Stub in more IPC attachment brokering functionality.

ipc/brokerable_attachment.cc

Index: ipc/brokerable_attachment.cc
diff --git a/ipc/brokerable_attachment.cc b/ipc/brokerable_attachment.cc
index be5d51b2069451e25eb1ff987fb48bfd32e9b166..54c905b661e60e69fce1e4ada9988bfd8c24dd55 100644
--- a/ipc/brokerable_attachment.cc
+++ b/ipc/brokerable_attachment.cc
@@ -4,9 +4,21 @@
 
 #include "ipc/brokerable_attachment.h"
 
+#include "crypto/random.h"
+
 namespace IPC {
 
-BrokerableAttachment::BrokerableAttachment() {
+namespace {
+
+BrokerableAttachment::AttachmentId GetRandomId() {
+  BrokerableAttachment::AttachmentId id;
+  crypto::RandBytes(id.nonce, BrokerableAttachment::kNonceSize);

[agl, 2015/06/24 00:59:56]

What are the security requirements around this? You've increased the nonce size
from 4 to 16 bytes. Is a global counter unsuitable?

The use of crypto::RandBytes suggests that there's some security property here
but that's unclear. If not, then base/rand_util.h should suffice.

[erikchen, 2015/06/24 01:06:02]

The nonce used to be 4 uint32_ts, now it's 16 uint8_ts, so there no change in
total size. The security requirement is that the nonce should be unguessable,
and there should be no nonce collisions. Based on the number of expected IPC
messages and the birthday paradox, 16 bytes seemed like the right size.

(This information is also available on my design doc:
https://docs.google.com/document/d/10OiW_YGxQsiTLaP1x3Yp0ymIEEJEBKqFPMVlFDdFZ...
)

[agl, 2015/06/24 01:28:57]

Ok. Please comment:

// In order to prevent mutually untrusted processes from stealing resources from
one another, the nonce must be secret. This generates a 128-bit,
cryptographicaly-strong random number.

[erikchen, 2015/06/24 17:50:25]

Done.

+  return id;
+}
+
+}  // namespace
+
+BrokerableAttachment::BrokerableAttachment() : id_(GetRandomId()) {
 }
 
 BrokerableAttachment::~BrokerableAttachment() {
@@ -16,4 +28,8 @@ BrokerableAttachment::AttachmentId BrokerableAttachment::GetIdentifier() const {
   return id_;
 }
 
+BrokerableAttachment::Type BrokerableAttachment::GetType() const {
+  return TYPE_BROKERABLE_ATTACHMENT;
+}
+
 }  // namespace IPC