Unkeyed

Unkeyed refers to algorithms or operations that do not require or use a secret key. In cryptography, the term is commonly applied to hash functions and other primitives that produce outputs from data without any secret material, in contrast to keyed constructs that rely on a secret key.

Unkeyed hash functions take an arbitrary input and return a fixed-size digest. They are designed to provide data integrity rather than authentication, since the digest alone does not prove who created or sent the data. A given input will always yield the same digest, and an attacker could alter the data and recompute a new digest if the channel or protocol does not provide additional protections.

Typical examples include widely used cryptographic hashes such as SHA-256 and SHA-3, as well as older schemes like MD5 and SHA-1, though many of these are considered weak by modern standards and should be avoided for security-sensitive applications. The primary security properties of unkeyed hashes are collision resistance and preimage resistance.

Keyed variants, such as message authentication codes (MACs) or HMAC, incorporate a secret key to provide authentication as well as integrity. Because they rely on a key, their outputs can be verified only by someone who possesses the same key.

Practical guidance: use unkeyed hashes for checksums, data integrity verification, and as building blocks inside larger protocols (for example, digital signatures rely on hashes but should be used with appropriate signing keys). Do not rely on unkeyed hashes for authentication in untrusted environments; for that, use MACs or digital signatures.

See also: hash function, HMAC, MAC, digital signature, cryptographic strength.