Secure Key Exchange over an Insecure Medium with Public Key Cryptography - Implementing SSL/TLS Using Cryptography and PKI - page 117

x 0

x 2 1 ¨ multiply

(x 2 ) 2 0
((x 2 ) 2 ) 2 1 ¨ multiply

multiplying the two “hits” together, you get x 2 (( x 2 ) 2 ) 2 or [(( x 2 ) 2 ) x ] 2 which is what
you got when you deconstructed decimal 10 in the fi rst place.
Listing 3-18 shows how you can implement this in code. Compare this to the
implementation of multiply in Listing 3-9.

Listing 3-18: “huge.c” exponentiate

/**
* Raise h1 to the power of exp. Return the result in h1.
*/
void exponentiate( huge *h1, huge *exp )
{
int i = exp->size, mask;
huge tmp1, tmp2;

set_huge( &tmp1, 0 );
set_huge( &tmp2, 0 );

copy_huge( &tmp1, h1 );
set_huge( h1, 1 );

do
{
i--;
for ( mask = 0x01; mask; mask <<= 1 )
{
if ( exp->rep[ i ] & mask )
{
multiply( h1, &tmp1 );
}

// Square tmp1
copy_huge( &tmp2, &tmp1 );
multiply( &tmp1, &tmp2 );
}
}
while ( i );

free_huge( &tmp1 );
free_huge( &tmp2 );
}