Hybrid Encryption Schemes

Although public key encryption is very secure, it does have some drawbacks. This method for encrypting and decrypting messages requires a lot of computation, so it is slower than other types of codes. Also, the encrypted message may be quite a bit longer than the original plaintext, perhaps twice as long, so it will take longer to transmit over a network. These considerations led people to develop hybrid schemes in which public key encryption is used to send a freshly-created key, called the session key, and then the actual message is encrypted using a symmetric encryption scheme like RC4 or DES, based on that session key. Because session keys are generated randomly and thrown away after one use, even if an eavesdropper manages to decrypt one message by brute force, the eavesdropper will be unable to decrypt any other messages between the same parties. So, the hybrid scheme is reasonably secure, and it takes advantage of the efficiency of the simpler symmetric encryption schemes.

Most public-key encryption schemes are actually hybrids of this sort. SSL (the Secure Socket Layer protocol used by web browsers) is a hybrid scheme. More about SSL will be discussed in section 6.2.3 Authentication.

6.2.2 Applications of Encryption

• Email

• Hard Drives

• DVD Movies

• Cellular Phones

Encryption technology is widespread in the digital world. Some important uses of encryption are discussed below.

Email

Email may travel through a dozen computers on its way from the sender to the intended recipient, so there is ample opportunity for eavesdropping. By default, email is not encrypted at present, but many users are opting to use PGP to secure their email communications. Some mail clients now include built-in PGP support.

Hard Drives

Some users are choosing to encrypt their entire hard drive, in order to protect against loss of secrets in the event that their computer is stolen or seized. An old laptop full of trade secrets or classified military information might be worth much more to a thief than a new laptop with an empty hard drive. While user directories can be protected with passwords, an intruder with physical access to the machine can use his own copy of the operating system to boot the computer, and thereby gain access to all the data on the computer. Or, the intruder can make a copy of the hard drive, a process known as mirroring, and thus steal the data it contained.

Encrypting the hard drive prevents others from reading your files, even if they steal or copy the drive. Each time your computer boots, you would be required to enter a pass phrase to unlock the drive.

Dvd Movies

Movies on DVD (Digital Video Disk) are encrypted using a scheme called CSS (Content Scrambling System), in order to prevent people from making unauthorized copies. However, the encryption scheme was not secure. In 1999, several groups of computer programmers broke the encryption by reverse-engineering a software DVD player. Although the motion picture studios filed several lawsuits to prevent the distribution of software for decrypting DVD movies, their efforts have been unsuccessful. Programmers claimed that computer code is a type of speech, and therefore should enjoy the same legal protections as books and music. Hence, they have spread the code all over the world, and even printed the code on T-shirts. You can learn more about this controversy by visiting theGallery of CSS Descramblers.