Introducing a net::GenerateMimeMultipartBoundary helper.

net/base/mime_util.cc

Index: net/base/mime_util.cc
diff --git a/net/base/mime_util.cc b/net/base/mime_util.cc
index 0d511201fd0cdfdda126ad480bc0157e5385e33f..97c7e001c53ad7d9fd8e498e6dbc3893bdb34c3b 100644
--- a/net/base/mime_util.cc
+++ b/net/base/mime_util.cc
@@ -7,9 +7,11 @@
 #include <map>
 #include <string>
 
+#include "base/base64.h"
 #include "base/containers/hash_tables.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
+#include "base/rand_util.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
@@ -561,6 +563,44 @@ void GetExtensionsForMimeType(
   HashSetToVector(&unique_extensions, extensions);
 }
 
+NET_EXPORT std::string GenerateMimeMultipartBoundary() {
+  // Based on RFC 1341, section "7.2.1 Multipart: The common syntax":
+  //   Because encapsulation boundaries must not appear in the body parts being
+  //   encapsulated, a user agent must exercise care to choose a unique
+  //   boundary. The boundary in the example above could have been the result of
+  //   an algorithm designed to produce boundaries with a very low probability
+  //   of already existing in the data to be encapsulated without having to
+  //   prescan the data.
+  //   [...]
+  //   the boundary parameter [...] consists of 1 to 70 characters from a set of
+  //   characters known to be very robust through email gateways, and NOT ending
+  //   with white space.
+  //   [...]
+  //   boundary := 0*69<bchars> bcharsnospace
+  //   bchars := bcharsnospace / " "
+  //   bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" /
+  //            "_" / "," / "-" / "." / "/" / ":" / "=" / "?"
+
+  char random_values[30];
+  base::RandBytes(random_values, sizeof(random_values));
+
+  // Allowed base64 characters are a subset of bcharnospace above:
+  //   base64char := DIGIT / ALPHA / "+" / "/" / "="
+  //   ("=" is used for padding which won't apply here).
+  std::string random_base64;
+  base::Base64Encode(base::StringPiece(random_values, sizeof(random_values)),
+                     &random_base64);
+
+  std::string result("----MultipartBoundary--");
+  result.append(random_base64);
+  result.append("----");
+
+  // Not a strict requirement - documentation only.
+  DCHECK_EQ(67u, result.size());
+
+  return result;
+}
+
 void AddMultipartValueForUpload(const std::string& value_name,
                                 const std::string& value,
                                 const std::string& mime_boundary,