Zig Zag Decryption - Computerphile

XOR encryption is flawed. Professor Brailsford explains the zig-zag method that can reveal the precious key stream.

The Professor's Supporting Documents:

This video was filmed and edited by Sean Riley.

[ButzPunk]

17 minutes of Prof Brailsford feels like 5 minutes of any other video. The way he explains this stuff is just so engaging. I love it!

[Kilroyan]

His voice is incedibly relaxing. Great job putting everything in understandable terms!

[David-lbte]

To watch this video is proof that teaching is a gift, and Prof Brailsford has it in bucket loads.

[itscomingoutofbothends]

"... then a very special technique could be used,
to try and disentangle what these messages were
without needing to know the key at all.
Now that's an amazing property of XOR,
you could perhaps say that it was a weakness or a flaw.
But, in wanting to explain it to you exactly how this worked,
I thought I'd better do it first of all, with a simple example"

So poetic!

[stephenphillip]

I could listen to Professor Brailsford all day- his enthusiasm for the subject is captivating and the clarity of his explanations is most welcome. MORE PLEASE!!

[Freakcent]

More Bletchley Park stuff with the great Professor Brailsford please!
I love this stuff and I love the way he explains and the passion he has telling about it.

[bmjames]

Forget Netflix, I am happy isolating and watching Professor Brailsford all day.

[iabervon]

One of my favorite problem sets from college (unfortunately now lost to history) consisted of two ASCII text documents XORed with the same random sequence, each about half a page from different books. Took a couple of hours to work out, including discovering a bunch of neat tricks that apply to D when P1 and P2 are ASCII text which make it a lot easier even when the plaintexts aren't otherwise related.

[thejskwared]

I recently found this channel, and I've mostly been binging the videos with Professor Brailsford. He's very good at explaining things, a very well-spoken and captivating speaker, and I quite enjoy the topics he covers :) Also, great work with the video editing/animation in this video especially - the visual representation of what he was saying was very helpful as I was trying to understand the concept!

[Dagrond]

I haven't seen green-bar paper since I was in college on a PDP-11.
Love these series in more way than one.

[DFXKX]

I took one of Stanford's online Crypto courses. And this was an amazingly easy way to put the method, never realized you could go back and forth like this, though, that's VERY clever!

[st-Impressions]

Instead of getting 2 messages with the same key why can't you simply divide one message into two halves and XOR the halves against each other?

[mikepodella]

Excellent teacher. Wish there were more of these at universities.

[robinwells]

To be able to “do” this stuff is human. To be able to teach it coherently is Devine!
Sadly I am unable to do either but I enjoy watching others that can 🤣

[bjornmu]

I see how this is a shortcut to decrypting the messages, but it doesn't actually give you more power. If you can guess parts of P1, you can use that to determine what the key would need to be to generate the corresponding C1, then try decrypting the same part of C2 using that key and see if it makes sense. It gives you the same result. Of course this technique with the combined key may be more convenient, though not necessarily faster. You still need two xor's per letter.

[devdeckardCain]

I would love to see more videos on encryption and compression!

[dasten]

4:19 ah I get it, so 'T' = 'A' ? So the guy in the plaintext is not David, it's Dtvid

[j.dietrich]

Professor Brailsford is consistently fascinating.

[DemetryRomanowski]

Its a great feeling of accomplishment when you break a cypher. The only cypher I have ever broken was for CATAPILLAR ECM passwords, I was overjoyed when I finally did it... I can only imagine how the people at Bletchley Park must have felt.

[cassia_cries]

Is there a Brailsfordettes fan club yet??!