153: Cryptography

Explain xkcd: It's 'cause you're dumb.
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If you got a big keyspace, let me search it.
Title text: If you got a big keyspace, let me search it.


Ambox notice.png This explanation may be incomplete or incorrect: Layout is ugly, content has to be verified. Explain the reversal part in Missy Elliott's song. Do we need an explain on the MD5 salt?
If you can address this issue, please edit the page! Thanks.

This comic refers to the study of cryptography. We can note the presence of the International Association for Cryptologic Research (IACR) logo in the podium, an association who organizes the most important conferences in the cryptology field. Cueball, at the podium, is describing a proposed crypto system in which a computer program that turns a very large number, called the "key", and a message into an encrypted form that can only be read by using the same key, based on the model of a Feistel cipher. Part of any Feistel cipher is the "round function", which determines how the key is applied to the original message; this is applied multiple times with a variety of tricks and techniques to insure the process can eventually be reversed. One common component of round functions is the S-box, a simple table that converts input bytes into output bytes, preferably in a way that doesn't correspond to any mathematical rules.

Here, the S-box would be implemented by doing the following (with the computer operation actually shown in the diagrams indicated in parentheses):

  1. Take the bitstring down (roll right by 1)
  2. Flip it (take its binary NOT)
  3. Reverse it (run the bits in the opposite order)

This would be run on each round of the cipher to further scramble the message for the next round. As the caption implies, the steps are based on a line from the Missy Elliott song Work It: "I put my thing down, flip it and reverse it." As with any encryption system there must be a way to decrypt the cipher text. In Missy Elliott's song, the phrase "I put my thing down, flip it and reverse it" is repeatedly played backward, sounding like gibberish. In the same way, a steps to a feistel cipher based algorithm are executed in reverse to obtain the original plain text from a cipher text.

The keyspace for a cryptographic algorithm is the number of possible keys the algorithm can possibly accept. For example, AES-256 has a keyspace of 2256 (roughly 1.1579209e+77) possible keys, simply because the algorithm specifies that each key is 256 bits wide. The title text is referring to "searching a keyspace", which is to say, simply trying every key until you find one that works. (For reference, a computer would require roughly the energy of a billion billion supernovas to even count to 2256, let alone actually try each one.) The precise wording "If you got a big keyspace, let me search it" is, of course, another reference to the same song: "If you got a big **** let me search ya" (The word "penis" is censored by the trumpeting of an elephant).

Inside references and real life shenanigans

Cueball is banned from multiple conferences for similar pranks in 541: TED Talk. In response to 541, PyCon organizers jokingly announced that Randall Munroe was banned from PyCon 2009 due to "last year's disgraceful keynote, 'Web Spiders vs. Red Spiders'." They also said they instructed their volunteers to refuse admission to him and "any stick figures who may attempt to register, particularly if they are wearing hats."

Messages on the PyCon-Organizers mailing list show that this joke was intended to get Randall to come to PyCon: (The links will only work if you're subscribed to the mailing list.)

PyCon mentioned briefly in today's xkcd:
We've still never gotten Randall Munroe to actually attend, have we? Anybody want to take charge of twisting his arm this time? I think we can still offer him a "press pass" (free registration). [...] [1]
An invite would seem most appropriate given the cartoon. :-)
We could also have an official PyCon blog post confirming his ban... [2]
How about a public blog post LIFTING the ban and inviting him? [3]
Confirming the ban is far funnier... He's definitely a disturbing influence on programmers. [4]
Agreed, especially if we invite him concurrently with confirming the ban. [5]
(a few posts later)
I'm happy to participate in actually throwing some thin guy out of the conference, and then get some graphics savvy person to animate a stick figure over that. :) [...][6]
here's a rough idea of what would show up if you invited him to actually do any speaking...
it's fairly long. he shows up in the beginning around 3:25. [7]
For the record, I did try to convince him to come when got the art for the tshirt last year. He didn't seem super psyched, but then again emotions are hard to read via IRC.
I love the idea of "officially" banning him, however. ;-) [8]
(a few posts later)
OK - posted to the PyCon blog, by the power vested in me as publicity chair. With Michael Foord's excellent sentence added.
Now let him know that since he's banned, he HAS to come. [9]


[Cueball at a podium.]
Cueball: My cryptosystem is like any Feistel cipher, except in the S-Boxes we simply take the bitstring down, flip it, and reverse it.
I've been barred from speaking at any major cryptography conferences ever since it became clear that all my algorithms were just thinly disguised Missy Elliot songs.

