Build 3.0.0.79 of xlsgen makes it possible to open encrypted XLSX files.

Just like encrypting files, decrypting takes a password. xlsgen takes care of everything else (AES, SHA, ...), including default encryption configurations used by Excel 2007, 2010, 2013 (they are different).

Here is how it works (C++ code follows) :

xlsgen::IXlsEnginePtr engine( __uuidof(xlsgen::CoXlsEngine) );

// pass "abcd" as open password
engine->Workbooks->OpenPassword = L"abcd";
// now we can attempt to open the XLSX encrypted file
xlsgen::IXlsWorkbookPtr workbook = engine->Open( L"input\\Book1_Excel_protected.xlsx", L"" );
//...
workbook->Close();