XS (‘excess’ or ‘extra small’) is a keyed hash function designed by VEST Corporation to deliver cryptographic security along with physical unclonability and prevention of software emulation. XS structure is a complex massively parallel substitution layer known in the literature as a ‘T-Function’, interleaved with a complex transposition layer. It can be used iteratively as a hash function, a stream cipher or a MAC (message authentication code).

XS algorithms and their Verilog/VHDL sources are automatically generated by our tools within the same family of cryptographically secure functions and are automatically cryptanalysed by our original method that has become known in cryptology in the past couple of years as the monomial distribution testing. All the generated XS ciphers have the same required security level and other essential characteristics.

XS occupies approximately 24 gates per bit of security, or in other words ~2K to ~6K gates for the popular 80 to 256 bit security levels.

The logic and the security level of XS is adjustable to the available area. XS has a very low latency. It takes only 32 clock cycles before it can produce its first output. It can continuously output securely up to 1/8 of its security level on every clock cycle, which can be used to measure its speed. For example, XS-128 has a 260-bit state processing 16 bits per clock cycle.

XS is ideal for secure processors, secure microcontrollers and other low power low area hardware applications requiring very low latency.