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armijnhemelfolkertdev
armijnhemel
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folkertdev
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add test case for files with extra data in between the last local file
header and the first central directory header
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‎testdata/readme.extra_data_before_central_directory‎

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Section 4.3.6 specifies the layout of a ZIP file:
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```
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4.3.6 Overall .ZIP file format:
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[local file header 1]
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[encryption header 1]
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[file data 1]
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[data descriptor 1]
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.
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.
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.
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[local file header n]
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[encryption header n]
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[file data n]
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[data descriptor n]
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[archive decryption header]
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[archive extra data record]
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[central directory header 1]
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.
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.
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.
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[central directory header n]
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[zip64 end of central directory record]
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[zip64 end of central directory locator]
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[end of central directory record]
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```
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According to this layout there can be no layout between the last local file
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header and the first central directory header, yet there are many files in
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which this happens. As an example, Android packages store a signing block right
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after the last local file header:
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<https://source.android.com/docs/security/features/apksigning/v2>
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Although this is not allowed APK files can still be unpacked, as programs use
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the central directory to find the offsets to each individual file and
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effectively skip the extra data. None of the regular ZIP programs seem to be
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able to extract this extra data.
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To create a file with extra data only one field needs to be adapted (not
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tested for ZIP64 files) in the "end of central directory" (section 4.3.16),
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namely:
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```
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offset of start of central directory with respect to the starting disk number
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```
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The file `test_with_extra_data_before_central_directory.zip` is a modified
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version of `test.zip` with an addition 1024 bytes added in between the last
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local file header and the first central directory header. This means that the
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central directory is moved an additional 1024 bytes. This can be shown by
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comparing the output of `hexdump -C` for both files:
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The position of the central directory in the original file is `ba 03`:
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```
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00000470 00 00 ba 03 00 00 1a 00 54 68 69 73 20 69 73 20 |........This is |
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```
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In the new file the central directory has been moved 1024 bytes and is now
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`ba 07`:
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```
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00000870 00 00 ba 07 00 00 1a 00 54 68 69 73 20 69 73 20 |........This is |
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```

‎testdata/test_with_extra_data_before_central_directory.zip‎

2.14 KB
Binary file not shown.

‎ziplinter/src/snapshots/ziplinter__test__test_with_extra_data_before_central_directory.zip.snap‎

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---
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source: ziplinter/src/lib.rs
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expression: result
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---
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{
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"comment": "This is a zipfile comment.",
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"contents": [
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{
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"central": {
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"comment": "",
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"compressed_size": 25,
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"crc32": 3287144384,
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"creator_version": {
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"host_system": "Unix",
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"version": 30
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},
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"disk_nbr_start": 0,
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"external_attrs": 2175008768,
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"extra": [
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85,
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84,
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5,
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0,
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3,
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113,
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252,
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130,
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76,
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117,
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120,
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0,
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1,
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20,
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0,
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0
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],
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"flags": 0,
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"header_offset": 0,
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"internal_attrs": 1,
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"method": "Deflate",
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"mode": 420,
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"modified": "2010-09-05T02:12:01Z",
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"name": "test.txt",
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"reader_version": {
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"host_system": "MsDos",
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"version": 20
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},
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"uncompressed_size": 26
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},
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"local": {
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"accessed": null,
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"compressed_size": 25,
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"crc32": 3287144384,
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"created": null,
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"extra": [
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85,
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84,
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9,
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0,
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3,
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113,
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0
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],
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"flags": 0,
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"gid": 501,
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"header_offset": 0,
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"method": "Deflate",
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"method_specific": "None",
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"mode": 0,
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"modified": "2010-09-05T02:12:01Z",
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"name": "test.txt",
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"reader_version": {
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"host_system": "MsDos",
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"version": 20
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},
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"uid": 501,
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"uncompressed_size": 26
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}
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},
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{
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"central": {
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"comment": "",
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"compressed_size": 785,
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"crc32": 1423258110,
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"creator_version": {
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"host_system": "Unix",
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"version": 30
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},
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"disk_nbr_start": 0,
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"external_attrs": 2175008768,
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"extra": [
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0
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],
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"flags": 0,
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"header_offset": 91,
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"internal_attrs": 0,
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"method": "Store",
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"mode": 420,
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"modified": "2010-09-05T05:52:58Z",
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"name": "gophercolor16x16.png",
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"reader_version": {
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"host_system": "MsDos",
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"version": 10
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},
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"uncompressed_size": 785
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},
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"local": {
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"accessed": null,
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"compressed_size": 785,
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"crc32": 1423258110,
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"created": null,
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"extra": [
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],
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"flags": 0,
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"gid": 501,
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"header_offset": 0,
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"method": "Store",
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"method_specific": "None",
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"mode": 0,
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"modified": "2010-09-05T05:52:58Z",
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"name": "gophercolor16x16.png",
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"reader_version": {
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"host_system": "MsDos",
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"version": 10
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},
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"uid": 501,
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"uncompressed_size": 785
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}
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}
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],
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"encoding": "Utf8",
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"eocd": {
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"dir": {
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"inner": {
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"dir_disk_nbr": 0,
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"dir_records_this_disk": 2,
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"directory_offset": 1978,
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"directory_records": 2,
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"directory_size": 168,
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"disk_nbr": 0
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},
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"offset": 2146
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},
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"dir64": null,
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"global_offset": 0
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},
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"parsed_ranges": [
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{
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"contains": "end of central directory record",
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"end": 2194,
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"start": 2146
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},
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{
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"contains": "central directory header",
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"end": 2056,
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"filename": "test.txt",
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"start": 1978
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},
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{
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"contains": "central directory header",
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"end": 2146,
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"filename": "gophercolor16x16.png",
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"start": 2056
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},
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{
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"contains": "local file header",
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"end": 66,
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"filename": "test.txt",
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"start": 0
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},
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{
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"contains": "file data",
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"end": 91,
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"filename": "test.txt",
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"start": 66
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},
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{
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"contains": "local file header",
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"end": 169,
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"filename": "gophercolor16x16.png",
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"start": 91
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},
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{
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"contains": "file data",
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"end": 954,
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"filename": "gophercolor16x16.png",
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"start": 169
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}
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],
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"size": 2194
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}

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