Foundations of Computer SecurityLecture 58: Cryptographic Protocols: Abstract ViewDr. Bill YoungDepartment of Computer SciencesUniversity of Texas at AustinLecture 58: 1 Cryptographic Protocols: Abstract ViewCryptographic ProtocolsA protocol involves a sequence of message exchanges of the form:A → B : Mmeaning that principal A sends to principal B the message M.There’s a “temporal” aspect to protocols. Until and unless Breceives the message, he can’t respond to it.In general, B won’t be expecting the message unless he has alreadyparticipated in earlier steps of the protocol.Lecture 58: 2 Cryptographic Protocols: Abstract ViewTaking an Abstract ViewThere is a lot involved in making a protocol work, particularly atthe implementation level.We’ll ignore issues like:What are the mechanisms of message transmission?How does a principal know when decryption has succeeded?How can you reliably parse a message of multiple components?If a message contains the name of a principal, what is theform of that name?How are public keys maintained and distributed?Those are all important issues, but we want to look at protocolsabstractly.Lecture 58: 3 Cryptographic Protocols: Abstract ViewProtocol QuestionsAn analysis of any protocol attempts to answer the following typesof questions:What are the goals of the protocol?What does the protocol actually achieve?Does it achieve its stated objective?Does it include unnecessary steps or messages?What assumptions are made?Does it encrypt items that could be sent in the clear?Is it susceptible to attack? What would an attack look like?Lecture 58: 4 Cryptographic Protocols: Abstract ViewLessonsWe want to look at protocols abstractly and ignore issues atthe implementation level.A standard set of questions can be asked of any cryptographicprotocol.Next lecture: Attacks on Cryptographic ProtocolsLecture 58: 5 Cryptographic Protocols: Abstract
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