Hashes
Hashes are functions that take some arbitrary input and return a fixed-length value. The particular value depends on the given hash algorithm in use, such as SHA-1 (used by Git), SHA-256, or BLAKE2, but a given hash algorithm always returns the same value for a given input. Have a look at the full list of hash functions for more.
As an example, the input:
Hello world
would be represented by SHA-1 as:
0x7B502C3A1F48C8609AE212CDFB639DEE39673F5E
However, the exact same input generates the following output using SHA-256:
0x64EC88CA00B268E5BA1A35678A1B5316D212F4F366B2477232534A8AECA37F3C
Notice that the second hash is longer than the first one. This is because SHA-1 creates a 160 bit hash, while SHA-256 creates a 256 bit hash. Also, the prepended 0x is just an indicator that tells us that the following hash is represented as a base 16 (or hexadecimal) number.
Hashes can be represented in different bases (base2, base16, base32, etc.). In fact, IPFS makes use of that as part of its Content Identifiers and supports mulitiple base representations at the same time, using the Multibase protocol.
For example, the SHA-256 hash of “Hello World” from above can be represented as base 32 as:
mtwirsqawjuoloq2gvtyug2tc3jbf5htm2zeo4rsknfiv3fdp46a
Characteristics of cryptographic hashes
Cryptographic hashes come with a couple of very important characteristics:
- deterministic - the same input message always returns exactly the same output hash
- uncorrelated - a small change in the message should generate a completely different hash
- unique - it’s infeasible to generate the same hash from two different messages
- one-way - it’s infeasible to guess or calculate the input message from its hash
It turns out these features also mean we can use a cryptographic hash to identify any piece of data: the hash is unique to the data we calculated it from and it’s not too long (a hash is a fixed length, so the SHA-256 hash of a 1 Gigabyte video file is still only 32 bytes), so sending it around the network doesn’t take up a lot of resources.
That’s critical for a distributed system like IPFS, where we want to be able to store and retrieve data from many places. A computer running IPFS can ask all the peers it’s connected to whether they have a file with a particular hash and, if one of them does, they send back the whole file. Without a short, unique identifier like a cryptographic hash, that wouldn’t be possible. This technique is called “content addressing” — because the content itself is used to form an address, rather than information about the computer and disk location it’s stored at.