Then the z value is just a double SHA256 of the transaction bytes after performing a little cosmetic surgery on it to add some bytes and substitute in a Ripemd160 hash

https://github.com/stateactor/Bitcoin-Key-Compression-Tool/blob/master/txnUtils.py#L31-L54

i have this error

assert(len(pub) == pubLen*2)                                                                                                                                       
AssertionError             

it's a btc transation not eth