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Can two strangers communicate securely without a friend?

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if you and your friend have both agreeded upon a cipher sure you can communicate securely, but that pre-supposes you already communicated before hand and was that secure?

So if the answer is "no, you need a trusted friend or a common secret", then I guess the real question of the thread is what are some practical ways of establishing a common secret or friend with a stranger?

 

If you're physically nearby, you can meet in person and be in close enough proximity that you can privately share some information without a MITM being able to intercept it. If that's not practical, are there other 'challenges' you can make which only the intended recipient can complete? e.g. something where the MITM would have to travel to the physical location of either party to know the answer, etc...?

 

you are once again putting words in my mouth

You missed the fact that the OP is asking about situations where you don't have a trusted 3rd party or a shared secret.
Samoth is putting words in your mouth because you're making suggestions that do require a shared secret, and he's assuming that you think these are valid in the OP's situation.
 
To decode Samoth's jab at you: One-time-pads are a shared secret. The OP's situation requires no shared secrets. Therefore sending a message encrypted with a one-time-pad is the same as sending random jibberish -- the recipient cannot possibly decode either.
 
@Samoth, stop picking at him, he obviously missed the bit where the OP wants to communicate without a shared secret, or alternatively, be more direct with your critique tongue.png 
 

The static background noise from radiowaves in space is also good and unless someone else recorded in the exact same direction as you, at the exact same time, with the exact same level of equipment, nobody else will ever have that key.

When I worked on gambling machines, this is what we used in order to ensure that no one could possibly guess the outcome of the games (along with ambient temperature, radiation, etc...). It's a genuine random number generator, instead of a PRNG.
It's impossible for someone outside the cabinet to take the exact same measurements, and the cabinet itself is secured with a pre-shared key -- literally, the government regulator is given a physical key in person.

Edited by Hodgman

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If you're physically nearby, you can meet in person and be in close enough proximity that you can privately share some information without a MITM being able to intercept it.

It is still very hard to guarantee that the person you are meeting is, in fact, the person you intended to meet, and not a skilled impersonator.

 

Unless you known this person already, or are introduced (in person) by a trusted 3rd party, the man-in-the-middle vector remains...

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If you're physically nearby, you can meet in person and be in close enough proximity that you can privately share some information without a MITM being able to intercept it.

It is still very hard to guarantee that the person you are meeting is, in fact, the person you intended to meet, and not a skilled impersonator.

 

Unless you known this person already, or are introduced (in person) by a trusted 3rd party, the man-in-the-middle vector remains...

Huh, yeah... if you ask someone to meet you at the town square to exchange keys, a MITM could intercept the message and change it to say that you want to meet at starbucks. They then meet you at the town-square, and meet 'the stranger' at starbucks, exchanging their own key with both of you, and gathering both of your own keys... They can then continue to act as a MITM, forwarding your messages to 'the stranger' via themselves.

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The static background noise from radiowaves in space is also good and unless someone else recorded in the exact same direction as you, at the exact same time, with the exact same level of equipment, nobody else will ever have that key.

When I worked on gambling machines, this is what we used in order to ensure that no one could possibly guess the outcome of the games (along with ambient temperature, radiation, etc...). It's a genuine random number generator, instead of a PRNG.

I think I got that from Bruce Schneier's 'Secrets and Lies' book, which was an interesting read for a layman like myself.

 

It is still very hard to guarantee that the person you are meeting is, in fact, the person you intended to meet, and not a skilled impersonator.
 
Unless you known this person already, or are introduced (in person) by a trusted 3rd party, the man-in-the-middle vector remains...

Huh, yeah... if you ask someone to meet you at the town square to exchange keys, a MITM could intercept the message and change it to say that you want to meet at starbucks. They then meet you at the town-square, and meet 'the stranger' at starbucks, exchanging their own key with both of you, and gathering both of your own keys... They can then continue to act as a MITM, forwarding your messages to 'the stranger' via themselves.

That happened in Mission Impossible: Ghost Protocol. rolleyes.gif

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Guys (and gals?), this has been an awesome thread to read. I'm wishing I hadn't posted it in the Lounge so that I could give upvotes galore! I just wanted to say thanks. Carry on :)

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Yea, the Two General's Problem has no perfect solution and I believe was proven to be impossible to solve, so only pragmatic mitigations exist: accept that you will never have security, and mitigate that as much as possible

Edited by Net Gnome

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I've been thinking about encryption this morning (and I have no clue why) and I started thinking about if it's at all possible for two strangers to establish a secure connection. I'm having my doubts, but I don't know a whole lot about encryption.

Yes.  There are several well-known algorithms that can be used to establish an encrypted communication channel without the cleartext exchange of secret keys.  A classic example is the use of Diffie-Hellman, in which a large number is agreed upon (exchanged in the clear) and then each party chooses a relatively prime factor and exchange some information using those factors to discover a mutually common secret key without actually revealing their secrets.  Very cunning.

 

Establishing an encrypted channel between two parties is a relatively simple and straightforward solved problem.

 

Where you might run into trouble is in mutual authentication.  This is an entirely different problem from encrypted communication, although many authentication solutions rely on the establishment of encrypted communication to be effective.  Almost all authentication mechanisms rely on a trusted third party somewhere along the line.

 

Security is a feeling, not a science.  Pretty much anything useful has some sort of security vulnerability, at least potentially.

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Where you might run into trouble is in mutual authentication.  This is an entirely different problem from encrypted communication, although many authentication solutions rely on the establishment of encrypted communication to be effective.  Almost all authentication mechanisms rely on a trusted third party somewhere along the line.

But the underlying problem is that encrypted communication is absolutely useless without mutual authentication - and integrity - because you have no guarantee that what you send will be received intact (or at all) by the other party and you don't know (and cannot detect) if someone is impersonating the party you are communicating with. So you really have no privacy nor integrity, in the technical sense, without authentication. DH does not "establish an encrypted communication channel" at all, it permits secure key exchange on an insecure channel if and only if the two parties can authenticate each other. Without this condition it does not provide any security at all and you cannot communicate securely on said insecure channel.

 

In the theoretical sense, anyway, but an MITM is really not that difficult to mount if you really wanted to.

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