Topics in Cryptography - Speaker Deck

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Special Topics in Cryptography Mohammad Mahmoody

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Cryptography’s main goals

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Secure communication

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Historic Ciphers

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Privacy / confidentiality

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Integrity / Authentication

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Integrity / Authentication

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Not loosing in chess to the grand master

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Tool: Pseudorandom (number) generators

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Public Key Encryption Revolution (70’s)

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Four main problems in cryptographic

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Main tool for public-key cryptography: Number theory and structured mathematics

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Beyond Encryption and Authentication

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Multi-party Computation

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Yao’s Billionaires Problem

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Poker over the phone

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Generalization: multi-party computation

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Multiparty consensus mechanisms (and block-chain protocols …)

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Weird useful tool: Zero Knowledge Proofs

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More powerful forms of Encryption

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Homomorphic Encryption

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Searchable Encryption

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And many more…

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Encryption : Perfect secrecy and its limitations

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Chapters 1-2 of Katz-Lindell book • Steganography and why it is not a good idea • Defining Perfect Secrecy • Problems with perfect secrecy

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Setting for private-key encryption schemes • Alice and Bob share some “secret information” in advance • They want to communicate a new message secretly. • The communication happens in public.

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Steganography • Art of concealing the message in “innocent-looking” messages.

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Steganography • Even the algorithm used by Alice and Bob is hidden… • When is it useful? • What is wrong with it?

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Kerckhoffs's principle

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Caesar cipher (weak variant) • the th letter is substituted with ( + ) th letter. • Is it secure?

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Caesar Cipher (strong variant) • Secret key: a random permutation over all letters. • Key size: log 26! > 88 bits • Is it now secure?