// Sign in with Keymail — a hand-written client for keymail's JSON-bodied
// OAuth 2.1 (authorization code + PKCE S256, no client registration:
// client_id IS the console's origin) and its app API (sealed sends into the
// user's Apps inbox).
//
// Wire contract implemented from the keymail source of truth:
// keymail oauth.go (handleOAuthToken, handleOAuthRevoke), appapi.go
// (handleAppSend), federation.go (envelope), crypto.go (seal / sign /
// key encodings), docs/apps.md. All bodies are JSON, never form-encoded —
// off-the-shelf oauth2 libraries do not speak this, hence by hand.
package console

import (
	"bytes"
	"context"
	"crypto/aes"
	"crypto/cipher"
	"crypto/ecdh"
	"crypto/ecdsa"
	"crypto/hkdf"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"
)

// ErrGrantRevoked reports a send refused with 401: the user revoked the
// console's grant in Keymail (or the token is otherwise dead), so the only
// road back is the OAuth ceremony, which rotates the grant.
var ErrGrantRevoked = errors.New("keymail: grant revoked or invalid")

// keymailError is a non-200 answer from Keymail. Error() keeps the text
// do() has always produced; Status and Msg let callers classify without
// matching strings.
type keymailError struct {
	Status       int
	Method, Path string
	Msg          string // Keymail's {"error": ...}, or the HTTP status line
}

func (e *keymailError) Error() string {
	return fmt.Sprintf("keymail: %s %s: %s", e.Method, e.Path, e.Msg)
}

// Keymail is a client bound to one keymail base URL and one console origin.
type Keymail struct {
	base    string // keymail base URL, no trailing slash
	origin  string // the console's origin — its OAuth client_id
	signKey *ecdsa.PrivateKey
	httpc   *http.Client
}

// NewKeymail builds a client. origin is the console's https origin (its
// client_id); baseURL the keymail deployment (https://keymail.dev).
func NewKeymail(baseURL, origin string) *Keymail {
	return &Keymail{
		base:   strings.TrimSuffix(baseURL, "/"),
		origin: strings.TrimSuffix(origin, "/"),
		httpc:  &http.Client{Timeout: 15 * time.Second},
	}
}

// Host is the hostname of the configured Keymail deployment (keymail.dev for
// https://keymail.dev), lowercased, no port. An address at this host is a
// Keymail address whatever a lookup says.
func (k *Keymail) Host() string {
	u, err := url.Parse(k.base)
	if err != nil {
		return ""
	}
	return strings.ToLower(u.Hostname())
}

// SetSigningKey installs the console's long-lived envelope-signing key
// (ECDSA P-256). With a key set the client asks for scope "identify send"
// and pins the key into the grant at exchange (keymail requires sign_pub
// whenever the grant can deliver mail); without one it asks for bare
// "identify" and cannot SendApp.
func (k *Keymail) SetSigningKey(priv *ecdsa.PrivateKey) { k.signKey = priv }

// SignPub is the base64 raw uncompressed P-256 point keymail pins in the
// grant ("" when no key is set).
func (k *Keymail) SignPub() string {
	if k.signKey == nil {
		return ""
	}
	pub, err := k.signKey.PublicKey.ECDH()
	if err != nil {
		return ""
	}
	return base64.StdEncoding.EncodeToString(pub.Bytes())
}

func (k *Keymail) scope() string {
	if k.signKey != nil {
		return "identify send"
	}
	return "identify"
}

// redirectURI is fixed by the console's route table.
func (k *Keymail) redirectURI() string { return k.origin + "/auth/callback" }

// stepUpRedirectURI is where a re-authentication lands. A separate route from
// the sign-in callback on purpose: a code minted for a step-up must not be
// spendable as a sign-in, and a sign-in must never be able to satisfy a
// step-up — the two flows never share a landing pad or a state cookie.
func (k *Keymail) stepUpRedirectURI() string { return k.origin + "/auth/stepup/callback" }

// AuthorizeURL is where the browser goes to approve the console.
func (k *Keymail) AuthorizeURL(state, challenge string) string {
	return k.authorizeURL(state, challenge, k.redirectURI(), 0)
}

