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\lstset{language=JavaScript}

\begin{document}

\fnote{\copyright{} Christian Urban, 2014, 2015}

\section*{Handout 1 (Security Engineering)}

Much of the material and inspiration in this module is taken

from the works of Bruce Schneier, Ross Anderson and Alex

Halderman. I think they are the world experts in the area of

security engineering. I especially like that they argue that a

security engineer requires a certain \emph{security mindset}.

Bruce Schneier for example writes:

\begin{quote} 

\it ``Security engineers --- at least the good ones --- see

the world differently. They can't walk into a store without

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}

\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

an intelligent and malicious adversary inside your system

\ldots, constantly trying new ways to

subvert it. You have to consider all the ways your system can

fail, most of them having nothing to do with the design

itself. You have to look at everything backwards, upside down,

and sideways. You have to think like an alien.''

\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

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

an attacker. This will include understanding techniques that

can be used to compromise security and privacy in systems.

This will many times result in insights where 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

rules, and it may be unethical. Under some circumstances, even

probing for weaknesses of a system may result in severe

penalties, up to and including expulsion, fines and

jail time. Acting lawfully and ethically is your

responsibility. Ethics requires you to refrain from doing

harm. Always respect privacy and rights of others. Do not

tamper with any of King's systems. If you try out a technique,

always make doubly sure you are working in a safe environment

so that you cannot cause any harm, not even accidentally.

Don't be evil. Be an ethical hacker.\medskip

\noindent In this lecture I want to make you familiar with the

security mindset and dispel the myth that encryption is the

answer to all security problems (it is certainly often a part

of an answer, but almost always never a sufficient one). This

is actually an important thread going through the whole

course: We will assume that encryption works perfectly, but

still attack ``things''. By ``works perfectly'' we mean that

we will assume encryption is a black box and, for example,

will not look at the underlying mathematics and break the

algorithms.\footnote{Though fascinating this might be.}

For a secure system, it seems, four requirements need to come

together: First a security policy (what is supposed to be

achieved?); second a mechanism (cipher, access controls,

tamper resistance etc); third the assurance we obtain from the

mechanism (the amount of reliance we can put on the mechanism)

and finally the incentives (the motive that the people

guarding and maintaining the system have to do their job

properly, and also the motive that the attackers have to try

to defeat your policy). The last point is often overlooked,

but plays an important role. To illustrate this lets look at

an example. 

\subsubsection*{Chip-and-PIN is Surely More Secure?}

The questions is whether the Chip-and-PIN system used with

modern credit cards is more secure than the older method of

signing receipts at the till. On first glance the answer seems

obvious: Chip-and-PIN must be more secure and indeed improved

security was the central plank in the ``marketing speak'' of

the banks behind Chip-and-PIN. The earlier system was based on

a magnetic stripe or a mechanical imprint on the cards and

required customers to sign receipts at the till whenever they

bought something. This signature authorised the transactions.

Although in use for a long time, this system had some crucial

security flaws, including making clones of credit cards and

forging signatures. 

Chip-and-PIN, as the name suggests, relies on data being

stored on a chip on the card and a PIN number for

authorisation. Even though the banks involved trumpeted their

system as being absolutely secure and indeed fraud rates

initially went down, security researchers were not convinced

(especially not the group around Ross Anderson). To begin with,

the Chip-and-PIN system introduced a ``new player'' into the

system that needed to be trusted: the PIN terminals and their

manufacturers. It was claimed that these terminals were

tamper-resistant, but needless to say this was a weak link in

the system, which criminals successfully attacked. Some

terminals were even so skilfully manipulated that they

transmitted skimmed PIN numbers via built-in mobile phone

connections. To mitigate this flaw in the security of

Chip-and-PIN, you need to be able to vet quite closely the

supply chain of such terminals. This is something that is

mostly beyond the control of customers who need to use these

terminals.

