How does two party encryption work?

Question

I am able, with pidgin otr for example, capable of communicating with anyone and it is secure. We haven't exchanged a secret passkey or anything else, the program just sets up the encryption and everyone assures me it is secure.

So what I have is this:

Person1 talking to Person2 (Assuming people really are who they say they are)

Since no info was ever exchanged between person1 and person2, what's to stop a badperson3 from intercepting all the traffic and deciphering all the messages?

I read about PGP, about how each machine has a private key and a public key that is shared between the two computers, but I don't fully get how this can work. It's not like person2 can encode messages using person1's private key, and if it uses the public key, anyone listening can decipher.

I'm a bit confused.

Answer 1

OTR does exchange information. You have to accept the other person's key and verify it's trusted. Then it all just goes back to normal asymmetric encryption. Which ... boils down to verifying that the one giving you the key and the one you want to talk to are the same person. Nothing magically happening here.

Answer 2

Messages encoded with someone's private key can only be decoded using the matching public key. Likewise, messages encoded with someone's public key can only be decoded with that person's private key.

If I encode a message with my private key and your public key, then you know that the message is guaranteed to be from me and only you can read it. You would decode it using my public key (to prove that the message came from me) and your private key (proving that only you can read it).

Answer 3

Here are the rules for public key encryption:

1. A public key may be used to encrypt a message that only the corresponding private key can decrypt.
2. A private key may be used to encrypt a message that only the corresponding public key can decrypt.

So, it works both ways. The public key being public isn't a problem.

The reason it works is a lot more complicated and involves some interesting math.

Now, OTR doesn't exactly use standard public key encryption. Their algorithms are based on public key encryption but have a small twist in them to allow either party to convincingly deny the conversation ever happened.

Also, OTR isn't secure if you don't actually talk to the person. It's susceptible to a man-in-the-middle attack. This isn't because the encryption can be broken or anything like that. No, a man-in-the-middle attack is where someone interposes themselves between you and the party you want to talk to.

If you are A and you are trying to talk to person B and there's a man in the middle, M, then the attack works like this.

You start talking to B, but unbeknownst to you, you start talking to M who is pretending to be B. When you start talking to M, M initiates a conversation with B pretending to be you. You encrypt all of your stuff to M (who you think is B). M decrypts it, and re-encrypts it to B (who thinks M is you).

If M can always stay in the middle, none of your conversations with B will be secure. If you can ever talk to B without M in the middle, OTR will complain that Bs public key has changed (which is basically because you were really using Ms public key and are just now getting Bs real public key). Of course, OTR might also be complaining because M is trying to interpose (him/her)self and the key you think is the right one really is.

The way to defeat a man-in-the-middle attack is for you and B to talk in some way where you can be certain for other reasons that you're talking to B. Then you can share information about your public key with B, and B can share information about h(is/er) public key with you. You can do this in a coffee shop or over the phone. It doesn't matter if anyone overhears the details, just that you know that it's the person you expect giving them to you.

There is also a fancier way to do this involving secrets that only you and B know. OTR has a mode for asking a question design to elicit a secret in such a way that M cannot provide the proper answer for you, only B can. M will be able to overhear the answer B provides, but will not be able to give it to you.

Answer 4

There is a good description written up on wikipedia: http://en.wikipedia.org/wiki/Public-key_cryptography

Answer 5

You can encode with a public key, but you have to have the private key to decode.

So if two people have public keys and share these with each other, then they can share information that can only be decoded with the receiver's private key.

Answer 6

Without getting into technical details, the whole idea of public key cryptography like RSA is that the public key can be used to encrypt data, but cannot be used to decrypt what it just encrypted (without breaking the encryption, of course -- in the case of RSA that normally means factoring the product of two large primes).

There is, of course, still some data exchanged: you still need to get a person's public key before you can encrypt data using that public key. It's also worth noting that public key cryptography is generally a lot slower than symmetric cryptography. That being the case, most protocols that use public key cryptography try to minimize the amount of data encrypted using the public key algorithm. Typically, the sender will pick a random number of the right size to use as a key with a symmetric algorithm, encrypt it with the public-key algorithm, and send it to the receiver. The rest of the data is encrypted with the symmetric algorithm, using that key.

As such, in a typical case you're retrieving a public key, then generating, encrypting, and sending a private key, then sending data encrypted with that private key.

Answer 7

Public key encryption works like this: You freely give out your public key. People use your public key to encrypt messages that can only be decrypted with your private key (which you do not share). Your private key acts like "the missing piece" that is used when decrypting the information insofar as it is the only part of the equation that can actually decrypt anything. If someone encrypts something with your public key, you cannot use your public key to decrypt the data afterwards. This is accomplished by using very large prime numbers and some very specific equations.

Answer 8

You have asked two questions:

1. There are ways to exchange keys securely, e.g. with the Diffie Hellman protocol. It's safe even if an attacker is listening to all the messages.

2. With public key cryptography each key has a public and a secret component. With the public component you can encrypt or verify a signature, with the secret component you can decrypt or create a signature. There's a lot of number theory to see how this works, but it's worth to learn it.