chapter-5 - CS549: Cryptography and Network Security by...

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Cryptography and Network Security 1 CS549: Cryptography and Network Security © by Xiang-Yang Li Department of Computer Science, IIT
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Cryptography and Network Security 2 Notice© This lecture note (Cryptography and Network Security) is prepared by Xiang-Yang Li. This lecture note has benefited from numerous textbooks and online materials. Especially the “Cryptography and Network Security” 2 nd edition by William Stallings and the “Cryptography: Theory and Practice” by Douglas Stinson. You may not modify, publish, or sell, reproduce, create derivative works from, distribute, perform, display, or in any way exploit any of the content, in whole or in part, except as otherwise expressly permitted by the author. The author has used his best efforts in preparing this lecture note. The author makes no warranty of any kind, expressed or implied, with regard to the programs, protocols contained in this lecture note. The author shall not be liable in any event for incidental or consequential damages in connection with, or arising out of, the furnishing, performance, or use of these.
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Cryptography and Network Security 3 Cryptography and Network Key Management and generation Xiang-Yang Li
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Cryptography and Network Security 4 Key Exchange Public key systems are much slower than private key system Public key system is then often for short data Signature, key distribution Key distribution One party chooses the key and transmits it to other user Key agreement Protocol such two parties jointly establish secret key over public communication channel Key is the function of inputs of two users
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Cryptography and Network Security 5 Distribution of Public Keys can be considered as using one of: Public announcement Publicly available directory Public-key authority Public-key certificates
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Cryptography and Network Security 6 Public Key Management Simple one: publish the public key Such as newsgroups, yellow-book, etc. But it is not secure, although it is convenient Anyone can forge such a announcement Ex: user B pretends to be A, and publish a key for A Then all messages sent to A, readable by B! Let trusted authority maintain the keys Need to verify the identity, when register keys User can replace old keys, or void old keys
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Cryptography and Network Security 7 Possible Attacks Observe all messages over the channel So assume that all plaintext messages are available to all Save messages for reuse later So have to avoid replay attack Masquerade various users in the network So have to be able to verify the source of the message
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Cryptography and Network Security 8 Public Announcement users distribute public keys to recipients or broadcast to community at large eg. append PGP keys to email messages or post to news groups or email list major weakness is forgery anyone can create a key claiming to be someone else and broadcast it until forgery is discovered can masquerade as claimed user
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chapter-5 - CS549: Cryptography and Network Security by...

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