CS Computer Networks 1: Authentication
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1 CS Computer Networks 1: Authentication Professor Patrick Traynor 4/14/11 Lecture 25
2 Announcements Homework 3 is due next class. Submit via T-Square or in person. Project 3 has been graded. Scores have been posted! Project 4 is due in 1 week nuff said... 2
3 Last Time What are the four (general) properties security tries to provide? The Caeser Cipher is an example of what kind of cryptographic cipher? What are the differences between symmetric and asymmetric (public key) cryptography? 3
4 Diffie-Hellman - Class Exercise Select a partner. Setup: Pick a prime number p and a base g (<p) p=13, g=4 Each partner chose a private value x (<p-1) Generate the following value and exchange it. y = g x mod p Now generate the shared secret z: z = y x mod p You should have both calculated the same value for z. This is your key! 4
5 Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message Integrity 8.4 End point Authentication 8.5 Securing 8.6 Securing TCP connections: SSL 8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS 5
6 Message Integrity Bob receives msg from Alice, wants to ensure: message originally came from Alice message not changed since sent by Alice Cryptographic Hash: takes input m, produces fixed length value, H(m) e.g., as in Internet checksum... but a bit different... computationally infeasible to find two different messages, x, y such that H(x) = H(y) equivalently: given m = H(x), (x unknown), can not determine x. note: Internet checksum fails this requirement! 6
7 Internet Checksum: Poor Crypto Hash Function Internet checksum has some properties of hash function: produces fixed length digest (16-bit sum) of message is many-to-one But given a message with given hash value, it is easy to find another message with same hash value: message ASCII format message ASCII format I O U B O B 49 4F E F 42 I O U B O B 49 4F E F 42 B2 C1 D2 AC different messages but identical checksums! B2 C1 D2 AC 7
8 Message Authentication Code (MAC) (shared secret) s (message) m append m H(m+s) public Internet m H(m+s) H(. ) H(m+s) compare H(. ) H(m+s) s (shared secret) 8
9 MACs in Practice MD5 hash function widely used (RFC 1321) computes 128-bit MAC in 4-step process. arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x recent (2005) attacks on MD5 SHA-1 is also used US standard [NIST, FIPS PUB 180-1] 160-bit MAC Brute-force attacks on SHA now require 2 63 operations to find a collision. 9
10 Digital Signatures Cryptographic technique analogous to hand-written signatures. sender (Bob) digitally signs document, establishing he is document owner/creator. verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document 10
11 Digital Signatures simple digital signature for message m: Bob signs m by encrypting with his private key KB, creating signed message, KB(m) Bob s message, m Dear Alice Oh, how I have missed you. I think of you all the time! (blah blah blah) Bob K B - public key encryption algorithm Bob s private key K B - (m) Bob s message, m, signed (encrypted) with his private key 11
12 Digital Signatures (more) Suppose Alice receives msg m, digital signature KB(m) Alice verifies m signed by Bob by applying Bob s public key KB to KB(m) then checks KB(KB(m) ) = m. + - If KB(KB(m) ) = m, whoever signed m must have used Bob s private key. Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not m. non-repudiation: Alice can take m, and signature KB(m) to court and prove that Bob signed m. 12
13 Digital Signature = signed MAC Bob sends digitally signed message: large message m + Bob s H: hash function private key K B - H(m) digital signature (encrypt) encrypted msg digest - K B (H(m)) Alice verifies signature and integrity of digitally signed message: large message m H: hash function H(m) Bob s public key K B + equal? encrypted msg digest - K B (H(m)) digital signature (decrypt) H(m) 13
14 Public Key Certification Public Key Problem: When Alice obtains Bob s public key (from web site, , diskette), how does she know it is Bob s public key, not Trudy s? Solution: Trusted certification authority (CA) 14
15 Certificate Authorities Certificate Authority (CA): binds public key to particular entity, E. E registers its public key with CA. E provides proof of identity to CA. CA creates certificate binding E to its public key. 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 identifying information digital signature (encrypt) CA private key K - CA K CA (K ) B K B + certificate for Bob s public key, signed by CA 15
