EEC 693/793 Special Topics in Electrical Engineering Secure and Dependable Computing Lecture 7 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University

2 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Outline Announcement –Feb 26, Monday, 3pm: CSU Data Center Tour; 10th floor of Rhodes Tower –Midterm#1: March 20 th, 4-6pm (moved from March 27 th ) Secure hash functions Message authentication code Public key management Authentication protocols

3 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Message Digests Message digest (MD): using a one-way hash function that takes an arbitrarily long piece of plaintext and from it computes a fixed-length bit string Requirement on the hash function: –Given P, it is easy to compute MD(P) –Given MD(P), it is effectively impossible to find P –Given P no one can find P’ such that MD(P’) = MD(P) –A change to the input of even 1 bit produces a very different output

4 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Digital Signatures Using Message Digests

5 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao MD5 One of the most widely used hash functions MD5 is the fifth in a series of message digests designed by Ronald Rivest (1992) It operates by mangling bits in a sufficiently complicated way that every output bit is affected by every input bit MD5 generates a 128-bit fixed value

6 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao SHA-1 SHA-1: Secure Hash Algorithm 1, developed by National Security Agency (NSA) and blessed by NIST. It generates 160-bit message digest Use of SHA-1 and RSA for signing nonsecret messages

7 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Message Authentication Code MACs are used between two parties that share a secret key in order to validate information transmitted between these parties The MAC mechanism that is based on cryptographic hash functions is called HMAC: –Append the key to the plaintext and generate a digest using a hash function –Ship the plaintext together with the digest

8 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Management of Public Keys Problem statement Certificates X.509 Public key infrastructure

9 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Problems with Public-Key Management If Alice and Bob do not know each other, how do they get each other’s public keys to start the communication process ? –It is essential Alice gets Bob’s public key, not someone else’s A way for Trudy to subvert public-key encryption

10 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Certificates Certification Authority (CA): an organization that certifies public keys –It certifies the public keys belonging to people, companies, or even attributes –CA does not need to be on-line all the time (in ideal scenarios) A possible certificate and its signed hash

11 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao X.509 Devised and approved by ITU The basic fields of an X.509 certificate

12 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Public-Key Infrastructures A Public-Key Infrastructure (PKI) is needed for reasons of –Availability, Scalability, Ease of management A PKI has multiple components –Users, CAs, Certificates, Directories A PKI provides a way of structuring these components and define standards for the various documents and protocols –A simple form of PKI is hierarchical CAs

13 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Public-Key Infrastructures Hierarchical PKI A chain of trust/certification path: A chain of certificates going back to the root

14 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Public-Key Infrastructures Revocation: sometimes certificates can be revoked, due to a number of reasons Reinstatement: a revoked certificate could conceivably be reinstated Each CA periodically issues a CRL (Certificate Revocation List) giving the serial numbers of all certificates that it has revoked –A user who is about to use a certificate must now acquire the CRL to see if the certificate has been revoked Having to deal with revocation (and possibly reinstatement) eliminates one of the best properties of certificates, namely, that they can be used without having to contact a CA

15 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authentication Protocols Authentication is the technique by which a process verifies that its communication partner is who it is supposed to be and not an imposter –Verifying the identity of a remote process in the face of a malicious, active intruder is surprisingly difficult and requires complex protocols based on cryptography Not to be confused with authorization –Authorization is concerned with what process is permitted to do

16 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao General Model for Authentication Protocols Alice starts out by sending a message either to Bob or to a trusted KDC (Key Distribution Center), which is expected to be honest Several other message exchanges follow in various directions Trudy may intercept, modify, or replay the messages transmitted to trick Alice and Bob When the protocol has been completed, Alice is sure she is talking to Bob and Bob is sure he is talking to Alice

17 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao General Model for Authentication Protocols In general, the authentication process also produce a secret session key for use in the upcoming conversation –For each new connection, a new, randomly-chosen session key should be used Public-key cryptography is widely used for the authentication protocols themselves and for establishing the session key

18 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Why Use a Session Key For performance reasons, symmetric key encryption is much faster than public-key encryption To minimize the amount of traffic that gets sent with the users’ secret keys or public keys To reduce the amount of ciphertext an intruder can obtain To minimize the damage done if a process crashes and its core dump falls into the wrong hands. Hopefully, the only key present then will be the session key –All the permanent keys should have been carefully zeroed out after the session was established

