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CMSC 414 Computer (and Network) Security Lecture 9

CMSC 414 Computer (and Network) Security Lecture 9. Jonathan Katz. Digital signatures. RSA signatures I. “Textbook RSA” Why textbook RSA is completely insecure! (Two attacks). RSA signatures for real. Hash functions… Collision-resistance Birthday attacks “Scrambling”

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CMSC 414 Computer (and Network) Security Lecture 9

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  1. CMSC 414Computer (and Network) SecurityLecture 9 Jonathan Katz

  2. Digital signatures

  3. RSA signatures I • “Textbook RSA” • Why textbook RSA is completely insecure! (Two attacks)

  4. RSA signatures for real • Hash functions… • Collision-resistance • Birthday attacks • “Scrambling” • How to fix RSA signatures • Why does this work? • Is it actually secure?

  5. Hash functions • SHA-1 • Proposed NIST standard • 160-bit output • MD5 • Developed by Rivest (RSA) • 128-bit output

  6. 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

  7. Signing long messages? • How…? • Hash-and-sign • Only need to assume that hash function is collision-resistant

  8. 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?

  9. A few words about PKI • Certification authorities; certificates • Single point of failure? • Certificate chains • More on this later…

  10. “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…

  11. “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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