crypto: special case ∞+a, a+∞ and a+a in p224.

crypto/p224_unittest.cc

Index: crypto/p224_unittest.cc
diff --git a/crypto/p224_unittest.cc b/crypto/p224_unittest.cc
index 1ab2ff763814c0d00c0105d1a5568d5bd01e50a6..5316d8a17e00f65040da9c72df3a19fbd2219a2b 100644
--- a/crypto/p224_unittest.cc
+++ b/crypto/p224_unittest.cc
@@ -806,3 +806,16 @@ TEST(P224, Addition) {
   p224::Add(minus_b, sum, &a_again);
   EXPECT_TRUE(a_again.ToString() == a.ToString());
 }
+
+TEST(P224, Infinity) {
+  char zeros[56];
+  memset(zeros, 0, sizeof(zeros));
+
+  // Test that x^0 = ∞.
+  Point a;
+  p224::ScalarBaseMult(reinterpret_cast<const uint8*>(zeros), &a);
+  EXPECT_TRUE(memcmp(zeros, a.ToString().data(), sizeof(zeros)) == 0);
+
+  // We shouldn't allow ∞ to be imported.
+  EXPECT_FALSE(a.SetFromString(std::string(zeros, sizeof(zeros))));
+}