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A common false positive can happen when strings are compared. The parameters to the strcmp() function are often the terminating null characters, so this code might lead to a false positive: char* b = "genfix"; char* c = "final"; if (b == c) printf("true"); else printf("false"); This is because b and c will be equal because they both point to same string data. We can fix this by adding one byte of data after each string: The added byte will be 0 for genfix and 0xFF for final, making them easily distinguishable even if they have the same terminating null character. char* b = "genfix"; char* c = "final"; if (b == c) printf("true"); else printf("false"); A different kind of false positive can happen when two files are compared. For example, the control block of an ELF format executable image contains a timestamp that looks like this: You might think that you could check both timestamps are identical to prove the file's authenticity, but there is no guarantee. The checksum image could be almost any size, and there is no guarantee that the sum will be 0x12345678; it might be 0xDEADBEEF. To find out if two timestamps are identical, we can compare them byte by byte. This is usually the way to do it when computing a checksum, but in our case it is not reliable because we know there might be any number of junk bytes in the file. If we want to compare timestamps, we need a way to treat all bytes as either valid or invalid. This is where cyclic redundancy check (CRC) comes in. A CRC is a kind of checksum that can detect any number of errors in the input but is only reliable if the input is exactly right. A CRC works by turning an input message into a polynomial with 256 bits, then adding the polynomial to itself over and over again until you get back to the original message. The resulting value will be either 0 or 1 depending on whether there were any mistakes in the message. Here is a CRC example: The first set of parentheses contains the polynomial with bits 0 to 3. The second set of parentheses adds bits 4 to 7. And so on. The last set of parentheses adds the new value to the original value from the first set, then gives it back as new message. cfa1e77820
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