16 Certificate Authority When Alice wants Bob s public key: gets Bob s certificate (Bob or elsewhere). apply CA s public key to Bob s certificate, get Bob s public key K B B K CA (K ) digital signature (decrypt) K B + Bob s public key CA public key K + CA 16
17 A Certificate Contains: Serial number (unique to issuer) info about certificate owner, including algorithm and key value itself (not shown) info about certificate issuer valid dates digital signature by issuer 17
18 Problems with PKI Why exactly do you trust a CA? Anyone have any idea how many you actually trust? If two CAs present you with a certificate for Microsoft, which one is right? What prevents a CA from making up a key for you? What happens when keys are compromised? 18
19 Chapter 8 Roadmap 8.1 What is network security 8.2 Principles of cryptography 8.3 Message Integrity 8.4 End point Authentication 8.5 Securing 8.6 Securing TCP connections: SSL 8.7 Network layer security: IPsec 8.8 Securing wireless LANs 8.9 Operational security: firewalls and IDS 19
20 Authentication Goal: Bob wants Alice to prove her identity to him Protocol ap1.0: Alice says I am Alice I am Alice Failure scenario?? 20
21 Authentication Goal: Bob wants Alice to prove her identity to him Protocol ap1.0: Alice says I am Alice I am Alice in a network, Bob can not see Alice, so Trudy simply declares herself to be Alice 21
22 Authentication: another try Protocol ap2.0: Alice says I am Alice in an IP packet containing her source IP address Alice s IP address I am Alice Failure scenario?? 22
23 Authentication: another try Protocol ap2.0: Alice says I am Alice in an IP packet containing her source IP address Trudy can create a packet spoofing Alice s address Alice s IP address I am Alice 23
24 Authentication: another try Protocol ap3.0: Alice says I am Alice and sends her secret password to prove it. Alice s IP addr Alice s password I m Alice Alice s IP addr OK Failure scenario?? 24
25 Authentication: another try Protocol ap3.0: Alice says I am Alice and sends her secret password to prove it. Alice s IP addr Alice s password I m Alice Alice s OK IP addr replay attack: Trudy records Alice s packet and later plays it back to Bob Alice s IP addr Alice s password I m Alice 25
26 Authentication: yet another try Protocol ap3.1: Alice says I am Alice and sends her encrypted secret password to prove it. Alice s IP addr encrypted password I m Alice Alice s IP addr OK Failure scenario?? 26
27 Authentication: another try Protocol ap3.1: Alice says I am Alice and sends her encrypted secret password to prove it. Alice s IP addr encrypted password I m Alice Alice s OK IP addr record and playback still works! Alice s IP addr encrypted password I m Alice 27
28 Authentication: yet another try Goal: avoid playback attack Nonce: number (R) used only once in-a-lifetime ap4.0: to prove Alice live, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key I am Alice Failures, drawbacks? R K A-B (R) Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice! 28
29 Authentication: ap5.0 ap4.0 requires shared symmetric key can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography R I am Alice - K A (R) send me your public key K A + Bob computes + K A - (K (R)) = R A and knows only Alice could have the private key, that encrypted R such that K A + - (K (R)) = R A 29
30 ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) I am Alice I am Alice - + m = K (K (m)) A A R - K (R) A Send me your public key + K (m) A K + A Trudy gets - + m = K (K (m)) T T R sends m to Alice encrypted with Alice s public key - K (R) T Send me your public key + K (m) T K + T 30
31 ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Difficult to detect: Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet one week later and recall conversation) problem is that Trudy receives all messages as well! 31
32 Remember Diffie-Hellman? How does Alice know Bob sent TA? Alice picks her x: SA TA=g SA mod p TB=g SB mod p Bob picks his x: SB Alice computes: K = TB SA mod p TB SA = (g SB ) SA = g SB SA = g SA SB = (g SA ) SB = TA SB mod p Encrypted communication with K Alice computes: K = TA SB mod p There is nothing to prevent a man-in-the-middle attack against this protocol. 32
33 Next Time Read Sections Read Security Problems in the TCP/IP Protocol Suite by Bellovin. Homework 3 is due at the beginning of next class. Show up late and it will be marked as late! 33
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