To make matters worse for Chip-and-PIN, around 2009 Ross

Anderson and his group were able to perform man-in-the-middle

attacks against Chip-and-PIN. Essentially they made the

terminal think the correct PIN was entered and the card think

that a signature was used. This is a kind of \emph{protocol

failure}. After discovery, the flaw was mitigated by requiring

that a link between the card and the bank is established at

every time the card is used. Even later this group found

another problem with Chip-and-PIN and ATMs which did not

generate random enough numbers (cryptographic nonces) on which

the security of the underlying protocols relies. 

The overarching problem with all this is that the banks who

introduced Chip-and-PIN managed with the new system to shift

the liability for any fraud and the burden of proof onto the

customer. In the old system, the banks had to prove that the

customer used the card, which they often did not bother with.

In effect, if fraud occurred the customers were either

refunded fully or lost only a small amount of money. This

taking-responsibility-of-potential-fraud was part of the

``business plan'' of the banks and did not reduce their

profits too much. 

Since banks managed to successfully claim that their

Chip-and-PIN system is secure, they were under the new system

able to point the finger at the customer when fraud occurred:

customers must have been negligent losing their PIN and

customers had almost no way of defending themselves in such

situations. That is why the work of \emph{ethical} hackers

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.

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

system. In case of the Chip-and-PIN system, no good security

engineer would dare to claim that it is secure beyond

reproach: the specification of the EMV protocol (underlying

Chip-and-PIN) is some 700 pages long, but still leaves out

many things (like how to implement a good random number

generator). No human being is able to scrutinise such a

specification and ensure it contains no flaws. Moreover, banks

can add their own sub-protocols to EMV. With all the

experience we already have, it is as clear as day that

criminals were bound to eventually be able to poke holes into

it and measures need to be taken to address them. However,

with how the system was set up, the banks had no real

incentive to come up with a system that is really secure.

Getting the incentives right in favour of security is often a

tricky business. From a customer point of view, the

Chip-and-PIN system was much less secure than the old

signature-based method. The customer could now lose

significant amounts of money.

\subsection*{Of Cookies and Salts}

Let us look at another example which will help with understanding how

passwords should be verified and stored.  Imagine you need to develop

a web-application that has the feature of recording how many times a

customer visits a page.  For example in order to give a discount

whenever the customer has visited a webpage some $x$ number of times

(say $x$ equals $5$). There is one more constraint: we want to store

the information about the number of visits as a cookie on the

browser. I think, for a number of years the webpage of the New York

Times operated in this way: it allowed you to read ten articles per

month for free; if you wanted to read more, you had to pay. My best

guess is that it used cookies for recording how many times their pages

was visited, because if I switched browsers I could easily circumvent

the restriction about ten articles.\footnote{Another online media that

  works in this way is the Times Higher Education

  \url{http://www.timeshighereducation.co.uk}. It also seems to 

  use cookies to restrict the number of free articles to five.}

To implement our web-application it is good to look under the

hood what happens when a webpage is displayed in a browser. A

typical web-application works as follows: The browser sends a

GET request for a particular page to a server. The server

answers this request with a webpage in HTML (for our purposes

we can ignore the details about HTML). A simple JavaScript

program that realises a server answering with a ``Hello

World'' webpage is as follows:

\begin{center}

\lstinputlisting{../progs/ap0.js}

\end{center}

\noindent The interesting lines are 4 to 7 where the answer to

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.

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

\emph{cookie}. The next time the browser makes another GET

request to the same webpage, this cookie can be read again by

the server. We can use cookies in order to store a counter

that records the number of times our webpage has been visited.

This can be realised with the following small program

\begin{center}

\lstinputlisting{../progs/ap2.js}

\end{center}

\noindent The overall structure of this program is the same as

the earlier one: Lines 7 to 17 generate the answer to a

GET-request. The new part is in Line 8 where we read the

cookie called \pcode{counter}. If present, this cookie will be

send together with the GET-request from the client. The value

of this counter will come in form of a string, therefore we

use the function \pcode{parseInt} in order to transform it

into an integer. In case the cookie is not present, we default

the counter to zero. The odd looking construction \code{...||

0} is realising this defaulting in JavaScript. In Line 9 we

increase the counter by one and store it back to the client

(under the name \pcode{counter}, since potentially more than

one value could be stored). In Lines 10 to 15 we test whether

this counter is greater or equal than 5 and send accordingly a

specially grafted message back to the client.

