ref: bd178b6dc6a6d6a81be05c0a1f7c03b352f833c1
dir: /sys/src/libsec/port/egdecrypt.c/
#include "os.h"
#include <mp.h>
#include <libsec.h>
mpint*
egdecrypt(EGpriv *priv, mpint *in, mpint *out)
{
EGpub *pub = &priv->pub;
mpint *gamma, *delta;
mpint *p = pub->p;
int plen = mpsignif(p)+1;
int shift = ((plen+Dbits-1)/Dbits)*Dbits;
if(out == nil)
out = mpnew(0);
gamma = mpnew(0);
delta = mpnew(0);
mpright(in, shift, gamma);
mpleft(gamma, shift, delta);
mpsub(in, delta, delta);
mpexp(gamma, priv->secret, p, out);
mpinvert(out, p, gamma);
mpmul(gamma, delta, out);
mpmod(out, p, out);
mpfree(gamma);
mpfree(delta);
return out;
}