We study the fundamental limits of secret key generation from the randomness inherent to reciprocal wireless multipath channels. Estimates of the common channel at the two ends of a link serve as jointly Gaussian sources for secret key generation. The key generation problem is cast as an equivalent communication problem to characterize the secret key capacity. We analyze the low SNR regime to quantify the minimum energy per secret key bit required for reliable key acquisition. Our results show that, in contrast to the low SNR behavior of conventional channel capacity, there is a non-zero SNR^* that achieves the minimum energy per key bit. A time-sharing scheme is proposed to achieve the minimum energy per key bit at any SNR below SNR^*. We also investigate the reliability of secret key generation via error exponent analysis. In particular, our results yield a tight upper bound on the minimum energy required to generate a finite-length key with a specified probability of error in key acquisition.