Open the sqlnet.ora and look for statements like
SQLNET.AUTHENTICATION_SERVICES = (NONE)
sqlnet.crypto_seed = "some text"
and make a copy 1st then comment out these lines using a "#" sign
at the head of the lines.
(from : http://www.orafaq.net/msgboard/sqlnet/messages/972.htm)
OR:
In your SQLNET.ORA, add the following line
SQLNET.AUTHENTICATION_SERVICES =(NONE)
Try it, please