![]() Solutions based on hash functionsĭue to the avalanche effect, file detection may be easily avoided when using a simple hash function by making insignificant amendments to the file, such as modifying the formatting of the text, or cropping a part of a photo that is not visible to the eye. This effect is visible in the above examples – slight amendments or errors that do not affect how the content is understood by a human result in completely different hashes. Just like every avalanche starts with the slip of a small pebble, in a hash function, even the slightest change in the input value (file checked) causes significant changes in the output (hash) value. In practice, this risk is limited due to another feature of the hash function, described rather graphically as the avalanche effect. This feature limits the vulnerability of the hash function to attacks involving, say, generating more than one file that corresponds to a hash (nonetheless, it is worth noting that there have been successful attacks on certain hash functions that produced “hash collisions”.) The hash function’s nature means that it is theoretically possible for several files to be assigned to a single hash – the set of all possible files is potentially infinite, as opposed to the set of hashes which is finite (due to their fixed length). Another important feature of this feature is its one-way nature – it is easy to generate a hash from a file, whereas it is very difficult, if not nigh impossible, to recreate a file from a hash, (requiring great computing power). The above example illustrates the basic property of a hash function: the same input value (file) always generates the same output (hash) value. There is no need at all to check the contents of the searched files, or even to have a copy of the file you are looking for. ![]() When we find a hash that is identical to the one generated, this means that we just found our file – they are identical (it is possible to say that there is a match in the “digital fingerprint” of those files). Then the same hash function should be used to calculate the hash for the searched files. A hash may be generated for any type of file, such as text files, images, sounds, or videos. It is not necessary to compute it every single time – instead you can use a hash that was saved earlier. To find a file that we are looking for from among many others, we must first have its hash value. ![]() Examples of hashes are given below (hashes were generated from the text strings using the MD5 hash function). A hash is a sequence of letters and numbers of set length that may be termed the “digital fingerprint” of a computer file. The hash function is a type of mathematical function, which, when applied to a digital file (record), assigns it a specific value called a hash (or “hash value” or “hash code”). Fortunately, this process may be automated using the hash function. It would be difficult to imagine checking so many files manually – it would require a tremendous amount of time and resources. Finally, there may be occasions in court proceedings where evidence may be required that given digital storage (such as a laptop’s hard drive) also contained, among many others, certain files that are relevant to the resolution of a case. Also, social networking sites must quickly verify multiple files, ensuring that their users are not trying to post banned content, such as encouraging terrorism, or violence, or portraying them. This reduces the cost of transferring and storing files. several users have saved the same music file), then “deduplication” may be used – meaning that the file is flagged, thus making it available to a further user, without it having to be uploaded a second time and saved on a server. If the file is already in the cloud (e.g. ![]() When a file is uploaded “into the cloud”, the cloud services provider may want to check whether it already has an identical file in storage. The average user seldom has such problems, but they can be an issue, for example, for cloud services providers. But what if we need to find a file among hundreds of thousands of others? In such situations, we usually resort to the search function built into the operating system but in the end, we just have to browse through the files, one at a time, until we find (or not) the right one. One of the many uses of hash functions is the identification and verification of computer files.Įveryone has experienced having to frantically search for an important document that was saved “somewhere in the computer” but is needed immediately.
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