/* RSA decryption with the private key */
public static void DECRYPT(rsa_private_key PRIV, sbyte[] G, sbyte[] F)
{
int n = PRIV.p.getlen();
FF g = new FF(2 * n);
FF.fromBytes(g, G);
FF jp = g.dmod(PRIV.p);
FF jq = g.dmod(PRIV.q);
jp.skpow(PRIV.dp, PRIV.p);
jq.skpow(PRIV.dq, PRIV.q);
g.zero();
g.dscopy(jp);
jp.mod(PRIV.q);
if (FF.comp(jp, jq) > 0)
{
jq.add(PRIV.q);
}
jq.sub(jp);
jq.norm();
FF t = FF.mul(PRIV.c, jq);
jq = t.dmod(PRIV.q);
t = FF.mul(jq, PRIV.p);
g.add(t);
g.norm();
g.toBytes(F);
}
/* Set r=this mod b */
/* this is of length - 2*n */
/* r,b is of length - n */
public FF dmod(FF b)
{
int k, n = b.length;
FF m = new FF(2 * n);
FF x = new FF(2 * n);
FF r = new FF(n);
x.copy(this);
x.norm();
m.dsucopy(b);
k = 256 * n;
while (k > 0)
{
m.shr();
if (comp(x, m) >= 0)
{
x.sub(m);
x.norm();
}
k--;
}
r.copy(x);
r.mod(b);
return(r);
}