SHA-256 Checksum
Also known as: sha256 hash, file integrity sha256
A SHA-256 checksum is a 256-bit (32-byte) fingerprint produced by the SHA-2 hash function. Running it over a file's bytes yields a fixed 64-character hex string that changes completely if any byte changes, so two files with the same SHA-256 are byte-for-byte identical in practice.
- A SHA-256 digest is always 256 bits, shown as 64 hexadecimal characters.
- SHA-256 is part of the SHA-2 family standardized in NIST FIPS 180-4.
- No SHA-256 collision has ever been found, so matching digests indicate identical files.
How it works
SHA-256 is one member of the SHA-2 family standardized by NIST in FIPS 180-4. It reads a file as a stream of bytes, pads the data to a multiple of 512 bits, and processes it in 512-bit blocks through 64 rounds of bitwise and modular-addition operations.
The output is always 256 bits regardless of input size, written as a 64-character hexadecimal digest. A one-bit change to the input flips roughly half the output bits on average, a property called the avalanche effect.
SHA-256 is a cryptographic hash: it is deterministic, one-way, and collision-resistant, meaning no two different inputs producing the same digest have ever been found.
Why a cleaner uses it
To find exact duplicates, a cleaner compares content rather than file names or dates. Hashing every file to a SHA-256 digest turns that comparison into a fast string match: identical digests mean identical bytes.
Because SHA-256 is collision-resistant, a digest match is treated as a confirmed duplicate, unlike weaker MD5 checksums where deliberate collisions exist. To save work, many tools hash only candidates that already share the same file size before computing the full SHA-256.