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pdfio/FAQ.md
Michael R Sweet 9f15e7397c
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Frequently Asked Questions

Why Don't You Support Writing a PDF File with Encryption?

PDF encryption offers very little protection:

  • PDF encryption keys are reused and derived from the user password (padded with a standard base string) and the object numbers in the file.
  • RC4 encryption (40- and 128-bit) was broken years ago.
  • AES encryption (128- and 256-bit) is better, but PDF uses Cipher Block Chaining (CBC) which enables attacks that allow the original encryption key to be recovered.

In addition, PDF usage controls (no print, no copy, etc.) are tied to this encryption, making them trivial to bypass.