Why does Bitcoin use two rounds of SHA256? Each blocked is hashed twice. Why isn't one application of SHA256 enough?
From Zooko's answer provided in Crypto StackExchange:


  SHA-256(SHA-256(x)) was proposed by Ferguson and Schneier in their
  excellent book "Practical Cryptography" (later updated by Ferguson,
  Schneier, and Kohno and renamed "Cryptography Engineering") as a way
  to make SHA-256 invulnerable to "length-extension" attack. They called
  it "SHA-256d".