package authsupport

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"fmt"
)

// csrfTokenBytes is the amount of entropy in a CSRF token before
// encoding. This isn't a secret that needs to resist offline brute
// force the way a password hash does — it just needs to be
// unguessable to a third-party site, so 128 bits is comfortably enough.
const csrfTokenBytes = 16

// NewCSRFToken returns a fresh, URL-safe random token suitable for the
// double-submit-cookie pattern: the same value is set as a cookie and
// embedded as a hidden form field, and a request is rejected unless the
// two match (see VerifyCSRF). Unlike session/verification tokens, the
// raw value itself is what's compared — there's no server-side lookup,
// so no hash-at-rest step is needed here.
func NewCSRFToken() (string, error) {
	b := make([]byte, csrfTokenBytes)
	if _, err := rand.Read(b); err != nil {
		return "", fmt.Errorf("auth: generate csrf token: %w", err)
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}

// VerifyCSRF reports whether a cookie value and a submitted form value
// match, in constant time. Both empty is never valid.
func VerifyCSRF(cookieValue, formValue string) bool {
	if cookieValue == "" || formValue == "" {
		return false
	}
	return subtle.ConstantTimeCompare([]byte(cookieValue), []byte(formValue)) == 1
}