1. v0.0CPSC415 Biometrics and Cryptography1 Placement of Encryption Function Lecture 3

2. v0.0CPSC415 Biometrics and Cryptography2 Points of Vulnerability Adversary can eavesdrop from a machine on the same LAN Adversary can eavesdrop by dialing into communication server Adversary can eavesdrop by gaining physical control of part of external links –twisted pair, coaxial cable, or optical fiber –radio or satellite links

3. v0.0CPSC415 Biometrics and Cryptography3

4. v0.0CPSC415 Biometrics and Cryptography4 Confidentiality using Symmetric Encryption have two major placement alternatives link encryption –encryption occurs independently on every link –All traffic over all communication links is secured –implies must decrypt traffic between links because the switch must read the address in the packet header –Each pair of nodes that share a unique key, with a different key used on each link, many keys. –Message is vulnerable at each switch –If working with a public network, the user has not control over the security of the nodes

5. v0.0CPSC415 Biometrics and Cryptography5 Confidentiality using Symmetric Encryption end-to-end encryption –encryption occurs between original source and final destination –need devices at each end with shared keys –Secure the transmission against attacks on the network links or switches –“end-to-end principle” –What part of each packet will the host encrypt? Header or user data? –A degree of authentication, only alleged sender shares the relevant key

6. v0.0CPSC415 Biometrics and Cryptography6

7. v0.0CPSC415 Biometrics and Cryptography7 Placement of Encryption Can place encryption function at various layers in OSI Reference Model –link encryption occurs at layers 1 or 2 –end-to-end can occur at layers 3, 4, 6, 7 If move encryption toward higher layer –less information is encrypted but is more secure –application layer encryption is more complex, with more entities and need more keys

8. v0.0CPSC415 Biometrics and Cryptography8 Scope of Encryption

9. v0.0CPSC415 Biometrics and Cryptography9 Traffic Analysis is monitoring of communications flows between parties –useful both in military & commercial spheres –can also be used to create a covert channel link encryption obscures header details –but overall traffic volumes in networks and at end-points is still visible traffic padding can further obscure flows –but at cost of continuous traffic

10. v0.0CPSC415 Biometrics and Cryptography10 Traffic Analysis when using end-to-end encryption must leave headers in clear –so network can correctly route information hence although contents protected, traffic pattern flows are not ideally want both at once –end-to-end protects data contents over entire path and provides authentication –link protects traffic flows from monitoring

11. v0.0CPSC415 Biometrics and Cryptography11 Key Distribution Center

12. v0.0CPSC415 Biometrics and Cryptography12 Symmetric Cryptographic System key encryptiondecryption M K cryptanalysis MKMK C M Secure channel Alice Bob Eve Ciphertext C = E K (M); Plaintext M = E K -1 (C) One of the greatest difficulties: key management Algorithms: DES, CAST, IDEA, RC2/4/5 (Rivest’s Code), AES, … Alice: sender Bob: receiver Eve: eavesdropper / Oscar : opponent Alice and Bob are the celebrities in cryptography.

13. v0.0CPSC415 Biometrics and Cryptography13 Symmetric Key Management Each pair of communicating entities needs a shared key –Why? –For a n-party system, there are n(n-1)/2 distinct keys in the system and each party needs to maintain n-1 distinct keys. How to reduce the number of shared keys in the system –Centralized key management –Public keys K1K1 K4K4 K2K2 K3K3 K5K5 K6K6 K7K7 K8K8 K9K9 K 10

14. v0.0CPSC415 Biometrics and Cryptography14 Centralized Key Management Online Central Server Only n keys, instead of n(n-1)/2 in the system. Central server may become the single-point-of-failure of the entire system and the performance bottleneck. AliceBob K1K1 K2K2 session key

15. v0.0CPSC415 Biometrics and Cryptography15 Key Distribution symmetric schemes require both parties to share a common secret key issue is how to securely distribute this key often secure system failure due to a break in the key distribution scheme

16. v0.0CPSC415 Biometrics and Cryptography16 Key Distribution given parties A and B have various key distribution alternatives: 1.A can select key and physically deliver to B 2.third party can select & deliver key to A & B 3.if A & B have communicated previously can use previous key to encrypt a new key 4.if A & B have secure communications with a third party C, C can relay key between A & B

17. v0.0CPSC415 Biometrics and Cryptography17 Key Distribution Scenario

18. v0.0CPSC415 Biometrics and Cryptography18 Key Distribution Issues hierarchies of KDC’s required for large networks, but must trust each other session key lifetimes should be limited for greater security controlling purposes keys are used for –lots of keys to keep track of –binding management information to key

19. v0.0CPSC415 Biometrics and Cryptography19 Key Distribution Center (KDC) Alice knows R1 Bob knows to use R1 to communicate with Alice Alice and Bob communicate: using R1 as session key for shared symmetric encryption Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? KDC generates R1 K B-KDC (A,R1) K A-KDC (A,B) K A-KDC (R1, K B-KDC (A,R1) )