CREATE TABLESPACE encrypt_ts
  DATAFILE '$ORACLE_HOME/dbs/encrypt_df.dbf' SIZE 1M
  ENCRYPTION USING 'AES256' ENCRYPT;

If you do not specify an encryption algorithm, then the default encryption algorithm, AES128, is used.

Example : Creating an Encrypted Tablespace That Uses AES192

CREATE TABLESPACE securespace_1
DATAFILE '/home/user/oradata/secure01.dbf'
SIZE 150M
ENCRYPTION USING 'AES192' ENCRYPT;

Example : Creating an Encrypted Tablespace That Uses the Default Algorithm

CREATE TABLESPACE securespace_2
DATAFILE '/home/user/oradata/secure01.dbf'
SIZE 150M
ENCRYPTION ENCRYPT;

Encrypting an Existing Tablespace with Online Conversion

Create and open a master encryption key.

For example:

ADMINISTER KEY MANAGEMENT CREATE KEYSTORE 'TDE_wallet_location' IDENTIFIED BY TDE_wallet_password;
ADMINISTER KEY MANAGEMENT SET KEYSTORE OPEN IDENTIFIED BY TDE_wallet_password;
ADMINISTER KEY MANAGEMENT SET KEY IDENTIFIED BY TDE_wallet_password WITH BACKUP;
ALTER TABLESPACE users ENCRYPTION ONLINE USING 'AES192' ENCRYPT FILE_NAME_CONVERT = ('users.dbf', 'users_enc.dbf');

Decrypting an Existing Tablespace with Online Conversion

ALTER TABLESPACE users ENCRYPTION ONLINE DECRYPT FILE_NAME_CONVERT = ('users_enc.dbf', 'users.dbf');