[Cryptography] Question re: Initialization Vector for AES Counter Mode…

[John Denker]

On 04/26/2017 03:28 PM, Jon Callas wrote:

> Personally, I would recommend that you *not* use counter mode or
> anything resembling it for writing files.

I agree with that 100%.

A cipher "mode" is a kludge that kinda maybe sorta allows people
to survive in situations where they can't (or won't) re-key ...
but you should always ask yourself, why not just re-key?  If
your cipher cannot be efficiently or securely re-keyed, maybe
you need a better cipher.

Constructive suggestion:  Use something like ChaCha20, which
is designed to do a good job with file encryption (and a lot
of other things).  It has an enormous keyspace, and can be
re-keyed efficiently.  Construct the key from at least:
  -- The master key.
  -- The block number.
  -- A sequence number, depending on how many times the
   block has been rewritten.  (This allows random access,
   as well as rewriting the whole file from the beginning.)

> Counter mode creates a stream cipher.

Right.

> it's like leaving a rake in the yard with the tines up.

Good analogy.