CSCE 465 Lecture 19

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Intrusion Detection System (IDS)

	Firewall	Network IDS
Monitoring	Passive	Active
If it fails	Fail-Close	Fail-Open

Network IDS Requirements:

• High-speed, large volume monitoring
• Real-time notification
• Separate mechanism from policy
• Extensible (add modules to detect new attacks)
• Broad array of protection
• Frugal with resources
• Resilient to Stress and attacks against IDS itself

Eluding IDS

Insertion Attack

Each letter represents a packet:

• End system sees "ATTACK"
• IDS Sees "ATXTACK"
• Attacker's data stream is "TXTCAAK"

Can be accomplished with "bad" checksum or carefully crafted TTL value.

Evasion Attack

Each letter represents a packet:

• End system sees "ATTACK"
• IDS Sees "ATTCK"
• Attacker stream is "TTCAAK"

This can be accomplished with fragmentation and overlapping packets

Summary

What IDS Sees may not be what the end system gets: IDS needs to perform full reassembly of packets in order to see for sure, but can bog down system.

Hash Functions

(See MATH 470 Lecture 20→)

A one-way function/transformation that produces a fixed-length unique identifier (message digest) for a longer sequence of data

E.g. MD5 = 128 Bits; SHA-1 = 160 Bits

Properties:

• Performance: Easy to compute $H(n)$
• One-Way: Given $H(m)$, but not $m$ it should be computationally infeasible to find $m$
• Weak Collision Resistance (Weak-collision-free): Given $H(m)$, it's computationally infeasible to find $m'$ such that $H(m)=H(m')$
• Strong Collision Resistance (Strong-collision-free): Computationally infeasible to find $m_{1}$ and $m_{2}$ such that $H(m_{1})=H(m_{2})$

Length of Hash Image:

• Too long: unnecessary overhead
• Too short: (birthday paradox) easy to find collisions

Application: Commitment Protocol

1. Alice picks $x$ and sends $z=H(x)$ to Bob
2. Bob picks his number $y$ and sends it to Alice.
3. Alice sends $x$
4. Bob verifies $z=H(x)$

if Hash function is not a good one, it can be easy for either Alice or Bob to cheat.

Application: Message Encryption

1. Alice and Bob share a secret key $k$, but don't want to use encryption of the message with $k$
2. Alice sends encrypted random number $r_{1}$ to Bob
3. Bob sends encrypted random number $r_{2}$ to Alice
4. Concatenation $r_{1}\mid r_{2}$ is used as IV into OFB, where $H$ is encryption function to produce a one-time pad

Is reverse possible? can encryption be used to generate hash?