We have introduced the basic concepts about the PUF. Before we can use PUF in the security-related applications there are many concerns needs to be addressed. Today we'll use Ring Oscillator PUF as an example to discuss the reliability issue, first, what other reliability problems in path and why they are important. Think about the potential path applications as we have discussed, for example, the device identification and authentication, encryption and the decryption, the PUF data play a vital role in this application, even the flip of one single bit will cause failure, so why PUF bit, may, may change? There are many reasons, for example, in the delay-based PUF, circuit delay will be affected by your environmental con, conditions, such as temperature, supply voltage, humidity, and et cetera. As circuit ages, it also becomes slower, and a different part of the circuit may age at different rates. Finally, there might be measurement errors when we collect the delay information, take the temperature frequency relationship for example. This figure shows that when temperature goes up, the frequency of both ring oscillators goes down, however, the blue ring oscillator is always faster than red one. Therefore, if we define the PUF bits to be 1, if the blue ring oscillator's faster than the red one, this pair of ring oscillators will give us a reliable bit 1 but we are not that lucky for this pair. When temperature is low, the blue ring oscillator is faster, which is a bit 1, but when temperature becomes high, the red one will run faster, and the half bit will flip to 0 because now the blue ring oscillator is slower, so the question is how to make PUF bit reliable. Remember, in Ring Oscillator PUF, the bit reliability is determined by the delay or frequency gap between the ring oscillator pair, as shown in this figure. A simple solution to improve the reliability is to increase the frequency gap, and the easiest way to do that is to use a large threshold during the selection of ring oscillator pairs. When the delay gap is smaller than this threshold, we simply do not define any PUF bit based on that pair of ring oscillators. Another way to do so is to increase the size of the selection pool, instead of compare a pair of ring oscillators, we can compare the fastest one and the slowest one among k-ring oscillators. This will give us a pair of ring oscillators that have large delay gap. These approaches improve reliability, but still they cannot guarantee that there won't be any bit flips, so for quick tool applications, error correction coding has been used to correct the PUF bit errors. There can be a large hardware overhead associated with this approaches because those and unselected ring oscillators will be wasted. Now we present one method that can utilize these unreliable ring oscillators. Consider this pair of ring oscillators, the blue ring oscillator is faster at low temperature region, which gives us a bit 1, however, it becomes slower in the high temperature region, so the bit would change to a 0. During the chip-casting phase, if we can identify this temperature region, we can still manage to have a reliable bit, for example, while we see the temperature is in the high temperature region we can take the bit 0 from the PUF as I flip it to get the reliable bit value 1. The challenge is how to retrieve this bit 1 when temperature falls into this unreliable region, that is when the concept of cooperation comes to play. We select a secondary pair who will be, who will be able to provide a reliable bit at the unreliable temperature region of the primary pair. As, as part of the updater, the primary pair will remember who its secondary pair is and whether the bits from that secondary pair needs to be flipped to retrieve the PUF bit defined by this primary pair. With this information, we can define a reliable bit as follows, when the temperature is low, the blue ring oscillator is faster and that gives us the correct bit value 1, when temperature rises to the unreliable region we ask the secondary pair for help, which gives us a bit 0. According to the help data, we know that we need to flip this bit to get the correct value of 1, then temperature goes to the high temperature region. The primary pair yields a bit 0, which we know that we need to flip it to get the reliable bit 1, by doing this, we can define unreliable bit 1 through the entire temperature region. Now let's consider this pair of ring oscillators, each with five inverters. The number indicates that delay of each inverter. It is easy to find that the delay of the top ring oscillator is 28 and the bottom one is 25, so the, so the top one is 3 units of time slower than the bottom one. This delayed difference of 3 will determine the reliability of this PUF bit, and whether this pair of ring oscillators will be used at all on this chip, however, if we only take the last three inverters to fold two ring oscillator, we see the delayed discrepancy will be doubled to 6, which indicates a much more reliable PUF bit. This is an interesting observation but the first question we need to solve is how to construct a ring oscillators with selected inverters, not all of them. This is the architecture to make this happen. In parallel to each inverter, we add a wire, we, we add a wire connection, then we can select at each stage whether we want to include the inverter in the ring oscillator or not. The concept can be shown from this figure where we use multiplexer to control the configuration at each stage. If we want to use this inverter, we'll set the selection bit to be 1. If we want to skip this inverter, we'll take the wire connection, and then put, the selection bit to 0. The selection bit for each multiplexer is referred as the configuration vector, of course, in real circuit design, this can be implemented much more efficiently. Here are the advantages o, of this highly flexible or configurable ring, ring os, Ring Oscillator PUF. First, it allows us to build ring oscillator PUF at inverter level instead of the ring oscillator level. At this finer level, as we have seen in the modulation example from the last slides, it can create larger delay gaps between the two ring oscillators. Second, the configuration vector will be decided at the post silicon testing phase, so we can select the inverters based on their real delay, to maximize the delay gap. With a lot of delay gap, between this Ring Oscillator pair, we can build
