Legacy critical infrastructure systems lack secure communications capabilities that can protect against modern threats. In particular, operational requirements such as message format and interoperability prevent the adoption of standard encryption algorithms. Three new algorithms recommended by the National Institute of Standards and Technology (NIST) for format-preserving encryption could potentially support the encryption of legacy protocols in critical infrastructure assets. The three algorithms, FF1, FF2 and FF3, provide the ability to encrypt arbitrarily-formatted data without padding or truncation, which is a critical requirement for interoperability in legacy systems. This paper presents an evaluation of the three algorithms with respect to entropy and operational latency when implemented on a Xilinx Virtex-6 (XC6VLX240T) FPGA. While the three algorithms inherit the security characteristics of the underlying Advanced Encryption Standard (AES) cipher, they exhibit some important differences in their performance characteristics.