organizational enviroment*

Added some additional requriments

@Delfin: extended the answer. In short: if it should not be stored locally it must be stored remotely and then retrieved in a secure way.

Not I mean, that it shall not depend in any specific hardware or the data stored in any specific device. Like stored in a Password Manager, but without needing to add extensions to the browser

@Delfin: a) you need to have some encryption key b) it needs to come from somewhere. Basically you have in impossible restriction for b) in that it should not be locally stored, not remotely stored, not derived from something stored locally, remotely, on a device, in the brain of the user .... This is impossible.

It can be derived from something else. Preferably for an interactive challenge process, where the server puts challenges to the client to be filled with help of the secrets, and the server may also to solve some challenges and in the process a secret can be derived that can be used by the user to encrypt their data

"such that the login secret can be stored in a password managed and entered in a webpage without the browser implementing any authentication extensions or standards" probably should work

@Delfin: And how do you think these challenges should work without the user either remembering something (which is needed to solve the challenge) or the user having some device which does this? Also, the server should never get access to the secret or something which can be used to get the secret, so how should the server ask useful challenges to derive the right secret?

The User can have to remember something.

What is the difference to a "password" then, apart from not calling it this way?

@SteffenUllrich Structure?

@Delfin ok, in this case: you can have an algorithm with parameters. The algorithms is fixed, the parameters need to be remembered. The structure is different from a password but it does not help much in my opinion, i.e. the problem is the same as with a password in that it needs to be easy to remember but hard to guess. Or it needs to be some "hardware" which contains part of the data, like a book. But you don't want such hardware if I remember correctly.

@SteffenUllrich You can use some software based password manager.

@Delfin This is what I consider "stored on a specific device" - which you explicitly excluded. Maybe we understand different things behind this term.

@SteffenUllrich You can always write it down in some paper.

@Delfin And how exactly is this different from writing down a password? I don't see any difference in security between writing down a password and writing down some "other structured" secret.

@SteffenUllrich Probably they would use the password manager rather than using paper, because of interactive procedure requires computer assistance, thus reducing error of mishandling of passwords.

Also some pishing attack to make you login in a fake login page would no get your secret, but just a one time secret.

(Probably)

@Delfin storing a password in a password manager or storing some "differently structured secret" in the password manager is also the same from the perspective of security. So why should be one of these acceptable but the other not? Also, how is this password manager then protected?

@SteffenUllrich Sure, but was always the complains about passwords about users not being responsible with the passwords? Thus why does not comply with the kill passwords agenda?

@Delfin They complain about insecure use of password since passwords are insecurely used. If you replace the password with "differently structured secret" but don't change the fundamental problem of insecure use, then they will complain about this new thing. The current movement for killing password is more focused on providing something which is both more secure and easier to use, i.e. Webauthn, biometry, ....

But, this is only about authentication, not about encryption. Your question is about client-side encryption though. Encryption needs some key to encrypt and thus something to derive the key from. Authentication just needs prove of identity, which can be simpler.

@SteffenUllrich I guess the answer is nothing