sure how much entropy to collect before using it to reseed the PRNG. This problem is solved by Fortuna (PRNG), an improvement of Yarrow. Fortuna has 32 pools to collect entropy and removed the entropy estimator completely. Yarrow's strength is limited by the size of the key. For example, Yarrow-160 has an effective key size of 160 bits. If the security requires 256 bits, Yarrow-160 is not capable of doing the job. References External links Yarrow algorithm page Yarrow-160: Notes on the Design and Analysis of the Yarrow Cryptographic Pseudorandom Number Generator, J. Kelsey, B. Schneier, and N. Ferguson "Yarrow implementation in Java" "Yarrow implementation in FreeBSD" "An implementation of the Yarrow PRNG for FreeBSD" Pseudorandom number generators Cryptographically secure pseudorandom number generators