8: Network Security8-1 Chapter 8 Network Security (some reviews and security protocols) These ppt slides are originally from the Kurose and Ross’s book. But some slides are deleted and added for my own purpose, and some of them are modified.

8: Network Security8-2 What is network security? Confidentiality: only sender, intended receiver should “understand” message contents 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 Message repudiation: sender cannot deny that he really sent the message. Access and Availability: services must be accessible and available to users

8: Network Security8-3 What we have to consider r Cryptography m Cryptography algorithms r Network security protocols r Security for individual attacks m Ex. Web security

8: Network Security8-4 The language of cryptography symmetric key crypto: sender, receiver keys identical public-key crypto: encryption key public, decryption key secret (private) plaintext ciphertext K A encryption algorithm decryption algorithm Alice’s encryption key Bob’s decryption key K B

8: Network Security8-5 Cryptography algorithms r Symmetric key algorithms m DES (Data Encryption Standard) m AES (Advanced Encryption Standard) r Asymmetric key algorithms m RSA r Diffie-Hellman m Two parties create a symmetric session key to exchange data without having to store the key for future use.

8: Network Security8-6 Symmetric key cryptography substitution cipher: substituting one thing for another m monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc E.g.: Q: How hard to break this simple cipher?:  brute force (how hard?)  other?

8: Network Security8-7 Symmetric key cryptography symmetric key crypto: Bob and Alice share know same (symmetric) key: K r e.g., key is knowing substitution pattern in mono alphabetic substitution cipher r Q: how do Bob and Alice agree on key value? plaintext ciphertext K A-B encryption algorithm decryption algorithm A-B K plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B

8: Network Security8-8 Symmetric key crypto: DES DES: Data Encryption Standard r US encryption standard [NIST 1993] r 56-bit symmetric key, 64-bit plaintext input r How secure is DES? m DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months m no known “backdoor” decryption approach r making DES more secure: m use three keys sequentially (3-DES) on each datum m use cipher-block chaining

8: Network Security8-9 Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation

8: Network Security8-10 AES: Advanced Encryption Standard r new (Nov. 2001) symmetric-key NIST standard, replacing DES r processes data in 128 bit blocks r 128, 192, or 256 bit keys r brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES

8: Network Security8-11 Public Key Cryptography symmetric key crypto r requires sender, receiver know shared secret key r Q: how to agree on key in first place (particularly if never “met”)? public key cryptography r radically different approach [Diffie- Hellman76, RSA78] r sender, receiver do not share secret key r public encryption key known to all r private decryption key known only to receiver

8: Network Security8-12 Public key cryptography 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 -

8: Network Security8-13 Public key encryption algorithms need K ( ) and K ( ) such that B B.. given public key K, it should be impossible to compute private key K B B Requirements: 1 2 RSA: Rivest, Shamir, Adelson algorithm + - K (K (m)) = m B B

8: Network Security8-14 RSA: Choosing keys 1. Choose two large prime numbers p, q. (e.g., 1024 bits each) 2. Compute n = pq, z = (p-1)(q-1) 3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”). 4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ). 5. Public key is (n,e). Private key is (n,d). K B + K B -

8: Network Security8-15 RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 1. To encrypt bit pattern, m, compute c = m mod n e (i.e., remainder when m is divided by n) e 2. To decrypt received bit pattern, c, compute m = c mod n d (i.e., remainder when c is divided by n) d m = (m mod n) e mod n d Magic happens! c

8: Network Security8-16 RSA example: Bob chooses p=5, q=7. Then n=35, z=24. e=5 (so e, z relatively prime). d=29 (so ed-1 exactly divisible by z. letter m m e c = m mod n e l c m = c mod n d c d letter l encrypt: decrypt:

8: Network Security8-17 RSA: Why is that m = (m mod n) e mod n d (m mod n) e mod n = m mod n d ed Useful number theory result: If p,q prime and n = pq, then: x mod n = x mod n yy mod (p-1)(q-1) = m mod n ed mod (p-1)(q-1) = m mod n 1 = m (using number theory result above) (since we chose ed to be divisible by (p-1)(q-1) with remainder 1 )

8: Network Security8-18 RSA: another important property The following property will be very useful later: K ( K (m) ) = m B B - + K ( K (m) ) B B + - = use public key first, followed by private key use private key first, followed by public key Result is the same!