Let us step back and analyse this program from a security

point of view. We store a counter in plain text on the

client's browser (which is not under our control). Depending

on this value we want to unlock a resource (like a discount)

when it reaches a threshold. If the client deletes the cookie,

then the counter will just be reset to zero. This does not

bother us, because the purported discount will just not be

granted. In this way we do not lose any (hypothetical) money.

What we need to be concerned about is, however, when a client

artificially increases this counter without having visited our

web-page. This is actually a trivial task for a knowledgeable

person, since there are convenient tools that allow one to set

a cookie to an arbitrary value, for example above our

threshold for the discount. 

There seems to be no simple way to prevent this kind of

tampering with cookies, because the whole purpose of cookies

is that they are stored on the client's side, which from the

the server's perspective is a potentially hostile environment.

What we need to ensure is the integrity of this counter in

this hostile environment. We could think of encrypting the

counter. But this has two drawbacks to do with the keys for

encryption. If you use a single, global key for all the

clients that visit our site, then we risk that our whole

``business'' might collapse in the event this key gets known

to the outside world. Then all cookies we might have set in

the past, can now be decrypted and manipulated. If, on the

other hand, we use many ``private'' keys for the clients, then

we have to solve the problem of having to securely store this

key on our server side (obviously we cannot store the key with

the client because then the client again has all data to

tamper with the counter; and obviously we also cannot encrypt

the key, lest we can solve an impossible chicken-and-egg

problem). So encryption seems to not solve the problem we face

with the integrity of our counter.

Fortunately, \emph{cryptographic hash functions} seem to be

more suitable for our purpose. Like encryption, hash functions

scramble data in such a way that it is easy to calculate the

output of a hash function from the input. But it is hard

(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

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

\begin{center}

\pcode{2aae6c35c94fcfb415dbe95f408b9ce91ee846ed}

\end{center}

\noindent Another handy feature of hash functions is that if

the input changes only a little, the output changes

drastically. For example \pcode{"iello world"} produces under

SHA-1 the output

\begin{center}

\pcode{d2b1402d84e8bcef5ae18f828e43e7065b841ff1}

\end{center}

\noindent That means it is not predictable what the output

will be from just looking at input that is ``close by''. 

We can use hashes in our web-application and store in the

cookie the value of the counter in plain text but together

with its hash. We need to store both pieces of data in such a

way that we can extract them again later on. In the code below

I will just separate them using a \pcode{"-"}. For the

counter \pcode{1} for example

\begin{center}

\pcode{1-356a192b7913b04c54574d18c28d46e6395428ab}

\end{center}

\noindent If we now read back the

cookie when the client visits our webpage, we can extract the

counter, hash it again and compare the result to the stored

hash value inside the cookie. If these hashes disagree, then

we can deduce that the cookie has been tampered with.

Unfortunately, if they agree, we can still not be entirely

sure that not a clever hacker has tampered with the cookie.

The reason is that the hacker can see the clear text part of

the cookie, say \pcode{3}, and also its hash. It does not take

much trial and error to find out that we used the SHA-1

hashing function and then the hacker can graft a cookie

accordingly. This is eased by the fact that for SHA-1 many

strings and corresponding hash-values are precalculated. Type,

for example, into Google the hash value for \pcode{"hello

world"} and you will actually pretty quickly find that it was

generated by input string \pcode{"hello world"}. Similarly for

the hash-value for \pcode{1}. This defeats the purpose of a

hashing function and thus would not help us with our

web-applications and later also not with how to store

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

keep the salt secret. As can be see in Figure~\ref{hashsalt},

we need to extract from the cookie the counter value and its

hash (Lines 19 and 20). But before hashing the counter again

(Line 22) we need to add the secret salt. Similarly, when we

set the new increased counter, we will need to add the salt

before hashing (this is done in Line 15). Our web-application

will now store cookies like 

\begin{figure}[p]

