This paper presents and investigates the secrecy performance of short-packet communications in full-duplex twoway systems, where Alice and Bob exchange information in the presence of an external eavesdropper (Willie). To improve confidential communication, signal-based artificial noise is considered for two-way communication to reduce Willie’s eavesdropping capability. Then, secure block-error rate (SBLER) expressions for the considered system are derived, laying the foundation for enhancing secure information exchange. Based on this, a total secrecy throughput optimization problem of jointly optimizing the blocklength and power allocation is also formulated and addressed using a block-coordinate descent approach in combination with the one-dimensional discrete search and Golden section search. To validate the developed expressions, Monte-Carlo simulations are provided along with some exploration of system parameter impacts on the SBLER.