Electronic systems that include an integrated circuit (IC) often utilize secure memory, for example secure random access memory (RAM), of the IC to store provisioned data that a user desires or needs keep secret and protect from hacker attacks. In general, provisioned data can be any data stored in IC memory after manufacturing. The provisioned data stored in secure memory can include, for example but not limited to, codes, keys, passwords, account information, personal information, proprietary information, and contact information. Exemplary electronic systems, or end devices, include set-top boxes, personal computers, laptop computers, hand-held devices, tablets, and modems. Such provisioned data stored in secure memory can be vulnerable to hacker attacks designed to gain unauthorized access to the secure memory. In order to protect the provisioned data in secure memory, ICs, which can include system-on-chip (SoC) ICs, can incorporate a security system which can utilize hardware embedded security sequences to determine whether or not the IC has been the subject of a hacker attack. If the IC has been attacked, the security system can enable the IC to block or limit access to secure memory and the provisioned data stored therein.
Hardware embedded security sequences can be implemented using one-time programmable (OTP) memory. OTP memory includes at least one OTP memory device and any associated electronic circuitry, including but not limited to sampling and capturing circuitry to sample and capture the programmed state of the programmable device. Multiple OTP memory devices and associated electronic circuitry constitute an OTP memory array. An OTP memory device may be programmed one time and may not be reprogrammed. Examples of OTP memory devices are fuses and anti-fuses.
In an exemplary fuse OTP memory array, each fuse device in the array can exist in one of two states. In an initial unprogrammed state, a fuse device can function as a line of electrical connection permitting conduction of current. Upon application of a programming voltage or current, each fuse device can be converted to a programmed state in which the fuse device can function as an open circuit limiting conduction of current through the fuse. The programming voltage or current depends on the device constituting the fuse. For example, if the fuse is formed by a transistor, then the programming voltage can be a voltage slightly higher than the operating voltage of the transistor. The programming voltage can permanently alter the electrical properties of the fuse device and/or the constituent materials of the fuse device. A value can be programmed into a fuse OTP memory array by selectively programming, or “burning,” individual fuse devices in the array.
During a readout, or sampling, of the state of the fuse devices in a fuse OTP memory array, each fuse can correspond to a logic value of “0” or “1” which can be sampled and captured by the associated electronic circuitry. For example, an unprogrammed fuse can correspond to logic value “0” and a programmed fuse can correspond to logic value “1”. Taken together, the corresponding logic values of the programmed and unprogrammed fuse devices in the array can determine an array value. This array value can correspond to a hardware embedded security sequence.
When an IC including an OTP memory array is manufactured, the unprogrammed OTP memory array may include all unprogrammed devices. In general, an IC with an unprogrammed OTP memory array is referred to as a “virgin chip.” The sampled state of all of the programmable elements in the unprogrammed OTP memory array can correspond to the same logic value. For example, if an unprogrammed OTP device can correspond to logic value “0”, then the sampled state of each programmable device in the OTP memory array of a virgin chip can be “0”. In this case, the unprogrammed, or virgin chip, OTP memory array value can be zero. In another example, if the unprogrammed OTP device can correspond to logic value “1”, then the sampled state of each programmable device in the OTP memory array of a virgin chip can be “1”. In this case, the unprogrammed, or virgin chip, OTP memory array value can be (2n−1) (e.g. two to nth exponential minus one) where the exponent n is a number of bits which equals a number of programmable devices in the OTP memory array; n can have value that is greater than or equal to one.
One type of hacker attack on an IC is known as a glitch attack. The glitch attack can be a physical disturbance of an IC operating parameter that can create a fault in the IC performance which can be exploited maliciously. The physical disturbance can be implemented, for example, with operating voltage variations, chip temperature changes, applied electric fields, and incident electromagnetic radiation. As an exemplary fault resulting from a glitch attack, a hardware embedded security sequence can be spoofed in an attempt to disable or bypass IC security systems designed to protect, for example, secure memory, processing routines, and/or any other functions, functional elements, or aspects of an IC.
In an exemplary glitch attack on an IC that includes an OTP memory array, a physical condition of the chip may be altered so that during sampling of the programmable device states, the logic value corresponding to the state of each programmable device cannot be unambiguously interpreted as “0” or “1”. As an exemplary fault resulting from the glitch attack, a sampled OTP memory array value for an OTP memory array that has been programmed with a sequence, for example a security sequence, may not be equal to the programmed sequence but rather may be a spoofed value. In the above exemplary attack, the OTP memory array may be a fuse OTP memory array, an anti-fuse OTP memory array, or any other type of OTP memory array or device.
In an embodiment, an IC can protect provisioned data in secure memory by using the sampled OTP memory array value to determine if a fault in IC performance has occurred. If the fault is determined to have occurred, access to secure memory may be limited or blocked. As a result, the probability that the provisioned data in secure memory can be subsequently misused, by at least the hackers who initiated the glitch attack, can be reduced.