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CMSC 414 Computer (and Network) Security Lecture 9 Jonathan Katz
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Digital signatures
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RSA signatures I “Textbook RSA” –Why textbook RSA is completely insecure! (Two attacks)
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RSA signatures for real Hash functions… –Collision-resistance Birthday attacks –“Scrambling” How to fix RSA signatures –Why does this work? –Is it actually secure?
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Hash functions SHA-1 –Proposed NIST standard –160-bit output MD5 –Developed by Rivest (RSA) –128-bit output
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DSA/DSS signatures “Digital signature standard” Security based on discrete logarithms –No (complete) proof of security Royalty-free Overall, neither RSA nor DSS has the advantage –Depends (in part) on relative strengths of assumptions
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Signing long messages? How…? –Hash-and-sign –Only need to assume that hash function is collision-resistant
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Non-repudiation Digital signatures achieve non-repudiation –In contrast to private-key case! Is this a good or a bad thing? –Sometimes you want deniability (e.g., no trace that you logged in) –Legal ramifications – do you really know what you are signing?
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A few words about PKI Certification authorities; certificates –Single point of failure? Certificate chains More on this later…
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“Why crypto fails” Two examples of bad crypto: –Replay of “ok” message from bank to ATM –PIN on ATM card was authenticated, but account number on ATM card was not…
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“Why crypto fails” Lack of information about previous failures Most frauds not caused by “bad” crypto, but by bad implementation/management –There is plenty of bad crypto, too! “Social engineering” attacks Importance of threat model (i.e., security policy) –Threat model may change… Dispute resolution
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