How can I encrypt a large file with OpenSSL using RSA keys?

Question

How can I encrypt a large file with a public key so that no one other than who has the private key be able to decrypt it? I don't want to use GPG!

Answer 1

This could be used to encrpyt a file mypic.png, given you already have a private/public keypair in ccbild-key.pem/ccbild-crt.pem. (You can find a guide to creating a keypair in this answer.)

# encrypt
openssl smime -encrypt -aes-256-cbc -binary -in mypic.png -outform DER -out mypic.png.der ccbild-crt.pem

# decrypt
openssl smime -decrypt -binary -in mypic.png.der -inform DER -out mypic.png -inkey ccbild-key.pem

Note that the settings may not reflect best practice in selection of crypto standard (in particular if you read this in the future), also it might not be a good choice performance-wise. (We only use it for sub-1M files in our application.)

Answer 2

Since you want to encrytpt a file - you can use GPG for this purpose.

Also, don't worry about the format of file - gpg can encrypt any file.

Short instructions:

1. Ensure gpg-agent is running:

   gpg-agent -s --daemon --write-env-file --use-standard-socket

2. Generate your key:

   gpg --gen-key

Select default ke, unless you know what you're doing, or are told otherwise by someone who does. If asked for keysize, enter 1024.(size in bits)

3. When GPG will say that it will generate your key, start pressing keyboard randomly(just avoid combinations that send signals, like CTRL + Z, etc.

4. You are ready to decrypt file. By default you use

   gpg -e -r user_id my_file

Replace user_id with email provided during key generation. Instructions based on Indiana University's article