Oracle 10gR2 provides Transparent Data Encryption (TDE), encryption that is transparent for the user.
When encrypting a column, Oracle creates an encryption key for the parent table and encrypts text data in the column with a user-specified encryption algorithm. The table key is encrypted using a master key and placed in the data dictionary.
The master key is stored in a secure location called a wallet, which can be a file on the database server. When a user enters data into an encrypted column, Oracle retrieves the master key from the wallet, decrypts the table key from the data dictionary, and uses it to encrypt the new data.
You can create one or more encrypted column in one or more tables. You can specify the encryption algorithm to be used, but all columns in a particular table must use the same algorithm. If you create a second encrypted column in a table, and specify a different algorithm, the last specified algorithm will be used for all columns in the table.