Puzzle for anyone except for eve

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Puzzle for anyone except for eve

Postby yy2bggggs » Wed Nov 01, 2006 10:00 am UTC

Alice needs to send a super secret solid object to her boyfriend Bob without nosy Eve knowing what it is. Alice has a box that, when locked, is impenetrable, a number of locks that are unpickable, and a number of corresponding keys that are copyable. Bob has nothing.

How should Alice send the secure object to Bob, given the following?
* Alice and Bob can send packages to each other through courier; however, these packages can be intercepted by Eve.
* Eve has been getting pretty cranky lately for some reason, and she occasionally steals packages.
* If Bob ever gets the object, glorious things will happen. Ideally, this is what we want.
* If Eve steals the package with the box, it's okay, so long as she can't open it.
* If Eve manages to open the box, dire things happen, especially if Alice and Bob don't know that she opened it.
* Alice and Bob can talk via cell phone.
* Eve can talk via cell phone as well
* Eve can also fake voices, and fake out the caller id system
* Eve cannot reroute cell phone calls
* Anything not otherwise stated above (within reason) is considered insecure

Edit:
Eve can send packages too.

Disambiguation: Alice and Bob's respective locations are considered secure.
Last edited by yy2bggggs on Wed Nov 01, 2006 4:05 pm UTC, edited 2 times in total.
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Postby ulnevets » Wed Nov 01, 2006 10:50 am UTC

AOL lol!
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Postby Gelsamel » Wed Nov 01, 2006 11:05 am UTC

Walking over and giving it to him?
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Postby Dashiva » Wed Nov 01, 2006 12:15 pm UTC

If Bob has nothing, it seems rather impossible to me:

* Any key Alice sends, Eve can copy
* Any key Bob receives, Eve already has
* Any lock Bob can open, Eve can open too
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Postby Oook » Wed Nov 01, 2006 2:26 pm UTC

Well, I think it's possible :)

I've started a solution thread here, which contains my answer: http://forums.xkcd.com/viewtopic.php?t=517
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Postby yy2bggggs » Wed Nov 01, 2006 3:38 pm UTC

Gelsamel wrote:Walking over and giving it to him?

By mentioning Alice and Bob can send packages to each other, it could be implied that I've already mentioned what could be done.
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Postby Oook » Wed Nov 01, 2006 3:45 pm UTC

Just to check I understand the problem correctly: can Eve only steal objects in transit, not when they are held by Bob or Alice?

Thanks.
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Postby Gelsamel » Wed Nov 01, 2006 3:50 pm UTC

yy2bggggs wrote:
Gelsamel wrote:Walking over and giving it to him?

By mentioning Alice and Bob can send packages to each other, it could be implied that I've already mentioned what could be done.


Or I wasn't being serious :P
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Postby yy2bggggs » Wed Nov 01, 2006 4:03 pm UTC

Oook wrote:Just to check I understand the problem correctly: can Eve only steal objects in transit, not when they are held by Bob or Alice?

Thanks.

Eve can only intercept packages in transit--those are the only packages she can steal.
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mail eve!

Postby gabrayle » Thu Nov 09, 2006 6:44 am UTC

I'd send Eve in the mail to somewhere far far away.
But.

What about sending Bob a package with all the keys, telling him (in person or phone) that he should be getting X number of keys. Knowing if some key is missing, the mission should be postponed.
Also, calling him at a later date to leave the DECOY box somewhere, hoping that Eve would intercept.
If she does, then she has a box that means nothing, and perhaps all the keys.
If she does not, Alice call Bob, who should be in plain site, and tell him she forgot to put something in there, or something else.

As for disguising the keys, she should find ways of disguising them that Eve wouldn't recognize. Bubble wrap all of them in separate pouches or perfume one of them. If Eve were sane (which from the likes of it, she's not), the odds of her smelling the keys is unlikely... Maybe use colors that she knows Bob likes, or shapes that are pleasing to him. They must have inside jokes, or they wake up in a very boring relationship.