8: Network Security8-19 Why is RSA Secure? r Suppose you know Alice’s public key (n,e). How hard is it to determine d? r Essentially need to find factors of n without knowing the two factors p and q. r Fact: factoring a big number is hard. Generating RSA keys r Have to find big primes p and q r Approach: make good guess then apply testing rules (see Kaufman)

8: Network Security8-20 RSA is slow r Exponentiation is computationally intensive r DES is at least 100 times faster than RSA r Session key, K S m Bob and Alice use RSA to exchange a symmetric key K S m Once both have K S, they use DES

8: Network Security8-21 Message authentication and integrity r Cryptographic algorithms are also used for message authentication and integrity.

8: Network Security8-22 Message Digests r Function H( ) that takes as input an arbitrary length message and outputs a fixed-length strength: “message signature” r Note that H( ) is a many- to-1 function r H( ) is often called a “hash function” r Desirable properties: m Easy to calculate m Irreversibility: Can’t determine m from H(m) m Collision resistance: Computationally difficult to produce m and m’ such that H(m) = H(m’) m Seemingly random output large message m H: Hash Function H(m)

8: Network Security8-23 Hash Function Algorithms r MD5 hash function widely used (RFC 1321) m computes 128-bit message digest in 4-step process. m arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x. r SHA-1 is also used. m US standard [ NIST, FIPS PUB 180-1] m 160-bit message digest

8: Network Security8-24 Message Authentication Code (MAC) message H( ) message H( ) compare r Notation: MD m = H(m) ; MAC = K(H(m)); send {m||MAC} MDm m MAC

8: Network Security8-25 MAC r Message digest hashed from a message provides the integrity of the message, but not the authenticity of the sender. r MAC is distinguished from message digest(MD) in the way that MAC takes message and symmetric key as inputs and generates the small block of data as output(so is called keyed hash).

8: Network Security8-26 Digital Signatures Cryptographic technique analogous to hand- written signatures. r sender (Bob) digitally signs document, establishing he is document owner/creator. r verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document r Digital signature uses the asymmetric key algorithms.

8: Network Security8-27 Digital Signatures 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)

8: Network Security8-28 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. -

8: Network Security8-29 Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length, easy- to-compute digital “fingerprint” r apply hash function H to m, get fixed size message digest, H(m). Hash function properties: r many-to-1 r produces fixed-size msg digest (fingerprint) r given message digest x, computationally infeasible to find m such that x = H(m) large message m H: Hash Function H(m)

8: Network Security8-30 large message m H: Hash function H(m) digital signature (encrypt) Bob’s private key K B - + Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: K B (H(m)) - encrypted msg digest K B (H(m)) - encrypted msg digest large message m H: Hash function H(m) digital signature (decrypt) H(m) Bob’s public key K B + equal ? Digital signature = signed message digest

8: Network Security8-31 Key distribution r In the symmetric key algorithm, how can only two parties have the key without it being known to others? r In the asymmetric key algorithm, if someone claims that it is my public key, how can I trust that the key is really his public key? r To solve this problem, we need to have the trust base (starting point).

8: Network Security8-32 Trusted Intermediaries Symmetric key problem: r How do two entities establish shared secret key over network? Solution: r trusted key distribution center (KDC) acting as intermediary between entities Public key problem: r 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: r trusted certification authority (CA)

8: Network Security8-33 Key Distribution Center (KDC) 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. 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

8: Network Security8-34 Key Distribution Center (KDC) 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 Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? KDC generates R1 K B-KDC (A,R1) K A-KDC (A,B) K A-KDC (R1, K B-KDC (A,R1) )

8: Network Security8-35 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 identifying information digital signature (encrypt) CA private key K CA - K B + certificate for Bob’s public key, signed by CA

8: Network Security8-36 Certification Authorities 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 Bob’s public key K B + digital signature (decrypt) CA public key K CA + K B +

8: Network Security8-37 A certificate contains: r Serial number (unique to issuer) r info about certificate owner, including algorithm and key value itself (not shown) r info about certificate issuer r valid dates r digital signature by issuer

8: Network Security8-38 Security protocols r PGP: secure r SSL(TSL): http vs. https r SSH: telnet vs. SSH r Ipsec r WEB: wireless LAN r And so on

8: Network Security8-39 Secure Alice:  generates random symmetric private key, K S.  encrypts message with K S (for efficiency)  also encrypts K S with Bob’s public key.  sends both K S (m) and K B (K S ) to Bob.  Alice wants to send confidential , m, to Bob. K S ( ). K B ( ) K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +

8: Network Security8-40 Secure Bob:  uses his private key to decrypt and recover K S  uses K S to decrypt K S (m) to recover m  Alice wants to send confidential , m, to Bob. K S ( ). K B ( ) K S (m ) K B (K S ) + m KSKS KSKS KBKB + Internet K S ( ). K B ( ). - KBKB - KSKS m K S (m ) K B (K S ) +

