How to get the application's encryption algorithm?

Question

Android

Hello.
Still working on automating actions in the application.
When registering, the following information is sent to the server:
{
"reg": {
"login": "...",
"password": "...",
"name": "Mark",
"gender": 1,
"birthday ": "1998-07-11"
},
"tp0": 468,
"tp1": 69,
"tp2": 412,
"client": {
"platform": "Android",
"userAgent": "... ",
"browserAgent": "...",
"osVersion": "6.0.1",
"uid": "...",
"udid": "...",
"appLocale": "ru",
"appPackage": ".."
},
"userId": "e5ab80aa903ec892cd667c96fa79191f"
}'
At the entrance it's almost the same, only login and password.
So, I need to get the userId encryption algorithm from the application.
As I understand it, this is only glued from the login and password, tk. all other data can be changed as you like. (I could be wrong)
How do I get it?
I tried to decompile the code, but there is almost every UserId file..