[NUT-devel] Incomplete description of checksum algorithm

[Rich Felker]

On Thu, Feb 16, 2006 at 04:44:02PM +0100, Michael Niedermayer wrote:
> Hi
> 
> On Thu, Feb 16, 2006 at 01:47:32AM +0100, Alexander Strasser wrote:
> [...]
> > > crc32 checksum
> > >     The checksum is choosen so that c(x) is a multiple of
> > >     x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1 over GF(2)
> > >     c(x) is the polynom coresponding to the block from and including the
> > >     forward ptr and upto and including the checksum
> > >     c(x)= (1&(d[0]>>8))*x^(n-1) + (1&(d[0]>>7))*x^(n- 2) + ...
> > >         + (1&(d[1]>>8))*x^(n-9) + (1&(d[1]>>7))*x^(n-10) + ...
> > >         + ...
> > >         + ... + (1&(d[n/8-1]>>1))*x^1 + (1&(d[n/8-1]>>0))*x^0
> > >     alternatively you can simply run
> > >     crc=0;
> > >     for(i=0; i<size; i++){
> > >         crc ^= buf[i]<<24;
> > >         for(j=0; j<8; j++)
> > >             crc= (crc<<1) ^ (0x04C11DB7 & (crc>>31));
> > >     }
> > >     over the data  
> > 
> >   ok, while we are at it, should we use non zero initial value? if yes which?
> 
> personally i would simply include the forward ptr, so the all zero case would
> naturally be gone, if the others are against this then we must either use a 

Is there an all-zero case? As far as I can tell, there's no NUT packet
that's valid as all zeros, except possibly the very first
syncpoint/header pair in the file.

Rich