Question: Recently I've been working on making some API calls to a 3rd party. I couldn't find anything available to use, so I had to do it from the ground up. Now I'm wondering whether it would be okay for me to rewrite all the platform specific parts and release that as open source (I'd say that would change ~50% of the code)?

Now the API doc that the other company gave me says "proprietary and confidential", but I assume that would apply only to the doc itself. Also when I asked them for help at one point they told me they don't support the language I am using. I have not signed a NDA specific to this, I have a general one in regards to client's data and info (and these are not clients). I am a bit hesitant to ask someone from my company about this since I have 0 experience when it comes to this sort of stuff, and I don't know whether they'd view it negatively..

**edit**

Thank you all for the advice. I do know that the code I write belongs to my employer, but I thought since most of what we do is platform dependent I'd have to majorly restructure it so it can work independently, and the rest could be considered a common way to do things. As for the proprietary API to which it would become a wrapper, it has wsdl and xsd schemas which can be accessed publicly (albeit you'd have to know where to get them), based on which a person can figure out how to use it. I guess the best thing really would be to speak to someone.. I'll let you know how it goes

Answer: 
While the answers provided thus far are good advice, I think they go way too far. The code you wrote belongs to your employer but they generally do not own the knowledge and skills you gained while implementing it.

Let's pretend you were tasked with implementing a random number generator using a proprietary API that is provided to you by a third party; for simplicity, this API consists of the single signature `public byte[] GetRandomBytes(int length)`. You're required to implement a new class `RandomNumberGenerator`, using the open source algorithm (such as PCG or Xoroshiro) of your choice, that exposes the APIs `public long GetNextInt64()` and `public long GetNextInt64(long lowerBound, long upperBound)`.

Your employer would own the specific code that you wrote but that's pretty much the end of it. Nothing prevents you from later implementing your own version of an RNG that depends on, say, `/dev/random` for the source of random bytes. It might be a wise defensive move to use a different algorithm and function signatures (where it makes sense to do so) but this matters less and less the more one depends on open source material.

There are definitely plenty of cases where one should follow the advice so far of **DON'T DO IT!** but the vast majority of the things that us programmers are asked to do fall into the category of general knowledge.
If you want the code to be usable open source, you need to get the license legally out there. Releasing code with an invalid source license so it's available is setting up legal time bombs for people who use it in good faith. They can be sued for damages, and can get injunctions against using the copyrighted material. You don't want to do this. You need to have the distribution be clearly legal.

To avoid this, you need some sort of release from the owner, allowing the program to be distributed under an Open Source license. This has to be done by someone with adequate authority, and you need to determine who that is. This should be written and signed. A verbal agreement is technically sufficient, but it's hard to prove such verbal agreements, and easy for the person saying it's OK to deny that he or she said such a thing or meant that, and so lawsuits are quite possible.

I'm happy to see someone trying to put software out under an Open Source license, but putting it out under an invalid license is a Bad Idea.