Protocol Analysis

Cryptographic Protocols Two or more parties Communication over insecure network Cryptography used to achieve goal Exchange secret keys Verify identity (authentication) Secure transaction processing CSCE 522 - Farkas 2

Emerging Properties of Protocols Greater interoperation Negotiation of policy Greater complexity Group-oriented protocols Emerging security threats CSCE 522 - Farkas 3

Protocols Good protocol characteristics: Established in advance Mutually subscribed Unambiguous Complete CSCE 522 - Farkas 4

Symmetric-Key Distribution: Symmetric-Key Techniques (repeat from lecture on 05/13/2014) Symmetric-Key without Server Symmetric-Key with Server CSCE 522 - Farkas 5

Symmetric-Key Distribution without Server Change encryption key E(Knew,K), where Knew is the session key, K is the master key New key Ciphertext C New key Encryption Decryption Sender Recipient K CSCE 522 - Farkas

Symmetric-Key Distribution with Server Knows KO and KR Server Originator (O,R,IO) E([(IO,R,KOR,E((KOR,O), KR)], KO) E((KOR,O), KR) Recipient Decrypts with KR Knows KOR Decrypts with KO Knows KOR Does not know E((KOR,O), KR) CSCE 522 - Farkas CSCE 522 - Farkas

Symmetric-Key Distribution: Public-Key Techniques Simple secret key distribution – insecure Secret key distribution with confidentiality and authentication Diffie-Hellman Key Exchange CSCE 522 - Farkas 8

Simple secret key distribution Public key of S KE-S ||ID-S 2. E KE-S(Ksession) Sender Recipient Secret Session key Vulnerable to active attack! HOW? CSCE 522 - Farkas 9

With confidentiality and authentication Assume: KE-R and KE-S are known in advance Nonce E KE-R[N1||ID-S] 2. E KE-S[N1||N2] 3. E KE-R[N2] 4. E KE-R E KD-S(Ksession) Sender Recipient Question: Why do we need reliable distribution of public keys? CSCE 522 - Farkas 10

Diffie-Hellman Key Exchange Proposed in 1976 First public key algorithm Allows group of users to agree on secret key over insecure channel Cannot be used to encrypt and decrypt messages CSCE 522 - Farkas 11

Diffie-Hellman Key Exchange Protocol for A and B want to agree on shared secret key: A and B agree on two large numbers n and g, such that 1<g<n A chooses random x and computes X=gx mod n and sends X to B B chooses random y and computes Y=gy mod n and sends Y to A A computes Yx mod n = gyx mod n B computer Xy mod n = gyx mod n Secret key: gyx mod n CSCE 522 - Farkas 12

Diffie-Hellman Key Exchange Requires no prior communication between A and B Security depends on difficulty of computing x given X=gx mod n Choices for g and n are critical: both n and (n-1)/2 should be prime, n should be large Susceptible to intruder in the middle attack (active intruder) CSCE 522 - Farkas 13

Intruder in the Middle Attack Eve Bob Alice Hi Alice, I’m Bob. Hi Alice, I’m Bob. Hi Bob, I’m Alice. Hi Bob, I’m Alice. Intruder and Bob Uses Diffie-Hellman To agree on key K. Intruder and Alice Uses Diffie-Hellman To agree on key K’. Question: the attacker may want to have K and K’ be the same, Why? CSCE 522 - Farkas 14

Public-Key Distribution Without server Broadcasting - insecure Publicly available directory With trusted server Public key distribution center Certificates CSCE 522 - Farkas 15

Public announcement KE-J.S. KE-J.S. KE-J.S. KE-J.S. John Smith KE-J.S. Question: What are the vulnerabilities of this approach? CSCE 522 - Farkas 16

Publicly available directory Better but not good enough  Directory could Be compromised Public Key Directory KE-J.S. KE-M.R.. John Smith Mary Rose CSCE 522 - Farkas 17

Public-key authority Public-Key Authority Sender Recipient Question1: What should the Authority, the Sender and the Recipient know before communication? Public-Key Authority 1. Request || Time1 4. Request || Time2 2. EKD-Auth[KE-R||Request||Time1] 5. EKD-Auth[KE-S||Request||Time2] 3. EKE-R(ID-S||N1) Sender 6. EKE-S(N1||N2) Recipient 7. EKE-R(N2) Exercise: After each message, show what the recipient of the message can do and what the Recipient know. CSCE 522 - Farkas 18

