Validate signup email and password strength

The password was only ever checked for length. It is now checked on both
sides, with each side doing the job it can actually do.

The browser scores it with zxcvbn, through WordPress's own
password-strength-meter script rather than a second opinion of our own, and
refuses to submit below "medium". That is the nuanced test — it knows
Tr0ub4dor&3 is weaker than it looks — but it is advice a client can decline
to take.

Auth\PasswordPolicy runs on the server and is the rule that holds. It does
not try to reproduce a strength score in PHP; it rejects the categorically
bad, which is what a server can check without shipping a dictionary: too
short, a well-known leaked password, fewer than four distinct characters, or
the user's own name or email inside it. No composition rules — NIST advises
against them, and they mostly produce predictable substitutions.

Both thresholds come from the same two constants, handed to JavaScript by
wp_localize_script, so the sides cannot drift into disagreeing about what
was accepted.

The verdict is attached to the field with setCustomValidity() rather than by
disabling a button. The form has up to three submits plus a "Next" that
already gates on checkValidity(), and an invalid field stops all of them
without any of them needing to know why.

Email validation moved ahead of the password check, since the password is
now checked against the email. A blank form therefore reports the email
first, which also matches the order the fields appear in.

Verified the browser half against a controllable scorer: each score band
blocks or allows as intended, the identity list reaches the meter, and the
gate stays open while zxcvbn's dictionary is still loading — the server
covers that window.

Closes #150

Co-Authored-By: Claude Opus 5 <[email protected]>

src/Auth/PasswordPolicy.php

@@ -0,0 +1,165 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Auth;
+
+/**
+ * What counts as an acceptable signup password.
+ *
+ * The check is deliberately split across the two sides, because the two sides
+ * can do different things:
+ *
+ * - **The browser** runs zxcvbn (WordPress ships it as `password-strength-meter`)
+ *   and gates the submit button on {@see MIN_SCORE}. That is the nuanced test —
+ *   it knows that `Tr0ub4dor&3` is weaker than `correct horse battery staple` —
+ *   but it is only advice, because anything in a browser can be turned off.
+ * - **This class** runs on the server and is the rule that actually holds. It
+ *   cannot score a password the way zxcvbn does without shipping a dictionary,
+ *   so it does not pretend to: it rejects the categorically bad — too short,
+ *   the user's own name or email, a password from the well-known lists, or one
+ *   built from almost no distinct characters.
+ *
+ * Neither half is sufficient alone, which is the point. A password that clears
+ * both is not guaranteed strong; one that fails either is definitely not.
+ */
+class PasswordPolicy {
+
+	/**
+	 * Minimum length. NIST SP 800-63B puts the floor at 8 and explicitly advises
+	 * against composition rules ("must contain a symbol") on the grounds that they
+	 * push people towards predictable substitutions. Length plus the checks below
+	 * does more for less annoyance.
+	 */
+	public const MIN_LENGTH = 8;
+
+	/**
+	 * The zxcvbn score the browser demands before it will let the form submit,
+	 * on WordPress's 0-4 scale: 0-1 weak, 2 medium, 3-4 strong. Two rejects the
+	 * passwords a stranger would guess while still accepting an ordinary
+	 * memorable one — a studio signup form is not a bank.
+	 */
+	public const MIN_SCORE = 2;
+
+	/**
+	 * How much of the user's own identity has to appear in the password before it
+	 * is refused. Short enough to catch a name inside a longer password, long
+	 * enough that a two- or three-letter coincidence does not trip it.
+	 */
+	private const IDENTITY_FRAGMENT_LENGTH = 4;
+
+	/** Fewest distinct characters a password may be built from. */
+	private const MIN_DISTINCT_CHARACTERS = 4;
+
+	/**
+	 * Why this password is unacceptable, or null when it passes.
+	 *
+	 * `$email` and `$displayName` are what the same submission is claiming as an
+	 * identity, so they can be checked against the password before either exists
+	 * as a user.
+	 */
+	public static function validate( string $password, string $email = '', string $displayName = '' ): ?string {
+		// Not trimmed: a leading or trailing space is a legitimate character, and
+		// silently changing what someone typed would lock them out later.
+		if ( strlen( $password ) < self::MIN_LENGTH ) {
+			return sprintf(
+				/* translators: %d: minimum number of characters. */
+				__( 'Please choose a password of at least %d characters.', 'unsupervised-schedular' ),
+				self::MIN_LENGTH
+			);
+		}
+
+		$lower = strtolower( $password );
+
+		if ( in_array( $lower, self::commonPasswords(), true ) ) {
+			return __( 'That password is one of the most commonly used ones. Please choose something less guessable.', 'unsupervised-schedular' );
+		}
+
+		if ( count( array_unique( str_split( $lower ) ) ) < self::MIN_DISTINCT_CHARACTERS ) {
+			return __( 'Please choose a password built from more than a few repeated characters.', 'unsupervised-schedular' );
+		}
+
+		if ( self::echoesIdentity( $lower, $email, $displayName ) ) {
+			return __( 'Please choose a password that does not contain your name or email address.', 'unsupervised-schedular' );
+		}
+
+		return null;
+	}
+
+	/**
+	 * Whether the password contains the user's display name, their email address,
+	 * or the part of it before the `@` — the first things anyone guessing would
+	 * try, and the reason "grace2019" is worse than its length suggests.
+	 */
+	private static function echoesIdentity( string $lowerPassword, string $email, string $displayName ): bool {
+		$email     = strtolower( trim( $email ) );
+		$localPart = '' !== $email ? (string) strstr( $email . '@', '@', true ) : '';
+
+		$fragments = [ $email, $localPart, strtolower( trim( $displayName ) ) ];
+
+		foreach ( $fragments as $fragment ) {
+			if ( strlen( $fragment ) >= self::IDENTITY_FRAGMENT_LENGTH && str_contains( $lowerPassword, $fragment ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	/**
+	 * Passwords common enough that a guess costs nothing. Only entries at least
+	 * {@see MIN_LENGTH} long are worth listing — anything shorter is already
+	 * refused — so this is the long tail of the usual leaked-password lists
+	 * rather than the whole of it. zxcvbn in the browser covers the rest.
+	 *
+	 * @return list<string>
+	 */
+	private static function commonPasswords(): array {
+		return [
+			'password',
+			'password1',
+			'password12',
+			'password123',
+			'passw0rd',
+			'p@ssword',
+			'p@ssw0rd',
+			'12345678',
+			'123456789',
+			'1234567890',
+			'123123123',
+			'qwertyui',
+			'qwertyuiop',
+			'qwerty123',
+			'qwerty12',
+			'1qaz2wsx',
+			'zaq12wsx',
+			'iloveyou',
+			'princess',
+			'sunshine',
+			'football',
+			'baseball',
+			'basketball',
+			'superman',
+			'batman123',
+			'trustno1',
+			'welcome1',
+			'welcome123',
+			'letmein1',
+			'letmein123',
+			'admin123',
+			'administrator',
+			'abc12345',
+			'abcd1234',
+			'monkey123',
+			'dragon123',
+			'michael1',
+			'jennifer',
+			'starwars',
+			'computer',
+			'whatever',
+			'freedom1',
+			'changeme',
+			'secret123',
+			'login123',
+		];
+	}
+}

