Add build flag for self-signed certs

This patch changes the standalone receiver and sender implementations to
use a new CAST_ALLOW_DEVELOPER_CERTIFICATE build flag/preprocessor
macro for gating usage of self-signed certificates. When false, the
cast_receiver app is disabled, and the cast_sender app will only connect
to receivers using a proper Google cert.

When the build flag is enabled, the cast sender can use any self-signed
certificate as long as the CA bit is set. The
cast receiver can either take a self-signed certificate and its private
key, or generate
both the private key and the certificate. The resulting private key and
certificate are written out to files, currently:
./generated_root_cast_receiver.(key|crt)

and can then be used with a
cast_sender to start a session.

Bug: b/169796278
Change-Id: I03675f85ac0b2bda76daf1bf11d9d7df064d0b8f
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/2443934
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Reviewed-by: Brandon Tolsch <btolsch@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Jordan Bayles <jophba@chromium.org>

1 files changed, 30 insertions, 13 deletions

diff --git a/cast/README.md b/cast/README.md
index a501703b..32de8c76 100644
--- a/cast/README.md
+++ b/cast/README.md
@@ -7,15 +7,38 @@ applications and streaming to Cast-compatible devices.
 
 To run the standalone sender and receivers together, first you need to install
 the following dependencies: FFMPEG, LibVPX, LibOpus, LibSDL2, as well as their
-headers (frequently in a seperate -dev package). From here, you need to generate
-a RSA private key and create a self signed certificate with that key.
+headers (frequently in a seperate -dev package). From here, you just need a
+video to use with the cast_sender, as the cast_receiver can generate a
+self-signed certificate and private key for each session. You can also generate
+your own RSA private key and either create or have the receiver automatically
+create a self signed certificate with that key. If the receiver generates a root
+certificate, it will print out the location of that certificate to stdout.
 
-From there, after building Open Screen the `cast_sender` and `cast_receiver`
-executables should be ready to use:
+Note that we assume that the private key is a PEM-encoded RSA private key,
+and the certificate is X509 PEM-encoded. The certificate must also have
+the CA bit set in the basic constraints. The easiest way to test with a key
+and certificate is to use ones generated by the cast_receiver standalone
+application.
+
+## Developer certificate generation and use
+
+The easiest way to generate a private key and certificate is to just run
+the cast_receiver with `-g`, and both should be written out to files:
+
+```
+  $ /path/to/out/Default/cast_receiver -g
+    [INFO:../../cast/receiver/channel/static_credentials.cc(161):T0] Generated new private key for session: ./generated_root_cast_receiver.key
+    [INFO:../../cast/receiver/channel/static_credentials.cc(169):T0] Generated new root certificate for session: ./generated_root_cast_receiver.crt
+```
+
+These generated credentials can be passed in to start a session, e.g.
 ```
-  $ /path/to/out/Default/cast_sender -s <certificate> <path/to/video>
-  ...
-  $ /path/to/out/Default/cast_receiver <interface> -p <private_key> -s <certificate>
+./out/Default/cast_receiver -d generated_root_cast_receiver.crt -p generated_root_cast_receiver.key lo0 -x
+```
+
+And then passed to the cast sender to connect and start a streaming session:
+```
+  $ ./out/Default/cast_sender -d generated_root_cast_receiver.crt ~/video-1080-mp4.mp4
 ```
 
 When running on Mac OS X, also pass the `-x` flag to the cast receiver to
@@ -25,9 +48,3 @@ Bonjour.
 When connecting to a receiver that's not running on the loopback interface
 (typically `lo` or `lo0`), pass the `-r <receiver IP endpoint>` flag to the
 `cast_sender` binary.
-
-An archive containing test running scripts, a video, and a generated RSA
-key and certificate is available from google storage. Note that it may require
-modification to work on your specific work environment:
-
-https://storage.googleapis.com/openscreen_standalone/cast_streaming_demo.tar.gz