12.3 Encryption

Encrypt clear text into cipher text.

Properties of good encryption technique:

    1. Relatively simple for authorized users to encrypt and decrypt data.

    2. Encryption scheme depends not on the secrecy of the algorithm but on a parameter of the

        algorithm called the encryption key.

    3. Extremely difficult for an intruder to determine the encryption key.

Data Encryption Standard substitutes characters and rearranges their order on the basis of an encryption key provided to authorized users via a secure mechanism.

Scheme only as secure as the mechanism.

Public-key encryption based on each user having two keys:

    1. public key – published key used to encrypt data.

    2. private key – key known only to individual user used to decrypt data.

Must be an encryption scheme that can be made public without making it easy to figure out the decryption scheme.

    1. Efficient algorithm for testing whether or not a number is prime.

    2. No efficient algorithm is know for finding the prime factors of a number.

Encryption Example - SSL

SSL – Secure Socket Layer

Cryptographic protocol that limits two computers to only exchange messages with each other.

Used between web servers and browsers for secure communication (credit card numbers)

The server is verified with a certificate.

Communication between each computers uses symmetric key cryptography.

Computer Security Classifications

U.S. Department of Defense outlines four divisions of computer security: A, B, C, and D.

D – Minimal security.

C – Provides discretionary protection through auditing.

Divided into C1 and C2. C1 identifies cooperating users with the same level of protection. C2 allows user-level access control.

B – All the properties of C, however each object may have unique sensitivity labels. Divided into B1, B2, and B3.

A – Uses formal design and verification techniques to ensure security.

Windows NT Example

Configurable security allows policies ranging from D to C2.

Security is based on user accounts where each user has a security ID.

Uses a subject model to ensure access security. A subject tracks and manages permissions for each program that a user runs.

Each object in Windows NT has a security attribute defined by a security descriptor. For example, a file has a security descriptor that indicates the access permissions for all users.