CS 537 Notes, Section #31: Security Improvements, Encryption

Solutions: nothing works perfectly, but here are some possibilities:

• Logging: record all important actions and uses of privilege in an indelible file. Can be used to catch imposters during their initial attempts and failures. E.g. record all attempts to specify an incorrect password, all super-user logins. Even better is to get humans involved at key steps (this is one of the solutions for EFT).
• Principle of minimum privilege ("need-to-know" principle): each piece of the system has access to the minimum amount of information, for the minimum possible amount of time. E.g. file system cannot touch memory map, memory manager cannot touch disk allocation tables. This reduces the chances of accidental or intentional damage. Note that capabilities are an implementation of this idea. It is very hard to provide fool-proof information containment: e.g. a trojan horse could write characters to a tty, or take page faults, in Morse code, as a signal to another process.
• Correctness proofs. These are very hard to do. Even so, this only proves that the system works according to spec. It does not mean that the spec. is necessarily right, and it does not deal with Trojan Horses.

The basic mechanism:

• Start with text to be protected. Initial readable text is called clear text.
• Encrypt the clear text so that it does not make any sense at all. The nonsense text is called cipher text. The encryption is controlled by a secret password or number; this is called the encryption key.

• 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 secret password 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), so cannot 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, or vice 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: 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 a phone 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.

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 of life."
• 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 an undergraduate (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 private key, 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 dederal Data Encryption Standard (DES). Is not public-key based, but as implemented efficiently and appeared to be relatively safe.

More recently, there is the Advanced Encryption Standard (AES), called Ryndal (pronounced "rine doll"). ***more on AES here****

General problem: how do we know that an encryption mechanism is safe? It is extremely hard to prove. This is a hot topic for research: 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 and more prevalent.