poly1305: add test vectors for edge cases

Often intermediate results of poly1305 calculations are only reduced to
the range [0, 2^130). These new test vectors exercise the code that
reduces the final output to the range [0, 2^130-5).

This improves the test coverage of CL 35294 and CL 32812.

Change-Id: Ifd2f64d4668c08a396ed81db3e88969a49baf777
Reviewed-on: https://go-review.googlesource.com/35918
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

poly1305/poly1305_test.go

@@ -39,6 +39,36 @@ var testData = []struct {
 		[]byte{0x3b, 0x3a, 0x29, 0xe9, 0x3b, 0x21, 0x3a, 0x5c, 0x5c, 0x3b, 0x3b, 0x05, 0x3a, 0x3a, 0x8c, 0x0d},
 		[]byte{0x6d, 0xc1, 0x8b, 0x8c, 0x34, 0x4c, 0xd7, 0x99, 0x27, 0x11, 0x8b, 0xbe, 0x84, 0xb7, 0xf3, 0x14},
 	},
+	{
+		// This test generates a result of (2^130-1) % (2^130-5).
+		[]byte{
+			0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		},
+		[]byte{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+		[]byte{4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+	},
+	{
+		// This test generates a result of (2^130-6) % (2^130-5).
+		[]byte{
+			0xfa, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		},
+		[]byte{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+		[]byte{0xfa, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff},
+	},
+	{
+		// This test generates a result of (2^130-5) % (2^130-5).
+		[]byte{
+			0xfb, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		},
+		[]byte{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+		[]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+	},
 }
 
 func testSum(t *testing.T, unaligned bool) {