Pure Python Data Mangling or Encrypting

[Randall Smith]

Chunks of data (about 2MB) are to be stored on machines using a 
peer-to-peer protocol.  The recipient of these chunks can't assume that 
the payload is benign.  While the data senders are supposed to encrypt 
data, that's not guaranteed, and I'd like to protect the recipient 
against exposure to nefarious data by mangling or encrypting the data 
before it is written to disk.

My original idea was for the recipient to encrypt using AES.  But I want 
to keep this software pure Python "batteries included" and not require 
installation of other platform-dependent software.  Pure Python AES and 
even DES are just way too slow.  I don't know that I really need 
encryption here, but some type of fast mangling algorithm where a bad 
actor sending a payload can't guess the output ahead of time.

Any ideas are appreciated.  Thanks.

-Randall