Blum started his teaching as an assistant mathematics professor at MIT. He later became a professor at the Univeresity of California at Berkeley, teaching electrical engineering and computer science. During Blum's career in computer science, he contributed many new ideas and theorems. Some of these include Blum complexity axioms, Blum's speedup theorem, Blum Blum Shub (pseudorandom number generator), and the Blum-Goldwasser cryptosystem. Most widely known, however, is CAPTCHA, a way to distinguish computers and humans on websites.