CTPolicy: always try to get SCTs from a tiled log first

ctpolicy/ctpolicy.go

@@ -111,7 +111,7 @@ func (ctp *CTPolicy) GetSCTs(ctx context.Context, cert core.CertDER, expiration
 	// Identify the set of candidate logs whose temporal interval includes this
 	// cert's expiry. Randomize the order of the logs so that we're not always
 	// trying to submit to the same two.
-	logs := ctp.sctLogs.ForTime(expiration).Permute()
+	logs := ctp.sctLogs.ForTime(expiration).Shuffle()
 	if len(logs) < 2 {
 		return nil, berrors.MissingSCTsError("Insufficient CT logs available (%d)", len(logs))
 	}

ctpolicy/loglist/loglist.go

@@ -226,13 +226,25 @@ func (ll List) ForTime(expiry time.Time) List {
 	return res
 }
 
-// Permute returns a new log list containing the exact same logs, but in a
-// randomly-shuffled order.
-func (ll List) Permute() List {
+// Shuffle returns a new log list containing the exact same logs, but in a
+// nearly-randomly-shuffled order. If possible, it ensures that the first two
+// logs consist of one tiled log and one non-tiled log, to boost the percentage
+// of certs which include an SCT from a static log.
+func (ll List) Shuffle() List {
 	res := slices.Clone(ll)
 	rand.Shuffle(len(res), func(i int, j int) {
 		res[i], res[j] = res[j], res[i]
 	})
+	if len(res) > 2 && res[0].Tiled == res[1].Tiled {
+		// If we have more than two logs, and both are either tiled or not,
+		// try to bring another log to the front of the list.
+		for i := 2; i < len(res); i++ {
+			if res[0].Tiled != res[i].Tiled {
+				res[0], res[i] = res[i], res[0]
+				break
+			}
+		}
+	}
 	return res
 }
 

ctpolicy/loglist/loglist_test.go

@@ -160,7 +160,7 @@ func TestForTime(t *testing.T) {
 	test.AssertDeepEquals(t, actual, expected)
 }
 
-func TestPermute(t *testing.T) {
+func TestShuffle(t *testing.T) {
 	input := List{
 		Log{Name: "Log A1"},
 		Log{Name: "Log A2"},
@@ -176,7 +176,7 @@ func TestPermute(t *testing.T) {
 	}
 
 	for range 100 {
-		actual := input.Permute()
+		actual := input.Shuffle()
 		for index, log := range actual {
 			foundIndices[log.Name][index]++
 		}
@@ -191,6 +191,30 @@ func TestPermute(t *testing.T) {
 	}
 }
 
+func TestShufflePrefersTiled(t *testing.T) {
+	input := List{
+		Log{Name: "Log A1"},
+		Log{Name: "Log A2"},
+		Log{Name: "Log T1", Tiled: true},
+	}
+
+	foundIndices := make(map[string]map[int]int)
+	for _, log := range input {
+		foundIndices[log.Name] = make(map[int]int)
+	}
+
+	for range 100 {
+		actual := input.Shuffle()
+		for index, log := range actual {
+			foundIndices[log.Name][index]++
+		}
+	}
+
+	if foundIndices["Log T1"][2] != 0 {
+		t.Errorf("Tiled log should have always been pulled into first two indices")
+	}
+}
+
 func TestGetByID(t *testing.T) {
 	input := List{
 		Log{Name: "Log A1", Id: "ID A1"},