Or.
(wait, are all these plans cheating?)
Trying to organize a little "find the key" search party. Like.. "the key to lock #1 is... *random puzzle here* or "go the the place where we met for a big surprise" And also things to through eve off that Bob would know.. like "The chair we shared at the skating rink" but they've never been to a skating rink. Again, I don't see how this coulnd't be done through person.

Ultimately..
Get Eve a boyfriend. Her time and the package(s) would then be secure.
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Postby V » Thu Nov 09, 2006 1:17 pm UTC

One stupid Question: Is Alice allowed to send the box first, and the key(s) later on ?

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Re: mail eve!

Postby yy2bggggs » Thu Nov 09, 2006 4:25 pm UTC

gabrayle wrote:I'd send Eve in the mail to somewhere far far away.
But. ...

Alice, Bob, and Eve are cryptography archetypes. As such, they never age, go away, etc. This means a lot:

If Alice can send n keys, Eve could steal all of them and send n duplicates.

I don't see the difference between Eve getting a decoy and not getting a decoy.

Alice and Bob have a very boring relationship in your terms, though for me it's kind of interesting. All they do is cryptography, and all Eve ever does is try to foil them. So they do in fact have inside jokes, but Eve knows them all.

Eve will never get a boyfriend.
V wrote:One stupid Question: Is Alice allowed to send the box first, and the key(s) later on ?

It's not forbidden, and reasonable, so yes.
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Re: Puzzle for anyone except for eve

Postby gconen » Wed Sep 24, 2008 8:53 pm UTC

Getting the middle solution is easy: Don't send any keys, ever.

I'm fairly certain that getting the object to Bob is impossible. Eve can simply deny service by stealing every package Alice sends.

Thus, getting either a "win" or a "loss" apparently requires one side to make a mistake.
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Re: Puzzle for anyone except for eve

Postby jestingrabbit » Wed Sep 24, 2008 9:15 pm UTC

gconen wrote:Getting the middle solution is easy: Don't send any keys, ever.

I'm fairly certain that getting the object to Bob is impossible. Eve can simply deny service by stealing every package Alice sends.

Thus, getting either a "win" or a "loss" apparently requires one side to make a mistake.


there is a solution thread

viewtopic.php?t=517

and it has a pretty nice answer.
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Re: Puzzle for anyone except for eve

Postby munchman » Thu Dec 04, 2008 1:00 pm UTC

Spoiler:
Alice locks object in box and keeps the key, she posts it to Bob who places his own lock on the box. Bob sends it back to Alice, she removes her lock and sends it to Bob
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Re: Puzzle for anyone except for eve

Postby codeFry » Tue Dec 09, 2008 10:09 pm UTC

Spoiler:
by munchman: Alice locks object in box and keeps the key, she posts it to Bob who places his own lock on the box. Bob sends it back to Alice, she removes her lock and sends it to Bob

EVE Method of Atack:

1. Eve Keeps Box Sent by Alice
2. Eve sends Box to Bob with her lock on it.
3. Eve waits for Bob to return box with his lock on it.
4. Eve returns Alice's box to Alice with her lock on it.
5. Eve waits for Alice to return the Box minus the Alice lock.
6. Eve unlocks the box


This is a classic man in the midlle attack... it really depends on Eve's resources and to what lenghts Eve is willing to go in order to intercept the contents in the box.

1. Eve has control over the physical flow of packages
2. Eve has all the information shared between Alice and Bob. This includes:
-- a. Encryption Methods
-- b. Handling Instructions
-- c. Shared Passwords

--- Eve can remain completely silent if she chooses to do so. i.e she never has to contact Bob or Alice in order to accomplish her goals.
--- If Eve is truly dedicated she can keep all originals and forward all fakes until either Alice or Bob make a mistake and sends the complimenting portion of the shipment. (regardless of which one was shipped first.
--- For Eve it is simply a waiting game.

3. If Bob and Alice decide to meet face to face... Eve simply needs to know what is actually being delivered in the Face to Face, to perform her next move.
--- Which would be timing when she needs to eliminate Bob and impersonate him.

4. Alternately using a Secure Courier Service fails if Eve has the resources to infiltrate the Courier Service with her own agents who will acquire the shipments for her.

