Principles of Network Security

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1 he Network Security Model Bob and lice want to communicate securely. rudy (the adversary) has access to the channel. lice channel data, control s Bob Kai Shen data secure sender secure receiver data rudy 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring Who might Bob and lice be? What can an adversary do? Web browser/server for electronic transactions (e.g., on line purchases/banking) DNS servers outers exchanging routing table updates well, real life Bobs and lices! Eavesdrop: understand the content of s ctively changing s Impersonation: fake (spoof) identity Denial of service: prevent service from being used by others (e.g., by overloading resources) 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring CSC 257/457 Spring

2 What is Network Security? Confidentiality: only sender, intended receiver should understand contents. uthentication: sender, receiver want to confirm identity of each other. Message Integrity: sender, receiver want to ensure not altered (in transit, or afterwards). Confidentiality: cryptography uthentication Integrity Key distribution and certification ccess and vailability: services must be accessible and available to (and only to) legitimate users. 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring he Language of Cryptography Symmetric Key Cryptography: Monoalphabetic Cipher plaintext First goal of cryptography: confidentiality. lice s K encryption encryption ciphertext decryption decryption plaintext Monoalphabetic cipher: substitute one letter for another. plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Example: Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Symmetric crypto: encryption and decryption s are identical. (both are secret) Public crypto: encryption is public, decryption is secret. 4/22/2013 CSC 257/457 Spring Q1: How hard to break this simple cipher? brute force? other? Q2: How to make it more difficult to break? 4/22/2013 CSC 257/457 Spring CSC 257/457 Spring

3 Symmetric Key Cryptography: DES DES: Data Encryption Standard US encryption standard [NIS 1993] 56 bit symmetric, 64 bit plaintext input encryption: initial permutation 16 rounds, each using different 48 bits of final permutation decryption: reverse operation using the same How secure is DES? DES Challenge (1999): 56 bit encrypted phrase decrypted (brute force) in 22 hours 15 minutes Making DES more secure: use three s sequentially (3 DES) use more bits 4/22/2013 CSC 257/457 Spring ES: dvanced Encryption Standard Newer (Nov. 2001) symmetric NIS standard, replacing DES Processes data in 128 bit blocks 128, 192, or 256 bit s Brute force decryption (try each ) taking 1 sec on DES, takes 149 trillion years for 128 bit ES 4/22/2013 CSC 257/457 Spring Public Key Cryptography Symmetric cryptography requires sender, receiver know shared secret Q: how to agree on in the first place? (particularly difficult if rudy is eavesdropping on all communication) Public cryptography encryption is different from decryption encryption is public,known to everyone, also called public decryption is secret, known only to receiver, also called private plaintext, m Public Key Cryptography encryption ciphertext K (m) B decryption public K private B plaintext m = K (K (m)) B B Principle for choosing the public/private pair: One should not be able to derive the private from the public. 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring CSC 257/457 Spring

4 Public Key Cryptography: S (on ivest, di Shamir and Len dleman) Choosing s: Choose two large prime numbers p, q. (e.g., 1024 bits each) Compute n = pq, z = (p 1)(q 1) Choose e (with e<n) that has no common factors with z Choose d such that ed 1 is exactly divisible by z Public is (n,e). Private is (n,d) o encrypt a, m (<n): do c = m e mod n o decrypt a received ciphertext, c: do m = c d mod n eason: for any m (relatively prime with n) m z mod n = 1; therefore m ed 1 mod n = 1 nother property: (m d mod n) e mod n = m S is much slower than the symmetric cryptos Confidentiality: cryptography uthentication Integrity Key distribution and certification 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring uthentication: version 1.0 uthentication: Bob wants lice to prove her identity to him. Protocol ap1.0: lice says I am lice. I am lice I am lice Failure scenario?? rudy can simply declare herself to be lice uthentication: version 2.0 Protocol ap2.0: lice says I am lice and sends her secret password to prove it. I m lice lice s password I m lice lice s password Failure scenario?? playback attack: rudy records lice s packet and later plays it back to Bob 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring CSC 257/457 Spring

5 uthentication: version 3.0 Goal: avoid playback attack Nonce: number () used only once in a lifetime ap3.0: Bob sends lice a nonce,. lice must return, encrypted with shared secret I am lice K () B only lice knows to encrypt nonce, so it must be lice! 4/22/2013 CSC 257/457 Spring uthentication: version 4.0 ap3.0 requires shared symmetric. Key distribution can be a problem. ap4.0: use nonce, public cryptography. I am lice Bob computes K (K ()) = K () and knows only lice could have the private, that encrypted such that K (K ()) = 4/22/2013 CSC 257/457 Spring Confidentiality: cryptography uthentication Integrity Key distribution and certification Integrity Digital Signatures: cryptographic technique to ensure document integrity. analogous to hand written signatures. Sender (Bob) digitally signs document, establishing he is document owner/creator. he recipient (lice) receives the document and the digital signature. he recipient can be sure that the document is verifiable: Bob signed the document. nonforgeable: the document hasn t been changed since Bob signed it. 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring CSC 257/457 Spring

