Python steemitimages.com Image Uploader (with secp256k1 problems)

Why I'm doing it?

I'm still new on steemit, and for my introduction post I had some hassles. Like I prepared my post offline because I don't like doing stuff in the browser only to find myself at the end loosing my work of hours. I wrote all in xhtml with css formated only to figure out that in the end I had to format everything once more. So I thought I would make me some script to make this process how I want it.

What I did.

I installed python-steem(Documentation), but I couldn't find how to upload the images. After some searching I figured out that there is now way with this library. But anyway I like python the last years and have still some plans with python.
But I found imagehoster which looks like the base of steemitimages. So I played around and got it managed to upload an image with the help sign.js and curl.

By the way shot by my wife Hive account@malamia :) Thank you for that!

What's wrong with it?

But the differences of the secp256k1 implementations between node.js and python driving me nuts. Has anyone an idea how to get the signature in DER format converted to lower S (whatever that means. Only thing what I understood is that it is bitcoin specific and little endian instead of big endian, and shorter. I guess it's the same for the steem blockchain!?)

More details.

And it took me now the whole day to read the following:

• secp256k1 node.js implementation
• dsteem node.js implementation
• dsteem node.js docs
• secp256k1 python implementation
• steem python implementation
• steem python docs
• and some others I discarded like two python bitcoin libraries

Help very welcome

Maybe I have overlooked something in the python steem implementation :/
So if someone have an idea, I would be very happy!

Some code

#!/usr/bin/python3                                                                                                                                                                              
from hashlib import sha256                                                                                                                                                                      
import sys                                                                                                                                                                                      
import secp256k1                                                                                                                                                                                
#first import of steembase.account fails. (at least on my machine)
try:                                                                                                                                                                                            
    import steembase.account                                                                                                                                                                    
except:                                                                                                                                                                                         
    import steembase.account                                                                                                                                                                    
from steem.utils import compat_bytes                                                                                                                                                            
                                                                                                                                                                                                
class SteemitImages:                                                                                                                                                                            
    def __init__(self):                                                                                                                                                                         
        pass                                                                                                                                                                                    
                                                                                                                                                                                                
    def imageSign(_data, _key):
        h = sha256()
        h.update(b"ImageSigningChallenge")
        h.update(_data)
# hash.digest() is the same as in sign.js
        key = steembase.account.PrivateKey(_key)
# key is also the same as in sign.js
        sk = secp256k1.PrivateKey(compat_bytes(key), raw=True)
        sig = sk.ecdsa_sign(h.digest(), raw=True)
        out = sk.ecdsa_signature_normalize(sig)
        return out.hex()

    def getDataFromFile(_file):
        with open(_file, mode='rb') as file:
            return file.read()

if __name__ == "__main__":
    print(SteemitImages.imageSign(SteemitImages.getDataFromFile(sys.argv[2]),sys.argv[1]))