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The Microsoft Office password protection is a security feature to protect Microsoft Office (Word, Excel, PowerPoint) documents with a user-provided password. Word and Excel 95 used a 32-bit key protection algorithm, later enhanced up to 40 bits in Excel 97, and then augmented to 128-bit Advanced Encryption Standard (AES) protection in Office 2007. Currently, the 40-bit key RC4 protection used in Office 97–2003 can typically be bypassed with password hacking software. The 128-bit key AES protection employed in newer Office 2007–2010 remains secure. In fact, the distributed.net RC5 project has been trying to brute force an RC5 72-bit key since 2002, and has not successfully done so yet. Furthermore, even utilizing all known breaks (that speed up brute force attacks by a factor of about four) it would take a typical computer millions of years or longer to break a 128-bit AES key of sufficient length and complexity. == Types == Microsoft Office applications offer the use of two main groups of passwords that can be set to a document depending on whether they encrypt a password-protected document or not. Passwords that do not encrypt a password-protected document have different security level features for each of Microsoft Office applications as mentioned below. * In Microsoft Word passwords restrict modification of the entire document. * In Microsoft Excel〔(【引用サイトリンク】url=http://desktopschool.com/how-to-password-protect-excel-workbooks-and-secure-your-data/ )〕 passwords (restrict modification of the workbook ), a worksheet within it, or individual elements in the worksheet. * In Microsoft PowerPoint〔 passwords restrict modification of the entire presentation. These password types are widely believed to be created for Microsoft Office document sharing rather than preventing other people from getting access to secret data that the document may contain. Because of the lack of document encryption, all the passwords mentioned above cannot reliably protect a document from a hacker. Most password-cracking software can remove such protection from a password-protected document in very little time. The password that encrypts a document also restricts the user from opening the document. It is possible to set this type of password in all Microsoft Office applications. If a user fails to enter a correct password to the field which appears after an attempt to open a password-protected document, viewing and editing the document will not be possible. Due to the encryption of a document protected by a password to open it, a hacker needs to decrypt the document to get access to its contents. To provide an improved security, Microsoft has been consistently enhancing the Office encryption algorithm strength. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Microsoft Office password protection」の詳細全文を読む スポンサード リンク
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