How to recover corrupted 7z archive How to recover corrupted 7z archive Try latest version of 7-Zip It's possible that new version of 7-Zip can solve your problems with 7z archives. So download latest version of 7-Zip and try to use that new version. You can try also latest alpha or beta version.

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If new version also doesn't help, read this manual. Required software: • 7-Zip (latest version, that can be stable, alpha or beta version). • Some program with hex viewer or editor, for example,. 7z archive structure 7z archive consists of 4 main blocks of data: • Start Header (32 bytes): it contains signature and link to End Header • Compressed Data of files • Compressed Metadata Block for files: it contains links to Compressed Data, information about compression methods, CRC, file names, sizes, timestamps and so on. • End Header: it contains link to Compressed Metadata Block. Note: If 7z archive contains only one file without encryption, 7-Zip stores Metadata for that file in End Header in uncompressed form, and there are only 3 main blocks in that case.

Archive example Archive example: a.7z (3740 bytes) that contains 5 files compressed with LZMA method. Start of archive:: 37 7A BC AF 27 1C 00 04 5B 38 BE F9 59 0E 000010: 00 00 00 00 23 00 00 00 00 00 00 00 7A 63 68 FD: 00 21 16 89 6C 71 3D AB 7D 89 E6 3C 2E BE 60 24 00: 6 bytes: 37 7A BC AF 27 1C - Signature 06: 2 bytes: 00 04 - Format version 08: 4 bytes: 5B 38 BE F9 - CRC of the following 20 bytes 0C: 8 bytes: 59 0E 00 00 00 00 00 00 - relative offset of End Header 14: 8 bytes: 23 00 00 00 00 00 00 00 - the length of End Header 1C: 4 bytes: 7A 63 68 FD - CRC of the End Header Relative offset of End Header is relative from the end of Start Header, that is at offset 0x20 (32 in decimal). Real offset of End Header in example archive = 0x20 + 0x0E59 = 0x0E79 20: 00 21 16 89. - start of compressed data.

Note: if the file was compressed with LZMA method, the first byte is always 00. If first byte is not 00, then archive uses another method (it can be LZMA2 or encrypted data with AES).

End of archive: End Header (offset = 0x0E59, length = 0x23): 0000000E70: 17 06 8D AD 01 000E80: AC 00 07 0B 01 00 01 23 03 01 01 05 5D 00 000E90: 00 0C 81 1A 0A 01 3C 70 52 F7 00 00 Possible values for first byte in End Header: 17 - End Header contains the link to Metadata Block. 01 - Metadata block is stored in End Header. Corruption types There are some possible cases when archive is corrupted: • You can open archive and you can see the list of files, but when you press Extract or Test command, there are some errors: Data Error or CRC Error. • When you open archive, you get message 'Can not open file 'a.7z' as archive' Corruption case: Data errors or CRC errors for files inside archive Here we describe the case, when you can open archive and you see the list of files, but when you press Extract or Test command, there are some errors: Data Error or CRC Error. It's pretty difficult to recover data for that case. If archive was compressed in 'Solid' mode, and you have exact copies of some files from archive, you can create similar archive with good copies of files with same settings and in same order, and replace 'bad' parts of bad.7z with 'good' parts from another good.7z. You must look listings of files in bad and good archives, logs of 'test' command, and think about ways to replace bad parts.

The are no more instructions here for that corruption case. Corruption case: Can not open file 'a.7z' as archive If you try to open or extract archive and you see the message 'Can not open file 'a.7z' as archive', it means that 7-Zip can't open some header from the start or from the end of archive. In that case you must open archive in hex editor and look to Start Header and End Header.