applied cryptography - protocols, algorithms, and source code in c

R1r2b 3 alice computes the one way function on the

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Unformatted text preview: & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book: Go! Previous Table of Contents Next ----------- To make this work, Alice had to switch envelopes at the end of the trick. However, cryptographic protocols can provide a method immune from any sleight of hand. Why is this useful? Here’s a more mundane story: Stockbroker Alice wants to convince investor Bob that her method of picking winning stocks is sound. Bob: “Pick five stocks for me. If they are all winners, I’ll give you my business.” Alice: “If I pick five stocks for you, you could invest in them without paying me. Why don’t I show you the stocks I picked last month?” Bob: “How do I know you didn’t change last month’s picks after you knew their outcome? If you tell me your picks now, I’ll know that you can’t change them. I won’t invest in those stocks until after I’ve purchased your method. Trust me.” Alice: “I’d rather show you my picks from last month. I didn’t change them. Trust me.” Alice wants to commit to a prediction (i.e., a bit or series of bits) but does not want to reveal her prediction until sometime later. Bob, on the other hand, wants to make sure that Alice cannot change her mind after she has committed to her prediction. Bit Commitment Using Symmetric Cryptography This bit-commitment protocol uses symmetric cryptography: (1) Bob generates a random-bit string, R, and sends it to Alice. R (2) Alice creates a message consisting of the bit she wishes to commit to, b (it can actually be several bits), and Bob’s random string. She encrypts it with some random key, K, and sends the result back to Bob. EK(R,b) That is the commitment portion of the protocol. Bob cannot decrypt the message, so he does not know what the bit is. When it comes time for Alice to reveal her bit, the protocol continues: (3) Alice sends Bob the key. (4) Bob decrypts the message to reveal the bit. He checks his random string to verify the bit’s validity....
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