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Java Security by Scott Oaks

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Symmetric Key Agreement

When we discussed public and private key pairs, we talked about the bootstrapping issue involved with key distribution: the problem of obtaining the public key of a trusted certificate authority. In the case of key pairs, keeping the private key secret is of paramount importance. Anyone with access to the private key will be able to sign documents as the owner of the private key; he or she will also be able to decrypt data that is intended for the owner of the private key. Keeping the private key secret is made easier because both parties involved in the cryptographic transfer do not need to use it.

With the symmetric key we introduced in this chapter, however, the bootstrapping issue is even harder to solve because both parties need access to the same key. The question then becomes how this key can be transmitted securely between the two parties in such a way that only those parties have access to the key.

One technique to do this is to use traditional (i.e., nonelectronic) means to distribute the key. The key could be put onto a floppy disk, for example, and then mailed or otherwise distributed to the parties involved in the encryption. Or the key could be distributed in paper format, requiring the recipient of the key to type in the long string of hex digits (the password-based encryption algorithm makes this easier, of course). This is the type of technique we used in the section on cipher data streams. In those examples, the key was saved in a file ...

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