Build 4.0.0.34 of xlsgen adds support for decrypting XLS files encrypted using the Microsoft Enhanced Cryptopgraphic provider, which uses SHA1 and RC4 algorithms. This comes in addition to the two other encryption modes which were already supported by xlsgen (XOR and regular RC4).
To decrypt and read such file, just pass the password :
xlsgen::IXlsEnginePtr engine( __uuidof(xlsgen::CoXlsEngine) );
engine->Workbooks->OpenPassword = L"knownpassword";
xlsgen::IXlsWorkbookPtr wbk = engine->Open( L"input\\protected.xls", L"output\\unprotected.xls" );
wbk->WorksheetByIndex[1]->Cell[1][1]->HtmlLabel = L"unprotected by <b>xlsgen</b>";
wbk->Close();