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03-30-2010-answers - CS 161 Spring 2010 Computer Security...

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CS 161 Computer Security Spring 2010 Paxson/Wagner Discussion 8 1. TLS (a) TLS provides end-to-end authentication, integrity, and confidentiality guarantees. Is that enough to make online commerce safe and secure? Why or why not? (b) Some client-side implementations of SSL checked the name field of a certificate by reading up to the first null character. How could this be exploited? (c) The TLS protocol includes a closure alert signal (close notify) that can be sent by either side to indicate the end of the connection. Why is this necessary? Couldn’t the two parties just stop sending new messages when they are done? Answer: (a) TLS provides secure communication between a client and server, but was not designed specifically for online transactions. For instance, the browser checks the name in the certificate against the site’s domain name, but this gives no assurance that the site is a bona fide merchant. Similarly, the online merchant has no way to check that the person making the purchase is authorized to use the credit card. Customer’s can repudiate purchases, claiming their credit card number was stolen. In these cases, the credit card company usually pays the price. (b) The client can be fooled into thinking an attacker’s certificate comes from a legitimate site. For ex- ample, yourbank.com \ 0.attacker.com would be read as a certificate from yourbank.com. This can be used to launch a MITM attack. See http://www.wired.com/threatlevel/2009/07/ kaminsky/ for more details. (c) If the protocol doesn’t include the close notify signal, an attacker that can forge a TCP FIN packet can trick the recipient into thinking the communication is over. If the protocol does include the close notify signal, the recipient of a forged TCP FIN signal will know the connection has not been legitimately closed by the other end. SSL v2 suffered from this vulnerability. 2. SYN Cookies The Wednesday before spring break, Professor Wagner described SYN cookies, a technique for choosing TCP initial sequence numbers (ISN’s) that helps mitigate denial of service (DOS) attacks. Specifically, SYN cookies help prevent SYN flooding with spoofed source addresses. (a) When using SYN cookies, a host computes part of its ISN as a MAC. Would using digital signatures instead offer any security advantages?
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