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CMSC 414 Computer (and Network) Security Lecture 2 Jonathan Katz

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Two papers linked from webpage “Reflections on trusting trust” “Managed security monitoring” Both leave a fairly negative impression of security… …at the very least, they show that security is not easy, and cannot just be applied as a “magic bullet”

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“Managed security monitoring” (Summarize article) –Is the state of network security really this bad? (Arguably, yes) –Although network monitoring and risk management are important, security is too –Security is not an ends unto itself If you really want to be secure, disconnect yourself from the Internet

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An Overview of Computer Security

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Basic components Confidentiality Integrity Availability

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Policy vs. mechanism Security policy –Statement of what is and is not allowed Security mechanism –Method for enforcing a security policy One is meaningless without the other… Problems when combining security policies of multiple organizations

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Security goals Prevention Detection Response/recovery

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Assumptions and trust Example: assume that all employees are trustworthy, and do not represent a threat Assumptions underlie any security mechanism –Important to recognize and evaluate these assumptions

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Example Assumption: locks cannot be picked –What if a locksmith is around? –What if this locksmith is trustworthy? –Why do we assume that she is trustworthy?

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More assumptions Two assumptions are (almost) always made: –Policy defines the intended level of security –Mechanism correctly implements policy

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System development Determine threats; develop policy Give specification of the system –Desired functionality of the system –If specification is ambiguous, vulnerabilities can result –An imprecise specification is useless… Design the system –Design system satisfying the specification –Difficult (but not impossible) to verify

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System development, cont’d… Implementation –Create a system satisfying the design –Impossible to fully verify correctness Software complexity Unknown inputs Unverified tools –“Testing” after the fact Subject to limitations of the tests

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System development (summary) 1. Threat analysis 2. Policy 3. Specification 4. Design 5. Implementation 6. (Operation/maintenance/monitoring?)

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Cost-benefit analysis Important to evaluate what level of security is necessary/appropriate –Cost of mounting a particular attack vs. value of attack to an adversary –Cost of damages from an attack vs. cost of defending against the attack –Likelihood of a particular attack

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Human factors E.g., passwords… Outsider vs. insider attacks Software misconfiguration Not applying security patches Social engineering

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Everything you wanted to know about cryptography * *But perhaps were afraid to ask…

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Caveat Everything I present will be (relatively) informal –But I will try not to say anything that is an outright lie… Cryptography is about precise definitions, formal models, and rigorous proofs of security (which we will not cover here) –If you want more details, take CMSC 456!

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Attacks Crypto deals primarily with three goals: –Confidentiality –Integrity (of data) –Authentication (of resources, people, systems) Other goals also considered –E.g., non-repudiation –E-cash (e.g., double spending) –General secure multi-party computation

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Private- vs. public-key For many security goals, there are two types of cryptographic algorithms –Private-key / shared-key / symmetric-key / secret-key –Public-key

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Private-key cryptography The parties communicating share a completely random and secret key –Main point: key is not known to an attacker –This key must be shared (somehow) before they communicate All “classical” cryptosystems are private- key based Can also be used for secure storage

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Private-key cryptography For confidentiality: –Private-key (symmetric-key) encryption For data integrity: –Message authentication codes –(sometimes called cryptographic checksums)

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Public-key cryptography One party (Alice) generates both a public key and a private key (or secret key) The public key is published; the private key is kept secret –An attacker knows the public key! The other communicating party (Bob) need not have any key of his own; knows Alice’s key Techniques for this first developed in the 70’s

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Public-key cryptography For confidentiality: –Public-key encryption For data integrity: –Digital signatures

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To review… Confidentiality: –Private-key encryption (schemes) –Public-key encryption (schemes) Integrity: –Message authentication (codes) –Digital signature (schemes) We will discuss authentication later

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Private- vs. public-key I Disadvantages of private-key –Need to securely share a key If you can share a key securely, why not just share the message itself? What if not possible? Need to know who you want to communicate with in advance! –O(n 2 ) blowup in storage

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Private- vs. public-key II Why study private-key at all? –Private-key is much more efficient (3 orders of magnitude) –Public-key crypto is “harder” to get right Needs stronger assumptions, more math –Can combine private-key with public-key to get the best of both worlds (for encryption)

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Private- vs. public-key III More disadvantages of public-key crypto –Public-key crypto still requires secure distribution and binding of public keys (PKI) May (sometimes) be just as hard as sharing a key –Not clear who you are communicating with (for public-key encryption)

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Confidentiality

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Overview Private-key encryption –Attack model –“Trivial” systems Show why the problem is hard Show methods of attack Convince you not to use “home-brewed” techniques –What do we mean by security? –Block ciphers and modern-day techniques

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Overview, continued… Public-key cryptography –A word about security –Some basic number theory –RSA and El Gamal –Some attacks…and some fixes

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Alice Bob shared info K K Alice K Bob K m C = E K (m) C1C1 m = D K (C)

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In more detail… Alice and Bob share a key K –Must be shared securely –Must be completely random –Must be kept completely secret from attacker –We don’t discuss (for now) how they do this Plaintext - encryption - ciphertext - decryption Decryption must recover the message!

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