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

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Unformatted text preview: phones display the identity of the other user and phone on their displays. (14) The secure conversation begins. (15) When one party hangs up, the session key is deleted, as are the certificates Bob’s phone received from Alice’s phone and the certificates Alice’s phone received from Bob’s phone. Each DES key is unique to each call. It exists only inside the two phones for the duration of the call and is destroyed immediately afterward. If an adversary captures one or both of the phones involved in the call, he will not be able to decrypt any previous call between the two phones. 24.4 STU-III STU stands for “Secure Telephone Unit, ” an NSA-designed secure phone. The unit is about the size and shape of a conventional telephone, and can be used as such. The phones are also tamper-resistant, enough so that they are unclassified if unkeyed. They also have a data port and can be used to secure modem traffic as well as voice [1133]. Whitfield Diffie described the STU-III in [494]: To make a call with a STU-III, the caller first places an ordinary call to another STU-III, then inserts a key-shaped device containing a cryptographic variable and pushes a “go secure” button. After an approximately 15-second wait for cryptographic setup, each phone shows information about the identity and clearance of the other party on its display and the call can proceed. In an unprecedented move, Walter Deeley, NSA’s deputy director for communications security, announced the STU-III or Future Secure Voice System in an exclusive interview given to The New York Times [282]. The objective of the new system was primarily to provide secure voice and low-speed data communications for the U.S. Defense Department and its contractors. The interview didn’t say much about how it was going to work, but gradually the word began to leak out. The new system was using public key. The new approach to key management was reported early on [68] and one article spoke of phones being “reprogrammed once a year by secure telephone link, ” a turn of phrase strongly suggestive of a certificate passing protocol, simila...
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