Re: AES counter mode

[Götz Babin-Ebell]

Hello Richard,

Richard Levitte - VMS Whacker wrote:

In message <[EMAIL PROTECTED]> on Thu, 26 Jun 2003 12:55:22 -0400, "Lee Dilkie" <[EMAIL PROTECTED]> said:

OK, I've been follownig this discussion for a while, and it's time I
ake action.  Basically, to provide for all the current and future ways
of handling the IV, I can see three alternatives:

- have the application provide a function that manipulates the IV.
- have the application specify exactly which part of the IV is the
  actual counter (in bit positions, or would byte positions be
  enough?).

The application specifies 4 datas:
1. a step size
2. a bit mask.
3. a (optional) pointer to a function that is called if the
   step bits that are not in the bit mask:
4. a (optional) pointer to a function doing the counting;

A quick draft of my idea:

typedef struct _CounterData CounterData;
struct _CounterData
{
   Counter_Number nActual;  /* the actual counter value  */
   Counter_Number nStep;    /* Step size added each step */
   Counter_Number nStepMask;/* Mask indikating when counter leaves
                               range */
   int (*Count)(CounterData *pCounter); /* optional: handle a step,
                                           returns: < 0: error
                                           returns: > 0: OK. */
   int (*Range)(CounterData *pCounter); /* optional: is called if
                                           Count() returns out of range.
                                           returns: < 0: error
                                           returns: > 0: OK */
};

int Count(CounterData *pCounter)
{
   if (!pCounter)
      return -1;
   pCounter->nActual+= pCounter->nStep;
   return 1;
}
int Range_restart(CounterData *pCounter)
{
   if (!pCounter)
      return -1;
   pCounter->nActual &= ~pCounter->nStepMask;
   return 1;
}
int Range_terminate(CounterData *pCounter)
{
   return -1;
}

int Do_Counter(CounterData *pCounter)
{
   int nRet;
   if (!pCounter)
      return -1;
   if (pCounter->Count)
      nRet = pCounter->Count(pCounter);
   else
      nRet = Count(pCounter);
   if (nRet < 0)
      return nRet;
   if (pCounter->nActual & ~pCounter->nStepMask)
   {
      if (pCounter->Range)
         return pCounter->Range(pCounter);
      else
         return Range_restart(pCounter);
   }
   return nRet;
}

OK.
The Mask could be an upper bound...

Any comments ?


Bye

Goetz

--
Goetz Babin-Ebell, TC TrustCenter AG, http://www.trustcenter.de
Sonninstr. 24-28, 20097 Hamburg, Germany
Tel.: +49-(0)40 80 80 26 -0,  Fax: +49-(0)40 80 80 26 -126

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