The Internet: Encryption & Public Keys

Summary

Mia Gil-Epner explains how encryption protects private information sent over the internet, an open system where data may travel across shared connections.

Encryption and decryption

Encryption scrambles a message so it cannot be read in its original form. Decryption reverses the process.

Early example: Caesar’s cipher

Caesar’s cipher shifts letters by a chosen number of positions. That number is the key. The method is easy to break because an attacker can try all 26 possible shifts.

Why stronger keys matter

Using a different shift for each letter creates many more possible keys, but computers can test enormous numbers of guesses quickly. Modern secure communications therefore use long keys, such as 256-bit keys, which create an impractically large number of possibilities to test. Longer keys greatly increase the work needed to crack a message.

Symmetric encryption

The sender and receiver use the same secret key to encrypt and decrypt. They must agree on the key in advance, which is difficult to do privately over the open internet.

Asymmetric, or public-key, encryption

Public-key encryption addresses the key-sharing problem by using two keys:

• A public key can be shared with anyone and used to encrypt a message.
• A private key is kept secret and is needed to decrypt messages.

The mailbox analogy illustrates the idea: anyone with the public “deposit” key can leave a message, but only the owner’s private key can open the mailbox. People can use one another’s public keys to exchange secure messages without first agreeing on a shared secret key.

Web security

Public-key cryptography underlies security protocols such as SSL and TLS. A browser’s lock icon or “https” indicates that it is using these protections to exchange data securely with a website.

Ongoing challenge

As computers become faster, encryption standards may need to evolve so that breaking them remains computationally impractical.

Speakers and sources

• Mia Gil-Epner — Presenter; identifies herself as a computer science student at UC Berkeley and a Department of Defense employee working to protect information.