client-side encrypted server side storage for private key

[brenzi]

similar to protonmail PGP key storage, we could sign TOPs with a client-side in-memory private key without the UX worries of key custody.

onboarding:

1. user picks a username and a strong passphrase
2. enclave ensures the username doesn't exist yet in the db
3. client generates a private key and caches it for the session (localstorage)
4. client encrypts private key with passphrase
5. client sends encrypted blob to enclave together with username
6. enclave encrypts both username and encrypted key again and stores them in a db (external to enclave)

later revisit (i.e.from different device):

1. user enters username
2. enclave looks up the encrypted username in db and returns encrypted private key
3. user enters passphrase
4. client decrypts private key and caches it in localstorage

notes:

• enclave/server never sees the passphrase
• enclave/server never sees the private key and could never sign messages

caveats:

• an attacker can guess usernames, retrieve the encrypted private key and brute-force passphrases