What is PDF encryption and how to encrypt PDF documents & files

Is PDF encryption secure?

PDF encryption prevents those who are not authorized from using the PDF document.  But those who are authorized to use it can decrypt it and then do whatever they wish with it – copy it, send it to friends, edit it, print it and so on.  In some instances, such as transferring financial data for computer processing, encryption on its own provides all the requirements.  But to protect information in published documents you need additional controls to actively prevent a user who is authorized to see the PDF from misusing the document.

DRM controls work in addition to encryption by providing protection once the document has been decrypted.  They limit what authorized users can do with a document and prevent them sharing it with others.  However, you also need:

1. a secure method of transferring the key such as public key technology
2. the ability to lock keys to devices to ensure they cannot be shared with others
3. decryption of PDF content in memory and not to temporary files so that decrypted content is never made available
4. to prevent simple ways of grabbing decrypted content such as screen grabbing and printing to file drivers

PDF encryption can be made secure, but it requires:

• a strong implementation that does not rely on passwords
• that decrypted content is not made available
• DRM controls to enforce what can be done with the document once it is decrypted

For more information on extending controls beyond PDF encryption see PDF security and PDF digital rights management.

Is Adobe PDF encryption secure?

Adobe PDF security uses encryption to protect PDFs and allows you to apply permissions to control what a user can do with a document (print or edit it).  The PDF is encrypted with a password (the encryption key) but weak passwords (short or non-complex passwords) can be easily removed in seconds with password recovery or removal software.

Many people think that using Adobe AES 256 bit encryption is much stronger than using 128 bit encryption.  However the Adobe implementation of 256 bit encryption (introduced in Acrobat 9) uses just one call to verify the password instead of the 71 calls for 128 bit encryption.  Brute-force password attacks therefore became much quicker – back in 2008 Elcomsoft said that a single GPU could process 100 million passwords per second.  Adobe released an update to resolve the issue using PDF 1.7 Extension Level 8 (Acrobat X and above) which became part of the PDF 2.0 specification.  When you encrypt a PDF it is therefore not a good idea to provide backwards compatibility for PDF v9 files (PDF 1.7 Extension Level 3 or below).

PDF permissions (restrictions) can be easily removed regardless of how strong the password is or the encryption algorithm used, and are therefore completely useless – see Removing PDF Passwords.

What PDF encryption algorithm should I use?

The US government approved an algorithm called AES for protecting US government information that is confidential.  It has two key lengths, 128 bit and 256 bit (16 or 32 bytes of data).  Other algorithms may be available in PDF encryption software but they do not have the recommendation of the US Government.

The PDF 2.0 specification recommends AES 256 bit.  However, either key length is perfectly fine – AES 128 bit has a stronger key schedule whereas AES 256 bit is more resitant to brute force attacks.  AES 128 bit is faster and more efficient, and cracking AES 128 with a quantum computer would take about 200 times longer than the universe has existed.  As they say, size does not always matter – it is what you do with it that counts…

The most important factor is the password length and complexity (a strong password will use multiple unicode characters).  Using AES 128 bit with a complex 32 character password is more secure than using AES 256 bit with a complex 16 character password.  This is because password crackers attack the password and not the encryption algorithm because they are much easier to break.

However, as mentioned above, even if the PDF encryption software uses AES encryption, if the implementation is not secure then the protection is useless.