TL DR Anyone can send a message and say it is from Alice if it is not signed there is no way for Alice to prove that she didn't send those unsigned messages. I am not aware of any deployed in the wild implementation of a cryptographic fair exchange protocol. Some people are running Web-based services in which they play the role of the trusted third-party I am not sure any of them has reached any level of significant commercial success. Standard emails follow neither method right now. This works because both Alice and Bob trust the postman: Bob knows that the postman will give him the letter, and thus has no problem with signing the receipt before having the letter in his hands Alice knows that the postman will not give the letter to Bob without receiving a receipt first, and the postman will send the receipt back to Alice. Alice gives the letter to the postal service the postman then ensures that Bob will get the letter only after having signed the receipt. This is the model of existing postal services around the World. There are a lot of details to care about. There is some remaining unfairness, but only a small one.įor such a protocol to work, some additional mathematics must be thrown at it, so that Alice and Bob may prove to each other that the "encrypted messages" they sent are the genuine things, and not some chunks of random junk. A cheater will then have an advantage over the other, but only by one bit in the resulting brute force. Either may bail out of the protocol at any time, with a partial key knowledge, and try to complete decryption through exhaustive search of the missing key bits. Alice and Bob then send each other the keys, bit by bit, one at a time. Bob sends an encrypted email to Alice, with a symmetric key K b. For instance, Alice sends an encrypted email to Bob, with a symmetric key K a. There are mostly two methods to get a fair exchange protocol: Now impossibility results have never discouraged people from trying. This is because of the lack of simultaneity. There can be no two-party protocol, however complex and convoluted, that ensures a fair exchange. What I mean here is that the flaw is fundamental.
In that case, Alice could cheat by getting a receipt from Bob, then refusing to complete the protocol and send the email. To fix that, you may want to design the protocol such that Bob sends the receipt first, and then Alice sends the email. In that case, Bob could get the email and refuse to send the receipt Alice would have no proof. To get some intuition about what I explain above: suppose a simple protocol by which Alice sends an email to Bob, then Bob sends a signed receipt. Bob will never get the element from Alice, while Alice got the one from Bob. Suppose that Alice gets her element first then she may simply stop responding at that point. Thus, Alice and Bob get all they need at necessarily distinct moments. You cannot ensure for two remote people (Alice and Bob) to send messages at the same time. The tricky point is that there is no such thing as simultaneity. Similarly, Bob will also obtain enough information to rebuild m b right after having received some message from Alice. That point will be immediately after reception by Alice of some protocol message from Bob before the message, Alice does not know enough to rebuild m a, and after she does. When the exchange protocol is played, at some point in the sequence of messages, Alice will obtain enough information to be able to rebuild m b. Let's call m a the element that is to be conveyed from Alice to Bob, and m b the element that should go from Bob to Alice. in your case an email from Alice to Bob, and a receipt from Bob to Alice but it also works as a model for payments), then Alice and Bob will send messages. If you consider a network protocol such that Alice and Bob want to send each other some data elements (e.g.
The underlying cryptographic problem is called fair exchange. In practice, unless Bob sends a signed receipt, you are out of luck.
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