(Disclaimer: My POV can be slightly biased by the protection I tried to crack, I sometime speak about stuff I don't know, I presume it work this way because I know similar DRM tools working like that)
Conor is right: Speaking about DRM in general is far too broad: You speak about a server in your question but one of the most used copyright content doesn't even require a server (DVD).
DRM tool purpose:
There is a lot of different goals for DRM (can be geographical restriction, user restriction, and many others) but most of the time it's to avoid private copy of a proprietary work.
How DRM tools work:
The principle is almost always the same:
You want to have the access X.
1) Get H[X] a cyphered copy of X. (By Download or a physical copy)
2) Get the key to convert H[X] to X. (Activating a licence, sometime automatic)
3) decypher H[X] to get X.
There is a ton of exceptions about how it works, sometime it's fully encrypted, sometime partially. Sometime it's a physical key, sometime virtual one...
You can also find a ton of different module to make each copy unique, unreadable using someone else key, harder to RE, temporary available etc..
Is DRM security through obscurity?
Why not 100%? Because (once again most of the time) you require someone that buy the content first to analyze the DRM and provide a workaround. In a certain way, DRM is fully secure until someone get an access to a copy.
Thus said, every DRM are breakable.
Understand and cracking DRM
Because it's unpossible to speak about every DRM, we can't say attack is purely a binary decryption of the CDM. Let's speak about how DRM are mostly attacked using some simple example:
Music and Video content (Deezer):
When downloading a song on Deezer, you will get unreadables files, and Deezer app has the key to open them. Keep it simple: Softwares like Audacity are able to grab the audio output. This vulnerability, working on every non-interactive content is called Analog hole
TV content is a radio wave. If the content is protected, it will be cyphered. You need to buy a decoder, and you have access to TV. To crack this, you need to reverse the decoder at hardware level: understand how the logical circuit work and reproduce it. (There is also Analog Hole here, but It's less relevant as it's harder to keep it live)
Here it's interesting, There is a lot of possibility. Firstly, you will have files encrypted specificaly for you and using the activation key (most of the time it's automatic) will connect your computer to the server, log with the key and it will give you a licence that will do verification and decypher your file. Here you got almost no choice to crack it: you will have to RE the binary. What to do then?
Well, it depends. Because files are uncyphered on client side during software usage, it's always* possible to provide them to anyone (create a crack). Another possibility is to create a software that will reproduce the licence behavior**.
*Generally really hard, but not impossible.
**It's not always possible, there is a lot of other protection.
How to protect content?
There is 2 way to avoid cracks
- Using the best security possible
As I said, It's hard but not impossible to crack a software, if you reach to produce an insanely efficient DRM tool, it can stand for months, years or decade.You need to evaluate how long your content will stay safe, and in how long it will be ok if your content is cracked. Because most of the content are not sold really often in their old day, a cracked version is not a problem at this time.
How to make a good protection?
(just my opinion) Hardware is really hard to understand, with physical protection and a lot of different implementation possibility, hardware protected content are probably among the hardest to revert.
Yes. It sounds weird but it "work". You are unable to protect your content at 100%? Very well, protect it and give a free modify copy of it! People won't bother creating a crack and people who didn't buy the content got a less pleasant version of it. Some games did this, honorable mention to Alan Wake that didn't affect the gameplay. It has some obvious disadvantage, but it's not that bad:
- You can save mony on the DRM
- People that use crack, probably wouldn't have buy the content even if no crack exist. (You don't lose much money)
TL;DR DRM is about obfuscation; wich is not 100% secure. Therefore, DRM implementation details are not revealed, even if there is models schema that are often used.