Check crc on tar header

src/SharpCompress/Common/Tar/Headers/TarHeader.cs

@@ -134,6 +134,12 @@ internal bool Read(BinaryReader reader)
             hasLongValue = false;
         } while (hasLongValue);
 
+        // Check header checksum
+        if (!checkChecksum(buffer))
+        {
+            return false;
+        }
+
         Name = longName ?? ArchiveEncoding.Decode(buffer, 0, 100).TrimNulls();
         EntryType = entryType;
         Size = ReadSize(buffer);
@@ -311,6 +317,42 @@ private static long ReadAsciiInt64(byte[] buffer, int offset, int count)
         (byte)' '
     };
 
+    internal static bool checkChecksum(byte[] buf)
+    {
+        const int eightSpacesChksum = 256;
+        var buffer = new Span<byte>(buf).Slice(0, 512);
+        int posix_sum = eightSpacesChksum;
+        int sun_sum = eightSpacesChksum;
+
+        foreach (byte b in buffer)
+        {
+            posix_sum += b;
+            sun_sum += unchecked((sbyte)b);
+        }
+
+        // Special case, empty file header
+        if (posix_sum == eightSpacesChksum)
+        {
+            return true;
+        }
+
+        // Remove current checksum from calculation
+        foreach (byte b in buffer.Slice(148, 8))
+        {
+            posix_sum -= b;
+            sun_sum -= unchecked((sbyte)b);
+        }
+
+        // Read and compare checksum for header
+        var crc = ReadAsciiInt64Base8(buf, 148, 7);
+        if (crc != posix_sum && crc != sun_sum)
+        {
+            return false;
+        }
+
+        return true;
+    }
+
     internal static int RecalculateChecksum(byte[] buf)
     {
         // Set default value for checksum. That is 8 spaces.

tests/SharpCompress.Test/Tar/TarArchiveTests.cs

@@ -287,4 +287,12 @@ public void Tar_Read_One_At_A_Time()
 
         Assert.Equal(2, numberOfEntries);
     }
+
+    [Fact]
+    public void Tar_Detect_Test()
+    {
+        var isTar = TarArchive.IsTarFile(Path.Combine(TEST_ARCHIVES_PATH, "false.positive.tar"));
+
+        Assert.False(isTar);
+    }
 }