sync crypto helpers: Generate smaller unique strings using the same

amount of randomness.  This will reduce the size of bookmark IDs by
20 bytes per object: meaning potential savings of 40-60 bytes per bookmark
SyncEntity.

Old and new server IDs (for example):
0DXKZkVqHIRcWYy00MzUzMjc1MDMzNjU1MDIzOTkyeDgxNjQ4NzEyNjczODUyMTk4NzB4fC02NTUzOQ
0DXKZkVqHIRcWYzBaWVRHN29zdzV4OFpTR1RWSE1STEE9PXwtMg

BUG=
TEST=

Review URL: http://codereview.chromium.org/3614015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@62162 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 24 insertions, 6 deletions

diff --git a/chrome/browser/sync/util/crypto_helpers.cc b/chrome/browser/sync/util/crypto_helpers.cc
index 9315d8b..36c5586 100644
--- a/chrome/browser/sync/util/crypto_helpers.cc
+++ b/chrome/browser/sync/util/crypto_helpers.cc
@@ -10,6 +10,7 @@
 #include "base/basictypes.h"
 #include "base/format_macros.h"
 #include "base/logging.h"
+#include "base/base64.h"
 #include "base/rand_util.h"
 #include "base/string_number_conversions.h"
 #include "base/string_util.h"
@@ -50,14 +51,21 @@ std::string MD5Calculator::GetHexDigest() {
 }
 
 void GetRandomBytes(char* output, int output_length) {
-  for (int i = 0; i < output_length; i++) {
-    // TODO(chron): replace this with something less stupid.
-    output[i] = static_cast<char>(base::RandUint64());
+  uint64 random_int;
+  const char* random_int_bytes = reinterpret_cast<const char*>(&random_int);
+  int random_int_size = sizeof(random_int);
+  for (int i = 0; i < output_length; i += random_int_size) {
+    random_int = base::RandUint64();
+    int copy_count = std::min(output_length - i, random_int_size);
+    memcpy(output + i, random_int_bytes, copy_count);
   }
 }
 
 string Generate128BitRandomHexString() {
-  int64 chunk1 = static_cast<int64>(base::RandUint64());
-  int64 chunk2 = static_cast<int64>(base::RandUint64());
-  return StringPrintf("%016" PRId64 "x%016" PRId64 "x", chunk1, chunk2);
+  const int kNumberBytes = 128 / 8;
+  std::string random_bytes(kNumberBytes, ' ');
+  GetRandomBytes(&random_bytes[0], kNumberBytes);
+  std::string base64_encoded_bytes;
+  base::Base64Encode(random_bytes, &base64_encoded_bytes);
+  return base64_encoded_bytes;
 }
diff --git a/chrome/browser/sync/util/crypto_helpers_unittest.cc b/chrome/browser/sync/util/crypto_helpers_unittest.cc
index 0277ae6..7be1270 100644
--- a/chrome/browser/sync/util/crypto_helpers_unittest.cc
+++ b/chrome/browser/sync/util/crypto_helpers_unittest.cc
@@ -15,3 +15,13 @@ TEST(ChecksumTest, MD5ChecksumTest) {
   std::string checksum("e2c865db4162bed963bfaa9ef6ac18f0");
   ASSERT_EQ(checksum, md5.GetHexDigest());
 }
+
+TEST(CryptoHelpers, GetRandomBytes) {
+  for (int i = 1; i < 25; ++i) {
+    std::string random_bytes(i+1, ' ');
+    do {
+      GetRandomBytes(&random_bytes[0], i);
+      ASSERT_EQ(random_bytes[i], ' ');
+    } while (random_bytes[i - 1] == ' ');
+  }
+}