Homework #8 SolutionsQuestion 1 – Fun w/ RevocationQuestion 1aSlide 4Question 1bSlide 6Question 1cSlide 8Question 2Slide 10Question 2 – Solution 1Slide 12Slide 13Question 1 – Solution 2Question 2 – Solution 2Slide 16Slide 17Question 3Slide 19Question 4 – Cert RolloverQuestion 4aSlide 22Question 4bSlide 24Homework #8Homework #8SolutionsSolutionsBrian A. LaMacchiaBrian A. [email protected]@[email protected]@microsoft.comPortions © 2002-2006, Brian A. LaMacchia. This material is provided without warranty of any kind including, without limitation, warranty of non-infringement or suitability for any purpose. This material is not guaranteed to be error free and is intended for instructional use only.February 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography22Question 1 – Fun w/ Question 1 – Fun w/ RevocationRevocationVeriSign’s RSASecureServer.crl. VeriSign’s RSASecureServer.crl. As of 3am Wed., Feb. 22:As of 3am Wed., Feb. 22:Valid from 2/22/06 to 3/8/06Valid from 2/22/06 to 3/8/06515,243 bytes in size515,243 bytes in size14,714 entries14,714 entriesAssume that all of the certs Assume that all of the certs listed on the CRL were issued listed on the CRL were issued within the past 12 months.within the past 12 months.VeriSign claims to have about VeriSign claims to have about 500,000 sites with “Secure 500,000 sites with “Secure Server IDs”, so assume that’s Server IDs”, so assume that’s the universe from which 14,714 the universe from which 14,714 certs have been revoked. certs have been revoked.February 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography33Question 1aQuestion 1aAssume 200,000,000 users who Assume 200,000,000 users who will negotiate an SSL/TLS will negotiate an SSL/TLS session with at least one of the session with at least one of the 500,000 sites over the next two 500,000 sites over the next two weeks. weeks. On average, how much On average, how much bandwidth is VeriSign going to bandwidth is VeriSign going to use use per dayper day distributing the distributing the RSASecureServer CRL? RSASecureServer CRL? You may assume user requests for You may assume user requests for CRLs are evenly distributed CRLs are evenly distributed throughout the CRL’s two-week throughout the CRL’s two-week validity period.validity period.February 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography44Question 1aQuestion 1a200M users, CRLs last 14 days, 200M users, CRLs last 14 days, so on average 1/14so on average 1/14thth of the of the users will have to download the users will have to download the CRL each day.CRL each day.200M/14 = 14.285714M 200M/14 = 14.285714M downloads/daydownloads/day515,243 bytes/download515,243 bytes/download ~7.360x10~7.360x101212 bytes of bytes of bandwidth bandwidth per dayper dayFebruary 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography55Question 1bQuestion 1bAssume there also exists an Assume there also exists an OCSP responder for the same OCSP responder for the same datadataIf the average size of an OCSP If the average size of an OCSP request/response message pair request/response message pair is 3KB, how many OCSP is 3KB, how many OCSP responses would the average responses would the average user have to request from the user have to request from the VeriSign OCSP responder per VeriSign OCSP responder per day in order to generate the day in order to generate the same about of bandwidth usage same about of bandwidth usage as the CRL downloading you as the CRL downloading you calculated in Question 1(a)?calculated in Question 1(a)?February 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography66Question 1bQuestion 1b~7.360x10~7.360x101212 bytes of bandwidth bytes of bandwidth per dayper day/ 3KB/OCSP request/response / 3KB/OCSP request/response pairpair 2.453x102.453x1099 OCSP round-trips OCSP round-trips/ 200,000,000 users/ 200,000,000 users ~12.267 OCSP ~12.267 OCSP requests/user/dayrequests/user/dayFebruary 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography77Question 1cQuestion 1cUSG wants to issue a cert to USG wants to issue a cert to each of 60 million passport each of 60 million passport holders.holders.VeriSign is experiencing about a VeriSign is experiencing about a 3% revocation rate; assume that 3% revocation rate; assume that the same rate would apply for the same rate would apply for these certs. these certs. Approximately how big would Approximately how big would the CRL be for the personal the CRL be for the personal certs issued by the US certs issued by the US Government?Government?You may assume that each CRL You may assume that each CRL entry requires 35 bytes of storage entry requires 35 bytes of storage when ASN.1 encoded. when ASN.1 encoded.February 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography88Question 1cQuestion 1c60 million passport holders * 3% 60 million passport holders * 3% revocation rate revocation rate 1.8 million 1.8 million revoked certs at any one time.revoked certs at any one time.1.8 million * 35 bytes/entry1.8 million * 35 bytes/entry 63x10 63x106 6 bytes in the CRLbytes in the CRLFebruary 7, 2006February 7, 2006Practical Aspects of Modern CryptographyPractical Aspects of Modern Cryptography99Question 2Question 2Design a certificate enrollment Design a certificate enrollment protocol for enrolling each user protocol for enrolling each user for two certificatesfor two certificatesLeverage the user’s Kerberos Leverage the user’s Kerberos credentials to authenticate the credentials to authenticate the certificate requests to the CA. certificate requests to the CA. You can choose whether users You can choose whether users enroll for both signing and enroll for both signing and encryption certificates encryption certificates simultaneously (in one execution simultaneously (in one execution of the protocol) or sequentially (in of the protocol) or sequentially (in two executions of the protocol). two executions of the protocol).February 7,
View Full Document
Unlocking...