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function old_key_candidates(string $k): array {
    // Try raw, trimmed, without wrapping quotes, and hex->bin
    $cands = [];
    $adds  = [];

    $adds[] = $k;
    $adds[] = trim($k);
    $adds[] = trim($k, " \t\n\r\0\x0B'\"");

    // If it looks like hex, also try binary form
    foreach ($adds as $a) {
        $cands[] = $a;
        if (ctype_xdigit($a) && (strlen($a) % 2 === 0)) {
            $bin = @hex2bin($a);
            if ($bin !== false) $cands[] = $bin;
        }
    }

    // For each, also try md5(key) in both binary+hex (some v2 forks used SECRET)
    $all = [];
    foreach (array_unique($cands, SORT_STRING) as $cand) {
        $all[] = $cand;                       // raw
        $all[] = md5($cand, true);            // md5 bin
        $all[] = md5($cand, false);           // md5 hex
    }

    // de-duplicate by string identity
    return array_values(array_unique($all, SORT_STRING));
}

// --- v2.x decrypt that tries many realistic permutations --------------------
function try_decrypt_v22(string $value, array $keys): ?string {
    $cipher = 'AES-256-CBC';
    $ivlen  = openssl_cipher_iv_length($cipher);
    $zeroIv = str_repeat("\0", $ivlen);

    $decoded = base64_decode($value, true); // default v2 produced base64

    foreach ($keys as $key) {
        // 1) Exact legacy style — PHP auto-base64, empty IV param (omitted)
        $pt = @openssl_decrypt($value, $cipher, $key);
        if ($pt !== false) return $pt;

        // 2) base64-decoded + RAW_DATA + zero IV (some stacks behaved like this)
        if ($decoded !== false) {
            $pt = @openssl_decrypt($decoded, $cipher, $key, OPENSSL_RAW_DATA, $zeroIv);
            if ($pt !== false) return $pt;
        }

        // 3) explicit empty IV with base64 input (older PHP tolerated empty IV)
        $pt = @openssl_decrypt($value, $cipher, $key, 0, '');
        if ($pt !== false) return $pt;

        if ($decoded !== false) {
            // 4) raw + empty IV, no RAW_DATA
            $pt = @openssl_decrypt($decoded, $cipher, $key, 0, '');
            if ($pt !== false) return $pt;
        }
    }
    return null;
}

function decrypt_v31_with_key(string $value, string $keyBin): ?string {
    $decoded = base64_decode($value, true);
    if ($decoded === false) return null;
    $cipher = 'AES-256-CBC';
    $ivlen  = openssl_cipher_iv_length($cipher);
    $hmacLen = 32;
    if (strlen($decoded) < $ivlen + $hmacLen) return null;
    $iv  = substr($decoded, 0, $ivlen);
    $hmc = substr($decoded, $ivlen, $hmacLen);
    $ct  = substr($decoded, $ivlen + $hmacLen);
    $calc = hash_hmac('sha256', $ct, $keyBin, true);
    if (!hash_equals($hmc, $calc)) return null;
    $pt = openssl_decrypt($ct, $cipher, $keyBin, OPENSSL_RAW_DATA, $iv);
    return ($pt !== false) ? $pt : null;
}

function encrypt_v31_with_key(string $plaintext, string $keyBin): string {
    $cipher = 'AES-256-CBC';
    $ivlen  = openssl_cipher_iv_length($cipher);
    $iv     = random_bytes($ivlen);
    $ct     = openssl_encrypt($plaintext, $cipher, $keyBin, OPENSSL_RAW_DATA, $iv);
    $h      = hash_hmac('sha256', $ct, $keyBin, true);
    return base64_encode($iv . $h . $ct);
}

// Re-key all rows with encrypt='1'
function migrate_encrypted_pastes(PDO $pdo, string $oldKeyInput, string $newKeyHex, bool $dryRun=false): array {
    $res = ['checked'=>0,'converted'=>0,'skipped'=>0,'failed'=>0,'errors'=>[]];

    if (!extension_loaded('openssl')) {
        $res['errors'][] = 'OpenSSL extension not available.';
        return $res;
    }
    $oldKeyCandidates = old_key_candidates($oldKeyInput);
    if (empty($oldKeyCandidates)) {
        $res['errors'][] = 'Old $sec_key not provided.';
        return $res;
    }
    $newKeyBin = @hex2bin(trim($newKeyHex));
    if ($newKeyBin === false || strlen($newKeyBin)!==32) {
        $res['errors'][] = 'New $sec_key is not valid 64-hex (32-byte) key.';
        return $res;
    }

    $total = (int)$pdo->query("SELECT COUNT(*) FROM pastes WHERE encrypt='1'")->fetchColumn();
    if ($total === 0) return $res;

    $batch = 500;
    for ($offset=0; $offset<$total; $offset+=$batch) {
        $q = $pdo->prepare("SELECT id, content FROM pastes WHERE encrypt='1' ORDER BY id ASC LIMIT :lim OFFSET :off");
        $q->bindValue(':lim', $batch, PDO::PARAM_INT);
        $q->bindValue(':off', $offset, PDO::PARAM_INT);
        $q->execute();
        $rows = $q->fetchAll(PDO::FETCH_ASSOC);
        if (!$rows) break;

        if (!$dryRun) $pdo->beginTransaction();
        foreach ($rows as $r) {
            $res['checked']++;
            $id  = (int)$r['id'];
            $enc = (string)$r['content'];

            // Already new-format under current key? Skip.
            $already = decrypt_v31_with_key($enc, $newKeyBin);
            if ($already !== null) { $res['skipped']++; continue; }

            // Try legacy decrypt with multiple key interpretations
            $plain = try_decrypt_v22($enc, $oldKeyCandidates);
            if ($plain === null) {
                $res['failed']++;
                $res['errors'][] = "ID $id: could not decrypt with any old key variant.";
                continue;
            }

            // Re-encrypt and verify
            $reb = encrypt_v31_with_key($plain, $newKeyBin);
            if (decrypt_v31_with_key($reb, $newKeyBin) !== $plain) {
                $res['failed']++;
                $res['errors'][] = "ID $id: verification after re-encrypt failed.";
                continue;
            }

            if (!$dryRun) {
                $u = $pdo->prepare("UPDATE pastes SET content=:c WHERE id=:id");
                $u->execute([':c'=>$reb, ':id'=>$id]);
            }
            $res['converted']++;
        }
        if (!$dryRun) $pdo->commit();
    }

    return $res;
}