All strings and streams in a protected document, except those in the Encryption
dictionary, are encrypted using the RC4 encryption algorithm. This prevents
unauthorized users from simply removing the password from a PDF file to gain
access to it. Strings in the Encryption dictionary are encrypted and decrypted by
the security handler itself, using whatever encryption algorithm it chooses.
RC4 is a proprietary algorithm of RSA Data Security, Inc. Adobe Systems has
licensed this algorithm for use in its products. Independent software vendors may
be required to license this algorithm to develop software that encrypts or decrypts
PDF documents. Further information can be obtained from RSA Data Security,
Inc., 100 Marine Parkway, Redwood City, California, 95065.