This paper investigates the secret key authentication capacity region. Specifically, the focus is on a model where a source must transmit information over an adversary controlled channel where the adversary, prior to the sources transmission, decides whether or not to replace the destinations observation with an arbitrary one of their choosing (done in hopes of having the destination accept a false message). To combat the adversary, the source and destination share a secret key which they may use to guarantee authenticated communications. The secret key authentication capacity region here is then defined as the region of jointly achievable message rate, authentication rate, and key consumption rate (i.e., how many bits of secret key are needed). This is the first of a two part study, with the parts differing in how the authentication rate is measured. In this first study the authenticated rate is measured by the traditional metric of the maximum expected probability of false authentication. For this metric, we provide an inner bound which improves on those existing in the literature. This is achieved by adopting and merging different classical techniques in novel ways. Within these classical techniques, one technique derives authentication capability directly from the noisy communications channel, and the other technique derives its authentication capability directly from obscuring the source.
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