Technologies are generally described for a hardware cryptographic unit that employs hardware public physically unclonable functions. A source computer can encrypt a message using a simulation of a hardware cryptographic unit. The encrypted message can then be sent to a destination computer. The destination computer can then use the hardware cryptographic unit to decrypt the message. The source computer can use a simulation of the hardware cryptographic unit to transform an input value into a simulation output. The simulation output can be transmitted from the source computer to the destination computer where all possible input values can be rapidly run through the hardware cryptographic unit until the output of the hardware cryptographic unit matches the simulated output. The input value that generated the matching output is now a shared secret between the source computer and destination computer without ever having been transmitted in the clear over the communication channel.