Transcript ppt
Firewalls 1 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 Untrusted Users Internet Router Intranet DMZ Public Accessible Servers & Networks Trusted Users 2 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 3 Firewalls can: • Restrict incoming and outgoing traffic by IP address, ports, or users • Block invalid packets 4 Firewalls Cannot Protect… • Traffic that does not cross it – routing around – Internal traffic • When misconfigured 5 Access Control ALERT!! Internet Security Requirement • Control access to network information and resources • Protect the network from attacks 6 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 7 Packet Filtering • Decisions made on a per-packet basis • No state information saved 8 Packet Filter Applications Applications Presentations Presentations Sessions Sessions Transport Transport Network Network DataLink DataLink DataLink Physical Physical Physical Router 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 10 Session Filtering Screens ALL attempts, Protects All applications Extracts & maintains ‘state’ information Makes an intelligent security / traffic decision Applications Applications Presentations Applications Presentations Sessions Presentations Sessions Transport Sessions Transport Network Transport Network Network DataLink DataLink DataLink Physical Physical Physical Dynamic Dynamic Dynamic State Tables State StateTables Tables 11 Proxy Firewalls • Relay for connections • Client <-> Proxy <->Server • Two flavors – Application Level – Circuit Level 12 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 13 Application Layer GW/proxy Telnet FTP HTTP Applications Applications Applications Presentations Presentations Presentations Sessions Sessions Sessions Transport Transport Transport Network Network Network DataLink DataLink DataLink Physical Physical Physical Application Gateway Encryption (VPNs) • Allows trusted users to access sensitive information while traversing untrusted networks • Useful for remote users/sites • IPSec • Encrypted Tunnels 15 PGP 16 Pretty Good Privacy (PGP) • • • • • widely used de facto secure email 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 17 PGP • Five services – Authentication, confidentiality, compression, email compatibility, segmentation • Functions – – – – – Digital signature Message encryption Compression Email compatibility segmentation 18 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 19 Pretty Good Privacy (PGP) - Message Integrity and Authentication Sender identity and message integrity confirmed if checksums match Calculate MD5 checksum over message contents Calculate MD5 checksum on received message and compare against received value Sign checksum using RSA w ith senderÕs private key Decrypt signed checksum w ith senderÕs public key Transmitted message 20 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 21 PGP Message Encryption Create a random secret key Original message k Encrypt message using DES with secret key Encrypt Decrypt message using k DES with secret key k using RSA with Decrypt recipient s‘ public key Encode message + E( k ) using RSA with my private key E (k ) k k Convert ASCII message in ASCII for transmission Transmitted message 22 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 23 Segmentation & Reassembly • Email systems impose maximum length – 50 Kb, for example • PGP provides automatic segmentation – Done after all other operations – Thus only one session key needed 24 PGP • Alice wants to provide secrecy, sender authentication, message integrity. - KA m . H( ) - . KA( ) - KA(H(m)) + KS . KS( ) + m KS + . K B( ) + Internet + KB(KS ) KB Alice uses three keys: her private key, Bob’s public key, newly created symmetric key 25 Folklore 26 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 27 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) 28 Continue.. • Use different keys in the two directions • Use different secret keys for encryption vs. integrity protection • Use different keys for different purposes 29 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 30 Continue… • Put Checksums at the end of data • Forward Compatibility • Negotiating Parameters – Different Algorithms 31