Transcript Internet Access - University of St. Thomas

CISC 210 - Class Today
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Homework Reminders
Recap
Finish up Public Key Crypto
Firewalls
Firewall Lab
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Homework Reminders
• DUE TODAY: Lab and Diagrams
• DUE Monday: Project Outline
– Requirements/Grading Rubric are posted on the Web
• DUE Following Monday: A10 Lab (Firewall)
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Recap: IP Security Protocol – IPSEC
• Security protection that’s IP routable
• We authenticate the IP addresses
• We encrypt everything inside the IP header
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Recap: Public Key Encryption
• Uses a pair of keys: the Private Key and the
Public Key
• Usually, one key of the pair decrypts what
the other key encrypts, and vice versa
• “Asymmetric Encryption”
Clear
Text
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Private
Key
Public
Key
Encryption
Procedure
Cipher
Text
Decryption
Procedure
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Clear
Text
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Public Key cryptography
• First successful version: Diffie Hellman
• ‘Distributive property’ of exponents
– (BX)Y = (BY)X
• Or, in Diffie-Hellman:
– (BX mod M)Y mod M = (BY mod M)X mod M
– (x) is Private Key; (BX mod M) is Public Key
• Why is it secure? Because…
– (BX mod M) * (BY mod M) mod M ! = (BY mod M)X mod M
– Modulus makes it impractical to reverse
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RSA
• More flexible variant
– Basic Math: Given M, My = Ciphertext; M = (My)-y
– y = Public Key; -y = Private Key (inverse of public key)
– RSA uses “Modular Inverse” instead of simple inverse
• Multiply two primes P x Q
– Product is the Modulus, part of the published key,
– 2 other numbers form rest of the key
• “Public” exponent “E” (often 3 or 65537)
• “Private” inverse “D” (computed from P, Q, and E)
• Works in both directions – encrypt and decrypt
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Using Public Key
• Diffie Hellman
– I can share one secret with another D-H user
• I use the other user’s PUBLIC key with my PRIVATE key
• RSA
– If I have a user’s PUBLIC key, I can send them a secret
• I encrypt the secret with THEIR public key
• They decrypt with their own private key
– I can use my PRIVATE key to “sign” things
• I encrypt a hash (checksum) with my PRIVATE key
• Others can check the result with my PUBLIC key
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IKE – Internet Key Exchange
• Sets Up “Security Associations” for IPSEC
– Assigns SPIs to connections between crypto
– Negotiates crypto selection and establishes secret keys
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How IKE Works
• Phase 1: Establish a shared secret (Diffie
Hellman)
– Set up the shared secret
– Authenticate each other
• How? Shared secrets or public keys
• “Challenge Response” protocols (next slide)
• Phase 2: Negotiate or update an association
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One asks for an association, specifying an SPI
The other says what crypto it supports
They agree on crypto to use
One provides a shared secret from which they produce keys
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Challenge Response Protocols
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Bob says “I’m Bob”
Alice says, “Prove it with this nonce: 1928”
Bob encrypts it
Alice verifies the encrypted nonce
• Crypto alternatives
– Use a shared secret
– Use public/private key pairs
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Firewalls
• Objectives
• Types of firewall traffic control
• Firewall Filtering
• Network Address Translation
• The Lab
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Firewall objectives
• Provide outbound Internet access
• Restrict/forbid inbound connections
• Detect and block malicious traffic
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Types of firewall traffic control
• Service control (allow specific protocols)
– Block unauthorized protocols
– Permit authorized ones
– Actually very hard to do
• Direction control (in/out)
– Allow outbound browsing
– Restrict access to internal servers
• User control (source/destination)
– User authorization, or perhaps subnet filtering
• Behavior control
– bandwidth, application specific cases
– Look in e-mail for malware
– Filter access to Web sites (China, Saudi, …)
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Network Access Architectures
Routers
Internal
Network
Router
Internet
Screened Subnet (Basic)
Screened Subnet
Internet
Router
Router
Internal
Network
Bastion
Host
Dual-Homed Gateway (Sophisticated)
Internet
Oct 2001
Router
DualHomed
Firewall
Router
Internal
Network
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Types of Firewall Filtering
Packet Filtering: based on packet header (Unsophisticated)
IP Header
TCP Data
Circuit Filtering: restricts connections (Common)
TCP Header
Application Data
+ Connection state
Application Proxy: restricts based on general policy (Refined)
Appl. Header
User Data
+ Connection state + application state
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Firewalls in Different Strengths
INTERNET
Application
IP
TCP/UDP
Link
IP
Packet Filter
• Control Based on Source /
Destination Internet
Addresses
TCP/UDP
IP
Link
Link
Application Gateway
• Control Based on Application
Type and Content
Circuit Gateway
• Hides Internal Network Details
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Proxies . . . . for the Application Gateway
M. A. Proxy
Proxies are small ( less than 2000 lines of code),
“minimal and modular”
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Proxies . . . for the Application Gateway.
User’s requests
CLIENT
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M. A. Proxy
SERVER
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Proxies . . . for the Application Gateway.
User’s requests
forwarded
User’s requests
Application
output
CLIENT
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M. A. Proxy
SERVER
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Proxies . . . for the Application Gateway.
CLIENT
User’s requests
User’s requests
forwarded
Application
output forwarded
Application
output
M. A. Proxy
SERVER
Logs maintained
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Internet Firewall
Application Level Gateway
Ethernet Card
Private
Network
http proxy
Public
nntp proxy
Network
smtp proxy
ftp proxy
telnet proxy
rlogin proxy
snmp proxy
X11 proxy
Ethernet Card
Router
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Audit
Logs
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Issues with using Firewalls
• All firewalls are NOT created equal
– Type and rigor of controls
– OS security
• Correct configuration is critical for any Firewall
– Many attacks exploit insecure default configurations
• Firewalls, even when functioning correctly,
open BIG holes in the security perimeter
– World-Wide Web (HTTP)
– Active content (Java, Java-Script, ActiveX)
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Network Address Translation
• Original purpose: more hosts & addresses
– Let “insiders” use restricted addresses
– Translate them on the way out
• A ‘multiplexing’ mechanism
– Users share a “real” Internet address
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Firewalls and LAN support
• Provide a few standard LAN services
–Router connection
–DHCP
–Network Address Translation
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Firewall Lab
• Overview
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Rewire the lab to use the firewall
Map the rewired lab
Demonstrate host blocking through the firewall
Demonstrate NAT through the firewall
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That’s it
• Questions?
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