Network Security

Network Security • Volkan Cambazoglu 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

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 (provided by Bob) 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

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. • such as • Discrete Logarithm • Factorizing large numbers f(x): efficient x f(x) f (x): hard -1 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 ciphertext f f m c = f(m,k) m k k Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

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 message m = KB-(KB+(m)) encryption algorithm decryption algorithm Plaintext message, m Ciphertext KB+(m) Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

Public Key Cryptography Public key: KB+(m) Private key: KB-(m) Plaintext message m = KB-(KB+(m)) encryption algorithm decryption algorithm Plaintext message, m Ciphertext KB+(m) • What could go wrong here? • 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) • 136064817260489928484113640026944941480975382962539945337862848254226224034275820538310008858403955437239102681465761388249980135083342434428721426840110617593953169835450968550730769430412845048185659381370857105323219453521491277894773367539216680431287506338710965204349119030528157752992551375455100484051 (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

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 hand-written 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 KB- 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

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

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

Security at Different Layers 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 • SYN flooding: attacker establishes many bogus TCP connections, no resources left for real connections • Prevent illegal modification/access of internal data • 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

Network Security