Digital envelope
From The Glossary of Digital Photography
A method of protecting a message or image by encrypting it. In this technique, two levels of encryption are used. The message or image is encrypted using a secret key. The secret key is then encrypted using a public key. Only the recipient has the private key to unlock the secret key. The advantage to this method is that the public/private key technique is much slower for opening larger messages or images. By using the faster secret key to encrypt the larger image and the slower public/private key technique to encrypt only the small, secret key, the encryption operation is substantially faster overall.
