Hello @ysncvk,
There are several discussions about creating a delegate identity such as this which would allow you to create an identity for them after they authenticate. Once they are authenticated, they can perform the transactions themselves.
To perform transactions for the user, you would need their private key which is very insecure.
On another note, Instead of using email and password, have you thought of using other easy-to-use authentication such as Internet Identity?
You should check out Oisy Wallet and the Oisy Wallet source code. It’s a good example of a web-based cross-chain wallet built using ICP.
The NNS wallet is also a good example of a web-based wallet for the ICP ecosystem.
Does this help?