Read/write Excel 2007 password-protected documents

Question

What method does Office 2007 use for encryption (when choosing Encrypt and setting a password from Office menu)?

My C# app needs to create and read encrypted Excel 2007 files (.xlsx). It is important that these files remain accessible from Excel, so I must use Microsoft's encryption method, can't brew my own.

Normal Excel 2007 file is a ZIP-compressed file, and I'm accessing it using ExcelPackage, which internally uses * System.Io.Packaging.Package* (part of .net 3.0).

However, the encryption in Office is not the standard ZIP encryption. The Package class seems not to support encryption, and reports a corrupted file. 7Zip opens the file (with no password provided) and shows a few binary files inside.

score 3 · Answer 1 · answered Oct 01 '09 at 09:21

3

Office 2007 uses an OLE document (the same container format that is used for the binary Office documents) to store encrypted documents.

[MS-OFFCRYPTO]: Office Document Cryptography Structure Specification should contain the specification of the used data structures and algorithms.

answered Oct 01 '09 at 09:21

Dirk Vollmar

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Thanks. That's a rather long spec. I'm still hoping for a black box :) – dbkk Oct 01 '09 at 09:33
Well, one black box you can use would be to automate Office. But I don't know about any third-part library that can handle encryption. – Dirk Vollmar Oct 01 '09 at 09:35

dbkk · Accepted Answer · 2009-10-07T08:43:46.680

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Based on several open source bits and pieces, I created a OoXmlCrypto stream wrapper to access Office 2007 encrypted files easily.

This is based on a blog post with source code which does OOXML encryption/decryption.

edited Oct 07 '09 at 08:43

answered Oct 01 '09 at 19:03

dbkk

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Have you used it since then? It is not so good documented, so I cannot figure out, when and how to use it ? – HeinrichStack Jun 03 '13 at 14:24
Any Usage Syntax for ooxmlcryto Stream library? – Raky Sep 10 '22 at 09:13

Eugene Mayevski 'Callback · Answer 3 · 2010-08-22T09:47:42.043

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JFYI: our SecureBlackbox product offers components for encryption and decryption of OOXML documents among many other functions.

edited Aug 22 '10 at 09:47

answered Jul 24 '10 at 21:50

Eugene Mayevski 'Callback

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Read/write Excel 2007 password-protected documents

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