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Currently, the 40-bit key protection used in Office 97–2003 can be easily cracked by the password-hacking software. The 128-bit key AES protection employed in Office 2007–2010 can still be considered as a relatively secure one. At the moment, however, cloud computing facilities are capable of unlocking a substantial number of the files saved in the Office 2007–2010 format.
Considerando que, se você quiser que seus dados fiquem realmente seguros, use um produto de terceiros para criptografar o documento.