Check that table evenly divides 256 when generating random string.
This prevents biased modulo arithmetic when selecting a character for a
random value from the provided table.
BUG=webrtc:5870
Review-Url: https://codereview.webrtc.org/2115793003
Cr-Commit-Position: refs/heads/master@{#13683}
diff --git a/webrtc/base/helpers.cc b/webrtc/base/helpers.cc
index 9674eb9..a8389d4 100644
--- a/webrtc/base/helpers.cc
+++ b/webrtc/base/helpers.cc
@@ -207,10 +207,15 @@
return str;
}
-bool CreateRandomString(size_t len,
+static bool CreateRandomString(size_t len,
const char* table, int table_size,
std::string* str) {
str->clear();
+ // Avoid biased modulo division below.
+ if (256 % table_size) {
+ LOG(LS_ERROR) << "Table size must divide 256 evenly!";
+ return false;
+ }
std::unique_ptr<uint8_t[]> bytes(new uint8_t[len]);
if (!Rng().Generate(bytes.get(), len)) {
LOG(LS_ERROR) << "Failed to generate random string!";
diff --git a/webrtc/base/helpers.h b/webrtc/base/helpers.h
index 2e72dfb..fcf77af 100644
--- a/webrtc/base/helpers.h
+++ b/webrtc/base/helpers.h
@@ -35,6 +35,8 @@
// Generates a (cryptographically) random string of the given length,
// with characters from the given table. Return false if the random
// number generator failed.
+// For ease of implementation, the function requires that the table
+// size evenly divide 256; otherwise, it returns false.
bool CreateRandomString(size_t length, const std::string& table,
std::string* str);
diff --git a/webrtc/base/helpers_unittest.cc b/webrtc/base/helpers_unittest.cc
index e4903f5..394e2b0 100644
--- a/webrtc/base/helpers_unittest.cc
+++ b/webrtc/base/helpers_unittest.cc
@@ -55,6 +55,17 @@
EXPECT_NE(0, memcmp(random1.data(), random2.data(), kRandomDataLength));
}
+TEST_F(RandomTest, TestCreateRandomStringEvenlyDivideTable) {
+ static std::string kUnbiasedTable("01234567");
+ std::string random;
+ EXPECT_TRUE(CreateRandomString(256, kUnbiasedTable, &random));
+ EXPECT_EQ(256U, random.size());
+
+ static std::string kBiasedTable("0123456789");
+ EXPECT_FALSE(CreateRandomString(256, kBiasedTable, &random));
+ EXPECT_EQ(0U, random.size());
+}
+
TEST_F(RandomTest, TestCreateRandomUuid) {
std::string random = CreateRandomUuid();
EXPECT_EQ(36U, random.size());
@@ -90,8 +101,8 @@
std::string str;
EXPECT_TRUE(CreateRandomString(16, "a", &str));
EXPECT_EQ("aaaaaaaaaaaaaaaa", str);
- EXPECT_TRUE(CreateRandomString(16, "abc", &str));
- EXPECT_EQ("acbccaaaabbaacbb", str);
+ EXPECT_TRUE(CreateRandomString(16, "abcd", &str));
+ EXPECT_EQ("dbaaabdaccbcabbd", str);
// Turn off test mode for other tests.
SetRandomTestMode(false);