PSA: A break in compatibility when using GCM crypto suites

[Peter Thatcher]

If you didn't enable GCM crypto suites , you can stop reading this message.

GCM crypto suites are off by default, but if you happened to enable, be aware that a breaking change in libsrtp (https://github.com/cisco/libsrtp/pull/259) is going into WebRTC (https://codereview.chromium.org/2723863003/, https://codereview.chromium.org/2743353002/).  As a result, if you have enabled GCM crypto suites, old clients will not be compatible with new clients. 

In case you're wondering where GCM mode is enabled, it's  via PeerConnectionFactoryInterface::SetOptions with options.crypto_options.enable_gcm_crypto_suites = true.  Again, it's off by default, so it's unlikely that you have it enabled and this probably doesn't affect you.

[Chen Cong]

Hi,

How can I enable GCM in the web by using js api?

在 2017年3月15日星期三 UTC+8下午11:47:14，Peter Thatcher写道：

[Cong Chen]

Hi,

How can I enable GCM crypto in the web by using js api?

[jba...@webrtc.org]

There is no API to control this from JavaScript, but you can enable it through "chrome://flags" ("Negotiation with GCM cipher suites for SRTP in WebRTC").

Making this the default is tracked by https://crbug.com/713701

[Jose Antonio Olivera Ortega]

Hi there!

Quick question about enabling GCM crypto suites. I am enabling GCM crypto suites (new clients M61, no old ones involved) for a quick test and I see in the stats this: `googComponent.srtpCipher=AEAD_AES_256_GCM`. The thing that I am only enabling GCM crypto suites *for only one side*. According to https://chromium.googlesource.com/external/webrtc/+/branch-heads/61/webrtc/rtc_base/sslstreamadapter.h#78 _"GCM will only be used if both sides enable it"_. That puzzles me a bit because I was assuming to see something like `googComponent.srtpCipher=AES_CM_128_HMAC_SHA1_80`. Could guys tell me how this is even possible?

Cheers,
jaoo

[Warren McDonald]

I think the negotiation logic was to use GCM if the offer contained and both sides supported it. You can safely ask for GCM and still negotiate and get non GCM with peers that don't support it.

[Jose Antonio Olivera Ortega]

Thanks! I figured it out that what I was seeing was because a SFU was involved in my test.