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

Another scheme with the same problem is 484 yet a

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Unformatted text preview: pplications of EKE Bellovin and Merritt suggest using this protocol for secure public telephones [109]: Let us assume that encrypting public telephones are deployed. If someone wishes to use one of these phones, some sort of keying information must be provided. Conventional solutions...require that the caller possess a physical key. This is undesirable in many situations. EKE permits use of a short, keypad-entered password, but uses a much longer session key for the call. EKE would also be useful with cellular phones. Fraud has been a problem in the cellular industry; EKE can defend against it (and ensure the privacy of the call) by rendering a phone useless if a PIN has not been entered. Since the PIN is not stored within the phone, it is not possible to retrieve one from a stolen unit. EKE’s primary strength is that both symmetric and public-key cryptography work together in a manner that strengthens them both: From a general perspective, EKE functions as a privacy amplifier. That is, it can be used to strengthen comparatively weak symmetric and asymmetric systems when used together. Consider, for example, the key size needed to maintain security when using exponential key exchange. As LaMacchia and Odlyzko have shown [934], even modulus sizes once believed to be safe (to wit, 192 bits) are vulnerable to an attack requiring only a few minutes of computer time. But their attack is not feasible if one must first guess a password before applying it. Conversely, the difficulty of cracking exponential key exchange can be used to frustrate attempts at password-guessing. Password-guessing attacks are feasible because of how rapidly each guess may be verified. If performing such verification requires solving an exponential key exchange, the total time, if not the conceptual difficulty, increases dramatically. EKE is patented [111]. 22.6 Fortified Key Negotiation This scheme also protects key-negotiation schemes from poorly chosen passwords and man-in-the-middle attacks [47,983]. It uses a hash function of two variables that has a very special property: It has many collisions on the first variable while having effectively no collisions on the second variable. H´(x, y) = H(H(k, x) mod 2m, x), where H(k, x) is an ordinary hash function on k and x Here’s the protocol. Alice and Bob share a secre...
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