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I think that executing that S-box twice would get you back where you started. gijobarts (talk) 05:28, 30 July 2013 (UTC)

Actually that's not true. Regardless of the bit in position 1 to begin with, you will always have a 1 in position 8 in the result. When you shift, you're adding a 0 in position 1 (assuming a 0 shift in), then the inverse is 1, and flipping would put the 1 in position 8. Jarod997 (talk) 14:06, 5 March 2014 (UTC)
I was expecting the last bit to wrap around to the front. It could go either way. gijobarts (talk) 05:57, 6 March 2014 (UTC)
I think it would actually take a few rounds, but yes will eventually get back to the same as the input. Remember that you aren't just doing this operation, you are doing it to one half of the block and then XORing with the other half of the block. But yes I think after a few rounds the XOR's would combine to the identity. (assuming that it wraps, which makes sense to me). Also it is not shown at all how the key would be incorporated into this... so maybe that would help? (or you just add a round key in after doing this operation?) 16:32, 18 November 2015 (UTC)

" and so is the author Randall Munroe at PyCon"

I think that post is a joke.
  • It links to 541: TED Talk.
  • It says "Registration volunteers have been instructed to refuse admission to Randall Munroe personally, and in fact, to any stick figures who may attempt to register"
  • There isn't anything on YouTube or Randall Munroe's Wikipedia page about it.
  • Another Python blog says that it was a publicity stunt, citing the organizers' mailing list archives. I didn't bother to sign up for access to the archive.
Catherine Devlin claims that she banned Randall, so we could try asking her if she's serious.
Another blog post about it
gijobarts (talk) 16:48, 2 September 2013 (UTC) (edited 20:29 UTC)
I've signed up for access to the mailing list, and am currently waiting for moderator approval. gijobarts (talk) 20:38, 2 September 2013 (UTC)

In the same way, a steps to a feistel cipher based algorithm are executed in reverse to obtain the original plain text from a cipher text. is not true. The whole point of a Feistel network is that you execute the same steps in the same order. The only thing that is reversed is the key. You can do almost any amount of mangling of the input, without having to worry about how to reverse it, because the magic of XOR ensures that All Will Be Well when you come to decrypt. There are limits to the kinds of mangling you can do, of course, but the basic principle is that the same function used for encryption is also used for decryption. It's quite startling, really. Horst Feistel - kudos! --BinaryDigit (talk) 15:08, 8 August 2014 (UTC)

+1 16:32, 18 November 2015 (UTC)

"This part needs editing" in what way? It looks fine to me. Clarify, or better yet, edit it in what way you think it needs editing. 16:08, 29 October 2017 (UTC)

Why is this page marked as an incomplete explanation? 16:59, 8 November 2017 (UTC)

The PyCon stuff happened because of 541: TED Talk, and is fully explained there, so is it necessary to have a full PyCon explanation here as well? It seems to me that this comic is not related to PyCon specifically, so would it be enough to just say this is the first comic that mentions getting banned from a conference (and link to Category:Banned from conferences)? If no one objects, I'll do that instead of the current "Inside references and real life shenanigans" section. -- Yfmcpxpj (talk) 13:19, 5 December 2019 (UTC)

I fully agree. Maybe we can use parts of this text to put it into the category instead? --Lupo (talk) 13:25, 5 December 2019 (UTC)
Yeah, it's hilarious they would (jokingly) implement the PyCon ban from comic 541 in real life, so I think it's worth a mention in the category page. (But I'm still a noob, so I'm not sure what belongs there.) -- Yfmcpxpj (talk) 14:15, 5 December 2019 (UTC)
For stuff regarding Categories, usually Kynde has a good feeling. I'll try to reach him on his talk page, to chime in here.--Lupo (talk) 14:21, 5 December 2019 (UTC)
I completely agree that this section did not belong here, mainly because it was a copy of text already in a section on 541: TED Talk. I have removed it from here adding a link to that section, as it is interesting here to mention given this was the first comic to use this angle, and TED Talk so far is the last. I have also added this text to the category, as TED talk is the one that makes the most out of this ban subject. Maybe the list of banned conferences on TED talk should also add a list of conferences Randall previously have been banned from, mentioning those, if they are not already the same as in that comic? --Kynde (talk) 08:28, 6 December 2019 (UTC)
Apart from Ted talk they all refer to crypto conferences, which is with one selected one mentioned in Ted Talk, siggraph, which is mentioned, and his math licence, which doesn't refer directly to a conference. So it doesn't need to be mentioned seperately I think. --Lupo (talk) 08:58, 6 December 2019 (UTC)