Key Management Session and Interchange Key Key Exchange Outline Session and Interchange Key Key Exchange Classical Cryptosystem Key Exchange and Authentication Kerberos Public key Cryptographic key Exchange and Authentication Cryptographic Key Infrasturcture Storing and Revoking Keys Digital Signature Classical Signatures Public Key Signatures

Session and Interchange Key Key Manegment refers to the distribution of keys: the Machanism used to bind identity to the key, generation , maintenance and revoking of such keys. Notation: X--> Y:{Z}k Session and Interchange Key An Interchange Key is a cryptographic key associated with a principal to a communication. A session key is a cryptographic key associated with the communication itself.

Key Exchange The goal of key Exchange is to enable Alice to communicate secretly to Bob, and vice versa, using a shared cryptographic key. Solution to this problem must meet following 3 criteria. 1. The key that Alice and Bob are to share cannot be transmitted in clear. 2. Alice and Bob may decide to trust third party (e.g. Cathy) 3. The cryptosystems and protocols are publicly Known. The Only secret data is to be the cryptographic keys involved.

Classical Cryptographic Key Exchange And Aythentication Suppose Alice and Bob wishes to communicate. They get the comman secret key from third party Cathy. 1. Alice ----> Cathy : { request for session key to Bob}K Alice 2. Cathy -----> Alice: { K Session } K Alice || { K session }KBob 3. Alice-------> Bob: { K Session} K Bob This protocol is having a problem that Bob does not know to whom he is talking . One such protocol is Needham-Schroeder Protocol.

Needham-Schroeder Protocol 1. Alice----> Cathy : { Alice|| Bob || rand1} 2. Cathy---->Alice:{ Alice || Bob || rand1|| K Session || {Alice || K Session } KBob} KAlice 3. Alice-----> Bob: {Alice || K Session } KBob 4. Bob----->Alice: { rand2} K Session 5. Alice---> Bob : { rand2-1} K Session

Public Key Cryptographic Key Exchange and Authentication Conceptually, public key cryptography makes exchanging keys very easy. 1. Alice -------> Bob :{ Ksession}eBob , Where, eBob is Bob's public key. Now Both can communicate using a classical cryptosystem. This protocol is having a flaw in it. If Alice do not have Bob's public key, she must get it from the public server, Peter.

2. Eve------> Peter: {send me Bob's Public key} Now, Attacker Eve can read Bob's message to Alice as given below: 1. Alice-----> Peter: {send me Bob's Public key} [intersepted by Eve] 2. Eve------> Peter: {send me Bob's Public key} 3. Peter----> Eve: eBob 4. Eve------> Alice: eEve 5. Alice-----> Bob: {kSession} eEve [intercepted by Eve] 6. Eve-------> Bob: {kSession} eBob Eve Now has the session key and can read any traffic between Alice and Bob. This is called Man-in-the-middle attack.