The way we solve server-side iterations with Standard Notes (which uses Argon2 and not PBKDF2) is to tie the derivation parameters (iterations, bytes, etc) to a hard-coded protocol version number. Accounts which register today for example have a protocol version of 004, which corresponds to specific, immutable derivation parameters.

For a given user, the client then receives from the server not key derivation parameters, but the version of the account. The client then maps that version to the precompiled derivation parameters.

Of course a server can then misreport a user's account version to something lower than it actually is. There are two solutions we implement here:

1. Deprecate older versions as quickly as possible after new protocol version rollouts. Older versions begin to get rejected by clients and clients will not allow sign in to proceed.

2. Allow an optional sign-in flag users can check called "Strict sign in" that forces the client to reject any server provided version that is not specifically the latest version. This means that if a user checks this option and the server reports a version != 004, the sign in will be rejected and the client will not perform any sort of handshake with the server.

More here: https://standardnotes.com/help/security/encryption