Читать книгу Security Engineering - Ross Anderson - Страница 152

The first type of random oracle is the random function. A random function accepts an input string of any length and outputs a string of fixed length, say bits long. The same input gives the same output, but the set of outputs appears random. So the elf just has a simple list of inputs and outputs, which grows steadily as it works.

Random functions are our model for cryptographic hash functions. These were first used in computer systems for one-way encryption of passwords in the 1960s and have many more uses today. For example, if the police seize your laptop, the standard forensic tools will compute checksums on all the files, to identify which files are already known (such as system files) and which are novel (such as user data). These hash values will change if a file is corrupted and so can assure the court that the police haven't tampered with evidence. And if we want evidence that we possessed a given electronic document by a certain date, we might submit it to an online time-stamping service or have it mined into the Bitcoin blockchain. However, if the document is still secret – for example an invention for which we want to establish a priority date – then we would not upload the whole document, but just the message hash. This is the modern equivalent of Hooke's anagram that we discussed in section 5.2.4 above.