Public Policy and the Internet

Course Syllabus

Privacy: How Public Key Encryption Works Alice Communicating with Bob Using Public Key Encryption Step 2: Authentication Alice also wants Bob to know for certain that her message is from her.   Alice uses her private key to encrypt a digital signature that she can append to the message she encrypted with Bob's public key.     Now the message is encrypted and also contains Alice's encrypted signature, which assures Bob the message is from Alice.           Alice sends the encrypted message to Bob, and she also sends Bob her public key to decrypt her signature.   Bob can now use Alice's public key to decrypt her digital signature, and then use his private key to decrypt the message.             Bob can now read the message. In this fashion, Bob is assured not only of confidentiality, but is also assured that the message came from Alice, because only Alice's public key can decrypt messages encrypted with her private key. In real-world practice, people rarely use public-key encryption for routine communication, even of sensitive documents. Instead, they use public-key encryption to exchange a pair of symmetric keys, or "session keys," which are then used to encrypt and decrypt communications. This is much faster, because the computational requirements for public-key encryption are very large. Systems that use both asymmetric and symmetric key functions are called "hybrid" systems.     Go to "What is a key?"   © The 21st Century Project, 2003-2008, All Rights Reserved