Operating Systems CS 5523
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This 8 page Class Notes was uploaded by Mireya Heidenreich on Thursday October 29, 2015. The Class Notes belongs to CS 5523 at University of Texas at San Antonio taught by Staff in Fall. Since its upload, it has received 7 views. For similar materials see /class/231395/cs-5523-university-of-texas-at-san-antonio in ComputerScienence at University of Texas at San Antonio.
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Date Created: 10/29/15
CS 5523 Lecture 19 Security Case Studies I Questions on laboratory 3 l Introduce laboratory 4 I Review scenarios I NeedhamSchroeder I Bn39ef summary of common algon39thms I Kerberos Simplified overview of secret key encryption Encrypted message EK M MK Decrypted message DK EK M DK MK M ltis hard to get M from MK without knowing K Scenario 1 Secret communication with shared secret key Alice and Bob share a secretkey KAB Alice wants to send a secret message M to Bob 1 Alice uses KAB and an agreed encryption function EKAB M to encrypt and send message M to Bob 2 Bob reads the encrypted messages using the corresponding decryption function DKAB M How can Bob and Alice safely get the shared key KAB How can Bob know that M wasn t a replay Scenario 2 Authenticated communication with a server Alice wants to access Bob s les on a local le server Sara is a trusted authentication server that holds passwords and current secret keys 1 Alice sends a message to Sara asking for a ticket to access Bob 2 Sara sends Alice a response encrypted with KA that is a ticket encrypted with KB and a new secret key KAB for communication ticketKB KAB KA 3 Alice decrypts response with KA 4 Alice sends ticket herD and request R to Bob ticketKB Alice R 5 Bob decrypts ticket using KB the ticket was KAB Alice KB This is the simpli ed scenario for Kerberos KAB is the session key Simplified overview of public key encryption Keys come in pairs K1 and K2 Keep one public and one private If you encrypt with K1 you can decrypt with K2 and vice versa DK2 EK1 M M and DK1 EK2 M M Scenario 3 Authenticated communication with public keys Bob has generated a publicprivate key pair There is a trusted authority that gives out key certi cates Alice accesses a key distribution center to obtain a public key certi cate with Bob s public key Alice extracts Bob s public key KBM Alice creates a new secret key KAB and encrypts KAB known string with KBM Alice sends unique keyname KAB known stringKBW to Bob A N Am Bob decrypts KAB known stringKBM using KB 5 Bob and Alice now communicate with KAB This is the scenario for the widely used hybrid cryptographic protocol prv Scenario 4 Digital signatures with a secure digest function Alice wants to sign document M so that any recipient can verify it came from Alice This assumes thatAice has a pn39vatepublic key pair A digest is like a checksum Alice computes a xedlength digest DigestM Alice encrypts DigestM with her private key certi cate with Bob s public key and makes M DigestMKAWa vailable Bob reads M DigestMKapJ extracts M and computes DigestM Bob applies K AM to DigestMKWv to obtain DigestM and compares the value with his computed value Ms Aw Figure 74 Alice s bank account certificate 1 Certi cate type Account number 2 Name Alice 3 Account 6262626 4 Certifying authority Bob s Bank 5 Signature Digest eld2 eld 3K 5an Mum39s om Ex lenthDulmme ma mm mama ysm comm new Edn 3 AddxsunWeslAyPnblnha 2mm Figure 75 Publickey certificate for Bob s Bank 1 Certi cate type Public key 2 Name Bob s Bank 3 Public key KBpub 4 Certifying authority Fred 7 The Bankers Federation 5 Signature DigesIO ield2 eld 3KF w Mum39s om Ex lenmsDulmnn ma mug Dism bntad ysm Cmepts AndDeslgn Edn 3 AddisonWeslAYPubl luE 2mm Figure 713 X509 Certificate format subject Distinguished Name Public Key Issuer Distinguished Name Signature Period of validity Not Before Date Not A er Date Administrative information Version Serial Number Fxtenrlerl Information Mum39s om Ex lenmsDulmnn ma mug Dism bntad ysm Cmepts AndDeslgn Edn 3 AddisonWeslAYPubl luE 2mm Some popular encryption schemes I TEA tiny encryption algorithm Wheeler and Needham 1994 uses 32 rounds with combinations of XOR text shifts I DES Data Encryption Standard National Bureau of Standards 1977 obsolete because of short keys successfully cracked by brute force attacks in 1997 and a machine was built in 1998 that could crack keys in 3 days Replaced by AES Advanced Encryption Standard NIST 1999 I RSA Rivest Shamir and Ademan public key encryption based on factoring products of large primes widely used RSA s patent just expired Some popular encryption schemes continued I 3DES tripleDES ANSI 1985 apply DES 3 times with two keys very slow I lDEA International Data Encryption Algorithm Lai and Massey 1990 based on group algebra with 8 rounds ofXOR I MD5 used for data digests Rivest 1992 four rounds applying 4 nonlinear functions to each of 16 32bit segments I SHA NIST 1995 based on Rivest s MD4 algorithm to produce a 160 bit digest Figure 714 Performance of encryption and secure digest algorithms Key sizehash size bits Extrapolated PRB optimized kbytess kbytesec TEA 128 700 DES 56 350 7746 TripleDES 112 120 2842 IDEA 128 700 4469 RSA 512 7 RSA 2048 1 MDS 128 1740 62425 SHA 160 750 25162 ummgamm campaign 33125335333 sum emmmsgn m 3 Figure 715 The Needham Schroeder secretkey authentication protocol Header Message Notes 1 AgtS A B N A requests S to supply a key for communication A with B 2 SgtA N B K S returns a message encrypted in A s secret key A K 15 containing a newly generated key K A 5 and a AB K5KA ticket encrypted in B s secret key The nonce N A demonstrates that the message was sent in response to the preceding one A believes that S sent the message because only S knows A s secret key 3 AgtB Kw AKB A sends the ticket to B 4 BgtA NBKAB B decrypts the ticket and uses the new key KAB to encrypt another nonce N 5 5 AgtB N5 UK A demonstrates to B that it was the sender ofthe previous message by returning an agreed 39 o N Mum39s om Ex lenmsDolmnn ma mug Dishian ysm Contains mama Edn 3 mismwtsiaymmm 2mm Kerberos I Follows Needham and Schroeder very closely I Uses time values as nonces I When userlogs in the login program sends user s name to the kerberos authentication server I If useris known server replies with a session key and a nonce encrypted in the user s password and a ticket for TGS I Afterlogin program authenticates the information it can erase the user s password from memory Figure 716 System architecture of Kerberos Kerberos Key Distribution Centre Step A 1 Request for TGS 39 server ticket 4 Server ticket quot Step C 5 Service request Request encrypted with session key ely encrypted with session ke nmmvs Gm Ex lenmsDulrmn ma mug stha nntad ysm Contains AridDeslgn Edn 3 AddxsunWeslAyPublnhm 2mm Login session setup 39 Server session setup DoOperation a Client Server 0 s
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