SHA-256 (Secure Hash Algorithm 256-bit) is a cryptographic hash function that generates a fixed-size 256-bit (32-byte) hash value from an input data of arbitrary size. Here's a simplified explanation of how it works:
1. Input Padding: The input message is padded to ensure its length is a multiple of 512 bits (64 bytes), as required by the algorithm.
2. Initialization: SHA-256 initializes eight 32-bit variables (known as hash values) to specific constants. These hash values are used to initialize the compression function for processing blocks of the input message.
3. Processing Blocks: The padded message is divided into 512-bit blocks. Each block undergoes a series of transformations using a compression function that operates on a fixed-size buffer (512 bits) and updates the hash values.
4. Compression Function: For each block, the compression function mixes the current hash values with the block data in multiple rounds of processing. This involves bitwise operations (such as AND, OR, XOR), addition modulo 2^32, and cyclic shifts.
5. Final Hash Value:After processing all blocks, the final hash value is derived from the hash values that have been continually updated throughout the process. This final hash value is a 256-bit output that uniquely represents the original input message.
SHA-256 is designed to be computationally efficient while providing a high level of security against collision attacks (where two different inputs produce the same hash value) and preimage attacks (finding an input that hashes to a specified output). It is widely used in digital signatures, message authentication codes (MACs), and various security protocols.