95-702 Distributed Systems 1!Master of Information System Management 95-702 Distributed Systems Lecture 14: Some Important Cryptographic Protocols95-702 Distributed Systems 2!Master of Information System Management This Week’s Topics • Secure Voting • Cryptographic notation • Four Scenarios from Colouris • Needham Schroeder • Kerberos • SSL • Using SSL Sockets95-702 Distributed Systems 95-702 Distributed Systems 3 Goals Of Secure Voting • Only Authorized Voters Can Vote • No one can vote more than once • No one can determine for whom anyone else voted • No one can duplicate anyone else’s vote • No one can change anyone else’s vote without being discovered • Every voter can make sure that his vote has been taken into account in the final tabulation.95-702 Distributed Systems 95-702 Distributed Systems 4 First Attempt • Each voter encrypts his vote with the public key of a Central Tabulating Facility (CTF) • Each voter send his vote in to the CTF • The CTF decrypts the votes, tabulates them, and makes the results public • What are some problems with this protocol?95-702 Distributed Systems 95-702 Distributed Systems 5 Second Attempt • Each voter signs his vote with his private key • Each voter encrypts his signed vote with the CTF’s public key • Each voter send his vote to the CTF • The CTF decrypts the votes, checks the signature, tabulates the votes and makes the results public • What are some problems with this protocol?95-702 Distributed Systems 6!Master of Information System Management Cast of Characters Alice! First participant!Bob! Second participant!Carol! Participant in three- and four-party protocols!Dave! Participant in four-party protocols!Eve! Eavesdropper!Mallory! Malicious attacker!Sara! A server!95-702 Distributed Systems 7!Master of Information System Management Cryptography Notation KA! Alice’s key that she keeps secret.!KB! Bob’s key that he keeps secret.!KAB! Secret key shared between Alice and Bob!KApriv! Alice’s private key (known only to Alice in asymmetric key crypto)!KApub! Alice’s public key (published by Alice for all to read)!{!M!}!K! Message! M! encrypted with key ! K![!M!]K! Message !M! signed with key! K!95-702 Distributed Systems 8!Master of Information System Management Categories of Encryption Algorithms Symmetric key encryption. Also called secret key crypto. Alice sends {M}Kab and Bob can read it. Bob knows Kab. Asymmetric key encryption. Also called public key crypto. Alice sends {M}KBpub and Bob can read it. Bob knows KBpriv. Public key encryption is typically 100 to 1000 times slower than secret key encryption.95-702 Distributed Systems 9!Master of Information System Management Scenario 1 Goal: Alice and Bob want to exchange messages using a shared and secret symmetric key. Alice and Bob share KAB. Alice computes E(KAB,Mi) for each message i. She sends these to Bob. Bob uses D(KAB, {Mi} KAB ) and reads each Mi. Problems: How do Bob and Alice communicate the key KAB? How does Bob know that {Mi} KAB isn’t a replay of an old message?95-702 Distributed Systems 10!Master of Information System Management Scenario 2 Goal: Authenticate Alice allowing her to access files held by Bob. Alice asks Sarah for a ticket to talk to Bob. Sarah knows Alice’s password so she can compute KA. Sarah send to Alice {{Ticket}KB,KAB}KA. A challenge! Alice knows her password and is able to compute KA. Note that the password is never placed on the network. Alice is able to compute {Ticket}KB and KAB. How? Alice sends a read request to Bob. She sends {Ticket}KB,Alice,Read. Another challenge! Bob uses KB to read the content of the Ticket. The Ticket is KAB,Alice. Bob and Alice then use this session key to communicate. Problems: Old tickets may be replayed by Mallory. Suppose she has an old session key. Does not scale well : Sarah must know KA, KB ….95-702 Distributed Systems 11!Master of Information System Management 11!Master of Information System Management Scenario 3 Goal: Non-repudiation. Alice signs a digital message M. She computes a digest of M, Digest(M). If the Digest method is a good one, it is very difficult to find another message M’ so that Digest(M) == Digest(M’). Alice makes the following available to the intended users: M,{Digest(M)}KApriv. Bob obtains the signed document, extracts M and computes Digest(M). Bob decrypts {Digest(M)}KApriv using KApub and compares the result with his calculated Digest(M). If they match, the signature is valid. Problems: Suppose Alice releases her private key to the world. She can now deny that she signed the message.95-702 Distributed Systems 12!Master of Information System Management Scenario 4 Bob and Alice wish to establish a shared secret KAB. Alice uses a key distribution service to get Bob’s public key. This key comes in a certificate. So, Bob’s public key has been signed by a trusted third party, Trent. Alice verifies that Trent signed the public key KBpub. Alice generates KAB and encrypts it with KBpub. Bob has many public keys and so Alice sends a key name along as well. Alice sends key name, {KAB}KBpub. Bob uses the key name to select the correct private key and computes {{KAB}KBpub} KBpriv == KAB. Problem: The man in the middle attack may be used when Alice first contacts the key distribution service. Mallory may return his own public key (also signed by Trent).95-702 Distributed Systems 13!Master of Information System Management Alice’s Bank Account Certificate 1. !Certificate type!:! Account number!2. !Name!:! Alice!3. !Account!:! 6262626!4. !Certifying authority!:! Bob’s Bank!5. !Signature!:! {Digest(field 2 + field 3)}!KBpriv!Quiz: What is being certified? How would you determine if Bob really signed this certificate?95-702 Distributed Systems 14!Master of Information System Management Public-Key Certificate for Bob’s Bank 1. !Certificate type!:! Public key!2. !Name!:! Bob’s Bank!3. !Public key!:! KBpub!4. !Certifying authority!:! Fred – The Bankers Federation!5. !Signature!:!{Digest(field 2 + field 3)}!KFpriv!Quiz: What is being certified? How would you determine if Fred really signed this public key certificate?95-702 Distributed Systems 15!Master of Information System Management Digital Signatures With Public Keys {h}KpriMSigningVerifyingE(Kpri, h)128 bitsH(M)hMhH(doc)D(Kpub,{h}) {h}Kprih'h =
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