src/Auth/RegistrationPage.php

@@ -124,6 +124,28 @@ class RegistrationPage {
 		// needed whenever the form itself is on screen.
 		if ( $canRegister && '' === $successType ) {
 			wp_enqueue_script( 'us-scheduler-register' );
+
+			// The browser gate reads the same numbers the server enforces, so the
+			// two cannot drift into disagreeing about what it accepted.
+			wp_localize_script(
+				'us-scheduler-register',
+				'usSchedulerPassword',
+				[
+					'minLength' => PasswordPolicy::MIN_LENGTH,
+					'minScore'  => PasswordPolicy::MIN_SCORE,
+					'strings'   => [
+						'short'    => sprintf(
+							/* translators: %d: minimum number of characters. */
+							__( 'At least %d characters, please.', 'unsupervised-schedular' ),
+							PasswordPolicy::MIN_LENGTH
+						),
+						'veryWeak' => __( 'Too weak — a stranger could guess this.', 'unsupervised-schedular' ),
+						'weak'     => __( 'Still too weak. Try a longer phrase.', 'unsupervised-schedular' ),
+						'medium'   => __( 'Good enough.', 'unsupervised-schedular' ),
+						'strong'   => __( 'Strong password.', 'unsupervised-schedular' ),
+					],
+				]
+			);
 		}
 
 		ob_start();
@@ -248,10 +270,6 @@ class RegistrationPage {
 		$password    = Val::string( wp_unslash( $_POST['password'] ?? '' ) );
 		$displayName = sanitize_text_field( Val::string( wp_unslash( $_POST['display_name'] ?? '' ) ) );
 
-		if ( strlen( $password ) < 8 ) {
-			return esc_html__( 'Please choose a password of at least 8 characters.', 'unsupervised-schedular' );
-		}
-
 		// The email is fixed by a personal invite; group-link signups and
 		// self-signups supply their own.
 		if ( $inviteValid && ! $invite->isGroup() ) {
@@ -263,6 +281,15 @@ class RegistrationPage {
 			}
 		}
 
+		// After the email, so the password can be checked against it. The browser
+		// scores the password with zxcvbn and refuses to submit a weak one, but
+		// that is advice a client can decline to take — this is the check that
+		// holds. See PasswordPolicy for why the two halves differ.
+		$passwordError = PasswordPolicy::validate( $password, $email, $displayName );
+		if ( null !== $passwordError ) {
+			return esc_html( $passwordError );
+		}
+
 		$policyForms = $this->signupPolicies();
 		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.MissingUnslash, WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- each element is coerced to a positive int in the array_map callback; slashes cannot survive integer coercion.
 		$accepted = array_map( static fn( mixed $v ): int => absint( Val::int( $v ) ), (array) ( $_POST['accept'] ?? [] ) );

src/ShortcodeRegistrar.php

@@ -88,7 +88,21 @@ class ShortcodeRegistrar {
 		wp_register_script( 'us-scheduler', USC_PLUGIN_URL . 'assets/js/booking.js', [ 'us-scheduler-pricing', 'us-scheduler-guardian' ], USC_VERSION, true );
 		wp_register_script( 'us-scheduler-group', USC_PLUGIN_URL . 'assets/js/group-classes.js', [ 'us-scheduler-pricing', 'us-scheduler-guardian' ], USC_VERSION, true );
 
-		// Progressive enhancement for the two-step registration form (no dependencies).
-		wp_register_script( 'us-scheduler-register', USC_PLUGIN_URL . 'assets/js/register.js', [], USC_VERSION, true );
+		/*
+		 * Progressive enhancement for the two-step registration form.
+		 *
+		 * `password-strength-meter` is WordPress's own wrapper around zxcvbn, so
+		 * the signup form scores a password exactly the way wp-admin does rather
+		 * than inventing a second opinion. It pulls in `zxcvbn-async`, which
+		 * fetches the (large) dictionary only once the page has loaded — hence
+		 * the guard in register.js for the window where it is not there yet.
+		 */
+		wp_register_script(
+			'us-scheduler-register',
+			USC_PLUGIN_URL . 'assets/js/register.js',
+			[ 'password-strength-meter' ],
+			USC_VERSION,
+			true
+		);
 	}
 }