I moved this to the Administration Forum where you might get an answer. Transparent Data Encryption is meant for data that resides directly in the database, not as a external table. You can use the DBMS_OBSFUSCATION toolkit to encrypt or decrypt data from external tables. But you need to write your own wrapper code to make it work and you need to store the encryption key somewhere, so that it can be accessed when needed.
Why do you want to encypt bfiles?
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