Oracle Database B10772-01 Administrator's Manual page 393

If you use this parameter to seed the random number
Note:
generator, then Oracle recommends that you enter as many
characters as possible, up to 70, to make the resulting key more
random and therefore stronger.
If you do not use this parameter, the system uses various sources of random
numbers, depending on your operating system, to seed the random number
generator.
Data Encryption and Integrity Parameters
Data Encryption and Integrity Parameters A-9