Public-key certificates Authority KE-R KE-S C-S=EKD-CAuth[Time1,ID-S,KE-S] CR=EKD-CAuth[Time2,ID-R,KE-R] 1. C-S Sender 2. C-R Recipient CSCE 522 - Farkas 19

Certificates Guarantees the validity of the information Establishing trust Public key and user identity are bound together, then signed by someone trusted Need: digital signature CSCE 522 - Farkas 20

Digital Signature Need the same effect as a real signature Un-forgeable Authentic Non-alterable Not reusable CSCE 522 - Farkas 21

Digital signature Direct digital signature: public-key cryptography based Arbitrated digital signature: Conventional encryption: Arbiter sees message Arbiter does not see message Public-key based CSCE 522 - Farkas 22

Digital Signatures in RSA Insecure channel Sign Verify Plaintext Plaintext Signed plaintext Decryption Alg. Encryption Alg. Recipient Sender S’s private key S’s public key (need reliable channel) CSCE 522 - Farkas 23

Lecture 8-9 CSCE 522 - Farkas Secret key (fast) Public key (slow) Hash   Secret key (fast) Public key (slow) Hash Confidentiality Integrity Availability Authentication (peers only) (third party) Non-repudiation Lecture 8-9 CSCE 522 - Farkas

Lecture 8-9 CSCE 522 - Farkas Secret key Public key Nonce Time stamp   Secret key Public key Nonce Time stamp Passive Eavesdropping Traffic monitoring Active Disruption Modification Fabrication Replay Traffic collection Lecture 8-9 CSCE 522 - Farkas

Protocol Analysis Exercise 1. Assume that Jane and Paul want to efficiently send very large files to each other. They also want to provide integrity verification, third- party message authentication (i.e., a third party can verify who the originator of the message is), and limit the scope of a compromise (i.e., providing forward secrecy). You can assume that Jane and Paul have public and secret key encryption capabilities, can generate a hash function, and they have a shared secret key K0 established before the communication. They do not have access to a mutually trusted server, and no other keys but K0 are known at the beginning of the communication. Propose a security protocol to establish necessary keys and show how Jane can send a file to Paul. Lecture 8-9 CSCE 522 - Farkas

Exercise 2. Message authentication and key agreement Alice wants to establish a secure communication with Bob. They agree to user the Yahalom protocol for mutual authentication and key agreement. The protocol uses symmetric key encryption only. Alice has a secret key shared with a trusted third party Server, KA and, similarly, Bob has a secret-key shared with Server, KB. NA and NB are nonces generated by Alice and Bob, respectively. E(M, K) indicates encryption of message M with key K, “||” means concatenation of messages. Explain after each protocol step what the recipient of the message knows based on the message and the properties of the encryption and what he/she is capable of doing. For example, Lecture 8-9 CSCE 522 - Farkas

Exercise 2. Message1: Alice  Server: IDA || E(“request for session key to Bob”, KA) Server: The server sees that that claimed sender of the message is Alice. The server can decrypt the message using KA that is shared between Alice and the Server. The message must have been sent by Alice because KA is only known by Alice and the server. The server knows that Alice is requesting a session key to be used by Alice and Bob. The server can generate a session key KS to be used by Alice and Bob and send the key to … Lecture 8-9 CSCE 522 - Farkas

Exercise 2. Message1: Alice  Bob: IDA || NA Bob knows/can do Message2: Bob  Server: IDB || E[(IDB || NA || NB), KB] Server knows/can do Message3: Server  Alice: E[(IDB || KS || NA || NB), KA] || E[(IDA || KS), KB] Alice knows/can do Message4: Alice  Bob: E[(IDA || KS), KB] || E(NB, KS)] Lecture 8-9 CSCE 522 - Farkas

Exercise 3. Secure communication Consider the following protocol. Ann wants to send a message M securely to Bob but there is no shared secret key between Ann and Bob, Ann does not even know Bob’s public key. However, using the properties of RSA (in particular the commutative property), Ann proposes the following protocol, where E(M, K) indicates encryption/decryption of message M with key K, “||” means concatenation of messages, KpubA means the public key of A, KprivA means private key of A. Lecture 8-9 CSCE 522 - Farkas

Exercise 3. Message1: Ann  Bob: IDA || E(M, KpubA) Message 2: Bob  Ann: IDB || E[(E(M, KpubA)), KpubB) Message3: Ann  Bob: IDA || E(M, KpubB) Show a man-in-the-middle attack against the above protocol. Lecture 8-9 CSCE 522 - Farkas

Next class Review for Test 1 Lecture 8-9 CSCE 522 - Farkas 32