Network Security Volkan Cambazoglu Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Communications Research Group Outlook • Secure channel • Principles of cryptography • Authentication, Integrity • Security at different layers • Firewalls and Intrusion Detection Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Communication Channels • Assume always that a communication channel is insecure! Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross Examples of Alice and Bob • • • • • E-commerce applications - Amazon, Spotify, etc. Online banking applications - Swedbank, Nordea, etc. Online chat applications - Skype, Google chat, etc. DNS servers - Exchange messages about where a website is located Routers - Exchange messages about routing tables (Routing Information Protocol) Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner What can Trudy do? • • • • • Eavesdrop - Sniff and record traffic between users (e.g. Alice and Bob) Insertion - Insert messages as if it comes from a specific user (Alice/Bob) Modification - Alter messages going from a user (Alice) to the other one (Bob) Deletion - Delete messages going from a user (Alice) to the other one (Bob) Denial of service - Prevent users (Alice) from reaching an existing service Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Properties of Secure Communication • • • • Confidentiality - Only the receiver should understand the message content Authentication - Receiver should be able to confirm sender’s identity Integrity - Receiver should be able to check that the message is not altered Availability - Receiver should be able to access services provided by the sender Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Outlook • Secure channel • Principles of cryptography • Authentication, Integrity • Security at different layers • Firewalls and Intrusion Detection Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Information Security • Conceptually, the way information is recorded has not changed dramatically over time. What has changed dramatically is the - • ability to copy and alter information. technological advancements change from physical to digital Cryptography is the study of mathematical techniques related to aspects of information security such as - confidentiality entity authentication data integrity data origin authentication Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner The Basic Idea • Mathematical functions f(x) that are efficient to compute. No efficient algorithm is known for the inverse function. f(x): efficient x f(x) -1 f (x): hard • such as • Discrete Logarithm • Factorizing large numbers Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Kerkhoff’s Principle An enemy knows the whole system including all transformations, but not the secret key(s). Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Principles of Cryptography • • • • Plaintext or cleartext - has some meaning Ciphertext - unintelligible content Encryption algorithm - encrypt (plaintext) = ciphertext Decryption algorithm - decrypt (ciphertext) = plaintext Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Encryption/Decryption • • • non-keyed - no secret parameters one-way functions e.g. MD5 secret key - two or more entities share some common secret values encrypt and decrypt with the same secret e.g. Caesar cipher, AES public key - no shared secret keys one secret for encryption and another secret for decryption e.g. RSA Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Symmetric Key Cryptography plaintext m ciphertext f c = f(m,k) k Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner f k m Gaius Julius Cæsar •Shared secret encryption/decryption •Secret is a number to shift the alphabet •abcdefghijklmnopqrstuvwxyz •k = 3 •defghijklmnopqrstuvwxyzabc Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Gaius Julius Cæsar • There will be a secret meeting in one of the Swedish cities. We obtained the ciphertext for it! Which city is it? uppsala toorzkz abcdefghijklmnopqrstuvwxyz Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Attacks on Symmetric Key Cryptography •ciphertext-only: • - statistical analysis (e,t most frequent) - typical words (the, in, it, ...ing, etc.) •known-plaintext - Uppsala, Alice, Bob, etc. •chosen-plaintext - “the quick brown fox jumps over the lazy dog” Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Improving Symmetric Key Cryptography • • • Monoalphabetic cipher - Caesar cipher Polyalphabetic cipher - e.g. combine two Caesar ciphers for one word Block cipher - e.g. 3-bit block cipher (000:110, 001: 101, 010: 000, ...) - DES: 64 bit input, 16 rounds of 48 bit key from 56 bit key, final permutation 64 bit output - AES: 128 bit blocks, accepts different key lengths (128, 192, 256) - brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Public Key Cryptography • How can Alice and Bob start secure communication, if they cannot come together in the physical world? - Send shared secret in plaintext? Send encrypted shared secret? Hide the secret somewhere in plaintext? Any other crazy ideas? Or shall we simply use public key cryptography? Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Public Key Cryptography Public key: KB+(m) Private key: KB-(m) Plaintext encryption Ciphertext decryption message, m algorithm KB+(m) algorithm Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Plaintext message m = KB-(KB+(m)) Public Key Cryptography Public key: KB+(m) Private key: KB-(m) Plaintext encryption Ciphertext decryption message, m algorithm KB+(m) algorithm What could go wrong here? Plaintext message m = KB-(KB+(m)) •Hint 1: Who can use the public key? •Hint 2: What happens when same text, algorithm and key are used? Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Public Key Cryptography • • • • • Prerequisite: Modular Arithmetic x mod n = remainder of x when divided by n facts: - [(a mod n) + (b mod n)] mod n = (a+b) mod n [(a mod n) - (b mod n)] mod n = (a-b) mod n [(a mod n) * (b mod n)] mod n = (a*b) mod n thus: - (a mod n)d mod n = ad mod n example: - a=14, n=10, d =2 (14 mod 10)2 mod 10 = 42 mod 10 = 6 142 mod 10 = 196 mod 10 = 6 Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner RSA: Encryption/Decryption • • • Encryption - c = me mod n c is ciphertext m is plaintext e is encryption key (n, e) is the public key Decryption - m = cd mod n = (me mod n)d mod n = me*d mod n d is decryption key (n, d) is the private key Do you notice something when m = me*d mod n? Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner RSA: Creating public/private key pair • • • Choose two large prime numbers p and q (1024 bits each) Compute (n = p * q) and (z = (p-1) * (q-1)) Choose e < n that has no common factors with z (relatively prime) - e.g. (3 and 7) and (5 and 12) are relatively prime. • Choose d that fulfills (e * d mod z = 1) • • Public key (n,e) Private key (n,d) Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner RSA Encryption p=5 q=7 n=35 z=24 e=5 d=29 Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross RSA Decryption p=5 q=7 n=35 z=24 e=5 d=29 Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross Why does RSA work? • • • m = cd mod n m = (me mod n)d mod n m = me*d mod n • fact: • - cd mod n = c(d mod z) mod n where n = p*q and z=(p-1)*(q-1) thus: - m = m((e*d) mod z) mod n - m = m1 mod n Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Why is RSA secure? • • • • • We know the public key (n,e). Can we compute d using n and e? We need to find the factors of n= p*q p and q are two very large prime numbers (at least 1024 bits) 13606481726048992848411364002694494148097538296253994533786 28482542262240342758205383100088584039554372391026814657613 88249980135083342434428721426840110617593953169835450968550 73076943041284504818565938137085710532321945352149127789477 33675392166804312875063387109652043491190305281577529925513 75455100484051 (309 digits) Factoring a big number is hard! Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner RSA in practice: Session keys • • • Exponentiation in RSA is computationally intensive Use public key crypto to establish secure connection Establish symmetric session key for encrypting data - Shared secret Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Outlook • Secure channel • Principles of cryptography • Authentication, Integrity • Security at different layers • Firewalls and Intrusion Detection Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Message Integrity • • • Apply hash function H to m and get fixed size message digest H(m). Good to rely on - MD5 (128 bit message digest) SHA-1 (160 bit message digest) (US standard) Bad to rely on - Internet checksum (16 bit digest) “IOU100.99BOB” and “IOU900.19BOB” have identical checksum (B2 C1 D2 AC) Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Message Integrity • • • If Alice sends (m, H(m)) to Bob, can Bob trust the message m comes from Alice? No; because Trudy can prevent Bob from receiving (m, H(m)) and instead send (m’, H(m’)). Bob will check that H(m’) is indeed digest/hash of m’. There is a solution to this problem: - Message Authentication Code (e.g. HMAC) Used together with a cryptographically secure hash function such as MD5 or SHA-1 There is a shared authentication key between Alice and Bob. So, Alice will send (m, H(m+s)) instead of (m, H(m)). Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Authentication • • • Bob wants Alice to “prove” her identity to him Bob wants to know that if he receives a message from Alice, the message actually comes from her. Bob wants to be sure that the message was not tampered with on its way to him. Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner RSA: Another important property • • KB-(KB+(m)) = m = KB+(KB-(m)) private(public(m)) = m = public(private(m)) • • Everyone can encrypt Only one can decrypt • • Only one can claim it Everyone can check it Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Digital Signatures • • • • • Cryptographic technique analogous to handwritten signatures Bob (sender) digitally signs document, establishing he document owner/creator Bob signs message m by encrypting with his private key KB-, creating signed message KB-(m). Verifiable, non-forgeable: Alice (recipient) can prove to someone that Bob and no one else must have signed the document Non-repudiation: - Alice can take m and signature KB-(m) to court and prove that Bob signed m - Only Bob possesses KBAdapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Digital Signature Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross Signed Message Digests • • • • • Computationally expensive to encrypt long messages with public key crypto Goal: - Fixed-length Easy-to-compute Digital fingerprint Apply hash function H to m and get fixed size message digest H(m). Sign H(m) Send (m, KB-(H(m))) Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Digital Signature Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross Impersonation Attack Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross Impersonation Attack Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross Replay Attack Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross Nonce (timeliness) • • Nonce: number R used only once-in-a-lifetime KA-B : Shared secret key Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Nonce (timeliness) Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross (Wo)Man-in-the-Middle Attack Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Source: Kurose Ross (Wo)Man-in-the-Middle Attack • Difficult to detect • Alice receives everything Bob sends • • Bob and Alice can meet later and still recall the last conversation Trudy receives all messages as well! Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Public Key Certification • • • • • Certification Authority (CA) - binds public key to particular entity (Bob) Bob provides proof of identity to CA CA creates certificate binding Bob to his public key Certificate containing Bob’s public key digitally signed by CA - CA says “this is Bob’s public key” When Alice wants Bob’s public key - gets Bob’s certificate (from Bob or elsewhere) apply CA’s public key to Bob’s certificate gets Bob’s public key Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Outlook • Secure channel • Principles of cryptography • Authentication, Integrity • Security at different layers • Firewalls and Intrusion Detection Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Security at Different Layers Mail: MIME/S, PGP Application TLS (Secure Socket Layer, SSL) Transport Network IP Security (IPSec) Packet Filter Frame Filter, WPA Link WEP Physical Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Outlook • Secure channel • Principles of cryptography • Authentication, Integrity • Security at different layers • Firewalls and Intrusion Detection Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Firewalls • Isolates organization’s internal network from larger Internet, allowing some packets to pass, blocking others Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Firewalls • • Prevent denial of service attacks - Prevent illegal modification/access of internal data - • • SYN flooding: attacker establishes many bogus TCP connections, no resources left for real connections Attacker replaces website’s homepage with something else Allow only authorized access to inside network - Set of authenticated users Three types of firewalls - Stateless packet filters Stateful packet filters Application gateways Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Stateless Packet Filtering • • Internal network connected to Internet via router firewall Router filters packet-by-packet, decision to forward/drop packet based on - • Source IP address, destination IP address TCP/UDP source and destination port numbers ICMP message type TCP SYN and ACK bits Example: - - Block incoming and outgoing datagrams with IP protocol field 17 All incoming and outgoing UDP flows are blocked Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Access Control Lists Action Source Address Dest Protoc Source Address ol Port Dest Port Flag Bit allow outside of 222.22/16 TCP 222.22/16 >1023 80 any allow outside of 222.22/16 TCP 222.22/16 80 >1023 ACK all all all deny all all all Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Stateful Packet Filtering • • Stateless packet filtering - Admits packets that makes no sense e.g. dest port=80, ACK bit set, even though no TCP connection established Stateful packet filtering tracks - Status of every TCP connection Connection setup (SYN) Connection teardown (FIN) Timeout inactive connections at firewall Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner Intrusion Detection Systems • • Packet filtering - operates on TCP/IP headers only no correlation check among sessions Intrusion Detection System - Deep packet inspection: Look at packet contents for viruses, attack patterns, etc. - Examine correlation among multiple packets for port scanning, network mapping, Denial of Service (DoS) attack, etc. Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

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# authentication key - Department of Information Technology