I'm considering using a key-derivation scheme for generating multiple keys from the same user secret. It is important that given all but one of these derived keys, it is not feasible to calculate the last one. Tunable difficulty would be a big plus.
Is there such a function, or set of functions, that are safe to use in parallel on the same secret?
This is not a duplicate of this question because I do not have any way to store a client-side salt between different devices. Using a constant would work, since that can be stored in the source code, e.g.
sha256(user_secret + "first_static_salt"), but I have no idea if such a scheme would be secure. I also don't have the same parameters and same key derivation function requirements of that question. Using three different functions would work fine for me.
EDIT: I only have the user password for entropy. Any user-specific salt used would have to be stored on a server, which means that it would be known for all parties involved (unless you encrypt it with the user password...).
The goal here is to hide sensitive data from the servers; they only need metadata to function properly, so there's no reason to send sensitive data through them. The current setup would use two different keys, one for authenticating with the servers, and one for encrypting/decrypting the actual data. Most servers would just push around uuid's, and these uuid's would be used to access a separate system where the actual sensitive encrypted data is stored. The client would then interact with these two separate systems.