Naor–Reingold pseudorandom function

In 1997, Moni Naor and Omer Reingold described efficient constructions for various cryptographic primitives in private key as well as public-key cryptography. Their result is the construction of an efficient pseudorandom function. Let p and l be prime numbers with l |p−1. Select an element g ∈ of multiplicative order l. Then for each (n+1)-dimensional vector a = (a0,a1, ..., an)∈ they define the function