Index: src/OFLHAArchive.m
==================================================================
--- src/OFLHAArchive.m
+++ src/OFLHAArchive.m
@@ -69,11 +69,11 @@
 	OFLHAArchive *_archive;
 	OFMutableLHAArchiveEntry *_entry;
 	OFStringEncoding _encoding;
 	OFSeekableStream *_stream;
 	OFStreamOffset _headerOffset;
-	uint32_t _bytesWritten;
+	uint64_t _bytesWritten;
 	uint16_t _CRC16;
 }
 
 - (instancetype)of_initWithArchive: (OFLHAArchive *)archive
 			    stream: (OFSeekableStream *)stream
@@ -552,28 +552,28 @@
 - (size_t)lowlevelWriteBuffer: (const void *)buffer length: (size_t)length
 {
 	if (_stream == nil)
 		@throw [OFNotOpenException exceptionWithObject: self];
 
-	if (UINT32_MAX - _bytesWritten < length)
+	if (UINT64_MAX - _bytesWritten < length)
 		@throw [OFOutOfRangeException exception];
 
 	@try {
 		[_stream writeBuffer: buffer length: length];
 	} @catch (OFWriteFailedException *e) {
 		OFEnsure(e.bytesWritten <= length);
 
-		_bytesWritten += (uint32_t)e.bytesWritten;
+		_bytesWritten += (uint64_t)e.bytesWritten;
 		_CRC16 = OFCRC16(_CRC16, buffer, e.bytesWritten);
 
 		if (e.errNo == EWOULDBLOCK || e.errNo == EAGAIN)
 			return e.bytesWritten;
 
 		@throw e;
 	}
 
-	_bytesWritten += (uint32_t)length;
+	_bytesWritten += (uint64_t)length;
 	_CRC16 = OFCRC16(_CRC16, buffer, length);
 
 	return length;
 }
 

Index: src/OFLHAArchiveEntry.m
==================================================================
--- src/OFLHAArchiveEntry.m
+++ src/OFLHAArchiveEntry.m
@@ -198,10 +198,26 @@
 	entry->_modificationDate = nil;
 
 	entry->_modificationDate = [[OFDate alloc]
 	    initWithTimeIntervalSince1970: modificationDate];
 }
+
+static void
+parseFileSizeExtension(OFLHAArchiveEntry *entry, OFData *extension,
+    OFStringEncoding encoding)
+{
+	uint64_t tmp;
+
+	if (extension.count != 17)
+		@throw [OFInvalidFormatException exception];
+
+	memcpy(&tmp, (char *)extension.items + 1, 8);
+	entry->_compressedSize = OFFromLittleEndian64(tmp);
+
+	memcpy(&tmp, (char *)extension.items + 9, 8);
+	entry->_uncompressedSize = OFFromLittleEndian64(tmp);
+}
 
 static bool
 parseExtension(OFLHAArchiveEntry *entry, OFData *extension,
     OFStringEncoding encoding, bool allowFileName)
 {
@@ -217,10 +233,13 @@
 		function = parseDirectoryNameExtension;
 		break;
 	case 0x3F:
 		function = parseCommentExtension;
 		break;
+	case 0x42:
+		function = parseFileSizeExtension;
+		break;
 	case 0x50:
 		function = parsePermissionsExtension;
 		break;
 	case 0x51:
 		function = parseGIDUIDExtension;
@@ -599,10 +618,11 @@
 	OFMutableData *data = [OFMutableData dataWithCapacity: 24];
 	const char *fileName, *directoryName;
 	size_t fileNameLength, directoryNameLength;
 	uint16_t tmp16;
 	uint32_t tmp32;
+	uint64_t tmp64;
 	size_t headerSize;
 
 	if ([_compressionMethod cStringLengthWithEncoding:
 	    OFStringEncodingASCII] != 5)
 		@throw [OFInvalidArgumentException exception];
@@ -610,11 +630,11 @@
 	getFileNameAndDirectoryName(self, encoding, &fileName, &fileNameLength,
 	    &directoryName, &directoryNameLength);
 
 	if (fileNameLength > UINT16_MAX - 3 ||
 	    directoryNameLength > UINT16_MAX - 3 ||
-	    _compressedSize > UINT32_MAX || _uncompressedSize > UINT32_MAX)
+	    _compressedSize > UINT64_MAX || _uncompressedSize > UINT64_MAX)
 		@throw [OFOutOfRangeException exception];
 
 	/* Length. Filled in after we're done. */
 	[data increaseCountBy: 2];
 
@@ -675,10 +695,24 @@
 		[data addItem: "\x3F"];
 		[data addItems: [_fileComment cStringWithEncoding: encoding]
 			 count: fileCommentLength];
 	}
 
+	/*
+	 * Always include the file size extension, as the header can be written
+	 * with size 0 initially and then rewritten with the actual size in
+	 * case the data to be archived is being streamed - but for that we
+	 * need to make sure we always have the space.
+	 */
+	tmp16 = OFToLittleEndian16(19);
+	[data addItems: &tmp16 count: sizeof(tmp16)];
+	[data addItem: "\x42"];
+	tmp64 = OFToLittleEndian64(_compressedSize);
+	[data addItems: &tmp64 count: sizeof(tmp64)];
+	tmp64 = OFToLittleEndian64(_uncompressedSize);
+	[data addItems: &tmp64 count: sizeof(tmp64)];
+
 	if (_POSIXPermissions != nil) {
 		tmp16 = OFToLittleEndian16(5);
 		[data addItems: &tmp16 count: sizeof(tmp16)];
 		[data addItem: "\x50"];