If you met error like "ORA-12578: TNS:wallet open failed", then one possibility is that the Oracle RAC Database is using a local wallet(created with parameter -auto_login_local, which is from 11.2 release, usually local wallet is used in a highly confidential system) but the wallet is migrated from another server.
The migrated local wallet can be opened and read without problems on the new host, but the information inside does not match the hostname and this leads to the error ORA-12578: TNS:wallet open failed. Be noted that even on the original host, the wallet cannot be used by another OS user.
Master encryption key is stored in wallet in TDE(transparent data encryption), it's the key that wraps(encrypts) the Oracle TDE columns and tablespace encryption keys. The wallet must be open before you can create the encrypted tablespace and before you can store or retrieve encrypted data. Also when recovering a database with encrypted tablespaces (for example after a SHUTDOWN ABORT or a catastrophic error that brings down the database instance), you must open the Oracle wallet after database mount and before database open, so the recovery process can decrypt data blocks and redo. When you open the wallet, it is available to all session, and it remains open until you explicitly close it or until the database is shut down.
Tablespace encryption encrypts at the physical block level, can perform better than encrypting many columns. When using column encryption for tables, there is only one table key regardless of the number of encrypted columns in a table, and the table key is stored in data dictionary. And when using tablespace encryption, the tablespace key is stored in the header of each datafile of the encrypted tablespace.
Below is from here:
TDE uses a two tier key mechanism. When TDE column encryption is applied to an existing application table column, a new table key is created and stored in the Oracle data dictionary. When TDE tablespace encryption is used, the individual tablespace keys are stored in the header of the underlying OS file(s). The table and tablespace keys are encrypted using the TDE master encryption key. The master encryption key is generated when TDE is initialized and stored outside the database in the Oracle Wallet. Both the master key and table keys can be independently changed (rotated, re-keyed) based on company security policies. Tablespace keys cannot be re-keyed (rotated); work around is to move the data into a new encrypted tablespace. Oracle recommends backing up the wallet before and after each master key change.