Tesla's April fools headline fooled stock trading algorithms causing $1.50 jump:
http://www.bloombergview.com/articles/2015-04-02/tesla-stockholders-can-t-take-a-joke …

Level 7 of the Stripe CTF involved running a length extension attack on the level 7 server's custom crypto code.

`@app.route('/logs/')`

@require_authentication

def logs(id):

rows = get_logs(id)

return render_template('logs.html', logs=rows)

...

def verify_signature(user_id, sig, raw_params):

# get secret token for user_id

try:

row = g.db.select_one('users', {'id': user_id})

except db.NotFound:

raise BadSignature('no such user_id')

secret = str(row['secret'])

h = hashlib.sha1()

h.update(secret + raw_params)

print 'computed signature', h.hexdigest(), 'for body', repr(raw_params)

if h.hexdigest() != sig:

raise BadSignature('signature does not match')

return True

The level 7 web app is a web API in which clients submit signed RESTful requests and some actions are restricted to particular clients. The goal is to view the response to one of the restricted actions. The first issue is that there is a logs path to display the previous requests for a user and although the logs path requires the client to be authenticatd, it doesn't restrict the logs you view to be for the user for which you are authenticated. So you can manually change the number in the '/logs/[#]' to '/logs/1' to view the logs for the user ID 1 who can make restricted requests. The level 7 web app can be exploited with replay attacks but you won't find in the logs any of the restricted requests we need to run for our goal. And we can't just modify the requests because they are signed.

However they are signed using their own custom signing code which can be exploited by a length extension attack. All Merkle–Damgård hash algorithms (which includes MD5, and SHA) have the property that if you hash data of the form (secret + data) where data is known and the length but not content of secret is known you can construct the hash for a new message (secret + data + padding + newdata) where newdata is whatever you like and padding is determined using newdata, data, and the length of secret. You can find a sha-padding.py script on VNSecurity blog that will tell you the new hash and padding per the above. With that I produced my new restricted request based on another user's previous request. The original request was the following.

`count=10&lat=37.351&user_id=1&long=%2D119.827&waffle=eggo|sig:8dbd9dfa60ef3964b1ee0785a68760af8658048c`

The new request with padding and my new content was the
following.
`count=10&lat=37.351&user_id=1&long=%2D119.827&waffle=eggo%80%02%28&waffle=liege|sig:8dbd9dfa60ef3964b1ee0785a68760af8658048c`

My new data in the new
request is able to overwrite the waffle parameter because their parser fills in a map without checking if the parameter existed previously.
Code review red flags included custom crypto looking code. However I am not a crypto expert and it was difficult for me to find the solution to this level.

Neat demo of Visvalingam’s line simplification algorithm in JavaScript applied to a map of the US.

To simplify geometry to suit the displayed resolution, various line simplification algorithms exist. While Douglas–Peucker is the most well-known, Visvalingam’s algorithm may be more effective and has a remarkably intuitive explanation: it progressively removes points with the least-perceptible change.

Breakdown of the STL’s implementation of next_permutation. Ever wondered how that works?

In my Intro to Algorithms course in college the Fibonacci sequence was used as the example algorithm to which various types of algorithm creation methods were applied. As the course went on we made
better and better performing algorithms to find the nth Fibonacci number. In another course we were told about a matrix that when multiplied successively produced Fibonacci numbers. In my linear
algebra courses I realized I could diagonalize the matrix to find a non-recursive Fibonacci function. To my surprise this worked and I
found a function.

Looking online I found that of course this same function was already well known. Mostly I was irritated that after all the
algorithms we created for faster and faster Fibonacci functions we were never told about a constant time function like this.

I recently found my paper depicting this and thought it would be a good thing to use to try out MathML, a markup language for displaying math. I went to the MathML implementations page and installed a plugin for IE to display MathML and then began writing up my paper in MathML. I wrote the MathML by hand and must say that's not how its intended to be created. The language is very verbose and it took me a long time to get the page of equations transcribed.

MathML has presentation elements and content elements that can be used separately or together. I stuck to content elements and while it looked great in IE with my extension when I tried it in FireFox which has builtin MathML support it didn't render. As it turns out FireFox doesn't support MathML content elements. I had already finished creating this page by hand and wasn't about to switch to content elements. Also, in order to get IE to render a MathML document, the document needs directives at the top for specific IE extensions which is a pain. Thankfully, the W3C has a MathML cross platform stylesheet. You just include this XSL at the top of your XHTML page and it turns content elements into appropriate presentation elements, and inserts all the known IE extension goo required for you. So now my page can look lovely and all the ickiness to get it to render is contained in the W3C's XSL.