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8-1 What is network security? Confidentiality: only sender, intended receiver should “understand” message contents m sender encrypts message m receiver decrypts message 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 Non-repudiation: receiver want to ensure message must have come from claimed sender Access and Availability: services must be accessible and available to (legitimate) users (e.g. no DoS)

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8-2 Friends and enemies: Alice, Bob, Trudy r Alice, Bob, and Trudy well-known short hands in network security world r Bob, Alice want to communicate “securely” r Trudy (intruder) may intercept, delete, add messages secure sender secure receiver channel data, control messages data Alice Bob Trudy Q: What can a “bad guy” do? m eavesdrop: intercept messages m actively insert messages into connection m impersonation: can fake (spoof) source address in packet (or any field in packet) m hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place m denial of service: prevent service from being used by others (e.g., by overloading resources)

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8-3 The language of cryptography symmetric key crypto: sender, receiver keys identical. Key used to encrypt and decrypt message. public-key crypto: encryption key public, decryption key secret (private) plaintext ciphertext K A-B encryption algorithm decryption algorithm K A-B plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B Symmetric key cryptography symmetric key crypto: Bob and Alice share same (symmetric) key: K r e.g., key is knowing substitution pattern in mono alphabetic substitution cipher r Bob and Alice agree on key value using the Diffie-Hellman Key Exchange

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8-4 Public Key Cryptography public key cryptography r sender, receiver do not share secret key r public encryption key known to all r private decryption key known only to receive plaintext message, m ciphertext encryption algorithm decryption algorithm Bob’s public key plaintext message K (m) B + K B + Bob’s private key K B - m = K ( K (m) ) B + B - need K ( ) and K ( ) such that B B 1 + - K (K (m)) = m B B - + given public key K, it should be impossible to compute private key K B B 2 + - Requirements

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8-5 Authentication Goal: Bob wants Alice to “prove” her identity to him Alice says “I am Alice” in an IP packet containing her source IP address Failure scenario?? K (m) B + Symmetric vs Public key encryption: There is no guarantee that the message came from Alice, since anyone can send public key encrypted to Bob, In contrast, in symmetric key encryption the sender is implicitly identified to the receiver. “I am Alice” Alice’s IP address Trudy can create a packet “spoofing” Alice’s address

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8-6 Authentication: r can we authenticate using public key techniques? use of nonce, public key cryptography “I am Alice” R Bob computes K (R) A - “send me your public key” K A + (K (R)) = R A - K A + and knows only Alice could have the private key, that encrypted R such that (K (R)) = R A - K A +

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8-7 Message integrity :Digital Signatures We all often signs documents (checks, credit card receipts, legal documents, …). How to sign electronic documents? Public key cryptographic technique analogous to hand-written signatures. r sender (Bob) digitally signs document, establishing he is document owner/creator. r verifiable, non-forgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document r Simple digital signature for message m: r Bob signs m by encrypting with his private key K B, creating “signed” message, K B (m) - - Dear Alice Oh, how I have missed you. I think of you all the time! …(blah blah blah) Bob Bob’s message, m Public key encryption algorithm Bob’s private key K B - Bob’s message, m, signed (encrypted) with his private key K B - (m)

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8-8 Digital Signatures (more) r Suppose Alice receives msg m, digital signature K B (m) r Alice verifies m signed by Bob by applying Bob’s public key K B to K B (m) then checks K B (K B (m) ) = m. r If K B (K B (m) ) = m, whoever signed m must have used Bob’s private key. + + - - -- + Alice thus verifies that: ü Bob signed m. ü No one else signed m. ü Bob signed m and not m’. Non-repudiation: Alice can take m, and signature K B (m) to court and prove that Bob signed m. -

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8-9 Trusted Intermediaries Symmetric key problem: r How do two entities establish shared secret key over network? Solution: trusted key distribution center (KDC) acting as intermediary between entities Public key problem: r When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: trusted certification authority (CA) r Alice, Bob need shared symmetric key. r KDC: server shares different secret key with each registered user (many users) r Alice, Bob know own symmetric keys, K A-KDC K B-KDC, for communicating with KDC.

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8-10 Key Distribution Center (KDC) K B-KDC K X-KDC K Y-KDC K Z-KDC K P-KDC K B-KDC K A-KDC K P-KDC KDC Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? Alice knows R1 Bob knows to use R1 to communicate with Alice Alice and Bob communicate: using R1 as session key for shared symmetric encryption KDC generates R1 K B-KDC (A,R1) K A-KDC (A,B) K A-KDC (R1, K B-KDC (A,R1) )

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8-11 Certification Authorities r Certification authority (CA): binds public key to particular entity, E. r E (person, router) registers its public key with CA. m E provides “proof of identity” to CA. m CA creates certificate binding E to its public key. m certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key” Bob’s public key K B + Bob’s identifyingi nformation digital signature (encrypt) CA private key K CA - K B + certificate for Bob’s public key, signed by CA Bob’s public key K B + digital signature (decrypt) CA public key K CA + K B + r When Alice wants Bob’s public key: m gets Bob’s certificate (Bob or elsewhere). m apply CA’s public key to Bob’s certificate, get Bob’s public key

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