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Check that table evenly divides 256 when generating random string.

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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}
This commit is contained in:
jbauch 2016-08-08 17:13:33 -07:00 committed by Commit bot
parent 912f46a348
commit e3eff7c062
3 changed files with 21 additions and 3 deletions
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7
webrtc/base/helpers.cc
View File

@ -207,10 +207,15 @@ std::string CreateRandomString(size_t len) {
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!";

2
webrtc/base/helpers.h
View File

@ -35,6 +35,8 @@ bool CreateRandomString(size_t length, std::string* str);
// 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);

15
webrtc/base/helpers_unittest.cc
View File

@ -55,6 +55,17 @@ TEST_F(RandomTest, TestCreateRandomData) {
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 @@ TEST_F(RandomTest, TestCreateRandomForTest) {
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);