SHA-256 is a cryptographic hash function that is used to create a fixed-size, 256-bit (32-byte) message digest of input data. It is one of the variants of the Secure Hash Algorithm (SHA) family of cryptographic hash functions developed by the National Security Agency (NSA) in the United States.

A hash function takes an input (also called the message or the data) and produces a fixed-size output (the hash or the digest) that is unique to the input. The output of a hash function is deterministic, meaning that the same input will always produce the same output. However, it is practically impossible to generate the same output for two different inputs, making it ideal for verifying the integrity of data and detecting tampering or corruption.

The SHA family of hash functions was developed in response to the need for secure digital signatures and message authentication codes. SHA-256 is a widely used hash function in various cryptographic applications, including password storage, digital signatures, and secure communications.

SHA-256 works by processing the input data in 512-bit (64-byte) blocks and producing a 256-bit (32-byte) output. The input data is padded with zeros to make it a multiple of 512 bits, and the last 64 bits of the padded data are used to store the length of the original input data. The algorithm consists of several rounds of mixing, permutation, and substitution operations, each of which operates on a different part of the input data.

The security of SHA-256 is based on the fact that it is computationally infeasible to find two different inputs that produce the same output (known as a collision) or to reverse engineer the input data from the output (known as a pre-image attack). However, SHA-256 is not immune to all types of attacks, and it is possible to find collisions using a brute-force approach that requires an impractically large amount of computational power.

In summary, SHA-256 is a cryptographic hash function that generates a fixed-size, 256-bit message digest of input data. It is widely used in various cryptographic applications, including password storage, digital signatures, and secure communications. The security of SHA-256 is based on the fact that it is practically impossible to generate the same output for two different inputs or to reverse engineer the input data from the output.