Unless Eve's resources can be limited in some way; the package can never be sent, without some risk of Eve acquiring its contents. There are methods to limit these risks while making it costly for Eve, but these still need to be recognized as unsecure.

From a signals perspective, if you are sitting on the repeater and recording everything going across the line before forwarding it, you own the communication channel. Anyone bouncing a signal on that repeater is technically at your mercy: Reasons for this type of control
-- a. you have a need to know and a need to control the com channel (National Security)
-- b. you decided to be malicious or highly curious that day, and you just happen to have the resources and ability to do it.
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Re: Puzzle for anyone except for eve

Postby Daywraith » Wed Dec 10, 2008 1:16 am UTC

I'm not sure but I think the following might work.
It does make the assumption that Eve can't duplicate the locks.

Spoiler:
Alice sends Bob the locked box with 5 locks (the more the better).
Alice calls to confirm bob got "the" box.
Alice then sends bob a key (for lock number 3).
Alice calls to confirm that the key arrived and tells Bob which lock the key should open.

If the key opens the correct lock then Alice can repeat the process with the remaining keys.
If it doesn't she must assume that Eve has the box and stop sending keys.


Actually I realized this doesn't work before I finished writing it.

Eve could get the box and then send a fake to replace it.
When keys are sent she could test them with each lock until she found the lock that the key opened.
She could then send a fake key that would open the corresponding lock on the box that she sent to Bob.
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Re: Puzzle for anyone except for eve

Postby mrpixkle » Thu Aug 11, 2011 9:49 am UTC

I probably should have read that solution forum before I went and posted my solution, shouldn't I have?
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Re: Puzzle for anyone except for eve

Postby jestingrabbit » Thu Aug 11, 2011 12:19 pm UTC

mrpixkle wrote:I probably should have read that solution thread before I went and posted my solution, shouldn't I have?

Or you might have at least considered when the last post in this thread was, and pondered whether resurrection of a thread dead for 2 1/2 years is the best use of your time.
ameretrifle wrote:Magic space feudalism is therefore a viable idea.
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Re: Puzzle for anyone except for eve

Postby mrpixkle » Mon Aug 15, 2011 9:36 am UTC

Oh, believe me, netcromancy is more than just a hobby where I come from ;)
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Re: Puzzle for anyone except for eve

Postby mrpixkle » Mon Aug 15, 2011 9:39 am UTC

Or...wait...the solution that I wrote didn't show up :|

@$!#&*
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Re: Puzzle for anyone except for eve

Postby jestingrabbit » Mon Aug 15, 2011 12:45 pm UTC

mrpixkle wrote:Or...wait...the solution that I wrote didn't show up :|

@$!#&*


I moved it to the other thread. Also, notice the edit button. Rather than making consecutive posts, use it.
ameretrifle wrote:Magic space feudalism is therefore a viable idea.
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Re: Puzzle for anyone except for eve

Postby mrpixkle » Tue Aug 16, 2011 7:41 am UTC

Right!

....sorry it's been a while
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Re: Puzzle for anyone except for eve

Postby mmmcannibalism » Thu Aug 18, 2011 6:08 am UTC

Since eve cannot reroute calls couldn't you just

Spoiler:
Alice sends Bob box with object then calls him to ask if he has it.
If he has the box she sends keys forever
If he doesn't she never sends a key

Alice can ensure she is talking to Bob by making the call to get into contact with him
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Re: Puzzle for anyone except for eve

Postby mrpixkle » Fri Aug 19, 2011 8:33 am UTC

mmmcannibalism wrote:Since eve cannot reroute calls couldn't you just

Spoiler:
Alice sends Bob box with object then calls him to ask if he has it.
If he has the box she sends keys forever
If he doesn't she never sends a key

Alice can ensure she is talking to Bob by making the call to get into contact with him


That's not nearly convoluted enough to be the answer to a logic puzzle of this scale :p
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Re: Puzzle for anyone except for eve

Postby rigwarl » Fri Aug 19, 2011 3:27 pm UTC

It also doesn't work for the reason Daywraith wrote a few posts up (Eve could get the box and then send a fake to replace it).
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