// StepUpURL is the same ceremony with one extra demand: a FRESH authenticator
// gesture, however live the user's Keymail session is. `prompt=login` is the
// ask; `max_age` is the same ask spelled for an IdP that only understands
// seconds, and both are sent because the tighter of the two wins at Keymail.
// The redirect lands on the step-up callback, not the sign-in one — a step-up
// must never be mistakable for a sign-in, in either direction.
func (k *Keymail) StepUpURL(state, challenge string, window time.Duration) string {
	return k.authorizeURL(state, challenge, k.stepUpRedirectURI(), window)
}

func (k *Keymail) authorizeURL(state, challenge, redirect string, window time.Duration) string {
	q := url.Values{
		"client_id":             {k.origin},
		"redirect_uri":          {redirect},
		"scope":                 {k.scope()},
		"state":                 {state},
		"code_challenge":        {challenge},
		"code_challenge_method": {"S256"},
	}
	if window > 0 {
		q.Set("prompt", "login")
		q.Set("max_age", strconv.Itoa(int(window.Seconds())))
	}
	return k.base + "/oauth/authorize?" + q.Encode()
}

// SupportsStepUp asks the deployment whether it can re-authenticate anyone at
// all: `/.well-known/keymail` advertises `reauth` where step-up exists (only
// the relay shape has an identity service behind it). An error here is NOT a
// "probably yes" — callers must treat both a false and an error as "cannot",
// because the alternative is walking a human through a ceremony that was never
// going to produce proof and then deciding what to do about it afterwards.
func (k *Keymail) SupportsStepUp(ctx context.Context) (bool, error) {
	var out struct {
		Reauth bool `json:"reauth"`
	}
	if _, err := k.do(ctx, http.MethodGet, "/.well-known/keymail", "", nil, &out); err != nil {
		return false, err
	}
	return out.Reauth, nil
}

// Grant is one completed ceremony: who the user is, the console's access
// token, and — for a step-up — when the authenticator gesture behind it
// happened.
type Grant struct {
	Address string
	Token   string
	// AuthTime is zero when Keymail proved no gesture. Zero is the only honest
	// representation of "it did not say", and the caller MUST treat it as a
	// refusal rather than substituting now: a fabricated freshness is worse
	// than no freshness, because it looks like proof.
	AuthTime time.Time
}

// Exchange trades the callback's code (plus the PKCE verifier) for the
// user's address and the console's access token, server-side.
func (k *Keymail) Exchange(ctx context.Context, code, verifier string) (address, token string, err error) {
	g, err := k.ExchangeGrant(ctx, code, verifier, false)
	return g.Address, g.Token, err
}

// ExchangeGrant is Exchange with the step-up answer attached. stepUp selects
// which redirect_uri is replayed — Keymail re-checks it against the one the
// authorization was minted for, so a code from the sign-in flow cannot be
// spent here and vice versa.
func (k *Keymail) ExchangeGrant(ctx context.Context, code, verifier string, stepUp bool) (Grant, error) {
	redirect := k.redirectURI()
	if stepUp {
		redirect = k.stepUpRedirectURI()
	}
	req := map[string]string{
		"code":          code,
		"code_verifier": verifier,
		"client_id":     k.origin,
		"redirect_uri":  redirect,
	}
	if sp := k.SignPub(); sp != "" {
		req["sign_pub"] = sp
	}
	var out struct {
		AccessToken string `json:"access_token"`
		TokenType   string `json:"token_type"`
		Scope       string `json:"scope"`
		Address     string `json:"address"`
		AuthTime    int64  `json:"auth_time"`
	}
	if _, err := k.do(ctx, http.MethodPost, "/api/oauth/token", "", req, &out); err != nil {
		return Grant{}, err
	}
	if !strings.EqualFold(out.TokenType, "bearer") {
		return Grant{}, fmt.Errorf("keymail: token exchange returned token_type %q, want bearer", out.TokenType)
	}
	if out.AccessToken == "" || out.Address == "" {
		return Grant{}, errors.New("keymail: token exchange returned an empty token or address")
	}
	g := Grant{Address: out.Address, Token: out.AccessToken}
	if out.AuthTime > 0 {
		g.AuthTime = time.Unix(out.AuthTime, 0)
	}
	return g, nil
}

