SHA-2

SHA-2 is a group of hash functions that produces a fixed-size digest from data of practically any length. NIST specifies six named variants: SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256.¹ The numbers identify the nominal digest length in bits, not a measure of the input that can be processed. A one-bit change in the message normally alters roughly half the digest bits, a property called the avalanche effect.

SHA-224 and SHA-256 use 512-bit message blocks and 32-bit words; SHA-384 and SHA-512 use 1024-bit blocks and 64-bit words. The SHA-512/224 and SHA-512/256 variants use the SHA-512 construction but truncate its output, with independently specified initial values. SHA-384 similarly derives a shorter result from the SHA-512 family, rather than simply running SHA-256 and cutting it down.¹

SHA-2 processes padded messages in blocks through an iterative compression construction. Each round combines additions modulo a power of two, bitwise Boolean functions, rotations, shifts, message-schedule words, and round constants; SHA-256 has 64 rounds, while SHA-512 and its derivatives have 80.¹ The final internal state is serialized as the digest.

Security discussions distinguish collision resistance from preimage resistance. For an ideal n-bit hash, generic collision search requires about 2^n/2 work, whereas finding a message for a specified digest requires about 2ⁿ work. These are design expectations, not guarantees against every attack or implementation mistake. SHA-2 has withstood extensive public cryptanalysis, but protocols still need correct domain separation, length handling, key management, and side-channel-aware implementations.

SHA-2 is widely used to authenticate data indirectly: a digital signature commonly signs a hash of a document, and a certificate or software-release mechanism can publish a digest for later comparison. SHA-256 and SHA-384 are among the hash algorithms used by modern Transport Layer Security cipher suites and signature schemes.³ The Internet Engineering Task Force also specifies SHA-2 implementations and test vectors for protocol developers.²

Typical applications include file-integrity verification, authenticated boot, password-storage constructions such as PBKDF2-HMAC-SHA-256, HMAC authentication, and content-addressed storage. A bare hash is not a password-protection scheme: passwords require a deliberately slow, salted password-based key-derivation function. Likewise, collision resistance does not prove that a downloaded file came from a trusted publisher; authenticity requires a trusted signature, secure channel, or equivalent provenance mechanism.

SHA-2’s shorter variants are not interchangeable labels for truncated output. SHA-224 and SHA-384 use distinct initial states and specified truncation rules, while SHA-512/224 and SHA-512/256 are SHA-512-based designs with their own initialization; this prevents accidental equivalence between differently named algorithms.¹

The family was developed as a successor line to SHA-1, whose declining security margin led standards bodies to discourage its use for new digital signatures and similar applications.⁴ SHA-2 can still be the wrong choice when a protocol needs a variable-length output, built-in tree hashing, or resistance to length-extension issues in an unkeyed construction; SHA-3 and keyed constructions such as HMAC address different requirements. SHA-2 also appears in less visible infrastructure, including certificate fingerprints, package indexes, version-control object identifiers, and cryptocurrency proof-of-work.

Digest

The fixed-length bit string produced by a hash function.

Collision resistance

The property that makes it computationally difficult to find two different inputs with the same digest.

Preimage resistance

The property that makes it difficult to recover an input matching a specified digest.

HMAC

A keyed message-authentication construction that combines a hash function with a secret key.

Length extension

An attack pattern affecting some iterative hash constructions when a digest is misused as an authentication value.