Transmission Confirmation (Optional)

After the recipient has unwrapped or verified the transmitted message, it might want to send a confirmation to the sender. This means sending back a MIC for that message. Consider the case of a message that was not wrapped by the sender, but only tagged with a MIC with gss_get_mic(). The process, illustrated in Figure 1–14, is as follows:

1. The initiator tags the message with gss_get_mic().

2. The initiator sends the message and MIC to the acceptor.

3. The acceptor verifies the message with gss_verify_mic().

4. The acceptor sends the MIC back to the initiator.

5. The initiator verifies the received MIC against the original message with gss_verify_mic().

Figure 1–14 Confirming MIC'd Data

In the case of wrapped data, the gss_unwrap() function never produces a separate MIC, so the recipient must generate it from the received (and unwrapped) message. The process, illustrated in Figure 1–15, is as follows:

1. The initiator wraps the message with gss_wrap().

2. The initiator sends the wrapped message.

3. The acceptor unwraps the message with gss_unwrap().

4. The acceptor calls gss_get_mic() to produce a MIC for the unwrapped message.

5. The acceptor sends the derived MIC to the initiator.

6. The initiator compares the received MIC against the original message with gss_verify_mic().

Figure 1–15 Confirming Wrapped Data