writeonce/docs/examples/writeonce-serve/http/auth.wo

-- http/auth.wo — the auth MECHANISM, framework core: parse the
-- Authorization header, split scheme from credentials, decode Basic's
-- base64, compare secrets in constant time, and attach the authenticated
-- principal to the request (req.principal) so downstream handlers see it.
-- POLICY stays in the app: which routes, which users, where secrets live.

-- ---- constant-time comparison -------------------------------------------
-- No early exit on the first differing byte: the accumulator visits every
-- byte, so a wrong secret costs the same time wherever it differs. Unequal
-- lengths answer false up front — length is not the secret.

pub fn ct_eq(a: Text, b: Text) -> Bool {
  if len(a) != len(b) { return false; }
  let diff = 0;
  let i = 0;
  while i < len(a) {
    let d = byte_at(a, i) - byte_at(b, i);
    diff = diff + d * d;
    i = i + 1;
  }
  return diff == 0;
}

-- ---- the Authorization header, split ------------------------------------

pub typedef AuthHeader = { scheme: Text, credentials: Text }

-- nil: no Authorization header, or no space between scheme and credentials.
-- The scheme comes back lowercased (schemes are case-insensitive, RFC 9110).
pub fn auth_header(req: Req) -> ?AuthHeader {
  let raw = req.headers["authorization"];
  if raw == nil { return nil; }
  let sp = index_of(raw, " ");
  if sp < 1 { return nil; }
  let scheme = to_lower(substr(raw, 0, sp));
  let creds = trim(substr(raw, sp + 1, len(raw) - sp - 1));
  if creds == "" { return nil; }
  return AuthHeader { scheme: scheme, credentials: creds };
}

-- nil unless the request carries `Authorization: Bearer <token>`.
pub fn bearer_token(req: Req) -> ?Text {
  let h = auth_header(req);
  if h == nil { return nil; }
  if h.scheme != "bearer" { return nil; }
  return h.credentials;
}

-- ---- base64 (RFC 4648, the Basic scheme's encoding) ----------------------

fn b64_val(c: Int) -> Int {
  if c >= 65 { if c <= 90 { return c - 65; } }        -- A-Z ->  0..25
  if c >= 97 { if c <= 122 { return c - 97 + 26; } }  -- a-z -> 26..51
  if c >= 48 { if c <= 57 { return c - 48 + 52; } }   -- 0-9 -> 52..61
  if c == 43 { return 62; }                           -- +
  if c == 47 { return 63; }                           -- /
  return 0 - 1;                                       -- anything else: bad
}

-- nil on anything malformed: length not a multiple of 4, a character
-- outside the alphabet, or padding anywhere but the last two positions.
pub fn base64_decode(s: Text) -> ?Text {
  let n = len(s);
  if n == 0 { return ""; }
  if n - (n / 4) * 4 != 0 { return nil; }
  let out = "";
  let i = 0;
  while i < n {
    let c0 = byte_at(s, i);
    let c1 = byte_at(s, i + 1);
    let c2 = byte_at(s, i + 2);
    let c3 = byte_at(s, i + 3);
    let last = i + 4 >= n;
    -- '=' (61) is legal only as the last one or two characters
    if c0 == 61 { return nil; }
    if c1 == 61 { return nil; }
    if c2 == 61 { if last == false { return nil; } if c3 != 61 { return nil; } }
    if c3 == 61 { if last == false { return nil; } }
    let v0 = b64_val(c0);
    let v1 = b64_val(c1);
    if v0 < 0 { return nil; }
    if v1 < 0 { return nil; }
    out = out .. char_of(v0 * 4 + v1 / 16);
    if c2 != 61 {
      let v2 = b64_val(c2);
      if v2 < 0 { return nil; }
      out = out .. char_of((v1 - (v1 / 16) * 16) * 16 + v2 / 4);
      if c3 != 61 {
        let v3 = b64_val(c3);
        if v3 < 0 { return nil; }
        out = out .. char_of((v2 - (v2 / 4) * 4) * 64 + v3);
      }
    }
    i = i + 4;
  }
  return out;
}

-- ---- Basic credentials ---------------------------------------------------

pub typedef BasicCreds = { user: Text, pass: Text }

-- nil unless `Authorization: Basic base64(user:pass)` decodes cleanly.
-- The password may itself contain ':' — the split is on the FIRST colon
-- (RFC 7617: the user-id must not contain one).
pub fn basic_credentials(req: Req) -> ?BasicCreds {
  let h = auth_header(req);
  if h == nil { return nil; }
  if h.scheme != "basic" { return nil; }
  let decoded = base64_decode(h.credentials);
  if decoded == nil { return nil; }
  let colon = index_of(decoded, ":");
  if colon < 0 { return nil; }
  return BasicCreds {
    user: substr(decoded, 0, colon),
    pass: substr(decoded, colon + 1, len(decoded) - colon - 1)
  };
}

-- ---- the two middlewares -------------------------------------------------
-- Mechanism only: one shared secret each. An app with a user table writes
-- its own Middleware on top of basic_credentials/bearer_token + ct_eq.

pub class BearerAuth {
  token:     Text   -- the shared secret
  principal: Text   -- attached to req.principal on success
  fn before(mut req: Req) -> ?Resp {
    let got = bearer_token(req);
    if got == nil { return unauthorized(); }
    if ct_eq(got, self.token) == false { return unauthorized(); }
    req.principal = "${self.principal}";
    return nil;
  }
}

pub class BasicAuth {
  user:  Text
  pass:  Text
  realm: Text   -- named in the WWW-Authenticate challenge
  fn before(mut req: Req) -> ?Resp {
    let c = basic_credentials(req);
    if c == nil { return self.challenge(); }
    let user_ok = ct_eq(c.user, self.user);
    let pass_ok = ct_eq(c.pass, self.pass);   -- both always compared
    if user_ok == false { return self.challenge(); }
    if pass_ok == false { return self.challenge(); }
    req.principal = "${c.user}";
    return nil;
  }
  fn challenge() -> Resp {
    let r = unauthorized();
    set_header(r, "www-authenticate", "Basic realm=\"${self.realm}\"");
    return r;
  }
}