:key.bin
# encrypt the random key with public key file
openssl rsautl -encrypt -inkey id_rsa.pub.pem -pubin -in key.bin -out key.bin.enc
# decrypt the random key with the private key file
openssl rsautl -decrypt -inkey id_rsa -in key.bin.enc -out key.bin
# decrypt the large file with the random key
openssl enc -d -aes-256-cbc -in secret.enc -out secret.txt -pass file:key.bin
# source https://raymii.org/s/tutorials/Encrypt_and_decrypt_files_to_public_keys_via_the_OpenSSL_Command_Line.html

# 3) encrypt large files without using priv/pub key at all but only OpenSSL and a password (you will be prompted)
openssl aes-256-cbc -salt -in secret.txt -out secret.txt.enc
openssl aes-256-cbc -d -in secret.txt -out secret.txt.enc
# source http://tombuntu.com/index.php/2007/12/12/simple-file-encryption-with-openssl/

See also http://www.czeskis.com/random/openssl-encrypt-file.html and
https://stackoverflow.com/questions/29010967/openssl-unable-to-load-public-key
and https://raymii.org/s/tutorials/Encrypt_and_decrypt_files_to_public_keys_via_the_OpenSSL_Command_Line.html

Remarks :
RSA is not suited for encrypting large files (If you create a key of n bits, then the file you want to encrypt must not larger than (n minus 11) bits)
GPG should be prefered to OpenSSL (more info https://stackoverflow.com/a/31552829/2309958)