1.8 Protection and security

• The encrypted text can be stored in a readable file, or transmitted over unprotected channels.
• To make sense of the cipher text, it must be decrypted back into clear text. This is done with some other algorithm that uses another secretpassword or number, called the decryption key.

All of this only works under three conditions:

• The encryption function cannot easily be inverted (cannot get back to clear text unless you know the decryption key).
• The encryption and decryption must be done in some safe place so the clear text cannot be stolen.
• The keys must be protected. In most systems, can compute one key from the other (sometimes the encryption and decryption keys are identical), socannot afford to let either key leak out.

Public key encryption: new mechanism for encryption where knowing the encryption key does not help you to find decryption key, orvice versa.

• User provides a single password, system uses it to generate two keys (use a one-way function, so cannot derive password from either key).
• In these systems, keys are inverses of each other: each one could just as easily encrypt with decryption key and then use encryption key to recover clear text.
• Each user keeps one key secret, publicizes the other. Cannot derive private key from public. Public keys are made available to everyone, in aphone book for example.

Safe mail:

• Use public key of destination user to encrypt mail.
• Anybody can encrypt mail for this user and be certain that only the user will be able to decipher it.

It is a nice scheme because the user only has to remember one key, and all senders can use the same key. However, how doesreceiver know for sure who it is getting mail from?

Digital signatures

Positive identification: can also use public keys to certify identity:

• To certify your identity, use your private key to encrypt a text message, e.g. "I agree to pay Mary Wallace $100 per year for the duration oflife."
• You can give the encrypted message to anybody, and they can certify that it came from you by seeing if it decrypts with your public key.Anything that decrypts into readable text with your public key must have come from you! This can be made legally binding as a form of electronic signature.

This is really even better than signatures: harder to forge, and can change if compromised. Note that this idea was developed by anundergraduate (Loren Kohnfelder) in 1978 as part of his undergraduate thesis (at MIT).

These two forms of encryption can be combined together. To identify sender in secure mail, encrypt first with your privatekey, then with receiver's public key. The encryption/decryption functions to send from B to A are:

Encryption appears to be a great way to thwart listeners. It does not help with Trojan Horses, though.

Old federal Data Encryption Standard (DES). Is not public-key based, but has been implemented efficiently and appears to berelatively safe.

New Advanced Encryption Standard (AES), called Ryndal.

General problem: how do we know that an encryption mechanism is safe? It is extremely hard to prove. This is a hot topic forresearch: theorists are trying to find provably hard problems, and use them for proving safety of encryption.

Summary of Protection: very hard, but is increasingly important as things like electronic funds transfer become more andmore prevalent.

Acknowledgment

We would like to thank Prof. Barton Miller for sharing as well as for his valued advice