1 Firewalls

2 What is a firewall? Device that provides secure connectivity between networks (internal/external; varying levels of trust) Used to implement and enforce a security policy for communication between networks Trusted Networks Untrusted Networks & Servers Firewall Router Internet Intranet DMZ Public Accessible Servers & Networks Trusted Users Untrusted Users

3 Firewalls From Webster’s Dictionary: a wall constructed to prevent the spread of fire Internet firewalls are more the moat around a castle than a building firewall Controlled access point

4 Firewalls can: Restrict incoming and outgoing traffic by IP address, ports, or users Block invalid packets

5 Firewalls Cannot Protect… Traffic that does not cross it –routing around –Internal traffic When misconfigured

6 Internet DMZ Net Web Server Pool Corporate Network ALERT!! Security Requirement Control access to network information and resources Protect the network from attacks Access Control

7 Filtering Packets checked then passed – typically route packets Inbound & outbound affect when policy is checked Packet filtering –Access Control Lists Session filtering –Dynamic Packet Filtering –Stateful Inspection –Context Based Access Control Fragmentation/reassembly Sequence number checking ICMP

8 Packet Filtering Decisions made on a per-packet basis No state information saved

Applications Presentations Sessions Transport DataLink Physical DataLink Physical Router Applications Presentations Sessions Transport DataLink Physical Network Network Packet Filter

10 Session Filtering Packet decision made in the context of a connection If packet is a new connection, check against security policy If packet is part of an existing connection, match it up in the state table & update table

11 Session Filtering Applications Presentations Sessions Transport DataLink Physical DataLink Physical Applications Presentations Sessions Transport DataLink Physical Network Presentations Sessions Transport Applications Dynamic State Tables l Screens ALL attempts, Protects All applications l Extracts & maintains ‘state’ information l Makes an intelligent security / traffic decision

12 Proxy Firewalls Relay for connections Client Proxy Server Two flavors –Application Level –Circuit Level

13 Application Gateway Understand specific applications –Limited proxies available –Proxy “impersonate” both sides of the connection Resource intensive –Process per connection HTTP proxies may cache we pages More appropriate for TCP Block all unless specifically allowed Must write a new proxy application to support new applications –Non Trivial

Applications Presentations Sessions Transport DataLink Physical Network DataLink Physical Applications Presentations Sessions Transport DataLink Physical Application Gateway Applications Presentations Sessions Transport Network Network TelnetTelnetHTTPHTTPFTPFTP Application Layer GW/proxy

15 Encryption (VPNs) Allows trusted users to access sensitive information while traversing untrusted networks Useful for remote users/sites IPSec Encrypted Tunnels

16 PGP

17 Pretty Good Privacy (PGP) widely used de facto secure developed by Phil Zimmermann selected best available crypto algs to use integrated into a single program available on Unix, PC, Macintosh and Amiga systems originally free, now have commercial versions available also

18 PGP Five services –Authentication, confidentiality, compression, compatibility, segmentation Functions –Digital signature –Message encryption –Compression – compatibility –segmentation

19 PGP Operation – Integrity and Authentication 1.Sender creates a message 2.SHA-1 used to generate 160-bit hash code of message 3.hash code is encrypted with RSA using the sender's private key, and result is attached to message 4.receiver uses RSA or DSS with sender's public key to decrypt and recover hash code 5.receiver generates new hash code for message and compares with decrypted hash code, if match, message is accepted as authentic

20 Pretty Good Privacy (PGP) - Message Integrity and Authentication

21 PGP Operation – Confidentiality 1.sender generates message and random 128-bit number to be used as session key for this message only 2.message is encrypted, using CAST-128 / IDEA/3DES with session key 3.session key is encrypted using RSA with recipient's public key, then attached to message 4.receiver uses RSA with its private key to decrypt and recover session key 5.session key is used to decrypt message

22 PGP Message Encryption Decrypt message using DES with secret keyk DecryptE(k) using RSA with my private keyk Convert ASCII message Encrypt k using RSA with recipient‘ s public key Encode message + E(k) in ASCII for transmission Encrypt message using DES with secret keyk Create a random secret key k Original message Transmitted message

23 PGP Operation – Compression by default PGP compresses message after signing but before encrypting –so can store uncompressed message & signature for later verification –& because compression is non deterministic uses ZIP compression algorithm

24 Segmentation & Reassembly systems impose maximum length –50 Kb, for example PGP provides automatic segmentation –Done after all other operations –Thus only one session key needed

25 PGP Alice wants to provide secrecy, sender authentication, message integrity. Alice uses three keys: her private key, Bob’s public key, newly created symmetric key H( ). K A ( ). - + K A (H(m)) - m KAKA - m K S ( ). K B ( ). + + K B (K S ) + KSKS KBKB + Internet KSKS

26 Folklore

27 Perfect Forward Security A protocol property that prevents someone who records an encrypted conversation from being able to later decrypt the conversation Keep the conversation secret from –Someone (an escrow agent, attacker..) who knows the long-term key Two ways –A Diffie-Hellman exchange, then forget DH information –Ephemeral public/private key pair

28 Change Keys Periodically The more examples of ciphertexts you can see, the more likely you can break the encryption and find the key Change keys (key rollover)

29 Continue.. Use different keys in the two directions Use different secret keys for encryption vs. integrity protection Use different keys for different purposes

30 Continue.. Have both sides contribute to the master key HMAC rather than Simple MD Key expansion Randomly Chosen IVs Use nonce in protocols Compress data before encrypting it Do not do encryption only Minimal vs. redundant designs

31 Continue… Put Checksums at the end of data Forward Compatibility Negotiating Parameters –Different Algorithms