A strong extractor takes as input:

A noisy source 𝑋 X: This could be an imperfect or random source, where some entropy is known to exist but cannot be reliably extracted directly. A random seed 𝑅 R: This is an additional input, assumed to be uniformly random and independent of 𝑋 X, which helps in amplifying the randomness in 𝑋 X and achieving the uniformity needed for security. The result of a strong extractor is a bit string that:

Is as close to uniformly random as possible, regardless of the biases or noise in 𝑋 X. Remains secure and unpredictable even to an observer who may have partial information about 𝑋 X.