/* 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);
}
/* U=1/a mod 2^m - Arazi & Qi */
private FF invmod2m()
{
int i, n = length;
FF b = new FF(n);
FF c = new FF(n);
FF U = new FF(n);
FF t;
U.zero();
U.v[0].copy(v[0]);
U.v[0].invmod2m();
for (i = 1; i < n; i <<= 1)
{
b.copy(this);
b.mod2m(i);
t = mul(U, b);
t.shrw(i);
b.copy(t);
c.copy(this);
c.shrw(i);
c.mod2m(i);
c.lmul(U);
c.mod2m(i);
b.add(c);
b.norm();
b.lmul(U);
b.mod2m(i);
c.one();
c.shlw(i);
b.revsub(c);
b.norm();
b.shlw(i);
U.add(b);
}
U.norm();
return(U);
}
/* Set return=1/this mod p. Binary method - a<p on entry */
public void invmodp(FF p)
{
int n = p.length;
FF u = new FF(n);
FF v = new FF(n);
FF x1 = new FF(n);
FF x2 = new FF(n);
FF t = new FF(n);
FF one = new FF(n);
one.one();
u.copy(this);
v.copy(p);
x1.copy(one);
x2.zero();
// reduce n in here as well!
while (comp(u, one) != 0 && comp(v, one) != 0)
{
while (u.parity() == 0)
{
u.shr();
if (x1.parity() != 0)
{
x1.add(p);
x1.norm();
}
x1.shr();
}
while (v.parity() == 0)
{
v.shr();
if (x2.parity() != 0)
{
x2.add(p);
x2.norm();
}
x2.shr();
}
if (comp(u, v) >= 0)
{
u.sub(v);
u.norm();
if (comp(x1, x2) >= 0)
{
x1.sub(x2);
}
else
{
t.copy(p);
t.sub(x2);
x1.add(t);
}
x1.norm();
}
else
{
v.sub(u);
v.norm();
if (comp(x2, x1) >= 0)
{
x2.sub(x1);
}
else
{
t.copy(p);
t.sub(x1);
x2.add(t);
}
x2.norm();
}
}
if (comp(u, one) == 0)
{
copy(x1);
}
else
{
copy(x2);
}
}