Source

File: wp-includes/pluggable.php

function wp_generate_auth_cookie( $user_id, $expiration, $scheme = 'auth', $token = '' ) {
		$user = get_userdata( $user_id );
		if ( ! $user ) {
			return '';
		}

		if ( ! $token ) {
			$manager = WP_Session_Tokens::get_instance( $user_id );
			$token   = $manager->create( $expiration );
		}

		$pass_frag = substr( $user->user_pass, 8, 4 );

		$key = wp_hash( $user->user_login . '|' . $pass_frag . '|' . $expiration . '|' . $token, $scheme );

		// If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
		$algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
		$hash = hash_hmac( $algo, $user->user_login . '|' . $expiration . '|' . $token, $key );

		$cookie = $user->user_login . '|' . $expiration . '|' . $token . '|' . $hash;

		/**
		 * Filters the authentication cookie.
		 *
		 * @since 2.5.0
		 * @since 4.0.0 The `$token` parameter was added.
		 *
		 * @param string $cookie     Authentication cookie.
		 * @param int    $user_id    User ID.
		 * @param int    $expiration The time the cookie expires as a UNIX timestamp.
		 * @param string $scheme     Cookie scheme used. Accepts 'auth', 'secure_auth', or 'logged_in'.
		 * @param string $token      User's session token used.
		 */
		return apply_filters( 'auth_cookie', $cookie, $user_id, $expiration, $scheme, $token );
	}