gotermix/internals/auth.go

package internals

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"net/http"
	"strings"
	"time"
)

// ── CSRF ──────────────────────────────────────────────────────────────

func newCSRFToken() string {
	b := make([]byte, 24)
	rand.Read(b) //nolint:errcheck
	return hex.EncodeToString(b)
}

// setCSRFCookie writes a fresh CSRF token to a short-lived cookie and returns
// the token value so it can be embedded in the rendered HTML form.
func setCSRFCookie(w http.ResponseWriter) string {
	tok := newCSRFToken()
	http.SetCookie(w, &http.Cookie{
		Name:     csrfCookieName,
		Value:    tok,
		Path:     "/",
		HttpOnly: true,
		Secure:   true,
		SameSite: http.SameSiteStrictMode,
		MaxAge:   900, // 15 min — covers slow typists
	})
	return tok
}

// checkCSRF returns true iff the submitted csrf_token form field matches the
// cookie value (constant-time compare to prevent timing side-channels).
func checkCSRF(r *http.Request) bool {
	c, err := r.Cookie(csrfCookieName)
	if err != nil || c.Value == "" {
		return false
	}
	formTok := r.FormValue("csrf_token")
	if formTok == "" {
		return false
	}
	return hmac.Equal([]byte(c.Value), []byte(formTok))
}

func checkCreds(username, password string) bool {
	if username != appCreds.Username {
		return false
	}
	got := hashPassword(password, appCreds.Salt)
	return hmac.Equal([]byte(got), []byte(appCreds.Hash))
}

func initAuthSecret() {
	authSecret = make([]byte, 32)
	rand.Read(authSecret)
}

func makeAuthToken() string {
	ts := fmt.Sprintf("%d", time.Now().Unix())
	mac := hmac.New(sha256.New, authSecret)
	mac.Write([]byte(ts))
	return ts + "." + hex.EncodeToString(mac.Sum(nil))
}

func validAuthToken(token string) bool {
	dot := strings.LastIndex(token, ".")
	if dot < 0 {
		return false
	}
	ts, sig := token[:dot], token[dot+1:]
	mac := hmac.New(sha256.New, authSecret)
	mac.Write([]byte(ts))
	if !hmac.Equal([]byte(sig), []byte(hex.EncodeToString(mac.Sum(nil)))) {
		return false
	}
	var t int64
	fmt.Sscanf(ts, "%d", &t)
	return time.Since(time.Unix(t, 0)) < authTokenTTL
}

func isAuthed(r *http.Request) bool {
	if nopwMode {
		return true
	}
	c, err := r.Cookie(authCookieName)
	return err == nil && validAuthToken(c.Value)
}