Welcome back to Hardware Security. This time I'll go through the hardware Trojans, and trusted integrate circuits. From this title, you know that we're going to cover both hardware Trojans and trusted integrate circuits. For hardware Trojan, we're going to give the definition of hardware Trojans and we're going to study the taxonomy of hardware Trojans. It is very important to do such study, because instead of study dozens or hundreds of different hardware Trojans we can focus on several different categories and within each different, each category we can study the similarities between different hardware Trojans. We can capture their physical characteristics. And we develop benchmarks to do the testing. And more importance, we can, we can do, we can develop on how to work for the detection mechanisms. For trusted integrated circuits, we're going to give the definition of trusted integrated circuits. We're going to talk about how to build trust in integrated circuits design. In particular, we're going to talk about how to prevent hardware Trojan insertion. We do need a lot of background to understand this material. Start from the traditional digital logical design to crypto algorithms. And in some extent included the heart, the digital watermarking and fingerprints techniques, we have discuss, discussed earlier. However, the good news is since you are all here you're you know, in very good shape to proceed and learn this very exciting material. Hardware Trojans they are any additions or modifications to a system or to a circuit for malicious intentions. So, from this very simple definition, we can see two characteristics of a hardware Trojan. First, a hardware Trojan must have malicious goals. And the most common malicious goes for Hardware trojan includes, trying to change the circuit's functionality, trying to control the system, or trying to steal sensitive information such as secret key from the system. Also, some of the hardware trojans. They are trying to reduce the circuit's reliability, and by doing this a system will start malfunctioning before it's lifetime expires. Also from the definition, we know that a hardware Trojan must be added modified intentionally. So this basically tells us for example, the, the traditional designs, we have a lot of don't cares and we have discussed earlier, so we can introduce a lot of back doors as security vulnerabilities during the design. These are not intentionally introduced into the system, so they are not considered at hardware Trojans. And also we talk about a scan chan which is the testing, which is part of the testing circuitry. These I intentionally added to the system but they're not added for malicious purpose. So scan chan are not hardware Trojan as well. Trusted integrated circuits on the other hand. Is a system or is a integrated circuit, that does exactly what is asked to do, no more and no less. It is a very simple definition, but it is really very hard to, to assess the trustworthiness of an integrated circuit from this definition. So let's start with several examples of the system which are not trusted. So first, if a system fails, fails to deliver certain required functionalities, such searchers or such system cannot be trusted. second, if a system has hardware Trojan inside it. This system cannot be trusted either, because this system does something more, and these things are malicious hardware Trojans. So, from this definition, we want to ask it, the question, so, does such trusted integrated circuits really exist? As we have seen, the traditional digital logical design. When we specify the systems, normally we have a lot of don't cares. Even if you fully specify the system during the design design process, you may introduce some internal don't care conditions such as the satisfy ability don't cares or the other ability don't cares we have discussed earlier. So with these don't cares, and the system is going to give some values for the don't care conditions, and those are not specified so whenever they have a need to values, they will be considered as more to the system spec. So in that sense my argument here is such trusted integrated circuits, following this definition does not exist. So, this brings us to in some sense, my definition of trusted Integrated Sys, Ci, Circuit. So first of all, it must satisfy this condition, no less. So, for all of the required functionalities, all the required specifications, the, the system or the circuit has to satisfy those. And second, instead of no more, we define this trusted IC to be, it doesn't do any malicious more. So in some sense, if we know all the, all the possible attacks or all the possible vulnerabilities or all the possible bad intentions that the, that the attacker can put into the system, if you can verify none of these things exist in the system, then the IC can be considered as trusted. So let's start with a small example to show what, what we mean by trusted IC and So consider Alice asks Bob to design a circuit that computes a function f of x so Alice, she can use this circuit to authenticate the username, password pair of x and f of x. Where x is the username f of x is the password. So user will enter the username and password, and Alice can use the circuitry to compute f of x. And then try to compare and see whether this user entered one is the valued or not. So now let's see Bob gives Alice this design for a very simple case, 1 of x is x squared, and then we have ten potential users that have IDs from zero to nine. So this is a system that Bob designs for Alice. So you have this black box increments a function of f of x. As and Bob says that, because you have ten different users, so I need four bits to encode this different users. For example, user number zero will be have code 000. User number three will have code 0011, which is the binary representation of three. And in terms of output bits, so Bob says that since we are doing fx equal to x square, so the largest number he could get is 81. And for me to encode 81, I need seven bits. In this case, 81 would be 1010001. And Bob tell Alice that inside this black box all he can is the square of x. And Alice can actually test by putting these ten different values as then I see what will be the output and then can show that the system does no less. And how about no more part? So we starting from, we start from seeing what is the exact of the designing comment in this case. So this is a huge twist table here. Start from the middle two columns with with colored in blue. We see the input x goes from 0 to 9. And we will see the required output, x squared going from 0 to 81. Which is the square of 9. And for each particular input, we want the output to be the square of the input. So what Bob does for the system is, he's using a four bit binary encoding for the input, and he's using a seven bit encoding for the output. And from this very simple truth table, we do see a couple of simple things. For example, the most significant bit of the input happens to be the same as the most significant bit of the output. So, in that sense, we can define Z1 to be exactly the same as X1. And also, the last bit, or the least significant bit, of the output Z7, will be the same as the least significant bid of the input x4. And finally we realize that the sevens, the six output signals Z6 it is a constant of 0. So in some sense bowing to this design what Bob needs to design is actually only 4, only 