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Class 19 Wrap-up and Review CIS 755: Advanced Computer Security Spring 2014 Eugene Vasserman http://www.cis.ksu.edu/~eyv/CIS755_S14/
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Administrative stuff No class during the last week of the semester (May 6 th and 8 th ) – No office hours either – I’m out of town – No presentations Remember exam on Thursday – Study guide is up on the class web page No office hours this Friday – email to meet Focus on your projects and reports
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The most important slide of the class What are the take-away messages? – Think like an adversary – Kerckhoffs’ principle and Shannon’s maxim – Be able to search for solutions – Read papers – Reuse, reuse, reuse (correctly!) – State assumptions (be sure they hold) – Be able to admit “I don’t know” – not everyone can engineer every solution
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I’m sure this is someone’s law… If a security system is too difficult to use, users will find a way to get around it – Corollary: Getting the job done is more important than security Has more immediate potentially bad outcomes
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Things to remember What does “secure” mean? Who is the adversary, and why? There is such a thing as too much security If too hard to use, users will bypass security Attacks only get better
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Some things to remember Theoretical to practical in ~10 years – Chosen ciphertext attack – HDMI – CBC chosen plaintext attack Attacks only get better – Look at history of MD5 – Look at history of SHA (e.g. SHA-0) Some things are a bad idea in the first place, e.g. “trusted” hardware
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NEVER BUILD YOUR OWN WHEN SOLUTION EXISTS!!! NEVER COMPOSE YOUR OWN WHEN LIBRARY EXISTS!!!
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Safety vs. security Think like an adversary! Random → malicious faults Engineering for security: “What’s the worst that can happen?” Assume it will… Always, always, ALWAYS state your assumptions!
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Security: Fundamental differences Real world: physical, intuitive – Risk assessment People are not even good at this in the real world! – Trusted vs. trustworthy – Forensics, physical evidence Forgery – Fail “evident,” e.g. theft – Scale of failures
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More basics Trusted vs. trustworthy – e.g. the recent SSL Certificate Authority fiasco Risk, hazard, vulnerability – Adversary, ROI, scale Assurance levels – “Rainbow” book series, Common Criteria Method of returning to secure states Fail-closed/secure or fail-open/insecure?
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Basic cryptographic primitives Confidentiality (encryption) – Symmetric (e.g. AES) – Asymmetric (e.g. RSA) Hash functions (e.g. SHA1) Integrity and authentication – Symmetric (message authentication codes) – Asymmetric (signatures) Key agreement Random numbers
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Block cipher modes of operation ECB, CBC, OFB, CTR, CFB, GCM, XEX, XTS Differences, i.e. why do we care? – Some are parallelizable (GCM) Also provides authentication! – Some are self-synchronizing (CFB) Trick question: Block ciphers vs. stream ciphers vs. pseudorandom number generators (PRNG)?
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Security (strength) Key size * – Commonly 2 256 for AES, 2 2048 for RSA – What is a [good] key? Underlying cryptosystem/primitives Composition e.g. MAC with broken underlying hash function may not itself be broken
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Modes of operation (ECB) Images borrowed from Wikipedia :)
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Modes of operation (CBC) Images borrowed from Wikipedia :)
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Recall: MACs “Keyed hash” (MAC from a cryptographically-secure hash function) – Hash Block cipher (CBC or CFB) MAC Hybrid modes e.g. CBC-MAC – Secrecy plus authenticity (2-party) Remember to use different keys for MAC and encryption… why?
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Modes of operation (CFB) Images borrowed from Wikipedia :)
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Modes of operation (CTR) Images borrowed from Wikipedia :) VS. ECB
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Giving, storing and wiping secrets Credentials Password security Storage security Input security – Ctrl-Alt-Del Forgetfulness security – Encryption? – https://citp.princeton.edu/research/memory/ https://citp.princeton.edu/research/memory/
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Access control Authentication → access No authentication → no access What are we protecting? Who is our adversary? – Threat model Who is trusted? Where does enforcement occur?
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Implementation considerations Kerckhoffs’ principle and Shannon’s maxim – Especially tempting to violate in case of “dirty” code – I’ve been there! Watch your (unstated) assumptions – Example: Unsanitized (untrustworthy) input Adversaries Side-channels Performance
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More considerations Correct tool for the job – Requirements (before, not after) – spend time on this Correct usage of the tool Documentation! Weakest links Pay attention to potential non-cryptographic issues such as side/covert channels – But you can never eliminate them: PROVABLE Think / test like an adversary
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Current state of symmetric encryption DES is too weak (56-bit key) 3DES is weak (168-bit keys but only 2 112 security – meet-in-the-middle attack) Recent weaknesses in AES: – AES-256 (2 254.4 ) AES-192 (2 189.7 ) AES-128 (2 126.1 ) http://research.microsoft.com/en- us/projects/cryptanalysis/aesbc.pdf
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Current state of hash functions MD5 is broken – http://www.win.tue.nl/hashclash/ http://www.win.tue.nl/hashclash/ SHA-1 is known to be weak – http://theory.csail.mit.edu/~yiqun/shanote.pdf (2 69 ) http://theory.csail.mit.edu/~yiqun/shanote.pdf – http://eprint.iacr.org/2004/304 (2 106, generalizable) http://eprint.iacr.org/2004/304 – SHA-256 (variant) is even weaker SHA-3 currently in “development” (NIST) – We have a winner: all hail Keccak (SHA-3)! – http://csrc.nist.gov/groups/ST/hash/sha-3/ http://csrc.nist.gov/groups/ST/hash/sha-3/
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Problems: Side channels Side-channel attacks VERY damaging – Power – Timing – Error messages Different errors in SSH leak information (mismatch between implementation and specification of CBC block cipher mode): http://portal.acm.org/citation.cfm?id=586112
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Distributed systems: Security Eliminating a single point of failure – Denial of service protection (robustness) Eliminating a single point of trust – What if your boss is malicious? If we want to reap benefits of distributed system designs, we have to take care of the “maybes” How?
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Distributed systems: Privacy Local system – local information Distributed system – more access to potentially private information Privacy vs. authentication Sometimes privacy is not a security requirement, sometimes it is Are there other potential security requirements related to privacy?
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Source routing with capabilities B, data S3 S2 S1 B S3 S2 S1 A
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eCash Broker WitnessClient Merchant
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Chaum Mixes Bob Alice Output in lexographic order
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Global Adversary Bob Alice
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Tor ABC TCP over TCP (UGH!)
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Tor hidden services ABCDEF
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Global adversary vs. Tor Bob Alice Entire Tor network
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Tor network positioning attack ABCM
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Tor linkability attack ABC
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Tor selective DoS attack ABC
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Tor and bridges
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Enumerating Freenet Run a Freenet node; wait for nodes to contact you Or just query random “locations”
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ISP Anonymity ISP AS1 AS2 Anonymizing Network
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ISP Censorship resistance ISP AS1 AS2 Anonymizing Network Membership Concealing Network
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secret Covert auth. !! Hi? Hi! XX Hi? ?? !!??
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Steganographic embedding Linux 2.6 TCP SYN packet header with embedded MAC
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Adeona
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Questions? Reading discussion
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