<?php
// Chappa Facturare — assertion endpoint in plain PHP (7.4+, ext-openssl).
// POST /chappa/assertion → text/plain JWS for the signed-in user.
// Your session decides who the user is; never take `sub` from the request.

declare(strict_types=1);
session_start();

const FACTURARE = 'https://facturare.chappa.ro';
const MY_ORIGIN = 'https://app.example.com';
const KEY_FILE = __DIR__ . '/../secrets/chappa-private.pem'; // outside the web root, mode 0600

function b64u(string $b): string { return rtrim(strtr(base64_encode($b), '+/', '-_'), '='); }

/** openssl_sign() returns DER; JWS wants r||s, 32 bytes each. */
function der_to_raw(string $der): string {
    $p = 4;
    $rl = ord($der[3]);
    $r = substr($der, $p, $rl);
    $p += $rl + 2;
    $sl = ord($der[$p - 1]);
    $s = substr($der, $p, $sl);
    $fix = fn(string $x): string => str_pad(ltrim($x, "\x00"), 32, "\x00", STR_PAD_LEFT);
    return $fix($r) . $fix($s);
}

if ($_SERVER['REQUEST_METHOD'] !== 'POST') { http_response_code(405); exit; }
if (($_SERVER['HTTP_ORIGIN'] ?? MY_ORIGIN) !== MY_ORIGIN) { http_response_code(403); exit; }
$user = $_SESSION['user'] ?? null;            // your login
if (!$user) { http_response_code(401); exit; }

$key = openssl_pkey_get_private(file_get_contents(KEY_FILE));
$d = openssl_pkey_get_details($key)['ec'];
$jwk = ['kty' => 'EC', 'crv' => 'P-256', 'x' => b64u($d['x']), 'y' => b64u($d['y'])];
$now = time();
$claims = [
    'aud' => FACTURARE,
    'sub' => (string)$user['id'],
    'name' => $user['name'] ?? null,
    'iat' => $now,
    'exp' => $now + 120,
    'jti' => bin2hex(random_bytes(16)),
    'ori' => MY_ORIGIN,
];
$input = b64u(json_encode(['alg' => 'ES256', 'typ' => 'JWT', 'jwk' => $jwk])) . '.' . b64u(json_encode(array_filter($claims, fn($v) => $v !== null)));
if (!openssl_sign($input, $der, $key, OPENSSL_ALGO_SHA256)) { http_response_code(500); exit; }

header('Content-Type: text/plain');
header('Cache-Control: no-store');
echo $input . '.' . b64u(der_to_raw($der));