I don't understand what you want I do.
I know what the algo do but I don't know how to reverse it.

I converted it in pseudo code, if it can help someone.

byte origine array [8];
byte final array [17];
int cpt1,cpt2,cpt3,cpt4;

for (cpt1=0;cpt1<8;cpt1++)
{
carac = origine[cpt1];
cpt4 = 0 ;
for (cpt2=0;cpt2<8;cpt2++)
{
carac2 = origine[cpt2];
carac2 = carac * carac2;
cpt3 = cpt4;
while(carac2!=0)
{
carac2 = carac2 + resultat[cpt3] ;
resultat[cpt3] = carac2 % 100 ;
carac2 = carac2 div 100 ;
cpt3++ ;
}
cpt4++;
}
}