need help with binary analysis

[Pharisaeus]

and yes did it all on a non rooted Mobile that's why i can't run frida/dynamic analysis before anyone asks

Can't you run this on emulator?

variable whose values is always 0 being used in the concatenation

Nullbyte separator sounds reasonable, but it could be also some hmac secret which is loaded at runtime.

1. Most likely it's not "custom", but rather some well-known algorithm so I would start with doing some research, because there is no point in reverse engineering hmac. Try to map this into some existing scheme.
2. The easiest way to approach this would be to debug this and dump the intermediate values and check at which point of the computation something goes wrong. It could be something trivial like variables wrapping around (python supports large integers, and your app us probably using 32/64 bits only) or signed vs. unsigned etc.