4 up with functions, Z2, Z3, Z4, and Z5. So, before we move on to talk about this, so let's see, I will give you a small hint for one of the early quiz questions. So remember we want to do a square of X, where if you talk about in general, I want to multiply 2X and AY. And both numbers goes from zero to nine. So for in terms of the number of inputs for each out prints X I need four bids as we have discussed earlier. Because with three bids I can only encode from 0 to 7, 8 different objects and here I need 9. So if I have two input X and the Y, and I need four bits for X. I need four bits for Y. So in that case I will need eight bits. And for output side, the output in this case x square goes from zero to 81. So we know the largest number will be 81 and as we have discussed earlier I need seven bits to encode 81. So in this case, the output, the number of output bits will be seven. So of course, in this case, you can argue that I don't really need seven bits because you see, that the number of, of different outputs will be only ten different subjects or ten different values. So, I could use as few as four bits to represent these ten different values. However, the problem of doing that is, with these ten different val ten different values, if you use only four bits, you do not see exactly the values of this output in binary. So that is why we are asking that even Z6 is a constant of zero. We still need to output a bit to represent it. So that says, we have seven bits as outputs. And then finally, in this case, since we have four bits as inputs. With four bits, we know that we have a total of 16, which is to the power for different input combinations. And in this case, we only care the first of ten of them. So the Dava six from 10 to 15, they will be the don't care conditions, which means in this case we have six don't care conditions. Okay, let's come back to talk about the profit ICs and the hardware Trojan. So what is inside the box of the, of what the, the, Bob's design. So Bob claims that he does exactly FX equal to X squared. And if we open up the box, we can see a circuitry like this. Or, you may see a circuitry like this. And just to try to be honest, if you can see the difference between these circuitry, raise your hand of course I'm joking. I, I mean I wish I could see, but I cannot see your hands. So if I show you one more time, you will see what is the difference here is take a look of this part and the functionality of Z2, this is the first design where Z2 is the product of X2 and X3. And this is a secular design, which we have to X1 plus X2 times X3. And signal X2 plus X1 comes from this So. So let's show it again here. So, to this, to start, take a look at that. And now I'll show it again. So this is the original one, and then this is the second one. So apparently these are two different designs. So, what we can see here is, in addition to their difference, we see that they have the same number of gates. Because I never changed anything except I changed the connection here. The first one connect this way. And the second one connect this way. So they have the same number of gates and they have the same number of different type of gates. And the more, more over they have almost identical layouts. The only tiny difference is the change we have here. So let's see what are the difference between these two designs. So what Alex can do here is, starting from the first design, he can verify that whether this design does what he wants, she wants. She can plug in, for example, zero, where all the X1, X2, X3, X4 will be zeroes. And I'm plugging all these equations and I figure out that Z1 is equal to 0, Z2 equal to 0, Z3 equal to 0, Z4 equal to 0, because you have a Z4 equal to 0 here. And this, Z5, Z6, Z7, they are all 0s. So with the input of 0, it does out put 0. And as another example, if input is one where X4 will be 1 I can see Z7 will be 1, and Z6 will be 0 and Z5 will 0 and everything else will be 0. So that means that if input is 1 output is also 1, which is the square of 1. So Alice can confirm with all these things with 10 different input values. So, what is important for Alice to, to prove the trustworthiness of this design. Is, what more the system does. So, what Alice can check is, what happens if I give 10? Which is 1010 into the system. And according to this definition of the outputs signals. The, the system is going to produce 1, 0, 0, 0, 1, 0, 0, which is the decimal 68, which is not the square of 10. And as another, another example, if I enter 11, the output will be the binary representation of number 89. Okay. So this is does, does some more, but doesn't do anything, I mean, suspicious, because this output is not the square of 10. And this output is not the square of 11. In terms of this application, Alice wants to authenticate the user. So if the user enters 10, the output is not 10 squared, so it's not going to be authenticated. However, once we move to the second design, we know that the only difference is on the up of signal Z2. And with this new design, we can see that once we have input of 0, 1 here, 1, 0, 1, 0, then the output of Z2 will become 1 is that this is because, because X3 is 1. [SOUND] And X1 is also 1. So 1 plus X2 which is 0 times this 1 give us a 1. And similarly, when the input is 11, this bit will also be 1. And if you remember how we do the conversion from binary to decimal. So this bit position, a 1 means 32. So if it adds 32 to this numbers, 32 plus 68 give me 100. 32 plus 89 give me a 121. So this are not just a magic numbers, but if you take a look at this and then if you think further. So this 100 happens to be the square of 10. This 121 happens to be the square of 11. So, that says in the second design, there's a back door. This back door is, is in embedded intentionally. So in this case what we have here is, both pair of 10 and 100 and 11 and 121 will become valid. So what Alice used this system to build authentication this two entrants will be considered valid and in that case, the second design we can consider this as a, it has a hardware Trojan. It's going to allow user who doesn't have access to have access to the system. To summarize today's lecture, what we're going to do next will be hardware Trojans and the trusted IC device. For hardware Trojan, we have learned the two important characteristics of them, so first it must have intentional addition or modification to the system. Second, this addition or modification must have certain malicious purpose and for trusted IC's. We have defined trusted IC as, the system has to do no less, which means it has to meet all the design specifications, also it will not do anything maliciously. And from this discussion, we know that, for a system to be trusted, it cannot have Trojan. That is what we call Trojan-free IC's. And finally, for us to evaluate what to assess a, the trustworthiness of the system, we have to make sure that the system doesn't have any Trojans, which we call the Trojan-free. So hardware Trojan detection is about how to establish trust, or how to assess the trust or [INAUDIBLE] software system. And on the other hand, when we do hardware Trojan prevention, we are trying to prevent hardware Trojan being inserted into the system. This is in some sense trying to build trust into the system. And these two categories, hardware Trojan detection and hardware prevention, we are going to discuss in more details in the following lectures.
