Class 19Wrap-up and Review

CIS 755: Advanced Computer SecuritySpring 2014

Eugene Vasserman

http://www.cis.ksu.edu/~eyv/CIS755_S14/

Administrative stuff

• No class during the last week of the semester (May 6th and 8th)–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

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

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

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

NEVER BUILD YOUR OWN WHEN

SOLUTION EXISTS!!!

NEVER COMPOSE YOUR OWN WHEN LIBRARY EXISTS!!!

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!

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

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?

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

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)?

Security (strength)

• Key size*

–Commonly 2256 for AES, 22048 for RSA–What is a [good] key?

• Underlying cryptosystem/primitives

• Composition• e.g. MAC with broken underlying hash function may

not itself be broken

Modes of operation (ECB)

Images borrowed from Wikipedia :)

Modes of operation (CBC)

Images borrowed from Wikipedia :)

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?

Modes of operation (CFB)

Images borrowed from Wikipedia :)

Modes of operation (CTR)

Images borrowed from Wikipedia :)

VS. ECB

Giving, storing and wiping secrets

• Credentials• Password security• Storage security• Input security–Ctrl-Alt-Del

• Forgetfulness security– Encryption?–https://citp.princeton.edu/research/memory/

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?

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

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

Current state of symmetric encryption

• DES is too weak (56-bit key)• 3DES is weak (168-bit keys but only 2112

security – meet-in-the-middle attack)

• Recent weaknesses in AES:–AES-256 (2254.4) AES-192 (2189.7) AES-128 (2126.1)

http://research.microsoft.com/en-us/projects/cryptanalysis/aesbc.pdf

Current state of hash functions

• MD5 is broken– http://www.win.tue.nl/hashclash/

• SHA-1 is known to be weak– http://theory.csail.mit.edu/~yiqun/shanote.pdf (269)– http://eprint.iacr.org/2004/304 (2106, generalizable)

– 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/

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

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?

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?

Source routing with capabilities

B, dataS3S2S1 B

S3

S2

S1

A

eCash

Broker

WitnessClient

Merchant

Chaum MixesBob

Alice

Output in lexographic order

Global AdversaryBob

Alice

Tor

A

B

C

TCP over TCP (UGH!)

Tor hidden services

A

B

C

D

E

F

Global adversary vs. TorBob

Alice

Entire Tor

network

Entire Tor

network

Tor network positioning attack

A

B

C

M

Tor linkability attack

A

B

C

Tor selective DoS attack

A

B

C

Tor and bridges

Enumerating Freenet

Run a Freenet node; wait for nodes to contact you

Or just query random “locations”

ISPISP

Anonymity

ISPISPAS1AS1

AS2AS2

Anonymizing Network

ISPISP

Censorship resistance

ISPISPAS1AS1

AS2AS2

Anonymizing Network

MembershipConcealingNetwork

secretsecret Covert auth.!!

Hi? Hi!

XX

Hi? ??

Steganographic embedding

Linux 2.6 TCP SYN packet header with embedded MAC

Questions?

Reading discussion