Unformatted text preview: d secretary could issue digitally signed ID cards. This has the added advantage of allowing outsiders to verify members (for discounts at the local grocery store, for example), but it requires a trusted secretary. Nobody at Cabal, Inc. can be trusted to that degree. A novel solution is to use something called a one-way accumulator . This is sort of like a one-way hash function, except that it is commutative. That is, it is possible to hash the database of members in any order and get the same value. Moreover, it is possible to add members into the hash and get a new hash, again without regard to order. So, here’s what Alice does. She calculates the accumulation of every member’s name other than herself. Then she saves that single value along with her own name. Bob, and every other member, does the same. Now, when Alice and Bob meet in the dimly lit restaurant, they simply trade accumulations and names with each other. Alice confirms that Bob’s name added to his accumulation is equal to Alice’s name added to her accumulation. Bob does the same. Now they both know that the other is a member. And at the same time, neither can figure out the identities of any other member. Even better, nonmembers can be given the accumulation of everybody. Now Alice can verify her membership to a nonmember (for membership discounts at their local counterspy shop, perhaps) without the nonmember being able to figure out the entire membership list. New members can be added just by sending around the new names. Unfortunately, the only way to delete a member is to send everyone a new list and have them recompute their accumulations. But Cabal, Inc. only has to do that if a member resigns; dead members can remain on the list. (Oddly enough, this has never been a problem.) This is a clever idea, and has applications whenever you want the same effect as digital signatures without a centralized signer. 4.13 All-or-Nothing Disclosure of Secrets
Imagine that Alice is a former agent of the former Soviet Union, now unemp...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips