Pseudorandom generators (PRG) are functions that takes a random small input (called the Seed) and maps it into much larger output .
However, the mapping process must be unpredictable in order for the PRG to be used securely in cryptography.
My question is: Are there any tool the does the opposite? i.e. takes a long random string and maps it to a smaller random unpredictable output?
And if not, can we use s-box for this purpose?