Add meta server description #25

We fully removed this from the app

I'm not sure what's the point of mentioning OM's implementation at all, as ours is a complete re-implementation.

I've reworded this. I think it's good and important to highlight ways that we're respecting privacy and security, to be transparent and also reduce fears anyone may have after digging around upstream.

The meta server would need to be extended to list compatible map versions for each app version, and provide secure verification info, to improve on this.

I'd drop this sentence, IMO its debatable.
IMO this repo's issues section is more suitable for discussing future plans, rather than stating them in the README.md

Isnt this incorrect?
Also, these servers can change over time, so they shouldn't be hardcoded into the app.
Since if this list gets another server added to it, will it even work on the app?
because its not in the private.h:

sry if i am getting something wrong

DEFAULT_URLS_JSON is used only when the metaserver is unreachable.

the full server list in private.h is a fallback which is used only if the app can't get the latest and georegion-tailored list from the metaserver

e.g. we might add new CDNs for South America region and clients will be able to download from them without waiting for the next app update (which will update the list in private.h)

oh nice, understood it wrong way around, that these where the only valid/safe cdn's, so if the metaserver would be compromised the app would not start requests to domains that are unsafe

actually you make a good point that metaserver could be potentially compromised (and CDN nodes too, of course)

we need to think of potential consequences and what we can do about it

@pastk wrote in #25 (comment):

actually you make a good point that metaserver could be potentially compromised (and CDN nodes too, of course)

we need to think of potential consequences and what we can do about it

Isn't app rejects MWMs that do not match the hash from countries.txt?

It does reject them.

But it'd try to download them first (hit the urls). So theoretically someone could replace CDN urls to make clients to hit them. DDOS attack by CoMaps users? :)))

Ultimately any online component to the app is a service, and the service needs to be trustworthy. A dns-based DoS due to HTTP GET requests for specific files that won't exist on most targets is a pretty small edge case I think.

A bigger issue is malicious MWMs crafted to buffer overflow a device, but it's correct that the checksum in countries.txt prevents this. So if we move to fully online updates, maybe what we do is embed a signing public key in the app instead, and then sign map updates? Or just sign a new countries.txt for the app to download, and then the files themselves probably don't need further checking.

Isn't app rejects MWMs that do not match the hash from countries.txt?

We use sha1 for hashing which was broken and is not considered cryptographically secure any more. But the computational effort for forging a file with the same hash is still immense.

"Upstream" sounds like this is a fork.

As @pastk said, it doesn't make sense to mention this at all.

I guess my question is, the metaserver concept exists due to OM, and it serves various purposes there, but these purposes are intentionally limited and privacy-preserving, and I don't know anywhere better to talk about that. I would be concerned, knowing the history, if I was a user

Then at least make it explicit Unlike Organic Maps meta-server
Because some people reading this won't understand what "upstream" is. Could be some 3party lib for example.