1. 1 Normal executable Infected executable Sequence of program instructions Entry Original program Entry Jump Replication and payload Viruses

2. 2 Firewall Bastion hosts Internal hosts Global Internet Second firewall Public hosts Protected enclave

3. 3 Communication Goals Remote authentication –Based on something you know Message confidentiality –Even if adversary eavesdrops Message integrity –Even if adversary intervenes Message non-repudiation –Even if sender changes her mind

4. 4 Very complicated encryption algorithm Message (bit string) Encrypted message (bit string) Key (bit string)

5. 5 Locking key (Identical) unlocking key Message Locking key (Different) unlocking key Symmetric lock and key Asymmetric lock and key

6. 6 Close and lock using sender’s locking key Recipient opens using unlocking key Put message in lockbox Transport to recipient Symmetric case Asymmetric case Locking and unlocking keys are different

7. 7 0 n-1 P = plaintext C = ciphertext Encrypt Decrypt RSA asymmetric algorithm C = P s mod n P = C t mod n t cannot be computed from (n,s) in reasonable time

8. 8 Confidentiality Bob must possess a secret not available to anyone else Alice (sender) Bob (recipient)

9. 9 Confidentiality (con’t) Bob must possess a secret not available to anyone else Alice (sender) Bob (recipient) Alice must be able to transform the message so that only the person possessing that secret can read it

10. 10 Confidentiality (con’t) Bob must possess a secret not available to anyone else Alice (sender) Bob (recipient) Alice must be able to transform the message so that only the person possessing that secret can read it or EncryptDecrypt

11. 11 Where to do encryption

12. 12 Non-repudiation Alice (sender) Bob (recipient) Alice must sign the message using a secret not revealed to anybody else Bob must be able to verify the signature using public information

13. 13 Authentication Alice (sender) Bob (recipient) Alice must possess a secret not available to anyone else

14. 14 Authentication (con’t) Alice (sender) Bob (recipient) Alice must possess a secret not available to anyone else Bob must be able to verify that Alice possesses that secret without Alice revealing it on the network or to Bob

15. 15 Authentication (con’t) Alice (sender) Bob (recipient) Alice must possess a secret not available to anyone else or Bob must be able to verify that Alice possesses that secret without Alice revealing it on the network (and possibly not to Bob) Challenge Response

16. 16 Digital certificate (ID,public_key) Encrypted with CA private_key CA_ID Certificate authority (CA) certifies (ID,key) binding

17. 17

18. 18 BobAlice CA  Alice convinces CA of her identity  Alice provides Bob with a replica of her digital certificate, which provides and certifies Alice’s public key  CA gives digital certificate and secret key to Alice  Bob verifies CA signature using CA public key

19. 19 Certificate authority Bank’s certificate issued by CA Merchant’s certificate issued by bank Verify signature Authority’s known public key Bank’s public key Verify signature Merchant’s public key Chain of trust

20. 20 CA Seller (server) Customer (client) Trusting CA public key, client can obtain authenticated public key of a seller Consumer electronic commerce

21. 21 Seller (server) Customer (client) Client can generate a random, secret “session key” and send confidentially to server Client can authenticate server using challenge response protocol

22. 22 Seller (server) Customer (client) Client and server can communicate confidentially