8: Network Security8-41 Secure (continued) Alice wants to provide sender authentication message integrity. Alice digitally signs message. sends both message (in the clear) and digital signature. H( ). K A ( ) H(m ) K A (H(m)) - m KAKA - Internet m K A ( ). + KAKA + K A (H(m)) - m H( ). H(m ) compare

8: Network Security8-42 Secure (continued) Alice wants to provide secrecy, sender authentication, message integrity. Alice uses three keys: her private key, Bob’s public key, newly created symmetric key H( ). K A ( ). - + K A (H(m)) - m KAKA - m K S ( ). K B ( ). + + K B (K S ) + KSKS KBKB + Internet KSKS

8: Network Security8-43 Pretty good privacy (PGP) r Internet encryption scheme, de-facto standard. r uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. r provides secrecy, sender authentication, integrity. r inventor, Phil Zimmerman, was target of 3-year federal investigation. ---BEGIN PGP SIGNED MESSAGE--- Hash: SHA1 Bob:My husband is out of town tonight.Passionately yours, Alice ---BEGIN PGP SIGNATURE--- Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ hFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- A PGP signed message:

8: Network Security8-44 SSL: Secure Sockets Layer r Most widely deployed security protocol m Supported by almost all browsers and web servers m https m Tens of billions $ spent per year over SSL r Originally designed by Netscape in 1993 r Number of variations: m TLS: transport layer security, RFC 2246 m SSL v3.0 = TLS v1.0 r Provides m Confidentiality m Integrity m Authentication r Original goals: m Had Web e-commerce transactions in mind m Encryption (especially credit-card numbers) m Web-server authentication m Optional client authentication m Minimum hassle in doing business with new merchant r Available to all TCP applications m Secure socket interface

8: Network Security8-45 SSL and TCP/IP Application TCP IP Normal Application Application SSL TCP IP Application with SSL SSL provides application programming interface (API) to applications C and Java SSL libraries/classes readily available

8: Network Security8-46 Could do something like PGP: But want to send byte streams & interactive data Want a set of secret keys for the entire connection Want certificate exchange part of protocol: handshake phase H( ). K A ( ). - + K A (H(m)) - m KAKA - m K S ( ). K B ( ). + + K B (K S ) + KSKS KBKB + Internet KSKS

8: Network Security8-47 Real SSL: Handshake (1) Purpose 1. Server authentication 2. Negotiation: agree on crypto algorithms 3. Establish keys 4. Client authentication (optional)

8: Network Security8-48 Real SSL: Handshake (2) 1. Client sends list of algorithms it supports, along with client nonce 2. Server chooses algorithms from list; sends back: choice + certificate + server nonce 3. Client verifies certificate, extracts server’s public key, generates pre_master_secret, encrypts with server’s public key, sends to server 4. Client and server independently compute encryption and MAC keys from pre_master_secret and nonces 5. Client sends a MAC of all the handshake messages 6. Server sends a MAC of all the handshake messages

8: Network Security8-49 handshake Client’s nonce Server’s nonce Pre-master secret generator Master secret generator Server’s MAC key Server’s encryption key Server’s IV client’s MAC key client’s encryption key client’s IV

8: Network Security8-50 Real SSL: Handshaking (3) Last 2 steps protect handshake from tampering r Client typically offers range of algorithms, some strong, some weak r Man-in-the middle could delete the stronger algorithms from list r Last 2 steps prevent this r Last two messages are encrypted

8: Network Security8-51 SSL Record Protocol data fragment data fragment MAC encrypted data and MAC encrypted data and MAC record header record header record header: content type; version; length MAC: includes sequence number, MAC key M x Fragment: each fragment 2 14 bytes

8: Network Security8-52 SSL Record Format content type SSL version length MAC data 1 byte 2 bytes Data and MAC encrypted (symmetric algo)

8: Network Security8-53 Content types in record header r application_data (23) r alert (21) m signaling errors during handshake m signal connection closure r handshake (22) m initial handshake messages are carried in records of type “handshake” r change_cipher_spec (20) m indicates change in encryption and authentication algorithms

8: Network Security8-54 handshake: ClientHello handshake: ServerHello handshake: Certificate handshake: ServerHelloDone handshake: ClientKeyExchange ChangeCipherSpec handshake: Finished ChangeCipherSpec handshake: Finished application_data Alert: warning, close_notify Real Connection TCP Fin follow

8: Network Security8-55 Key derivation r Client random, server random, and pre-master secret input into pseudo random-number generator. m Produces master secret r Master secret, client and server random numbers into another random-number generator m Produces “key block” r Key block sliced and diced: m client MAC key m server MAC key m client encryption key m server encryption key m client initialization vector (IV) m server initialization vector (IV)