\lstinputlisting{../progs/App4.js}

\caption{A Node.js web-app that sets a cookie in the client's

browser for counting the number of visits to a page.\label{hashsalt}}

\end{figure}

\begin{center}\tt

\begin{tabular}{l}

1 + salt - 8189effef4d4f7411f4153b13ff72546dd682c69\\

2 + salt - 1528375d5ceb7d71597053e6877cc570067a738f\\

3 + salt - d646e213d4f87e3971d9dd6d9f435840eb6a1c06\\

4 + salt - 5b9e85269e4461de0238a6bf463ed3f25778cbba\\

...\\

\end{tabular}

\end{center}

\noindent These hashes allow us to read and set the value of

the counter, and also give us confidence that the counter has

not been tampered with. This of course depends on being able

to keep the salt secret. Once the salt is public, we better

ignore all cookies and start setting them again with a new

salt.

There is an interesting and very subtle point to note with

respect to the 'New York Times' way of checking the number

visits. Essentially they have their `resource' unlocked at the

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

not work, because then this newspaper will cut off any new

readers, or anyone who gets a new computer. In contrast, 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

unlocked. Apart from thinking about the difference very

carefully, I do not know of any good ``theory'' that could

help with solving such security intricacies in any other way.  

\subsection*{How to Store Passwords Properly?}

While admittedly quite silly, the simple web-application in

the previous section should help with the more important

question of how passwords should be verified and stored. It is

unbelievable that nowadays systems still do this with

passwords in plain text. The idea behind such plain-text

passwords is of course that if the user typed in

\pcode{foobar} as password, we need to verify whether it

matches with the password that is already stored for this user

in the system. Why not doing this with plain-text passwords?

Unfortunately doing this verification in plain text is really

a bad idea. Alas, evidence suggests it is still a

widespread practice. I leave you to think about why verifying

passwords in plain text is a bad idea.

Using hash functions, like in our web-application, we can do

better. They allow us to not having to store passwords in

plain text for verification whether a password matches or not.

We can just hash the password and store the hash-value. And

whenever the user types in a new password, well then we hash

it again and check whether the hash-values agree. Just like

in the web-application before.

Lets analyse what happens when a hacker gets hold of such a

hashed password database. That is the scenario we want to

defend against.\footnote{If we could assume our servers can

never be broken into, then storing passwords in plain text

would be no problem. The point, however, is that servers are

never absolutely secure.} The hacker has then a list of user names and

associated hash-values, like 

\begin{center}

\pcode{urbanc:2aae6c35c94fcfb415dbe95f408b9ce91ee846ed}

\end{center}

\noindent For a beginner-level hacker this information is of

no use. It would not work to type in the hash value instead of

the password, because it will go through the hashing function

again and then the resulting two hash-values will not match.

One attack a hacker can try, however, is called a \emph{brute

force attack}. Essentially this means trying out exhaustively

all strings

\begin{center}

\pcode{a},

\pcode{aa},

\pcode{...},

\pcode{ba},

\pcode{...},

\pcode{zzz},

\pcode{...}

\end{center}   

\noindent and so on, hash them and check whether they match

with the hash-values in the database. Such brute force attacks

are surprisingly effective. With modern technology (usually

GPU graphic cards), passwords of moderate length only need

seconds or hours to be cracked. Well, the only defence we have

against such brute force attacks is to make passwords longer

and force users to use the whole spectrum of letters and keys

for passwords. The hope is that this makes the search space

too big for an effective brute force attack.

Unfortunately, clever hackers have another ace up their

sleeves. These are called \emph{dictionary attacks}. The idea

behind dictionary attack is the observation that only few

people are competent enough to use sufficiently strong

passwords. Most users (at least too many) use passwords like