I think Theymos can answer my question, how to get a private key from a custom built Public Key ?
What is a custom public key, anyway? The public key is essentially a concatenation of an index that describes what we are looking at (either compressed or uncompressed public key) and, depending on this index, a concatenation of X-coordinate and Y-coordinate. There is nothing particularly interesting or "custom" about these coordinates because these are only numbers expressed in a certain format. Humans can't read these numbers easily, let alone they can comprehend how big they are. You are probably talking about public addresses, not public keys, but a public address is a derivative, it is an unpredictable output of using a public key as an input in two one-way hash functions, namely sha256 and RIPED160. You can't know in advance what a public address will be, you must first generate a random number (aka a private key), then calculate the coordinates using a previously generated private key, and only then can you start calculating your desired public address. Given that hash functions are one-way (you can't figure out input from output unless by brute-forcing all possible input values, which is practically impossible), you can't calculate the corresponding public key from your address, and, therefore, you also can't start to brute force corresponding private key.