diff --git a/autotests/karchivetest.h b/autotests/karchivetest.h
--- a/autotests/karchivetest.h
+++ b/autotests/karchivetest.h
@@ -77,6 +77,8 @@
     void testTarRootDir();
     void testTarDirectoryTwice();
     void testTarIgnoreRelativePathOutsideArchive();
+    void testTarLongNonASCIINames();
+    void testTarShortNonASCIINames();
 
     void testCreateZip();
     void testCreateZipError();
diff --git a/autotests/karchivetest.cpp b/autotests/karchivetest.cpp
--- a/autotests/karchivetest.cpp
+++ b/autotests/karchivetest.cpp
@@ -769,6 +769,55 @@
     QCOMPARE(listing.count(), 10);
 }
 
+void KArchiveTest::testTarLongNonASCIINames() // bug 266141
+{
+    const QString tarName = QString("karchive-long-non-ascii-names.tar");
+    const QString longName =
+        QString("раз-два-три-четыре-пять-вышел-зайчик-погулять-вдруг-охотник-"
+                "выбегает-прямо-в-зайчика.txt");
+
+    {
+        KTar tar(tarName);
+        QVERIFY(tar.open(QIODevice::WriteOnly));
+        QVERIFY(tar.writeFile(longName, "", 0644, "user", "users"));
+        QVERIFY(tar.close());
+    }
+
+    {
+        KTar tar(tarName);
+
+        QVERIFY(tar.open(QIODevice::ReadOnly));
+        const KArchiveDirectory *dir = tar.directory();
+        QVERIFY(dir != nullptr);
+
+        const QStringList listing = recursiveListEntries(dir, QString(""), 0);
+
+        const QString expectedListingEntry =
+            QString("mode=644 path=") + longName + QString(" type=file size=0");
+
+        QCOMPARE(listing.count(), 1);
+
+        QCOMPARE(listing[0], expectedListingEntry);
+        QVERIFY(tar.close());
+    }
+}
+
+void KArchiveTest::testTarShortNonASCIINames() // bug 266141
+{
+    KTar tar(QFINDTESTDATA(QString("tar_non_ascii_file_name.tar.gz")));
+
+    QVERIFY(tar.open(QIODevice::ReadOnly));
+    const KArchiveDirectory *dir = tar.directory();
+    QVERIFY(dir != nullptr);
+
+    const QStringList listing = recursiveListEntries(dir, QString(""), 0);
+
+    QCOMPARE(listing.count(), 1);
+    QCOMPARE(listing[0], QString("mode=644 path=абвгдеёжзийклмнопрстуфхцчшщъыьэюя.txt"
+                                 " type=file size=0"));
+    QVERIFY(tar.close());
+}
+
 void KArchiveTest::testTarDirectoryTwice() // bug 206994
 {
     KTar tar(QFINDTESTDATA(QLatin1String("tar_directory_twice.tar.gz")));
diff --git a/autotests/tar_non_ascii_file_name.tar.gz b/autotests/tar_non_ascii_file_name.tar.gz
new file mode 100644
index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000
GIT binary patch
literal 0
Hc$@<O00001

literal 0
Hc$@<O00001

diff --git a/src/ktar.cpp b/src/ktar.cpp
--- a/src/ktar.cpp
+++ b/src/ktar.cpp
@@ -196,12 +196,12 @@
 
             int check = 0;
             for (uint j = 0; j < 0x200; ++j) {
-                check += buffer[j];
+                check += static_cast<unsigned char>(buffer[j]);
             }
 
             // adjust checksum to count the checksum fields as blanks
             for (uint j = 0; j < 8 /*size of the checksum field including the \0 and the space*/; j++) {
-                check -= buffer[148 + j];
+                check -= static_cast<unsigned char>(buffer[148 + j]);
             }
             check += 8 * ' ';
 
@@ -698,7 +698,7 @@
     // Header check sum
     int check = 32;
     for (uint j = 0; j < 0x200; ++j) {
-        check += buffer[j];
+        check += static_cast<unsigned char>(buffer[j]);
     }
     s = QByteArray::number(check, 8);   // octal
     s = s.rightJustified(6, '0');
@@ -773,8 +773,8 @@
     const QByteArray uname = user.toLocal8Bit();
     const QByteArray gname = group.toLocal8Bit();
 
-    // If more than 100 chars, we need to use the LongLink trick
-    if (fileName.length() > 99) {
+    // If more than 100 bytes, we need to use the LongLink trick
+    if (encodedFileName.length() > 99) {
         d->writeLonglink(buffer, encodedFileName, 'L', uname.constData(), gname.constData());
     }
 
@@ -838,8 +838,8 @@
     QByteArray uname = user.toLocal8Bit();
     QByteArray gname = group.toLocal8Bit();
 
-    // If more than 100 chars, we need to use the LongLink trick
-    if (dirName.length() > 99) {
+    // If more than 100 bytes, we need to use the LongLink trick
+    if (encodedDirname.length() > 99) {
         d->writeLonglink(buffer, encodedDirname, 'L', uname.constData(), gname.constData());
     }
 
@@ -894,11 +894,11 @@
     QByteArray uname = user.toLocal8Bit();
     QByteArray gname = group.toLocal8Bit();
 
-    // If more than 100 chars, we need to use the LongLink trick
-    if (target.length() > 99) {
+    // If more than 100 bytes, we need to use the LongLink trick
+    if (encodedTarget.length() > 99) {
         d->writeLonglink(buffer, encodedTarget, 'K', uname.constData(), gname.constData());
     }
-    if (fileName.length() > 99) {
+    if (encodedFileName.length() > 99) {
         d->writeLonglink(buffer, encodedFileName, 'L', uname.constData(), gname.constData());
     }