Network Security Network Security Srinidhi Varadarajan
Network security Network security Foundations: � what is security? � cryptography � authentication � message integrity � key distribution and certification Security in practice: � application layer: secure e-mail � transport layer: Internet commerce, SSL, SET 2
Friends and enemies: Alice, Bob, Trudy Friends and enemies: Alice, Bob, Trudy Figure 7.1 goes here � well-known in network security world � Bob, Alice want to communicate “securely” � Trudy, the “intruder” may intercept, delete, add messages 3
What is network security? What is network security? Secrecy: only sender, intended receiver should “understand” msg contents – sender encrypts msg – receiver decrypts msg Authentication: sender, receiver want to confirm identity of each other Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection 4
Internet security threats Internet security threats Packet sniffing: – broadcast media – promiscuous NIC reads all packets passing by – can read all unencrypted data (e.g. passwords) – e.g.: C sniffs B’s packets C A src:B dest:A payload B 5
Internet security threats Internet security threats IP Spoofing: – can generate “raw” IP packets directly from application, putting any value into IP source address field – receiver can’t tell if source is spoofed – e.g.: C pretends to be B C A src:B dest:A payload B 6
Internet security threats Internet security threats Denial of service (DOS): – flood of maliciously generated packets “swamp” receiver – Distributed DOS (DDOS): multiple coordinated sources swamp receiver – e.g., C and remote host SYN-attack A C A SYN SYN SYN SYN SYN B SYN SYN 7
The language of cryptography The language of cryptography K K plaint ext plaint ext B A ciphert ext Figure 7.3 goes here symmetric key crypto: sender, receiver keys identical public-key crypto: encrypt key public , decrypt key secret 8
Symmetric key cryptography Symmetric key cryptography substitution cipher: substituting one thing for another – monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq E.g.: Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Q: How hard t o break t his simple cipher?: •brut e f orce (how hard?) •ot her? 9
Symmetric key crypto: DES Symmetric key crypto: DES DES: Data Encryption Standard � US encryption standard [NIST 1993] � 56-bit symmetric key, 64 bit plaintext input � How secure is DES? – DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months – no known “backdoor” decryption approach � making DES more secure – use three keys sequentially (3-DES) on each datum – use cipher-block chaining 10
Symmetric key Symmetric key crypto: DES crypto: DES DES operat ion initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation 11
Public Key Cryptography Public Key Cryptography symmetric key crypto public key � requires sender, cryptography receiver know shared secret key � radically different approach [Diffie- � Q: how to agree on Hellman76, RSA78] key in first place (particularly if never � sender, receiver do “met”)? not share secret key � encryption key public (known to all) � decryption key private (known only to receiver) 12
Public key cryptography Public key cryptography Figure 7.7 goes here 13
Public key encryption algorithms Public key encryption algorithms Two int er-relat ed requirement s: need d ( ) and e ( ) such that . . 1 B B d (e (m)) = m B B 2 need public and private keys . . for d ( ) and e ( ) B B RSA: Rivest , Shamir, Adelson algor it hm 14
Authentication Authentication Goal: Bob wants Alice to “prove” her identity to him Prot ocol ap1.0: Alice says “I am Alice” Failure scenario?? 15
Authentication: another try Authentication: another try Prot ocol ap2.0: Alice says “I am Alice” and sends her I P address along t o “prove” it . Failure scenario?? 16
Authentication: another try Authentication: another try Prot ocol ap3.0: Alice says “I am Alice” and sends her secret password t o “prove” it . Failure scenario? 17
Authentication: yet another try Authentication: yet another try Prot ocol ap3.1: Alice says “I am Alice” and sends her encrypt ed secret password t o “prove” it . I am Alice encrypt (password) Failure scenario? 18
