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An extendable-output function (XOF) is similar to a hash function, but uses the internal state to output a stream of bits instead of a fixed-length octet string.

A XOF is an extendable-output function. While a standard hash function uses the internal state to output a fixed length bit or octet string, a XOF uses the internal state to output a stream of bits or octets.

The first family of extendable-output functions that was standardized as such is SHAKE128 and SHAKE256, which are variants of and defined in FIPS 202.