irule to decode using base64

Hi All,

Need help in irule. Currently using the below irule to capture username and password which is in plain text. The username/password is sent as query string in uri and this is working fine for me.

Query sent is like this : https://tryloginsso.help.com.ca?username=67580&password=login@123

Current irule is as below:

when ACCESS_SESSION_STARTED {
set username [URI::query [HTTP::uri] "username"]
set password [URI::query [HTTP::uri] "password"]
ACCESS::session data set session.logon.last.username $username
ACCESS::session data set session.logon.last.password $password
log local0. "vvvvv $username"
log local0. "vvvvv $password"
log local0. session.logon.last.usernam 
}

As credentials sent is in plain text and concern about security. Query string is encoded using base64 as below from code. I want to write irule to decode the query string using base64 pass it for authentication.

var useridBytes = Encoding.UTF8.GetBytes(userId);
string userId_Encoded = Convert.ToBase64String(useridBytes);

var pwdBytes = Encoding.UTF8.GetBytes(pwd);
string pwd_Encoded = Convert.ToBase64String(pwdBytes);

Let me know what irule can be written to achieve this. Thanks in advance.