Authentication: yet another try Authentication: yet another try Goal: avoid playback at t ack Nonce: number (R) used onlyonce in a lif et ime ap4.0: t o prove Alice “live”, Bob sends Alice nonce, R. Alice must ret urn R, encrypt ed wit h shared secret key Figure 7.11 goes here Failures, drawbacks? 19
Authentication: ap5.0 Authentication: ap5.0 ap4.0 requires shared symmetric key – problem: how do Bob, Alice agree on key – can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography Figure 7.12 goes here 20
ap5.0: security hole ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Figure 7.14 goes here Need “cert if ied” public keys (more lat er … ) 21
Digital Signatures Digital Signatures Cryptographic technique Simple digital signature analogous to hand- for message m: written signatures. � Bob encrypts m with his � Sender (Bob) digitally private key d B , creating signs document, signed message, d B (m). establishing he is � Bob sends m and d B (m) to document owner/creator. Alice. � Verifiable, nonforgeable: recipient (Alice) can verify that Bob, and no one else, signed document. 22
Digital Signatures (more) Digital Signatures (more) � Suppose Alice Alice thus verifies that: receives msg m , and – Bob signed m . digital signature d B (m) – No one else signed m . � Alice verifies m – Bob signed m and not signed by Bob by m’ . applying Bob’s public Non-repudiation: key e B to d B (m) then – Alice can take m , and checks e B (d B (m) ) = m. signature d B (m) to � If e B (d B (m) ) = m , court and prove that whoever signed m Bob signed m . must have used Bob’s private key. 23
Message Digests Message Digests Computationally expensive to public-key-encrypt long Hash function properties: messages � Many-to-1 Goal: fixed-length,easy to � Produces fixed-size msg compute digital signature, digest (fingerprint) “fingerprint” � Given message digest x, computationally infeasible � apply hash function H to m , to find m such that x = get fixed size message H(m) digest, H(m). � computationally infeasible to find any two messages m and m’ such that H(m) = H(m’). 24
Digital signature = Signed message digest Digital signature = Signed message digest Bob sends digitally signed Alice verifies signature and message: integrity of digitally signed message: 25
Hash Function Algorithms Hash Function Algorithms � MD5 hash function widely � Internet checksum used. would make a poor – Computes 128-bit message digest. message digest in 4-step – Too easy to find process. two messages with – arbitrary 128-bit string x, same checksum. appears difficult to construct msg m whose MD5 hash is equal to x. � SHA-1 is also used. – US standard – 160-bit message digest 26
Trusted Intermediaries Trusted Intermediaries Problem: Problem: – How do two entities – When Alice obtains establish shared Bob’s public key secret key over (from web site, e- network? mail, diskette), how does she know it is Solution: Bob’s public key, – trusted key not Trudy’s? distribution center Solution: (KDC) acting as intermediary – trusted certification between entities authority (CA) 27
Key Distribution Center (KDC) Key Distribution Center (KDC) � Alice,Bob need shared symmetric key. � KDC: server shares different secret key with each registered user. � Alice communicates with � Alice, Bob know own KDC, gets session key R1, symmetric keys, K A- and K B-KDC (A,R1) KDC K B-KDC , for � Alice sends Bob communicating with K B-KDC (A,R1), Bob extracts R1 KDC . � Alice, Bob now share the symmetric key R1. 28
Certification Authorities Certification Authorities � Certification authority (CA) binds public key to particular entity. � Entity (person, router, etc.) can register its public key with CA. – Entity provides “proof of identity” to CA. – CA creates certificate � When Alice wants Bob’s binding entity to public public key: key. � gets Bob’s certificate (Bob or – Certificate digitally elsewhere). signed by CA. � Apply CA’s public key to Bob’s certificate, get Bob’s public key 29
Secure e- -mail mail Secure e • Alice want s t o send secret e-mail message, m, t o Bob. • generat es random symmet ric privat e key, K S . • encrypt s message wit h K S • also encrypt s K S wit h Bob’s public key. • sends bot h K S (m) and e B (K S ) t o Bob. 30
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