// Package authtoken signs the short tokens hammerhead hands to visitors and blacktip verifies.
package authtoken

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"strings"
	"time"
)

const (
	KindCallback = "cb"   // minted by hammerhead after a successful login, valid for minutes
	KindSession  = "sess" // minted by blacktip for the visitor's cookie
)

type Token struct {
	Mapping string `json:"m"`
	Sub     string `json:"sub"` // who: an email, "password", "pin", "share:name"
	Kind    string `json:"k"`
	Exp     int64  `json:"exp"`
	// Epoch is the resource's login generation when this was issued. Hammerhead
	// bumps it to sign every visitor out (a member removed, a password changed);
	// tokens from an older epoch are then refused without any per-token state.
	Epoch int64 `json:"ep"`
}

var enc = base64.RawURLEncoding

func mac(key, payload string) string {
	m := hmac.New(sha256.New, []byte(key))
	m.Write([]byte(payload))
	return enc.EncodeToString(m.Sum(nil))
}

func Sign(key string, t Token) string {
	b, _ := json.Marshal(t)
	p := enc.EncodeToString(b)
	return p + "." + mac(key, p)
}

// Verify checks the signature, kind, mapping, epoch and expiry.
func Verify(key, tok, kind, mapping string, epoch int64) (Token, bool) {
	var t Token
	p, sig, found := strings.Cut(tok, ".")
	if !found || !hmac.Equal([]byte(sig), []byte(mac(key, p))) {
		return t, false
	}
	b, err := enc.DecodeString(p)
	if err != nil || json.Unmarshal(b, &t) != nil {
		return t, false
	}
	if t.Kind != kind || t.Mapping != mapping || t.Epoch != epoch || time.Now().Unix() > t.Exp {
		return t, false
	}
	return t, true
}