little refactor of decoding

lib/decoding.cpp

@@ -1,14 +1,15 @@
 
 #include "decoding.h"
 
-bool syndrome_decode (bvector&syndrome, gf2m&fld, polynomial& goppa,
-                      std::vector<polynomial>& sqInv, bvector&ev,
-                      bool check_failure)
-
+void compute_error_locator (bvector&syndrome, gf2m&fld, polynomial& goppa,
+                            std::vector<polynomial>& sqInv, polynomial&out)
 {
-	ev.clear();
-	ev.resize (fld.n, 0);
-	if (syndrome.zero() ) return true;
+	if (syndrome.zero() ) {
+		//ensure no roots
+		out.resize (1);
+		out[0] = 1;
+		return;
+	}
 
 	polynomial v;
 	syndrome.to_poly (v, fld);
@@ -28,20 +29,26 @@ bool syndrome_decode (bvector&syndrome, gf2m&fld, polynomial& goppa,
 	a.add (b, fld); //new a = a^2 + x b^2
 
 	a.make_monic (fld); //now it is the error locator.
+	out = a;
+}
+
+bool evaluate_error_locator_dumb (polynomial&a, bvector&ev, gf2m&fld)
+{
+	ev.clear();
+	ev.resize (fld.n, 0);
 
 	for (uint i = 0; i < fld.n; ++i) {
 		if (a.eval (i, fld) == 0) {
 			ev[i] = 1;
 
-			if (!check_failure) continue;
-			//check if the error locator splits over GF(2^m).
-			//We simplify it to the assumption that all roots are
-			//also roots of linear factors.
+			//divide the polynomial by (found) linear factor
 			polynomial t, q, r;
 			t.resize (2, 0);
 			t[0] = i;
 			t[1] = 1;
 			a.divmod (t, q, r, fld);
+
+			//if it doesn't divide, die.
 			if (r.degree() >= 0) {
 				ev.clear();
 				return false;
@@ -50,7 +57,8 @@ bool syndrome_decode (bvector&syndrome, gf2m&fld, polynomial& goppa,
 		}
 	}
 
-	if (check_failure && a.degree() > 0) {
+	//also if there's something left, die.
+	if (a.degree() > 0) {
 		ev.clear();
 		return false;
 	}

lib/decoding.h

@@ -5,11 +5,14 @@
 #include "codecrypt.h"
 
 using namespace ccr;
-bool syndrome_decode (bvector&syndrome,
+
+void compute_error_locator (bvector&syndrome,
                             gf2m&fld,
-                      polynomial& gp,
+                            polynomial&goppa,
                             std::vector<polynomial>& sqInv,
-                      bvector&ev,
-                      bool check_failure = true);
+                            polynomial&loc);
+
+bool evaluate_error_locator_dumb (polynomial&el, bvector&ev, gf2m&fld);
+bool evaluate_error_locator_trace (polynomial&el, bvector&ev, gf2m&fld);
 
 #endif

lib/mce.cpp

@@ -79,13 +79,18 @@ int privkey::decrypt (const bvector&in, bvector&out)
 	h.mult_vec_right (canonical, syndrome);
 
 	//decode
+	polynomial loc;
+	compute_error_locator (syndrome, fld, g, sqInv, loc);
+
 	bvector ev;
-	if (!syndrome_decode (syndrome, fld, g, sqInv, ev) )
+	if (!evaluate_error_locator_dumb (loc, ev, fld) )
 		return 1; //if decoding somehow failed, fail as well.
 
 	// check the error vector, it should have exactly t == deg (g) errors
 	if ( (int) ev.hamming_weight() != g.degree() )
 		return 1;
+	//TODO cryptoanalysis suggests omitting this check for preventing
+	//bit-flipping attack
 
 	//correct the errors
 	canonical.add (ev);
@@ -115,6 +120,7 @@ int privkey::sign (const bvector&in, bvector&out, uint delta, uint attempts, prn
 	bvector p, e, synd, synd_orig, e2;
 	std::vector<uint> epos;
 	permutation hpermInv;
+	polynomial loc;
 
 	s = hash_size();
 
@@ -143,7 +149,9 @@ int privkey::sign (const bvector&in, bvector&out, uint delta, uint attempts, prn
 			synd.add (h[epos[i]]);
 		}
 
-		if (syndrome_decode (synd, fld, g, sqInv, e2, true) ) {
+		compute_error_locator (synd, fld, g, sqInv, loc);
+
+		if (evaluate_error_locator_dumb (loc, e2, fld) ) {
 
 			//create the decodable message
 			p.add (e);

lib/nd.cpp

@@ -58,8 +58,11 @@ int privkey::decrypt (const bvector&in, bvector&out)
 	bvector unsc; //unscrambled
 	Sinv.mult_vec_right (in, unsc);
 
+	polynomial loc;
+	compute_error_locator (unsc, fld, g, sqInv, loc);
+
 	bvector ev;
-	if (!syndrome_decode (unsc, fld, g, sqInv, ev) )
+	if (!evaluate_error_locator_dumb (loc, ev, fld) )
 		return 1;
 
 	if ( (int) ev.hamming_weight() != g.degree() )
@@ -75,6 +78,8 @@ int privkey::sign (const bvector&in, bvector&out, uint delta, uint attempts, prn
 
 	bvector synd_unsc, synd, e;
 
+	polynomial loc;
+
 	s = hash_size();
 	if (in.size() != s) return 2;
 
@@ -88,7 +93,9 @@ int privkey::sign (const bvector&in, bvector&out, uint delta, uint attempts, prn
 
 		Sinv.mult_vec_right (synd, synd_unsc);
 
-		if (syndrome_decode (synd_unsc, fld, g, sqInv, e, true) ) {
+		compute_error_locator (synd_unsc, fld, g, sqInv, loc);
+
+		if (evaluate_error_locator_dumb (loc, e, fld) ) {
 
 			Pinv.permute (e, out);
 			return 0;