Add tests for |BN_mod_inverse| with modulus 1.

Zero is only a valid input to or output of |BN_mod_inverse| when the
modulus is one. |BN_MONT_CTX_set| actually depends on this, so test
that this works.

Change-Id: Ic18f1fe786f668394951d4309020c6ead95e5e28
Reviewed-on: https://boringssl-review.googlesource.com/8922
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/bn/bn_test.cc

@@ -584,6 +584,33 @@ static bool TestModSqrt(FileTest *t, BN_CTX *ctx) {
   return true;
 }
 
+static bool TestModInv(FileTest *t, BN_CTX *ctx) {
+  ScopedBIGNUM a = GetBIGNUM(t, "A");
+  ScopedBIGNUM m = GetBIGNUM(t, "M");
+  ScopedBIGNUM mod_inv = GetBIGNUM(t, "ModInv");
+  if (!a || !m || !mod_inv) {
+    return false;
+  }
+
+  ScopedBIGNUM ret(BN_new());
+  if (!ret ||
+      !BN_mod_inverse(ret.get(), a.get(), m.get(), ctx) ||
+      !ExpectBIGNUMsEqual(t, "inv(A) (mod M)", mod_inv.get(), ret.get())) {
+    return false;
+  }
+
+  BN_set_flags(a.get(), BN_FLG_CONSTTIME);
+
+  if (!ret ||
+      !BN_mod_inverse(ret.get(), a.get(), m.get(), ctx) ||
+      !ExpectBIGNUMsEqual(t, "inv(A) (mod M) (constant-time)", mod_inv.get(),
+                          ret.get())) {
+    return false;
+  }
+
+  return true;
+}
+
 struct Test {
   const char *name;
   bool (*func)(FileTest *t, BN_CTX *ctx);
@@ -601,6 +628,7 @@ static const Test kTests[] = {
     {"ModExp", TestModExp},
     {"Exp", TestExp},
     {"ModSqrt", TestModSqrt},
+    {"ModInv", TestModInv},
 };
 
 static bool RunTest(FileTest *t, void *arg) {

crypto/bn/bn_tests.txt

@@ -10689,3 +10689,19 @@ P = 9df9d6cc20b8540411af4e5357ef2b0353cb1f2ab5ffc3e246b41c32f71e951f
 ModSqrt = a1d52989f12f204d3d2167d9b1e6c8a6174c0c786a979a5952383b7b8bd186
 A = 2eee37cf06228a387788188e650bc6d8a2ff402931443f69156a29155eca07dcb45f3aac238d92943c0c25c896098716baa433f25bd696a142f5a69d5d937e81
 P = 9df9d6cc20b8540411af4e5357ef2b0353cb1f2ab5ffc3e246b41c32f71e951f
+
+ModInv = 00
+A = 00
+M = 01
+
+ModInv = 00
+A = 01
+M = 01
+
+ModInv = 00
+A = 02
+M = 01
+
+ModInv = 00
+A = 03
+M = 01

crypto/bn/check_bn_tests.go

@@ -247,6 +247,11 @@ func main() {
 					}
 				}
 			}
+		case "ModInv":
+			if checkKeys(test, "A", "M", "ModInv") {
+				r := new(big.Int).ModInverse(test.Values["A"], test.Values["M"])
+				checkResult(test, "A ^ -1 (mod M)", "ModInv", r)
+			}
 		default:
 			fmt.Fprintf(os.Stderr, "Line %d: unknown test type %q.\n", test.LineNumber, test.Type)
 		}