Showing Revision 1 created 06/29/2012 by Amara Bot.

1. Title:
   07-25 Security
2. Description:

   Other units in this course below:
   Unit 1:http://www.youtube.com/playlist?list=PL3FEF9299137945CC
   Unit 2:http://www.youtube.com/playlist?list=PLBF6FC32358457242
   Unit 3:http://www.youtube.com/playlist?list=PL30EF9EB86B4D2E54
   Unit 4:http://www.youtube.com/playlist?list=PL6881C8D2E9D63242
   Unit 5:http://www.youtube.com/playlist?list=PL62AF1F6D3B654E14
   Unit 6:http://www.youtube.com/playlist?list=PL8FCC9D9C45A6640E
   Unit 7:http://www.youtube.com/playlist?list=PL9533BCB71FD88E38

   Q&A: http://www.youtube.com/playlist?list=PLE0EB375FA373A866

   To gain access to interactive quizzes, homework, programming assignments and a helpful community, join the class at http://www.udacity.com

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   Let's take a break and talk a bit about computer security--
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   a topic we really haven't touched on much in this unit so far.
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   Security can sometimes be defined as computing in the presence of an adversary--
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   someone else using a computer or a network or resources
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   who means you hard or hopes to exploit or take advantage of resources
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   resources that you've put out there.
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   It just so happens--you'll be surprised to discover I am breaking this news--
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    that the internet is not secure.
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    Malicious agents or adversaries can write malicious webpages or JavaScript fragments.
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    The simplest way to do this would be to write a JavaScript fragment that loops forever--
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    some sort of version of Fibonacci that has no stopping condition
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    that just keeps going and going and going.
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    Then as soon as you've directed your browser to that page, it would stall,
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    and you'd be denied some server. You wouldn't be able to use your computer.
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    In practice, browsers often put some sort of timeout on script execution.
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    I'll run your script for 3 or 4 seconds, and after that if it hasn't finished, I'd better stop.
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    that's one of the reasons why optimization is so important, but it also has this security angle.
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    If we make any mistakes when we're designing our web browser,
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    then outside agents might break in to our computers and gain access
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    to sensitive documents--our tax returns, our personal email, or whatnot.
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    We want to make sure that that doesn't happen.
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    Let's listen to someone who knows quite a bit more about security than I do
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    talk about how this plays out in the real world with web browsers.
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    The web can be a great place but it can also be a dangerous place.
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    Nothing stops users from posting malicious code or websites
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    or writing scripts that will trying to take advantage of your computer or your browser.
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    There might be security holes or exploits, they're sometimes called, that allow this.
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    I was wondering if you might talk a bit about security at the language or implementation level
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    from the perspective of Mozilla.
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    Absolutely. Security is a prevalent concern at Mozilla.
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    When I was growing up, the idea of getting a piece of software was that you actually
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    drove to a store and you looked at a shelf and you pulled a box off the shelf
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    and you paid somebody and you took it home and that piece of software was sold to you
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    by a company that you trusted, by a third party that you trusted,
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    so you had some sense of this piece of software that I'm about to put on my computer
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    is something that I can believe is going to do something on behalf,
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    not something against me.
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    The web doesn't work like that way at all.
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    The way web applications work is you can go to any website anywhere in the world,
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    and somebody you've never met and never seen and that nobody can vouch for you
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    is going to run some of their code that they wrote on your computer.
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    So that changes the game.
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    That means that in order to build a serious platform where programs can do important things
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    on your behalf, we need to make sure that they can't also do important things against you.
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    The more people put valuable parts of their lives onto the web like their bank account,
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    for example, the more we have important assets to defend.
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    One of the concepts people talk about a lot in security is the notion of the trust computing base.
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    When you download some code from some third party that you don't know,
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    if we're being kind of maximally pessimistic we say, well, I don't trust this code completely.
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    I'd like it to do something for me but I don't know for sure that it's not malicious.
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    However, that code is running inside of a web browser like Firefox,
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    and I do trust the code that was written by Mozilla. I do trust Firefox.
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    In order to be able to say as much as we can about the security of a system,
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    we'd like for the parts of the system that we need to trust the most
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    to be as small as possible, so we talk about shrinking the trusted computing base,
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    as being a goal of having the smallest possible amount of software where if it goes wrong
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    disaster can ensue--like somebody can steal or credit card information or your bank account.
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    All modern web browsers are implemented in C++.
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    Now, C++ is a very powerful language. There are a lot of things that you can do with it.
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    But it's also not a particularly safe language.
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    It's a language where if you make one wrong move, if you make one little programming mistake,
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    you can actually leave parts of your system open to malicious programs.
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    For example, if you write a program in C++ and you have an array of data
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    and you forget to make sure that the program doesn't go beyond the end of that array,
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    in most programming languages you'll get a runtime error that says,
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    "Oops. You tried to access beyond the end of the array."
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    C++ doesn't give you that protection.
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    What C++ does is it just keeps reading what ever happens to be in memory
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    at the end of that array.
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    Well, whatever happens to be in memory could actually be some part of the browser
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    that has internal information like a password.
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    It could also be some other program running on the system,
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    and there are a lot of people out there who work on finding ways to exploit
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    these kinds of bugs to use them to take control of your computer or to get access to your private data.
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    The project of building a web browser that people can trust
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    and building a web browser that operates on user's behalf
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    is also one of building software that they can trust.
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    In order to build software that they can trust,
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    it needs to be built on top of programming technologies
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    that we know we can work with effectively to build software that's not unsafe.
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    We've been discussing malicious code like JavaScript written by evildoers
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    that we would run in our interpreter. Running evil code seems really bad.
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    Can't I just look at the source code and tell if it's bad before I run it
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    and then not run the bad code? Why doesn't Mozilla do something like that.
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    Unfortunately, it turns out that there's a lot of good theoretical computer science
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    that shows us that that is a provably impossible problem.
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    I don't know if you've discussed the halting program. >>We may have.
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    It turns out that if I were able to prove that any particular piece of code was not malicious
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    I would also be able to solve the halting problem,
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    and we already know that that's an unsolvable problem in computer science.
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    Looking at this maybe from a more positive side,
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    that means that I'll always have a job.
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    The law of compiler writer employability. >>Exactly.