Digital Envelope

Public key cryptography is computationally slow. If one used 128- or 256-bit keys to encode large documents — such as this chapter or the entire book — significant declines in transmission speeds and increases in processing time would occur. Symmetric key cryptography is computationally faster, but it has a weakness: the symmetric key must be sent to the recipient over insecure transmission lines.

One solution is to use the more efficient symmetric encryption and decryption for large documents, but public key cryptography to encrypt and send the symmetric key. This technique is called using a digital envelope. A digital envelope is a technique that uses symmetric encryption for large documents, but public key cryptography to encrypt and send the symmetric key.

In a digital envelope, a document is encrypted using a symmetric key. The symmetric key — which the recipient will require to decrypt the document — is itself encrypted using the recipient's public key. So we have a 'key within a key' (a digital envelope). The encrypted report and the digital envelope are sent across the Web.

The recipient first uses his or her private key to decrypt the symmetric key, and then uses the symmetric key to decrypt the report. This method saves time because both encryption and decryption are faster with symmetric keys. The digital envelope combines the speed of symmetric encryption with the secure key-exchange of public key encryption, making it the practical choice for large-scale e-commerce transactions.