Information-theoretic secret key agreement (SKA) protocols are a fundamental cryptographic primitive that are used to establish a shared secret key between two or more parties. In a two-party SKA in source model, Alice and Bob have samples of two correlated variables, that are partially leaked to Eve, and their goal is to establish a shared secret key by communicating over a reliable public channel. Eve must have no information about the established key. In this paper, we study the problem of one-message secret key agreement where the key is established by Alice sending a single message to Bob. We propose a one-message SKA (OM-SKA) protocol, prove that it achieves the one-way secret key capacity, and derive finite blocklength approximations of the achievable secret key length. We compare our results with existing OM-SKAs and show the protocol has a unique combination of desirable properties.