[OTR-dev] AES128 CTR?

[Ian Goldberg]

On Tue, Jan 18, 2005 at 08:51:29PM -0500, alex323 wrote:
> "This should monotonically increase (as a big-endian value) for
>      each message sent with the same (sender keyid, recipient keyid)
>      pair , and must not be all 0x00."
> 
> 
> So I am assuming that whenever the sender keyid and recipient keyid are 
> the same, i add one to the counter. For example:

That's correct, where "the sender keyid and recipient keyid are the same"
means "they're both the same as last time", not "they're the same as
each other".

> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03
> ...
> 
> "The initial counter is a 16-byte value whose first 8
>     bytes are the above "top half of counter init" value, and whose
>     last 8 bytes are all 0x00."
> 
> And this should be:
> 
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 
> 0x00, 0x00, 0x00, 0x00
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 
> 0x00, 0x00, 0x00, 0x00
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x00, 
> 0x00, 0x00, 0x00, 0x00
> 
> Correct?

Yes.

   - Ian