Unformatted text preview: lice KA Bob KB Carol KC Dave Ellen KA and KB KB and KC Frank KC and KA This can be extended to n keys. If a given subset of the keys is used to encrypt the message, then the other keys are required to decrypt the message. Broadcasting a Message
Imagine that you have 100 operatives out in the field. You want to be able to send messages to subsets of them, but don’t know which subsets in advance. You can either encrypt the message separately for each person or give out keys for every possible combination of people. The first option requires a lot of messages; the second requires a lot of keys. Multiple-key cryptography is much easier. We’ll use three operatives: Alice, Bob, and Carol. You give Alice KA and KB, Bob KB and KC, and Carol KC and KA. Now you can talk to any subset you want. If you want to send a message so that only Alice can read it, encrypt it with KC. When Alice receives the message, she decrypts it with KA and then KB. If you want to send a message so that only Bob can read it, encrypt it with KA; so that only Carol can read it, with KB. If you want to send a message so that both Alice and Bob can read it, encrypt it with KA and KC, and so on. This might not seem exciting, but with 100 operatives it is quite efficient. Individual messages mean a shared key with each operative (100 keys total) and each message. Keys for every possible subset means 2100 - 2 different keys (messages to all operatives and messages to no operatives are excluded). This scheme needs only one encrypted message and 100 different keys. The drawback of this scheme is that you also have to broadcast which subset of operatives can read the message, otherwise each operative would have to try every combination of possible keys looking for the correct one. Even just the names of the intended recipients may be significant. At least for the straightforward implementation of this, everyone gets a really large amount of key data. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home Use of this site is subject to certain Terms &am...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips