1. 6.033 Quiz3 Review Spring 2007

2. How can we achieve security? Authenticate agent’s identity Verify the integrity of the request Check the agent’s authorization Complete mediation – answer all three questions for every request

3. Design Principles Open Design Principle – need help spotting security holes Minimize secrets: hard to keep them! Economy of mechanism: fewer things to get right Minimize common mechanism- fewer unintended communication paths Fail-safe defaults: most users won’t change them Least-privilege principle: limit the damage of an accident Complete mediation- check every operation

4. Authentication Establish the origin and integrity of the message Sign and Verify: Sender creates an authentication tag.[ T= sign(M, K1)] Receiver verifies it. [Result = Verify(M’, T’, K2)]

5. Implementation Cryptographic transformations used – without knowing K, it should be “impossible” to construct a different message and tag that verifies correctly. Algorithms are public, secret is just a key (longer keys harder to break) Shared key : Sign and verify using the same key Public-Key : Use private key to sign, public key to verify

6. Key Distribution Trusted physical delivery Use mutually-trusted third party “3” forms a certificate and Charles is CA.

7. Attacks on authentication systems Modifications to M and T Reordering M Extending M by appending information Splicing several messages and tags Attacks on cryptographic transformations Sign and verify construction is hard!

8. Confidentiality Encrypt – message to ciphertext Decrypt – ciphertext to plain text Can use shared key or public-key cryptography Combining authentication and confidentiality- encrypt and then sign the encrypted message