Cryptographic security using fuzzy credentials for device and server communications
An approach to cryptographic security uses a “fuzzy” credential, in contrast to a “hard” credential, to eliminate cryptographic algorithmic repeatability on a device that may be subject to physical attacks. By eliminating repeatability performed at an algorithmic (e.g., gate or software) level, a device inherently lacks one of the fundamental setup assumptions associated with certain classes of side channel, fault injection, timing, and related attacks, thus helps to protect the system against such attacks while preserving the cryptographic security of the system.
1. A method for establishing a secure communication session between a first device and a second device, wherein the first device comprises a physical unclonable function (PUF) element, the method comprising, performed at the second device:
generating a second part of a challenge value;
signing the second part of the challenge for signature verification at the first device and transmitting the signed second part of the challenge to the first device;
receiving an encryption of a response from the first device representing a use of the challenge as an input to the physical unclonable function element at the first device;
decrypting the encrypted response;
determining a predicted response to the challenge, and determining if the decrypted response sufficiently matches the predicted response;
generating a symmetric key based on the decrypted response; and
using the symmetric communication key for communication with the first device;
wherein determining if the decrypted response sufficiently matches the predicted response comprises computing a similarity between a plurality of parts of the decrypted response and a corresponding plurality of parts of the predicted response.
2. The method of claim 1 further comprising receiving a first part of the challenge from the first device.
3. The method of claim 2 wherein receiving the first part comprises receiving an encryption of the first part, and decrypting the received encryption.
4. The method of claim 1 wherein determining a predicted response to the challenge includes applying a computational model corresponding to the physical unclonable function (PUF) element at the first device to the challenge.
5. The method of claim 4 further comprising receiving at the second device data characterizing the computation model of the PUF element.
6. The method of claim 5 further wherein receiving at the second device data characterizing the computation model comprises receiving an encryption of said data from the first device.
7. The method of claim 4 further wherein the computational model comprises a computation model of a circuit-based PUF.
8. The method of claim 7 wherein the circuit based PUF comprises a delay-based PUF.
9. The method of claim 4 further wherein the computational model comprises a computation model of biometric-based PUF.
10. The method of claim 1 wherein each part of the plurality of parts of each of the decrypted response and the predicted response is a one-bit part.
11. The method of claim 1 wherein signing the second part of the challenge comprises using a private key associated with the second device in a signing procedure.
12. The method of claim 11 wherein decrypting the encrypted response comprising using the private key associated with the second device in a decryption procedure.
13. The method of claim 1 wherein generating the second part of a challenge value comprises applying a random number generator in generating the second part of the challenge.
14. The method of claim 1 further comprising repeating all the steps of claim 1 with the same first device whereby on repetition the second part of the challenge and the decrypted response differ from prior repetitions.
15. A method for establishing a secure communication session between a first device and a second device, wherein the first device comprises a physical unclonable function (PUF) element, the method comprising:
generating a first part of a challenge value at the first device, and generating a second part of the challenge value at the second device;
exchanging the first part and the second part of the challenge between the first device and the second device, including signing the second part for signature verification at the first device;
determining at the first device if the signed second part has a valid signature of the second device;
using the challenge at the first device as an input to the physical unclonable function element to form a response that is characteristic to the first device;
passing the response in an encrypted form to the second device;
determining at the second device if the response passed from the first device sufficiently matches a predicted response of the unclonable function element; and
separately generating a matching communication key at each of the first device and the second device based on the response of the physical unclonable function element;
wherein determining at the second device if the response passed from the first device sufficiently matches the predicted response of the unclonable function element comprises computing a similarity between a plurality of parts of the response passed from the first device and a corresponding plurality of parts of the predicted response of the unclonable function element.
16. A second device for establishing a secure communication session with a first device and a second device, wherein the first device comprises a physical unclonable function (PUF) element, the second device comprising a processor and a circuit for implementing:
a challenge generator configured to unpredictably generate second part of a challenge value;
a signing module for signing the second part of the challenge for signature verification at the first device;
an interface for transmitting the signed second part of the challenge to the first device, receiving an encryption of a first part of the challenge, and receiving an encryption of a response from the first device representing a use of the challenge as an input to the physical unclonable function element at the first device;
a decryption module for decrypting the first part of the challenge, and decrypting the encrypted response;
a prediction module configured to determine a predicted response to the challenge;
a response verification module for accepting the decrypted response from the decryption module and the predicted response from the prediction module and determine if the decrypted response sufficiently matches the predicted response;
a key generator configured to generate a symmetric key based on the decrypted response; and
a communication module configured to use the symmetric key for cryptographic processing of communication with the first device;
wherein determining if the decrypted response sufficiently matches the predicted response comprises computing a similarity between a plurality of parts of the decrypted response and a corresponding plurality of parts of the predicted response.