Understanding PDF Encryption and Password Protection

By default, Adobe PDF documents are saved in plaintext with no password and can be accessed and viewed by anyone for any purpose.You may, however, want a little more control over your PDF document and the information it contains and only allow certain people to view it, and possibly even prevent it from being copied or printed.

This comes in handy id you wish to allow only certain people to be able to read your information when it contains confidetial information such as financial information, health information, legal information and more.

PDF control and protection requires a password which is used for the PDF encryption key. Without this password the content of the PDF file is unintelligible. This is because the encryption algorithm scrambles the data using a complicated mathematical algorithm that is dependent on the key or password you had selected, and that data may be un-scrambled using the same key.

Typically, PDF documents are encrypted by 40-bit keys (low security) and 128-bit keys (for high security). 40-bit encryption is not considered very safe these days as powerful computers can crack them in a matter of hours. 128-bit encryption is much more secure (it would take 2^88 times longer to crack them) so the same brute force cracking methods could take thousands of years. In other words, the longer key increases the number of possible encryption keys by 309,485,009,821,345,068,724,781,056. So if a 40-bit encrypted PDF took 1 day to crack, it would take up to 309,485,009,821,345,068,724,781,056 days to crack a 128-bit encrypted document. 128-bit is thefore highly recommended and is safe for this forseeable future.

You can set two passwords to any PDF document: an "owner" password and a "user" password. Either of these passwords on their own allow the PDF to be decrypted. When the owner password is used the document can be read, copy-paste may be used and it may be printed out. When the user password is used, the PDF creator can apply various restrictions on the PDF document. Implementing these latter restrictions is up to the PDF viewer application, so a different viewer may ignore these restrictions provided the document had already been decrypted. The restrictions can restrict a user from doingfrom printing the document, from selecting content, performing copy-paste operations, etc.