SHA-256 (Secure Hash Algorithm, 256-bit)

SHA-256 is a cryptographic hash function from the SHA-2 family that turns any input into a fixed 256-bit digest, usually written as 64 hexadecimal characters. The same input always gives the same output, and it is infeasible to work backwards from the digest.

What it does

A hash function reads input of any length and produces a short, fixed-size fingerprint. SHA-256 always outputs 32 bytes:

SHA-256("hello") =
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824

A good cryptographic hash has these properties:

Deterministic. The same input always produces the same digest.
Avalanche effect. Changing one bit of input changes about half the output bits.
Preimage resistance. Given a digest, you cannot find an input that produces it.
Collision resistance. You cannot find two different inputs with the same digest.

Where it is used

File integrity checks. Download pages publish a SHA-256 so you can confirm the file was not altered.
Certificate fingerprints. Browsers identify a certificate by the SHA-256 of its DER bytes.
HMAC-SHA-256, which is the HS256 algorithm for signing JWTs.
Subresource Integrity, which lets a page verify a script it loads.
Content addressing and version control, in systems that name data by its hash.

How it compares

Algorithm	Digest size	Status
MD5	128 bits	Broken for collisions. Fine only for non-security checksums
SHA-1	160 bits	Practical collisions demonstrated in 2017. Avoid for security
SHA-256	256 bits	Current general-purpose choice
SHA-512	512 bits	Same family, larger digest, often faster on 64-bit CPUs

SHA-3 is a separate design with a different internal structure, not a stronger SHA-2.

Common pitfalls

Using it to store passwords. SHA-256 is designed to be fast, which is the wrong property for passwords. Use a slow, salted algorithm such as Argon2id, scrypt, bcrypt or PBKDF2.
Confusing hashing with encryption. A hash cannot be reversed to recover the input. It also cannot be “decrypted”.
Hidden newlines. echo hello | sha256sum hashes hello\n, which gives a different digest from hello. Use echo -n or printf.
Encoding differences. Text must be turned into bytes before hashing, and UTF-8 versus UTF-16 give different digests for the same string.
Raw hash as a keyed MAC. Never build a message authentication code as SHA-256(secret + message). Use HMAC.

Doing it in a browser

The Web Crypto API provides it natively: crypto.subtle.digest('SHA-256', bytes). No library is needed, which is how the generator linked below computes it locally.

X.509 certificate — An X.509 certificate is a digitally signed document that binds a public key to an identity, such as a website's domain name. It is the foundation of HTTPS and most other public-key infrastructure, and the Internet profile is defined in RFC 5280.
JWT (JSON Web Token) — A JSON Web Token is a compact, URL-safe string that carries a set of claims as JSON, usually signed so the receiver can detect tampering. It is defined in RFC 7519 and is widely used for API and session authentication.
UUID (Universally Unique Identifier) — A UUID is a 128-bit identifier written as 32 hexadecimal digits in an 8-4-4-4-12 pattern, designed so that values can be generated independently anywhere with a negligible chance of collision. The current specification is RFC 9562.