  Muchnik's theorem about simple conditional descriprion states that for all
words $a$ and $b$ there exists a short program $p$ transforming $a$ to $b$ that
has the least possible length and is simple conditional on $b$. This paper
presents a new proof of this theorem, based on extractors. Employing the
extractor technique, two new versions of Muchnik's theorem for space- and
time-bounded Kolmogorov complexity are proven.
