All the bytes are changed using the key – unless they are altered back using the same key, they are rubbish and the file cannot be opened. The only way to work it out is to try random combinations – and there are a lot of them…Įncryption is actually done using a key-value (usually 32 or 128 bits in size). But it is much harder to have some data and workout the password from it. Encryption works in a similar way – given a password, it is easy to encode and decode some data. But it is much harder to do in reverse – given 42 as a value, there is an infinite number of sums. For example, the question ‘what is 6 times 7?’ is easy – it’s 42 (indeed the answer to life, the Universe and Everything according to the writer Douglas Adams). PDF Encryption relies on the fact that some mathematical functions are much harder to reverse than to do. If you do not have the password, you can still copy the file but not open it. So Acrobat files can be protected with 2 passwords – an Owner password which also allows the user to alter the file and a more limited User password. One of the many reasons people do not put their content online is worries about security – they worry that the material can be repeatedly copied and they will have no control over it. He has an MA in Medieval History and a passion for reading. Mark Stephens Mark has been working with Java and PDF since 1999 and is a big NetBeans fan.
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