Class 5 Channels and Preview CIS 755: Advanced Computer Security Spring 2014 Eugene Vasserman eyv/CIS755_S14

Class 5Channels and Preview

CIS 755: Advanced Computer SecuritySpring 2014

Eugene Vasserman

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

Administrative stuff

Last time: Basic primitives

• Confidentiality (encryption)– Symmetric (e.g. AES)– Asymmetric (e.g. RSA)

• Hash functions• Integrity and authentication– Symmetric (authentication codes)– Asymmetric (signatures)

• Random numbers

Preview of Math in Asymmetric Crypto

• Diffie-Hellman–Discrete logarithm is “hard”–Computational, decisional (“flavors”)

• RSA–Prime factorization is “hard”

• Quantum computing and Shor’s algorithm• Elliptic Curves• Bilinear Maps

Person-in-the-middle

Alice

Bob

Alice

Confidential

NOT Authenticated

Bob

?

Muahaha!

Person-in-the-middle

Alice

Bob

Alice?

NOT Confidential

NOT Authenticated

Bob

Certificates

Alice

Bob

Alice!

Confidential

Authenticated

Bob

CRAP!

Confidential?

Authenticated?

PKI Example: Confidential email

BobAlice

BobBob

Alice?

Confidential

Authenticated

PKI Example: Confidential email

BobAlice

BobBob

Alice!

Questions?

In practice: Optimizations

• Asymmetric encryption:–Password Secret Key ESK(K), EK(M)

• Signatures:–Password Secret Key M, SigSK(h(M))

• Why do this? Why is this safe?• Symmetric:–Password Key

derivation/stretching/strengthening function K

In practice: Problems

• Composability:http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html

• Attack on PKCS #1 v2 standard-compliant RSA OAEP leaks plaintext bits:

http://www.springerlink.com/content/tw5tuqb3hxbn9grq/

• This attack also leaks plaintext bits in a lot of systems that use CBC block cipher mode:

http://lasecwww.epfl.ch/pub/lasec/doc/Vau02a.psxkcd.com

• Example: WEP– IV, RC4(IV, k) (M, c(M))–Claim: 24-bit IV + 40-bit key = 64-bit

security

• Example: WEP– IV, RC4(IV, k) (M,

c(M))– Claim: 24-bit IV + 40-

bit key = 64-bit security

• On the right: text from Jonathan Katz

Problems: Composability• Is this secure against chosen-plaintext attacks?

– It is randomized…

• 40-bit key (in some implementations)!– Claims that, with IV, this gives a 64-bit effective key(!)

• And how is the IV chosen?– Only 24 bits long -- IV repetitions are a problem!– Reset to 0 upon re-initialization– Some implementations increment the IV as a counter

• A repeating IV allows the attacker to compute the XOR of two plaintexts– We have discussed already how this can be damaging

• Small IV space means the attacker can build a dictionary of (IV, RC4(IV, k)) pairs– If portions of some plaintexts known, this enables determination of other

plaintexts

• Known-plaintext attacks discovered on this usage of RC4– Possible because the first byte of plaintext is a fixed, known header!

• Chosen-plaintext attacks– Send IP traffic/e-mail to the mobile host and watch it get forwarded– Transmit broadcast messages to access point– Authentication spoofing

• No cryptographic integrity protection– The checksum is linear (i.e., c(xy) = c(x)c(y)) and unkeyed, and therefore

easy to attack– Allows IP redirection attack– Allows TCP “reaction” attacks

• Look at whether TCP checksum is valid• Form of chosen-ciphertext attack

• Encryption used to provide authenticationof mobile station (access point sends nonce; station returns an encryption of the nonce)– Allows easy spoofing after eavesdropping

Problems: Side channels

• Side-channel attacks VERY damaging–Power– Timing• See news (2013) and cool stuff (2014) pages

– 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

Questions?

Cool stuff

• Elliptic curves– y2 = x3 + ax + b

• Secure multiparty computation–General existence result• Communication complexity

• Threshold cryptography– Encryption, signatures, secret sharing

More cool stuff

• Identity-based encryption (IBE)– Time period-based

• Attribute-based encryption (ABE)• Zero-knowledge (ZK) proofs–General existence result in NP– Interactive or non-interactive (NZIK)• Strength from number of rounds or predefined

• Homomorphic encryption

Yet more cool stuff

• Key management–Key trees• Hierarchical, time-based access

• One-time use tokens–Compare to capabilities

• Blind signatures• Compact signature aggregation• Commitments (vs. hashes)

Questions?