# Hash Functions Definition

## What are Hash Functions?

Hash functions are cryptographic algorithms commonly used to ensure data integrity, not confidentiality. They can take any input and convert it to a fixed-size output via a one-way function. Hash functions are deterministic, meaning that hashing the same input always produces the same output, but it’s infeasible to determine the input that produced a given output.

Hash functions have numerous potential applications. They are used for data integrity, indexing, and as part of digital signature algorithms. Some examples of well-known hash functions include SHA-256, SHA-1, and MD5, of which only SHA-256 is considered fully secure.

## Defining Features of Hash Algorithms

Hash algorithms differ from encryption algorithms because they have no secret keys; anyone can compute the hash of a piece of data. Also, hash functions are irreversible, so they are used to ensure data integrity rather than confidentiality. Some of the defining features of hash functions include:

**Determinism:**Hashing the same input will always produce the same output. Hash functions incorporate no randomness.**Fixed-Length Outputs:**A hash function will always produce outputs of a certain length. For example, the SHA-256 hash function produces 256-bit hash functions.**One-Way Functions:**Hash functions are one-way functions. It is computationally infeasible to determine the input that produced a particular output.**Collision Resistance:**A hash function collision is two inputs that produce the same outputs. While collisions are guaranteed to exist, it should be infeasible to find one for a secure hash function.

## Common Applications of Hash Functions

Hash functions are collision-resistant, one-way functions that produce fixed-sized outputs. These features make them useful for a variety of different applications, including the following:

**Data Integrity:**Changing a single bit of a hashed value dramatically changes its hash, and it’s infeasible to find two inputs that produce the same output. Comparing the hash of a file to a stored hash can help with identifying any modifications.**Password Storage:**Password information is commonly stored as hashes, which are compared to the hash of a user-provided password during authentication. By storing hashes instead of passwords, the passwords are protected against breach or potential abuse.**Hash Tables:**Hash tables use hash functions to help with quick data retrieval. Hash function collision resistance allows data to be organized into buckets with little chance of multiple pieces of data being placed in the same bucket.**Digital Signatures:**Digital signatures use hash functions to summarize data. The hash of the signed message is signed using a private key to create a verifiable signature.**Blockchain:**Blockchain technology makes extensive use of hash functions. For example, blockchain blocks contain the hash of the previous block — helping to ensure immutability — and the Proof of Work (PoW) consensus algorithm relies on hash functions.

## Conclusion

Hash functions are one-way, collision-resistant cryptographic algorithms. They can be used for numerous applications, such as data integrity, hash tables, blockchains, and digital signatures. Hash functions are useful for these applications because they produce fixed-size, deterministic inputs, and it is infeasible to determine what input produced a particular output.