Extendable-output function
Cryptographic hash extension
From Wikipedia, the free encyclopedia
Extendable-output function (XOF) is a type of cryptographic hash function that allows its output to be arbitrarily long, allowing it to be used as a cryptographically secure pseudo-random number generator.[1]
One particular hash construction, the sponge construction, makes any sponge hash a natural XOF: the squeeze operation can be repeated thus resulting in an XOF (the regular hash functions with a fixed-size result are obtained from a sponge mechanism by stopping the squeezing phase after obtaining the fixed number of bits).[2]
A secure XOF is collision, preimage and second preimage resistant. While technically any XOF can be turned into a cryptographic hash by truncating the result to a fixed length, in the real world hashes and XOFs tend to be defined differently using domain separation.[3]) Examples of sponge construction XOFs include the algorithms from the Keccak family: SHAKE128, SHAKE256, and a variant with higher efficiency, KangarooTwelve.[1]
There are other XOFs which are not sponge constructions, such as Skein and RadioGatún.
XOFs are used as key derivation functions (KDFs), stream ciphers[1], and mask generation functions.[4]
Related-output issues
By their nature, XOFs can produce related outputs (a longer result includes a shorter one as a prefix). The use of KDFs for key derivation can therefore cause related-output problems. As a "naïve" example, if the Triple DES keys are generated with an XOF, and there is a confusion in the implementation that causes some operations to be performed as 3TDEA (3 × 56 = 168-bit key), and some as 2TDEA (2 × 56 = 112 bit key), comparing the encryption results will lower the attack complexity to just 56 bits; similar problems can occur if hashes in the NIST SP 800-108 are naïvely replaced by the KDFs.[5]
