The default crypt method for Type 4 is V2, which uses RC4
algorithm. Crypt method settings are in effect only when Type is 4 or above.
The CryptMethodType enumeration can take any of the following
- None – (Not supported.) An exception is thrown when it is
- Identity – No encryption.
- V2 – Uses the RC4 algorithm.
- AESV2 – Uses the AES algorithm with 128-bit encryption
- AESV3 – Uses the AES algorithm with 256-bit encryption
Because Adobe Reader does not support using more than one crypt
methods per document (i.e. stringMethod≠streamMethod), an exception
is thrown if you try to use multiple crypt methods. However,
Identity is degenerate and can be used with other crypt
The PDF 2.0 standard deprecates a number of these options as
being potentially insecure. The recommended set for PDF 2.0
compliant documents is Type
5 with SetCryptMethods AESV3.