6 Digital Signatures Bob signs m by encrypting with his private, creating a digital signature (m), m private K B (m) Dear lice, Oh, how I have missed Public m, signed you. I think of you all the time! (blah blah blah) encryption (encrypted) with his private Bob Suppose lice receives msg m and its digital signature (m) lice applies public to (m) then checks whether ( (m)) = m. If so, whoever signed m must have used private. Problem: computationally expensive to public encrypt long s. 4/22/2013 CSC 257/457 Spring Bob sends digitally signed (small) digest: large m Signed Message Digest H: Hash function private H(m) digital signature (encrypt) encrypted msg digest (H(m)) lice verifies signature and integrity of digitally signed : large m H: Hash function H(m) public equal? encrypted msg digest (H(m)) digital signature (decrypt) H(m) 4/22/2013 CSC 257/457 Spring Message Digests pply a hash function H to m, get a much smaller digest H(m). Public encrypt the digest to generate the digital signature (H(m)). Good/bad hash functions? Hint: given a hash function, it is possible for many s sharing the same digest. Internet Checksum: Poor Hash Function for Generating Message Digests Given a and its Internet checksum, it is easy to find another with same checksum. I O U B O B SCII format 49 4F E D2 42 B2 C1 D2 C I O U B O B different s but identical checksums! SCII format 49 4F E D2 42 B2 C1 D2 C Hash function property: given digest x for m, computationally infeasible to find another m that shares the same digest. 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring CSC 257/457 Spring

7 Good Hash Functions for Generating Message Digests MD5 computes 128 bit digest in 4 step process. appears difficult to construct m whose MD5 hash is equal to x. SH 1 [NIS, FIPS PUB 180 1] 160 bit digest Confidentiality: cryptography uthentication Integrity Key distribution and certification 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring Key Distribution and Certification Symmetric distribution problem: How do lice and Bob establish shared secret over network without rudy s knowledge? Public distribution problem: When lice obtains public (from web site, e mail, diskette), how does she know it is public, not rudy s? Secret Key Distribution: Key Distribution Center (KDC) KDC: server shares different secret with each registered user (many users). lice, Bob know own symmetric s, K KDC KDC, for communicating with KDC. KDC KDC K K PKDC KDC K XKDC K YKDC K KDC KDC K ZKDC 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring CSC 257/457 Spring

8 lice knows 1 Key Distribution using KDC Q: How does KDC allow Bob, lice to determine shared symmetric secret to communicate with each other? K KDC (,B) KDC generates 1 K KDC (1, KDC (,1)) KDC (,1) lice and Bob communicate: using 1 as session for shared symmetric encryption Bob knows to use 1 to communicate with lice 4/22/2013 CSC 257/457 Spring Security Vulnerability with Public Key Distribution case example for public based authentication. I am lice Bob computes K (K ()) = K () and knows only lice could have the private, that encrypted such that K (K ()) = What if Bob doesn t know lice s public ahead of time? 4/22/2013 CSC 257/457 Spring Security vulnerability when public s are not well known Man (woman) in the middle attack: rudy poses as lice (to Bob) and as Bob (to lice) m = K (K (m)) I am lice K () Send me your public K K (m) rudy gets m = K (K (m)) sends m to lice ennrypted with lice s public I am lice K () Send me your public K K (m) 4/22/2013 CSC 257/457 Spring Public Key Distribution: Certification uthorities Certification authority (C): trustable by everyone; every one knows its public. E (person, router) registers its public with C. identifying information E provides proof of identity to C. C creates certificate binding E to its public. certificate is C signed document saying E s public is public digital signature (encrypt) C private K C 4/22/2013 CSC 257/457 Spring certificate for public, signed by C CSC 257/457 Spring

9 Certification uthorities (cont.) Key Certification Methods When lice wants to verify public : gets certificate (Bob or elsewhere). apply C s public to certificate, verify public. digital signature (decrypt) C public K C public Public certificate signed by a certification authority Peer certification: If knows B personally, they can verify each other s public s using offline means and sign them; Certificate chain leading to a certificate authority C signs s public certificate signs B s public certificate B signs C s public certificate 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring Summary: Principles of Network Security Cryptography: symmetric s: protocols? weakness? public s: protocol? weakness? Confidentiality: only sender, intended receiver should understand contents uthentication: sender, receiver want to confirm identity of each other Message Integrity: sender, receiver want to ensure not altered (in transit, or afterwards) Key distribution? Disclaimer Parts of the lecture slides contain original work of James Kurose, Larry Peterson, and Keith oss. he slides are intended for the sole purpose of instruction of computer networks at the University of ochester. ll copyrighted materials belong to their original owner(s). 4/22/2013 CSC 257/457 Spring /22/2013 CSC 257/457 Spring CSC 257/457 Spring

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