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

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Unformatted text preview: not reveal their hands can be found in [390]. Don Coppersmith discusses two ways to cheat at mental poker using the RSA algorithm [370]. Anonymous Key Distribution While it is unlikely that anyone is going to use this protocol to play poker via modem, Charles Pfleeger discusses a situation in which this type of protocol would come in handy [1244]. Consider the problem of key distribution. If we assume that people cannot generate their own keys (they might have to be of a certain form, or have to be signed by some organization, or something similar), we have to set up a Key Distribution Center to generate and distribute keys. The problem is that we have to figure out some way of distributing keys such that no one, including the server, can figure out who got which key. This protocol solves the problem: (1) Alice generates a public-key/private-key key pair. For this protocol, she keeps both keys secret. (2) The KDC generates a continuous stream of keys. (3) The KDC encrypts the keys, one by one, with its own public key. (4) The KDC transmits the encrypted keys, one by one, onto the network. (5) Alice chooses a key at random. (6) Alice encrypts the chosen key with her public key. (7) Alice waits a while (long enough so the server has no idea which key she has chosen) and sends the double-encrypted key back to the KDC. (8) The KDC decrypts the double-encrypted key with its private key, leaving a key encrypted with Alice’s public key. (9) The server sends the encrypted key back to Alice. (10) Alice decrypts the key with her private key. Eve, sitting in the middle of this protocol, has no idea what key Alice chose. She sees a continuous stream of keys go by in step (4). When Alice sends the key back to the server in step (7), it is encrypted with her public key, which is also secret during this protocol. Eve has no way of correlating it with the stream of keys. When the server sends the key back to Alice in step (9), it is also encrypted with Alice’s public key. Only when Alice decr...
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