// Revoke retires an access token (RFC 7009 manners server-side: always 200,
// revealing nothing). Callers treat failures as best-effort.
func (k *Keymail) Revoke(ctx context.Context, token string) error {
	_, err := k.do(ctx, http.MethodPost, "/api/oauth/revoke", "", map[string]string{"token": token}, nil)
	return err
}

// ---- app send (the returning-sign-in confirmation channel) ------------------

// envelope is keymail's message-in-flight shape (federation.go): plaintext
// routing headers, subject+body sealed to the recipient's box key in blob,
// sig the sender's ECDSA signature over headers+blob.
type envelope struct {
	V       string `json:"v"`
	ID      string `json:"id"`
	From    string `json:"from"`
	To      string `json:"to"`
	Date    string `json:"date"`
	SignPub string `json:"sign_pub"`
	Blob    string `json:"blob"`
	Sig     string `json:"sig"`
}

const (
	protocolVersion = "keymail1"
	envelopeInfo    = "keymail-envelope-v1"
	envSigContext   = "keymail-env-v1"
)

// sealEnvelope seals {subject, body} to the recipient's box key and signs
// the envelope with the console's key. SendApp and SendFederated share it,
// so the two wires cannot drift apart.
func (k *Keymail) sealEnvelope(from, to, boxPub, subject, body string) (envelope, error) {
	payload, err := json.Marshal(map[string]string{"subject": subject, "body": body})
	if err != nil {
		return envelope{}, err
	}
	blob, err := sealToBox(boxPub, payload)
	if err != nil {
		return envelope{}, fmt.Errorf("keymail: seal to %s: %w", to, err)
	}
	idb := make([]byte, 16)
	if _, err := rand.Read(idb); err != nil {
		return envelope{}, err
	}
	id := hex.EncodeToString(idb)
	date := time.Now().UTC().Format(time.RFC3339)
	sig, err := signRaw(k.signKey, envelopeMsg(id, from, to, date, blob))
	if err != nil {
		return envelope{}, err
	}
	return envelope{
		V: protocolVersion, ID: id, From: from, To: to, Date: date,
		SignPub: k.SignPub(), Blob: blob, Sig: sig,
	}, nil
}

// SendApp delivers one sealed message (kind "app" → the user's Apps inbox)
// to the granting user, using the stored grant token. It looks up the
// recipient's box key, seals {subject, body} to it, signs the envelope with
// the console's pinned key, and posts it. Returns the envelope id (the
// sender mints it, so a deep link is knowable up front). A 401 comes back
// as ErrGrantRevoked.
func (k *Keymail) SendApp(ctx context.Context, token, to, subject, body string) (msgID string, err error) {
	if k.signKey == nil {
		return "", errors.New("keymail: SendApp requires a signing key")
	}
	// From is pinned server-side to the console's own origin host — the
	// console writes as itself, never as anyone the user might trust.
	from := "console@" + OriginHost(k.origin)
	var lr struct {
		BoxPub string `json:"box_pub"`
	}
	lu := "/api/federation/lookup?addr=" + url.QueryEscape(to) + "&from=" + url.QueryEscape(from)
	if _, err := k.do(ctx, http.MethodGet, lu, "", nil, &lr); err != nil {
		return "", fmt.Errorf("keymail: lookup %s: %w", to, err)
	}
	env, err := k.sealEnvelope(from, to, lr.BoxPub, subject, body)
	if err != nil {
		return "", err
	}
	status, err := k.do(ctx, http.MethodPost, "/api/app/send", token,
		map[string]any{"envelope": env, "kind": "app"}, nil)
	if status == http.StatusUnauthorized {
		return "", ErrGrantRevoked
	}
	if err != nil {
		return "", err
	}
	return env.ID, nil
}

// OriginHost strips the scheme off an origin: the host (with port, if any)
// that keymail treats as the app's identity.
func OriginHost(origin string) string {
	h := strings.TrimPrefix(origin, "https://")
	return strings.TrimPrefix(h, "http://")
}

