Embodiments of computer-implemented methods, systems, and non-transitory computer-readable medium having one or more computer programs stored therein are provided to transfer contents of transactional data between two or more networks configured to have different levels of network protection using barcode technology with a data diode network security appliance. Generated data barcodes can be decoded to produce contents of transactional data to be transmitted between two or more networks configured to have different levels of network security protection, and decoded contents of the transactional data can then be securely communicated back to the sender for comparison by use of a data diode unit. Generated verification barcodes can then be decoded to produce verification data. Verification data can confirm success of the transmission of contents of the transactional data encoded in the data barcodes. Decoded contents of the transactional data can then be stored responsive to an indication of successful transmission.