a

libmp/crt.c

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+#include "os.h"
+#include <mp.h>
+#include <libsec.h>
+
+// chinese remainder theorem
+//
+// handbook of applied cryptography, menezes et al, 1997, pp 610 - 613
+
+struct CRTpre
+{
+	int	n;		// number of moduli
+	mpint	**m;		// pointer to moduli
+	mpint	**c;		// precomputed coefficients
+	mpint	**p;		// precomputed products
+	mpint	*a[1];		// local storage
+};
+
+// setup crt info, returns a newly created structure
+CRTpre*
+crtpre(int n, mpint **m)
+{
+	CRTpre *crt;
+	int i, j;
+	mpint *u;
+
+	crt = malloc(sizeof(CRTpre)+sizeof(mpint)*3*n);
+	if(crt == nil)
+		sysfatal("crtpre: %r");
+	crt->m = crt->a;
+	crt->c = crt->a+n;
+	crt->p = crt->c+n;
+	crt->n = n;
+
+	// make a copy of the moduli
+	for(i = 0; i < n; i++)
+		crt->m[i] = mpcopy(m[i]);
+
+	// precompute the products
+	u = mpcopy(mpone);
+	for(i = 0; i < n; i++){
+		mpmul(u, m[i], u);
+		crt->p[i] = mpcopy(u);
+	}
+
+	// precompute the coefficients
+	for(i = 1; i < n; i++){
+		crt->c[i] = mpcopy(mpone);
+		for(j = 0; j < i; j++){
+			mpinvert(m[j], m[i], u);
+			mpmul(u, crt->c[i], u);
+			mpmod(u, m[i], crt->c[i]);
+		}
+	}
+
+	mpfree(u);
+
+	return crt;		
+}
+
+void
+crtprefree(CRTpre *crt)
+{
+	int i;
+
+	for(i = 0; i < crt->n; i++){
+		if(i != 0)
+			mpfree(crt->c[i]);
+		mpfree(crt->p[i]);
+		mpfree(crt->m[i]);
+	}
+	free(crt);
+}
+
+// convert to residues, returns a newly created structure
+CRTres*
+crtin(CRTpre *crt, mpint *x)
+{
+	int i;
+	CRTres *res;
+
+	res = malloc(sizeof(CRTres)+sizeof(mpint)*crt->n);
+	if(res == nil)
+		sysfatal("crtin: %r");
+	res->n = crt->n;
+	for(i = 0; i < res->n; i++){
+		res->r[i] = mpnew(0);
+		mpmod(x, crt->m[i], res->r[i]);
+	}
+	return res;
+}
+
+// garners algorithm for converting residue form to linear
+void
+crtout(CRTpre *crt, CRTres *res, mpint *x)
+{
+	mpint *u;
+	int i;
+
+	u = mpnew(0);
+	mpassign(res->r[0], x);
+
+	for(i = 1; i < crt->n; i++){
+		mpsub(res->r[i], x, u);
+		mpmul(u, crt->c[i], u);
+		mpmod(u, crt->m[i], u);
+		mpmul(u, crt->p[i-1], u);
+		mpadd(x, u, x);
+	}
+
+	mpfree(u);
+}
+
+// free the residue
+void
+crtresfree(CRTres *res)
+{
+	int i;
+
+	for(i = 0; i < res->n; i++)
+		mpfree(res->r[i]);
+	free(res);
+}