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AES has no modes — you pick one

Level: 201 · working knowledge

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

One line: AES encrypts exactly sixteen bytes with one key. Everything that encrypts a file is a mode wrapped around that, and the mode is where the security property lives.

The questions this page has to answer

  • What is a block cipher, and why is 128 bits the block size rather than the key size?
  • What does each mode actually do — ECB, CBC, CTR, GCM — in one line and one diagram each?
  • What is an IV, how does it differ from a nonce, and which modes need it unpredictable rather than merely unique?
  • What is padding for, and why is CTR mode free of it?
  • Why is the modern answer 'do not choose a mode, choose an AEAD'?

See also