I found that the password in Excel to unlock worksheet or workbook is not
unique. That means I can use password B, C or D to unlock a worksheet or a
workbook that was locked with password A. Let me quote some examples:

Password Equivalent Unlock Passwords (just quote some for
reference)
1234 <- 9080, 1480, 3580, ¡K
3456 <- 9380, 7480, 5580, ¡K

I just tested it with MS Excel Xp SP2 / SP3. Not sure if this problem also
exists in other versions of Excel.

Perhaps the password authority is checked by certain non-one-to-one
calculations so that many inputs can result true. I also know that this can
minimize the risk of guest to find out the original password that may be
used in many other applications. However, this will also greatly increase
the possibility of guest to unlock the worksheet or workbook and do
something unexpectedly.

Could anyone tell me whether it is a bug of Excel or it is by-design?

Best regards,
Danny