steps- 

1) created tablespace and user to the respective tablespace.
2) created table by issuing command as "CREATE TABLE TEST1 (SR_NO NUMBER(3), NAME VARCHAR2(30) ENCRYPT) DEFAULT TABLESPACE ENC". (by user - enc_user)
3) Inserted rows on to the table.
4) entry made in sqlnet.ora as 
ENCRYPTION_WALLET_LOCATION=
  (SOURCE=(METHOD=FILE)(METHOD_DATA=
    (DIRECTORY=C:\app\Administrator\admin\test\encrypted_wallet/)))
5) issued command as sysdba -
ALTER SYSTEM SET ENCRYPTION KEY IDENTIFIED BY "ASHISH123";
6) restarted the database.
7) since i want to made it autologin. so I open wallet through the below mentioned path
   program file >>oracle_home>> integrated management tool >> wallet
   But when it asked the password I typed the same password but it was not accepting. So i made new wallet with the same specified path. Also I clicked on auto login. 
8) Now, i have restarted the database and tried to issue the command "ALTER SYSTEM SET ENCRYPTION KEY IDENTIFIED BY "suresh123";

While issuing the command, getting error "ora-28374 typed master key not found in wallet". "

ENCRYPTION_WALLET_LOCATION=
  (SOURCE=(METHOD=FILE)(METHOD_DATA=
    (DIRECTORY=C:\app\Administrator\admin\test\encrypted_wallet/)))

ENCRYPTION_WALLET_LOCATION=
  (SOURCE=(METHOD=FILE)(METHOD_DATA=
    (DIRECTORY=C:\app\Administrator\admin\test\encrypted_wallet\)))