I tried this with the query string
query =
"key=...&signature=...&nonce=1394469877081"

I replaced here the key and signature text by dots

I got as return string
json =
"{"error": "API key not found"}"

I verified that the API key in the query string is correct (32 characters).
So what is wrong?

edit: I solved this - the API key activation was missing, the code is ok.