// ---- HTTP plumbing ----------------------------------------------------------

// do runs one JSON round-trip. Non-200 answers become an error carrying the
// server's {"error": ...} text; the status is returned either way so callers
// can classify (401 → revoked grant).
func (k *Keymail) do(ctx context.Context, method, path, bearer string, in, out any) (int, error) {
	var body io.Reader
	if in != nil {
		raw, err := json.Marshal(in)
		if err != nil {
			return 0, err
		}
		body = bytes.NewReader(raw)
	}
	req, err := http.NewRequestWithContext(ctx, method, k.base+path, body)
	if err != nil {
		return 0, err
	}
	if in != nil {
		req.Header.Set("Content-Type", "application/json")
	}
	if bearer != "" {
		req.Header.Set("Authorization", "Bearer "+bearer)
	}
	res, err := k.httpc.Do(req)
	if err != nil {
		return 0, err
	}
	defer res.Body.Close()
	raw, err := io.ReadAll(io.LimitReader(res.Body, 1<<20))
	if err != nil {
		return res.StatusCode, err
	}
	if res.StatusCode != http.StatusOK {
		var e struct {
			Error string `json:"error"`
		}
		msg := res.Status
		if json.Unmarshal(raw, &e) == nil && e.Error != "" {
			msg = e.Error
		}
		return res.StatusCode, &keymailError{Status: res.StatusCode, Method: method, Path: path, Msg: msg}
	}
	if out != nil {
		if err := json.Unmarshal(raw, out); err != nil {
			return res.StatusCode, fmt.Errorf("keymail: %s %s: bad response body: %v", method, path, err)
		}
	}
	return res.StatusCode, nil
}

// ---- PKCE -------------------------------------------------------------------

// NewVerifier mints a PKCE code verifier (43 base64url chars — inside the
// RFC 7636 window). Also serves as the state generator.
func NewVerifier() (string, error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}

// ChallengeS256 derives the S256 code challenge for a verifier.
func ChallengeS256(verifier string) string {
	sum := sha256.Sum256([]byte(verifier))
	return base64.RawURLEncoding.EncodeToString(sum[:])
}

// ---- envelope crypto (ported from keymail crypto.go, byte-compatible) -------

// envelopeMsg is what a sender signs per message: routing headers and the
// ciphertext, so tampering with any of them breaks the signature.
func envelopeMsg(id, from, to, date, blob string) []byte {
	return []byte(envSigContext + "\n" + id + "\n" + from + "\n" + to + "\n" + date + "\n" + blob)
}

// signRaw produces a WebCrypto-compatible raw r||s ECDSA P-256 signature.
func signRaw(priv *ecdsa.PrivateKey, msg []byte) (string, error) {
	h := sha256.Sum256(msg)
	r, s, err := ecdsa.Sign(rand.Reader, priv, h[:])
	if err != nil {
		return "", err
	}
	sig := make([]byte, 64)
	r.FillBytes(sig[:32])
	s.FillBytes(sig[32:])
	return base64.StdEncoding.EncodeToString(sig), nil
}

// sealToBox encrypts payload to a recipient box key: ephemeral ECDH P-256 →
// HKDF-SHA256("keymail-envelope-v1") → AES-GCM-256, blob = ephPub(65) ||
// iv(12) || ciphertext, base64 (std).
func sealToBox(recipientBoxPubB64 string, payload []byte) (string, error) {
	raw, err := base64.StdEncoding.DecodeString(recipientBoxPubB64)
	if err != nil {
		return "", errors.New("bad recipient key encoding")
	}
	pub, err := ecdh.P256().NewPublicKey(raw)
	if err != nil {
		return "", fmt.Errorf("bad recipient key: %w", err)
	}
	eph, err := ecdh.P256().GenerateKey(rand.Reader)
	if err != nil {
		return "", err
	}
	shared, err := eph.ECDH(pub)
	if err != nil {
		return "", err
	}
	key, err := hkdf.Key(sha256.New, shared, nil, envelopeInfo, 32)
	if err != nil {
		return "", err
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	iv := make([]byte, 12)
	if _, err := rand.Read(iv); err != nil {
		return "", err
	}
	ct := gcm.Seal(nil, iv, payload, nil)
	blob := append(append(eph.PublicKey().Bytes(), iv...), ct...)
	return base64.StdEncoding.EncodeToString(blob), nil
}