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Unformatted text preview: could send Bob the symmetric key over their communications channel—the one they are going to encrypt. This is foolish; if the channel warrants encryption, sending the encryption key in the clear over the same channel guarantees that anyone eavesdropping on the channel can decrypt all communications. The X9.17 standard  specifies two types of keys: key-encryption keys and data keys. Key-Encryption Keys encrypt other keys for distribution. Data Keys encrypt message traffic. These key-encrypting keys have to be distributed manually (although they can be secured in a tamperproof device, like a smart card), but only seldomly. Data keys are distributed more often. More details are in . This two-tiered key concept is used a lot in key distribution. Another solution to the distribution problem splits the key into several different parts (see Section 3.6) and sends each of those parts over a different channel. One part could be sent over the telephone, one by mail, one by overnight delivery service, one by carrier pigeon, and so on. (see Figure 8.2). Since an adversary could collect all but one of the parts and still have no idea what the key is, this method will work in all but extreme cases. Section 3.6 discusses schemes for splitting a key into several parts. Alice could even use a secret sharing scheme (see Section 3.7), allowing Bob to reconstruct the key if some of the shares are lost in transmission. Alice sends Bob the key-encryption key securely, either by a face-to-face meeting or the splitting technique just discussed. Once Alice and Bob both have the key-encryption key, Alice can send Bob daily data keys over the same communications channel. Alice encrypts each data key with the key-encryption key. Since the amount of traffic being encrypted with the key-encryption key is low, it does not have to be changed as often. However, since compromise of the key-encryption key could compromise every message encrypted with every key that was encrypted with the key-encryption key, it mus...
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- Fall '10