A file-backed store of user accounts for a served application.

Passwords are kept as PBKDF2-HMAC-SHA256 hashes with a random salt per account; the file never holds a password. Usernames are unique without regard to case and are returned in the spelling they were registered with. Each account carries a props map the application owns.

{:ok, accounts} = Drafter.Accounts.start_link(path: "/var/lib/myapp/accounts.terms")
:ok = Drafter.Accounts.register(accounts, "alice", "correct horse")
{:ok, %{username: "alice", props: %{}}} = Drafter.Accounts.authenticate(accounts, "alice", "correct horse")

authenticate/4 costs the same for an unknown name as for a wrong password, and refuses a peer outright, without hashing, once it has failed five times in fifteen minutes.

Rules

• a username is 1 to 32 characters from A-Z, a-z, 0-9, _ and -
• a password is at least 8 characters
• every change is written to the file before the call returns, by writing a sibling file and renaming it over the old one
• the file is text: one Erlang term per account, in registration order, that :file.consult/1 reads — %{username: "alice", number: 0, props: %{…}, hash: <<…>>, salt: <<…>>, iterations: 300000} — so it can be read and searched as it is. It is UTF-8: a binary that is printable text is written as one, any other as its bytes. A file written as :erlang.term_to_binary/1 by an earlier version is read and written back as text, as is one an earlier version wrote as Latin-1