19 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authentication Protocols Authentication Based on a Shared Secret Key Establishing a Shared Key: Diffie-Hellman Authentication Using a Key Distribution Center Authentication Using Public-Key Cryptography

20 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authentication Based on a Shared Secret Key Two-way authentication using a challenge- response protocol –Challenge-response: one party sends a random number to the other, who then transforms it in a special way and then returns the result –Nonces: random numbers used just once in challenge-response protocols –Assume that Alice and Bob already share a secret key, K AB

21 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authentication Based on a Shared Secret Key

22 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authentication Based on a Shared Secret Key A shortened two-way authentication protocol. Is this new protocol an improvement over the original one ? –It is shorter –But it is also wrong –Under certain circumstances, Trudy can defeat this protocol by using what is known as a reflection attack

23 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Reflection Attack The reflection attack: Trudy can break it if it is possible to open multiple sessions with Bob at once This attack can be defeated by encrypting R B with K AB in message 2

24 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao General Rules for Authentication Protocols Design Rule#1: Have the initiator prove who she is before the responder has to –In the previous case, Bob gives away valuable information before Trudy has to give any evidence of who she is Rule#2: Have the initiator and responder use different keys for proof, e.g., K AB and K' AB Rule#3: Have the initiator and responder draw their challenges from different sets –E.g., the initiator uses even numbers, the responder uses odd numbers Rule#4: Be aware of parallel sessions (no info flows across different sessions)

25 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Establishing a Shared Key: The Diffie-Hellman Key Exchange A protocol that leads to the establishment of a shared secrete key is called key agreement protocol or key exchange protocol Diffie-Hellman key exchange –Two large numbers, n and g, where n is a prime, (n - 1)/2 is also a prime and certain conditions apply to g

26 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Establishing a Shared Key: The Diffie-Hellman Key Exchange

27 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Establishing a Shared Key: The Diffie-Hellman Key Exchange Example: n = 47 and g = 3. Alice picks x = 8 and Bob picks y = 10. Both of these are kept secret –Alice's message to Bob is (47, 3, 28) because 3 8 mod 47 is 28. Bob's message to Alice is (17) –Alice computes 17 8 mod 47, which is 4 –Bob computes mod 47, which is 4 –Alice and Bob have independently determined that the secret key is now 4 –Trudy has to solve the equation 3 x mod 47 = 28

28 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Establishing a Shared Key: The Diffie-Hellman Key Exchange The man-in-the-middle attack –When Bob gets (47, 3, 28), how does he know it is from Alice and not from Trudy? There is no way he can know –Trudy can exploit this fact to deceive both Alice and Bob

29 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Man-In-The-Middle Attack A man-in-the-middle attack (MITM) is an attack in which an attacker is able to read, insert and modify at will, messages between two parties without either party knowing that the link between them has been compromised The attacker must be able to observe and intercept messages going between the two victims

30 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authentication Using a Key Distribution Center Each user has a single key shared with the KDC. Authentication and session key management now goes through the KDC The following protocol is subject to replay attack

31 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Needham-Schroeder Authentication Protocol Needham-Schroeder protocol: a multi-way challenge-response protocol –By having each party both generate a challenge and respond to one, the possibility of any kind of replay attack is eliminated

32 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Needham-Schroeder Authentication Protocol Message 1: R A is a nonce Message 2: –K B (A, K S ) is ticket Alice will send to Bob –R A : so that message 2 is not a replay –B: so that if Trudy replaces B with her id in message 1, it will be detected –Ticket is encrypted using Bob’s key K B so that Trudy cannot replace it with something else on the way back to Alice

33 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Needham-Schroeder Authentication Protocol Message 3: a new nonce R A2 is used Message 4: Bob sends back K S (R A2 -1) instead of K S (R A2 ) so that Trudy cannot steal K S (R A2 ) from message 3 and replay it here Message 5: to convince Bob he is talking to Alice and no replays are being used

34 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authentication Using Public-Key Cryptography

35 Spring 2007EEC693: Secure & Dependable ComputingWenbing Zhao Authorization Authentication: Verify the claim that a subject says it is S: verifying the identity of a subject Authorization: Determining whether a subject is permitted certain services from an object Note: authorization makes sense only if the requesting subject has been authenticated