Excel Workbook File Open passwords to prevent unathorized users from opening and viewing the workbook.
Recovering and removing a password from a protected and closed Excel file is more time-consuming with a lower probability of success.
Most Excel recovery software uses a brute-force approach trying every possible combination of letters, numbers and symbols. A more refined approach is the dictionary-attack that only looks for “real” words and ignores nonsense combinations.
“For example, the fastest program gives about 170,000 password/second on Pentium III/800. To find an 8-character password consisting of lowercase Latin letters and digits you’ll need about 200 days.”
Brute force and dictionary attacks are appropriate if you are confident that the passwords are under 8 characters, use all letters (no numbers or symbols) and are based on ‘real’ words. (Un)fortunately most Excel passwords are not sophisticated, are too short and do not incorporate numbers or symbols.
A relatively quicker approach is “key encryption recovery”. Because there are fewer encryption keys than letters and symbols (a many to one relationship), fewer combinations have to be searched, and the recovery time can be reduced to under 30 days.
Encrypted passwords should be harder to crack but US crypto export regulations limit the key length to 40 bits.
You may either purchase the software yourself or use a professional recovery service which will apply multiple dedicated computers to the task reducing turnaround time to 2-14 days.
