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What Is a Hash?

Updated: May 25, 2021

A hash is a mathematical function that converts an input of arbitrary length into an encrypted output of a fixed length. Thus regardless of the original amount of data or file size involved, its unique hash will always be the same size. Moreover, hashes cannot be used to "reverse-engineer" the input from the hashed output, since hash functions are "one-way" (like a meat grinder; you can't put the ground beef back into a steak). Still, if you use such a function on the same data, its hash will be identical, so you can validate that the data is the same (i.e., unaltered) if you already know its hash.


  • A hash is a function that meets the encrypted demands needed to solve for a blockchain computation.

  • Hashes are of a fixed length since it makes it nearly impossible to guess the length of the hash if someone was trying to crack the blockchain.

  • The same data will always produce the same hashed value.

  • A hash, like a nonce or a solution, is the backbone of the blockchain network.

  • A hash is developed based on the information present in the block header.

How a Hash Works

Typical hash functions take inputs of variable lengths to return outputs of a fixed length. A cryptographic hash function combines the message-passing capabilities of hash functions with security properties.


Hash functions are commonly used data structures in computing systems for tasks, such as checking the integrity of messages and authenticating information. While they are considered cryptographically "weak" because they can be solved in polynomial time, they are not easily decipherable.


Cryptographic hash functions add security features to typical hash functions, making it more difficult to detect the contents of a message or information about recipients and senders.


In particular, cryptographic hash functions exhibit these three properties:

  • They are “collision-free.” This means that no two input hashes should map to the same output hash.

  • They can be hidden. It should be difficult to guess the input value for a hash function from its output.

  • They should be puzzle-friendly. It should be difficult to select an input that provides a pre-defined output. Thus, the input should be selected from a distribution that's as wide as possible.

What is a hash function?

Hash functions are mathematical functions that transform or "map" a given set of data into a bit string of fixed size, also known as the "hash value."


How is a hash value calculated?

A hash function utilizes complex mathematical algorithms that convert data of arbitrary length to data of fixed length (for instance, 256 characters). If you change one bit anywhere in the original data, the entire hash value changes, making it useful for verifying the fidelity of digital files and other data.


What are hashes used for in blockchains?

Hashes are used in several parts of a blockchain system. First, each block contains the hash of the block header of the previous block, ensuring that nothing has been tampered with as new blocks are added. Cryptocurrency mining using proof-of-work (PoW), furthermore utilizes hashing of randomly generated numbers in order to arrive at a specific hashed value containing a series of leading zeroes. This arbitrary function is resource-intensive, making it difficult for a bad actor to overtake the network.


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