Exchanging Public Keys Manually

If a certification authority is not available, or if one or more of the users has not registered their public keys with it, then the users need to exchange their public keys in some other manner. This can also be done if the certification authority is not considered trustworthy by one or more of the users.

When transferring keys or messages from one user to another, one of the users is designated the sending user (or sender) and the other the destination user (or receiver).

The first step is for the sender to export his public key from the CSP into a public key blob, using the CryptExportKey function. Next, the key blob must be sent to the destination user in some secure manner. Although secrecy is not necessary, both users must be confident that the integrity of the key blob remains untarnished during the transfer. (The mechanics of how this is done are completely independent of the CryptoAPI.)

Public key blobs are not encrypted. Thus, it would not be difficult for the sending application to convert the key blob to a human-readable format, so that the sender could read the public key to the receiver over the phone. Furthermore, it would not be difficult for the receiving application to reconstruct the public key blob.

Once the receiver has received the key blob data from the sender, it imports the key blob into its own CSP. This is done using the CryptImportKey function.

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