a

libsec/rsadecrypt.c

@@ -0,0 +1,37 @@
+#include "os.h"
+#include <mp.h>
+#include <libsec.h>
+
+// decrypt rsa using garner's algorithm for the chinese remainder theorem
+//	seminumerical algorithms, knuth, pp 253-254
+//	applied cryptography, menezes et al, pg 612
+mpint*
+rsadecrypt(RSApriv *rsa, mpint *in, mpint *out)
+{
+	mpint *v1, *v2;
+
+	if(out == nil)
+		out = mpnew(0);
+
+	// convert in to modular representation
+	v1 = mpnew(0);
+	mpmod(in, rsa->p, v1);
+	v2 = mpnew(0);
+	mpmod(in, rsa->q, v2);
+
+	// exponentiate the modular rep
+	mpexp(v1, rsa->kp, rsa->p, v1);
+	mpexp(v2, rsa->kq, rsa->q, v2);
+	
+	// out = v1 + p*((v2-v1)*c2 mod q)
+	mpsub(v2, v1, v2);
+	mpmul(v2, rsa->c2, v2);
+	mpmod(v2, rsa->q, v2);
+	mpmul(v2, rsa->p, out);
+	mpadd(v1, out, out);
+
+	mpfree(v1);
+	mpfree(v2);
+
+	return out;
+}