I've looked with the whole group of "Oracle Database 11g Release 2 (11.2) Documentation" trying to find the particulars from the master file encryption key employed for transparent data file encryption (TDE) with HSM. There is merely a single minor line from the whole group of documentation that states it utilizes AES.

What surprised me was that not sure was pointed out from the AES mode (CBC, ECB, etc) and also the key length for that master key which was produced while using following command if this might have been information for many. Did I miss something? Does anybody has any ideas?

SQL> ALTER SYSTEM SET ENCRYPTION KEY IDENTIFIED BY "user_Id:password"

The next response is as cited from Oracle Forum.

"Oracle TDE asks the HSM via PKCS#11 particularly to create an AES256 key."

Obtain the Oracle Database Advanced Security Administrator Guide 11g R2 (E10746-02): http://www.foodpicky.com/download/docs/Oracle/Oracle_ASO_AdminGuide_11gR2_e10746.pdf