In most cases Compositors in production request RGBA mattes for CG elements from the CG Artists. Depending on the complexity of the scene this could become quite the task as your limited to 4 channels per image. you can spend hours if not days of artists time creating mattes for comp. Cryptomattes eliminates all of that by providing one pass that includes a matte for everything that is rendered. in other words it’s magic… my jaw litterally dropped the first time I used it. Having worked with Andy and Jonah at Psyop and having used this on “Hail, Ceaser!” (Cohen brothers latest flick) and other projects i can speak for it empirically. Kinda the falls into the category of the best things that ever happened to CG along with blender 
Isolating out meshes in comp, with DoF / Motion blur applied… Pretty much a fullsample matte in which you can select different objects.
If this could be added in Cycles and the compositor, it would be a great step for CG rendering with Blender! 
Very interesting feature to implement into Cycles’ Compositor. Thank you Lukas for the Proof of concept patch.
So has there been any updates or cleaner implementation of Cryptomatte in Cycles?
This is a must for Blender/ Cycles. I am primarily a Maxwell Render user, but really want to use Cycles more. But Maxwell Render has a very simple and easy way to generate masks for compositing. I can generate a mask by object or by material. The object can be made up of mixed materials as well. It is so handy. Cryptomatte sounds like a great easy way to get all the masks you need for compositing. So, can this be used in Blender now? Someone on another thread pointed this out to me and suggested I compile blender with this patch (https://github.com/Psyop/Cryptomatte). Or is it code from this link? https://developer.blender.org/D2106 How do I compile this into blender?
Thanks
I’m looking forward for full integration of Cryptomatte, right now the ID system for materials and objects is rather useless.
So much enthusiasm and interest from our dear developers in July 2016, and its almost June 2017 - nothing happened. Where has all this gone? Isn’t it going to happen to Cycles to have production quality mattes?
As much as I’d like to have Cryptomatte in Cycles as soon as possible I think most developers are busy getting 2.79 ready and awesome. 
Really Impressive guys, thank you for sharing it, would be one of the best addon for Blender.
@Alex.Petrov this is explained on first page - Cycles has no multisample AA support, and it is needed for Cryptomatte. Multisample AA was a candidate for this year’s GSOC but did not get picked.
removed because of irrelevance
If you want to edit what you render in post production you need an easy way of distinguishing different things in your render, so you can adjust separate elements you just rendered. Masks are used for that. If you render in passes for example, you can adjust A LOT of things in post production even things like textures. You can carry on developing the look of your image without the need to wait for it to render or you can fix mistakes or change something your client thought of the last minute so those masks are quite a cool thing. This would make the process of generating those masks a lot more precise, easier to work with and automatic. This would be amazing as it would save thousands of hours of work for a lot of people like myself who work with it and would make the process easy and enjoyable. The world would instantly become a better place and people would worship the developers who made it happen as if they were gods! I think.
Spot on Martin…Blender needs this ASAP.
Sadly it does not seem like this is moving forward really fast. I am getting used to going through the trouble of assigning emission shaders to all objects in order to render masks.
Thanks to lukas for making this happen for blender!
I’m busy writing code instead of forum posts, so I apologise for the silence. Work on it is progressing, it’s about to get put under some rigorous testing. I hope I can make the time to release an up-to-date patch for 2.79 soon.
Here my belated “thank you” to all who explained what this is about…
Didn’t even wait a year? 
I only realized when it was too late.