Massachusetts Institute of Technology 6 857 Network and Computer Security Professor Ronald L Rivest Handout 5 September 11 2003 Problem Set 1 This problem set is due on paper in room 6 120 on Thursday September 18 at the beginning of class You are to work on this problem set in groups of three or four people Problems turned in by individuals pairs pentuples etc will not be accepted Be sure that all group members can explain the solutions See Handout 1 Course Information for our policy on collaboration If you do not have a group seek partners by emailing 6 857 students public mit edu Homework must be typed Each problem answer must appear on separate sheets of paper Mark the top of each sheet with your name s the course number 6 857 the problem set number and question and the date Homework must be typed and clear We have provided templates for LATEX and Microsoft Word on the course website Grading and Late Policy Each problem is worth 10 points Late homework will not be accepted without prior approval Homework should not be submitted by email except with prior approval Somebody from your group should be in class on the day that the homework is due With the authors permission we will distribute our favorite solution to each problem as the official solution this is your chance to become famous If you do not wish for your homework to be used as an official solution or if you wish that it only be used anonymously please note this on your homework Problem 1 1 PGP This problem has both individual and group elements to it Your group should turn in one write up answering each of the parts labelled Group but all key pairs emails etc should be created and sent individually Read Alma Whitten s paper Why Johnny Can t Encrypt There is a link to it on the Lectures and Handouts page of the course website Locate and install a fresh version of PGP or GPG There are versions for Unix flavors Windows and the Macintosh See http web mit edu network pgp html for downloads If for some reason you are not able to download from this site http www pgpi org may be of use Find the PGP public keys for as many of the 6 857 staff as you can Part of your assignment is figuring out how to locate PGP keys Searching the Internet for PGP key servers may be of help But beware there may be fake keys out there a Group Reflections on Trust PGP s web of trust model allows users to sign each others public keys Suppose Alice signs Bob s key what in effect is Alice declaring when she does this Why is it useful for people to sign each other s keys What precautions should one take before signing someone else s key and why are these measures appropriate b Individual Getting started Create a new public private key pair for yourself you may use an existing key pair if you already have one Sign each of your group members public keys and have them sign yours When all of your group members have signed your public key email it to 6 857 tas mit edu in ASCII armored format with the subject My public key c Individual Encrypting email Send an encrypted signed email to 6 857 tas mit edu with the subject PGP is fun Do not send the mail to the TAs individually In the body of the message Tell us what operating system and version of PGP you are using Show us the public keys you found for the 6 857 staff PGP fingerprints are sufficient In a few sentences explain why you do or do not believe that these keys do indeed belong to the 6 857 staff If you do not trust a public key explain what would convince you otherwise 2 6 857 Handout 5 Problem Set 1 Your mail should be protected with PGP such that the 6 857 TAs and only the 6 857 TAs can obtain the plaintext contents You must also sign the mail with your private key We will only accept your first message so make sure to get it right the first time Are you able to finish the assignment in fewer than 90 minutes as in Whitten s experiment Remember to cite all your sources books manuals friends etc according to the guidelines in Handout 1 d Group Acting Presidential Find a PGP key for president whitehouse gov on a PGP key server Based on your findings explain one useful feature and one drawback of PGP key servers Limit your answer to two paragraphs Remember to cite all your sources Problem 1 2 Password Sniffing Group Discuss the opportunities for password sniffing eavesdropping in one of the following scenarios Consider software hardware network based and electronic means among any others you may think up Find at least five vulnerabilities Alice visits her friend Bob s office While she is there she sits down at Bob s computer and uses it to access the MIT WebMail server To use WebMail Alice needs to enter her username and password which are sent over an SSL protected link to the web server Louis Reasoner takes his laptop to the local Starbuck s to enjoy an espresso and free Wi Fi wireless net access He uses telnet to log in to his Linux machine back in his dorm room Eve sitting at home wants to make a PPP connection over an analog telephone line to her ISP Her ISP uses PAP authentication see RFC 1334 for a discussion of PPP authentication protocols The ISP s modem bank communicates over Ethernet to an authentication server that stores usernames and passwords Eve types her username and password into a Windows dialog box her modem calls a local phone number and the ISP authenticates her Problem 1 3 Exercises in Hashing Group a Ideally a hash function will have the properties of both collision resistance and pseudorandomness Is one property stronger than the other In other words if a hash function is collision resistant must is also be pseudorandom And vice versa For each implication either argue that it is true or give a counterexample b Bitdiddle Security Inc proposes the following authentication scheme using a hash function h the client and server systems both maintain a secret string s which is initialized to some 16 word random value Whenever the client wants to authenticate itself the server chooses a fresh random challenge string r of length 16 words and sends it to the client The client sends h s r to the server If the server receives the expected value it accepts the client and both the client and the server update s by appending r to it i e s s r Otherwise the server rejects the client and leaves s unchanged After a cursory look over pages 218 9 of her 6 857 textbook Alyssa P Hacker was overheard to say That s all well and good if h is a random oracle but they …
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