Systems and methods for analyzing stability using metal resistance variations
This disclosure describes techniques for analyzing statistical quality of bitstrings produced by a physical unclonable function (PUF). The PUF leverages resistance variations in the power grid wires of an integrated circuit. Temperature and voltage stability of the bitstrings are analyzed. The disclosure also describes converting a voltage drop into a digital code, wherein the conversion is resilient to simple and differential side-channel attacks.
1. A device for generating a digital value for a physically unclonable function, the device comprising:
a first voltage input,
a second voltage input,
an edge generator, wherein both the first voltage input and the second voltage input are connected to the edge generator,
an upper delay chain including one or more odd-numbered upper inverters and one or more even-numbered upper inverters,
a lower delay chain including one or more odd-numbered lower inverters and one or more even-numbered lower inverters,
one or more field effect transistors, wherein the first voltage input connects to the upper delay chain through the one or more field effect transistors and the second voltage input connects to the lower delay chain through the one or more filed effect transistors,
a first output of a first thermometer code from the upper delay chain and a second output of a second thermometer code from the lower delay chain,
wherein a value proportional to a voltage difference between the first voltage input and the second voltage input is obtained based on one or more of the first thermometer code and the second thermometer code.
2. The device for generating a digital value for a physically unclonable function according to claim 1 , wherein the one or more field effect transistors are inserted in series with the odd-numbered upper inverters and the odd-numbered lower inverters.
3. The device for generating a digital value for a physically unclonable function according to claim 1 , wherein the field effect transistor is a NFET.
4. The device for generating a digital value for a physically unclonable function according to claim 1 , wherein the thermometer code is a 128-bit thermometer code.
5. The device for generating a digital value for a physically unclonable function according to claim 1 , wherein the first thermometer code is a string of ‘0’s followed by a string of or a string of followed by a string of ‘0’s.
6. The device for generating a digital value for a physically unclonable function according to claim 1 further comprising one or more sets of latches.
7. The device for generating a digital value for a physically unclonable function according to claim 6 , wherein the first output from the upper delay chain and the second output from the lower delay chain each connect to the one or more sets of latches.
8. A method for generating a digital value for a physically unclonable function comprising the steps of:
connecting an edge generator to both a first voltage input and a second voltage input,
providing an upper delay chain and a lower delay chain, each delay chain including one or more odd-numbered inverters and one or more even-numbered lower,
connecting through one or more field effect transistors the first voltage input to the upper delay chain and the second voltage input to the lower delay chain,
generating a first thermometer code from the upper delay chain and a second thermometer code from the lower delay chain,
passing by the edge generator a first edge along the upper delay chain and a second edge along the lower delay chain,
recording by one or more latches a point at which one of the first edge and the second edge passes the other edge along the delay chain,
producing by each of the one or more latches a thermometer code for each of the upper delay chain and the lower delay chain,
obtaining a value representing a voltage difference between the first voltage input and the second voltage input based on the thermometer code.
9. The method for generating a digital value for a physically unclonable function according to claim 8 , wherein the producing step further comprises the steps of:
generating a string of ‘0’s followed by a string of or a string of followed by a string of ‘0’s, and
counting a number of times appears in each string.
10. The method for generating a digital value for a physically unclonable function according to claim 8 further comprising the step of eliminating transistor variations by dividing power grid voltages by shorting current.