sen-material: comparison handouts/ho01.tex

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-:cceb3d2dcba0
+:67d7d239c617
 noticing how they might shoplift. They can't use a computer
 without wondering about the security vulnerabilities. They
 can't vote without trying to figure out how to vote twice.
 They just can't help it.''
 \end{quote}
+\noindent
+and
 \begin{quote}
 \it ``Security engineering\ldots requires you to think
 differently. You need to figure out not how something works,
 but how something can be made to not work. You have to imagine
 \end{quote}
 \noindent In this module I like to teach you this security
 mindset. This might be a mindset that you think is very
 foreign to you---after all we are all good citizens and not
-hack into things. I beg to differ: You have this mindset
+hack into things. However, I beg to differ: You have this
-already when in school you were thinking, at least
+mindset already when in school you were thinking, at least
 hypothetically, about ways in which you can cheat in an exam
 (whether it is by hiding notes or by looking over the
 shoulders of your fellow pupils). Right? To defend a system,
-you need to have this kind of mindset and be able to think like
+you need to have this kind of mindset and be able to think
-an attacker. This will include understanding techniques that
+like an attacker. This will include understanding techniques
-can be used to compromise security and privacy in systems.
+that can be used to compromise security and privacy in
-This will many times result in insights where well-intended
+systems. This will many times result in insights where
-security mechanisms made a system actually less
+well-intended security mechanisms made a system actually less
 secure.\medskip
 \noindent
 {\Large\bf Warning!} However, don’t be evil! Using those
 techniques in the real world may violate the law or King’s
 like Ross Anderson's group was so important, because they and
 others established that the banks' claim that their system is
 secure and it must have been the customer's fault, was bogus.
 In 2009 the law changed and the burden of proof went back to
 the banks. They need to prove whether it was really the
-customer who used a card or not.
+customer who used a card or not. The current state of affairs,
+however, is that standing up for your right requires you
+to be knowledgeable\ldots{}if not, the banks are happy to
+take advantage of you.
 This is a classic example where a security design principle
 was violated: Namely, the one who is in the position to
 improve security, also needs to bear the financial losses if
 things go wrong. Otherwise, you end up with an insecure
 the GET request is generated\ldots in this case it is just a
 simple string. This program is run on the server and will be
 executed whenever a browser initiates such a GET request. You
 can run this program on your computer and then direct a
 browser to the address \pcode{localhost:8000} in order to
-simulate a request over the internet.
+simulate a request over the internet. You are encouraged
+to try this out\ldots{}theory is always good, but practice is
+better.
 For our web-application of interest is the feature that the
 server when answering the request can store some information
 on the client's side. This information is called a
 (i.e.~practically impossible) to calculate the input from
 knowing the output. This is often called \emph{preimage
 resistance}. Cryptographic hash functions also ensure that
 given a message and a hash, it is computationally infeasible to
 find another message with the same hash. This is called
-\emph{collusion resistance}. Because of these properties hash
+\emph{collusion resistance}. Because of these properties, hash
 functions are often called \emph{one-way functions}: you
 cannot go back from the output to the input (without some
 tricks, see below).
+There are several such hashing function. For example SHA-1
+would hash the string \pcode{"hello world"} to produce the
+hash-value
-There are several such
-hashing function. For example SHA-1 would hash the string
-\pcode{"hello world"} to produce the hash-value
 \begin{center}
 \pcode{2aae6c35c94fcfb415dbe95f408b9ce91ee846ed}
 \end{center}
 \begin{center}
 \pcode{1-356a192b7913b04c54574d18c28d46e6395428ab}
 \end{center}
-\noindent If we now read back the
+\noindent If we now read back the cookie when the client
-cookie when the client visits our webpage, we can extract the
+visits our webpage, we can extract the counter, hash it again
-counter, hash it again and compare the result to the stored
+and compare the result to the stored hash value inside the
-hash value inside the cookie. If these hashes disagree, then
+cookie. If these hashes disagree, then we can deduce that the
-we can deduce that the cookie has been tampered with.
+cookie has been tampered with. Unfortunately, if they agree,
-Unfortunately, if they agree, we can still not be entirely
+we can still not be entirely sure that not a clever hacker has
-sure that not a clever hacker has tampered with the cookie.
+tampered with the cookie. The reason is that the hacker can
-The reason is that the hacker can see the clear text part of
+see the clear text part of the cookie, say \pcode{3}, and also
-the cookie, say \pcode{3}, and also its hash. It does not take
+its hash. It does not take much trial and error to find out
-much trial and error to find out that we used the SHA-1
+that we used the SHA-1 hashing function and then the hacker
-hashing function and then the hacker can graft a cookie
+can graft a cookie accordingly. This is eased by the fact that
-accordingly. This is eased by the fact that for SHA-1 many
+for SHA-1 many strings and corresponding hash-values are
-strings and corresponding hash-values are precalculated. Type,
+precalculated. Type, for example, into Google the hash value
-for example, into Google the hash value for \pcode{"hello
+for \pcode{"hello world"} and you will actually pretty quickly
-world"} and you will actually pretty quickly find that it was
+find that it was generated by input string \pcode{"hello
-generated by input string \pcode{"hello world"}. Similarly for
+world"}. Similarly for the hash-value for \pcode{1}. This
-the hash-value for \pcode{1}. This defeats the purpose of a
+defeats the purpose of a hashing function and thus would not
-hashing function and thus would not help us with our
+help us with our web-applications and later also not with how
-web-applications and later also not with how to store
+to store passwords properly.
-passwords properly.
 There is one ingredient missing, which happens to be called
 \emph{salts}. Salts are random keys, which are added to the
 counter before the hash is calculated. In our case we must
 beginning and lock it only when the data in the cookie states
 that the allowed free number of visits are up. As said before,
 this can be easily circumvented by just deleting the cookie or
 by switching the browser. This would mean the New York Times
 will lose revenue whenever this kind of tampering occurs. The
-quick fix to require that a cookie must always be present does
+`quick fix' to require that a cookie must always be present
-not work, because then this newspaper will cut off any new
+does not work, because then this newspaper will cut off any
-readers, or anyone who gets a new computer. In contrast, our
+new readers, or anyone who gets a new computer. In contrast,
-web-application has the resource (discount) locked at the
+our web-application has the resource (discount) locked at the
 beginning and only unlocks it if the cookie data says so. If
 the cookie is deleted, well then the resource just does not
 get unlocked. No major harm will result to us. You can see:
 the same security mechanism behaves rather differently
 depending on whether the ``resource'' needs to be locked or
 \begin{center}
 \url{http://archive.wired.com/politics/security/commentary/securitymatters/2007/04/securitymatters_0419}
 \end{center}
 \noindent
+A recent research paper about surveillance using cookies is
+\begin{center}
+\url{http://randomwalker.info/publications/cookie-surveillance-v2.pdf}
+\end{center}
+\noindent
 A slightly different point of view about the economies of
 password cracking:
 \begin{center}
 \url{http://xkcd.com/538/}
 better in remembering (securely) longer and longer passwords.
 The big question is which technology can replace
 passwords\ldots
 \medskip
-\noindent
-A recent research paper about surveillance using cookies is
-\begin{center}
-\url{http://randomwalker.info/publications/cookie-surveillance-v2.pdf}
-\end{center}
 \end{document}
 %%% fingerprints  vs. passwords (what is better)
 https://www.youtube.com/watch?v=VVxL9ymiyAU&feature=youtu.be

changeset 443	67d7d239c617
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child 445	9ad6445a0354