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# Part04 key exchange protocols

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### Part04 key exchange protocols

1. 1. 2/24/2012LOGO Key Exchange Protocols IT Faculty – DaLat University February - 2012 Encryption For a plaintext M, a crypto algorithm A and a cryptographic key K , the ciphertext M‟ is calculated as follows:  M‟ = A(K,M) = {M}K Perfect Encryptioni) Without the key K (in the case of a symmetric cryptosystem), or the matching private key of K (in the case of an asymmetric cryptosystem), the ciphertext {M}K does not provide any cryptanalytic means for finding the plaintext message M. 1
2. 2. 2/24/2012 Perfect Encryptionii) The ciphertext {M}K and maybe together with some known information about the plaintext message M do not provide any cryptanalytic means for finding the key K (in the case of a symmetric cryptosystem), or the matching private key of K (in the case of an asymmetric cryptosystem). The Dolev-Yao Threat Model In that model  Malice (can ): • can obtain any message passing through the network • is a legitimate user of the network, and thus in particular can initiate a conversation with any other user • will have the opportunity to become a receiver to any principal • can send messages to any principal by impersonating any other principal The Dolev-Yao Threat Model Malice (cannot ): • cannot guess a random number which is chosen from a sufficiently large space • without the correct secret (or private) key, cannot retrieve plaintext from given ciphertext, and cannot create valid ciphertext from given plaintext, wrt. the perfect encryption algorithm • cannot find the private component, i.e., the private key, matching a given public key • while he may have control of a large public part of our computing and communication environment, in general, he is not in control of many private areas of the computing environment, such as accessing the memory of a principals offline computing device 2
3. 3. 2/24/2012 The Dolev-Yao Threat ModelSuppose that two principals Alice and Bob wish to communicate with each other in a secure mannerƒSuppose also that Alice and Bob have never met before, so they do not already share a secret key between them and do not already know for sure the other partys public keyƒThen how can they communicate securely over completely insecure networks? The Dolev-Yao Threat Model The Dolev-Yao Threat Model 3
4. 4. 2/24/2012 The Dolev-Yao Threat Model1. Alice generates K at random, creates {K}KAT, and sends to Trent: Alice, Bob, {K}KAT2. Trent finds keys KAT, KBT, decrypts {K}KAT to reveal K, creates {K}KBT and sends to Bob: Alice, Bob, {K}KBT3. Bob decrypts {K}KBT to reveal K, forms and sends to Alice: {Hello Alice, Im Bob!}K. The Dolev-Yao Threat ModelProblem: K created by Alice is not strong enough  Bob is unhappy about thisNew protocol: “Session key from Trent” 4
5. 5. 2/24/2012 An attack The Dolev-Yao Threat ModelThis attack will only succeed if Malice is a legitimate user known to Trent  a realistic assumption – an insider attacker is often more of a threat than outsiders A fix: 1. Alice sends to Trent: Alice, {Bob}KAT; 5
6. 6. 2/24/2012 The Dolev-Yao Threat Model Another attack1. Alice sends to Trent: Alice, {Bob}KAT;1. Malice("Alice") sends to Trent: Alice, {Malice}KAT The Dolev-Yao Threat ModelInstead, Malice can alter the message from Trent to Alice (message line 2 in Protocol "Session Key From Trent) into the following: Malice("Trent") sends to Alice: {K}KAT The Dolev-Yao Threat ModelMalice can alter some protocol messages without detection.This suggests that the protocol needs a security service which can guard against tampering of messages. This brings us to the following security service “Protocol with message authentication” 6
7. 7. 2/24/2012 Protocol with Message AuthenticationMalice has always been able to alter some protocol messages without detection None of the protocols designed so far has provided any cryptographic protection against message alteration. Thus, one way to fix these protocols is to provide such protection Protocol with Message AuthenticationThe protection should enable legitimate principals who have the right cryptographic keys to detect any unauthorized alteration of any protected protocol messages. Such protection or security service is called message authentication (is also called data integrity) Protocol with Message AuthenticationWe observe that Malice‟s alteration of the protocol messages has caused the following two effects:  a session key is shared between wrong principals  a wrong session key get established message authentication protection should provide a cryptographic binding between the session key to be established and its intended users. 7
8. 8. 2/24/2012 Protocol with Message Authentication This lead to a new protocol: “Message Authentication”, where the identities of Alice and Bob are included in the encrypted message parts sent by Trent. Perfect encryption for message authentication service Perfect Encryption with Notation {M}K (for message authentication service) iii) Without the key K, even with the knowledge of the plaintext M, it should be impossible for someone to alter {M}K without being detected by the recipient during the time of decryption 8
9. 9. 2/24/2012Attack on Protocol "Message Authentication"Problem: message replay attack.ƒMalice intercepts Alices request, then:  1. Alice sends to Malice(“Trent”)  2. Malice(“Trent”) sends to Alice: {Bob,K}KAT ,{Alice,K} KBTƒTwo ciphertext blocks containing K are a replay of old messages which Malice has recorded from a previous run of the protocol (between Alice and Bob)Attack on Protocol "Message Authentication"This attack will cause Alice & Bob to reuse the old session key K„ƒSince K is old, it may be possible for Malice to have discovered its value Protocol “challenge-response" Using this method Alice will generate a new random number NA at the start of the protocol and send this to Trent with the request for a new session keyThe random number NA created by Alice for enabling the challenge-response mechanism is called a nonce which stands for a number used once 9
10. 10. 2/24/2012 Protocol “challenge-response" An attack on the Needham-Schroeder symmetric key authentication protocol:  Bob thinks he is sharing a new session key with Alice while actually the key is an old one and may be known to Malice 10
11. 11. 2/24/2012 SolutionA Protocol Using Public-key Cryptosystems Called the Needham-Schroeder Public- key Authentication Protocol Alice public key is KA, Alice private key is K-1A 11
12. 12. 2/24/2012A Protocol Using Public-key CryptosystemsAn attack on public key authentication protocol  Found after 17 years  Result: Bob thinks he is sharing secrets NA, NB with Alice while actually sharing them with Malice  Method: Malice makes use of Alice as she is trying to establish a connection with himA Protocol Using Public-key Cryptosystems Malice may ask for a session key and Bob may believe that this request is from AliceƒThen, an example if Bob is a bank, Malice(“Alice”) sends to Bob the following command:  { NA, NB, Transfer 5000\$ from my account to Malices“}KB 12
13. 13. 2/24/2012A Protocol Using Public-key CryptosystemsIt is fairly easy to change the protocol so as to prevent the attack. If we include the responders identity in message 6 of the protocol  2-6. Bob sends to Malice("Alice"): {Bob, NA, NB}KAA Protocol Using Public-key Cryptosystems This is what we are using nowadays 13