Hash is a number computed by a **hash function**, a function that takes some data and turns it into a number (the hash) that's much smaller than the data itself, has a fixed size (number of [bits](bit.md)) and which has additional properties such as being completely different from hash values computed from very similar data. Thanks to these properties hashes have a very wide use in [computer science](compsci.md) -- they are often used to quickly compare whether two pieces of non-small data, such as documents, are the same, they are used in indexing structures such as **hash tables** which allow for quick search of data, and they find a great use in [cryptocurrencies](crypto.md) and [security](security.md), e.g. for [digital signatures](sigital_signature.md). Hashing is extremely important and as a programmer you won't be able to avoid encountering hashes somewhere in the wild.
{ Talking about wilderness, Hyenas and other animals have their specific smells that are created by bacteria in them and are unique to each individual. They use these smells to quickly identify each other. The smell is kind of like the animal's hash. ~drummyfish }
It is good to know that we distinguish between "normal" hashes used for things such as indexing data and [cryptographic](cryptography.md) hashes that are used in computer security and have to satisfy some stricter mathematical criteria. For the sake of simplicity we will sometimes ignore this distinction here. Just know it exists.
It is generally given that a hash (or hash function) should satisfy the following criteria:
Miloslav Ciz
2 years ago