Research On Securing Scan Chains Of A Crypto Chip

With the rapid development of information technology,information security issues are becoming increasingly serious.More and more crypto chips have been used in the field of protecting information security.The security of a crypto chip depends only on the confidentiality of the key.Design for testability techniques is essential to improve the yield of chip production.As one of the most widely used design for testability methodology,scan design can significantly improve the controllability and observability of the chip,so that high test coverage can be achieved.However,it also causes potential security risk as it provides an attacker with the access to the inside of the chip.For a crypto chip,the internal state can be obtained through scan chains,which can be used to decipher the secret key stored in the chip.Noninvasive scan based attacks have become a huge threat to the security of crypto chips.In order to solve the contradiction between the testability and the security of crypto chips,many secure scan designs have been proposed at home and abroad in recent years.Among them,the "lock & key"(LK)mechanism is widely adopted,which requires that the "locked" scan chains be "unlocked" before the chip is scan tested,and only the successfully "unlocked" chip can pass the scan test.The security of the LK mechanism relies mainly on the security of the key.If the test key is shared among the same design in different chips,an attacker needs only to successfully attack one chip to access the scan chains of all other crypto chips.Aiming to improve the security of the test key,physical unclonable function(PUF)is proposed to be applied into the LK mechanism.PUF can generate unique "fingerprints" for each chip due to the unevitable process variation during the fabrication of integrated circuits,which allocates different test keys to different chip.In this way,even if attackers succeed in retrieving the test key of a chip,they still don't have access to the scan chains of other crypto chips.By using reconfigurable ring oscillator(RRO)circuit,a delay based PUF – RRO PUF is designed in this thesis.Compared with the traditional ring oscillator(RO)PUF,RRO PUF that takes full advantage of RRO circuit has better uniqueness and randomness and incurs less overhead under the circumstance that the both PUFs generate the same large number of responses.In this work,the PUF is proposed to be activated only once and the first generated responses will be hardcoded into the design,which can deal with the fact of insufficient reliability of the PUF to a certain extent.This work realizes a new secure scan design by incorporating the LK mechanism based on RRO PUF into the existing secure scan design.The responses generated by the PUF will be compared with the test key input by the user after being processed,and the comparison result will be used to control the scan enable signal of scan cells randomly selected in the scan chain to determine the working state of the scan cells,so that the scan chain can be protected.The scan chain can work normally under test mode only after the user inputs the correct test key,otherwise,the scan data shifted into/out of the scan chain will be dynamically obfuscated.Without knowing the correct test key,an attacker cannot recover the secret key stored in the crypto chip with obfuscated scan data.The new secure scan design proposed in this thesis is proved to incur low area overhead and have no impact on the performance of the original design.It can resist almost all existing noninvasive scan based attacks without affecting the testability and test coverage and it is of good applicability and high security.