For single sign-on, the values of the Skew Time and the SSO Validity Duration determine how long an assertion is valid. The Policy Server applies the skew time to the generation and consumption of assertions. In the assertion document, the NotBefore and NotOnOrAfter values represent the beginning and end of the validity interval.
At the asserting party, the Policy Server sets the assertion validity. The Policy Server determines the beginning of the validity interval by taking the system time when the assertion is generated. The software sets the IssueInstant value in the assertion from this time. The Policy Server then subtracts the skew time value from the IssueInstant value. The resulting time becomes the NotBefore value.
NotBefore=IssueInstant - Skew Time
To determine the end of the validity interval, the Policy Server adds the Validity Duration value and the skew time to the IssueInstant value. The resulting time becomes the NotOnOrAfter value.
NotOnOrAfter=Validity Duration + Skew Time + IssueInstant
Times are relative to GMT.
For example, an assertion is generated at the asserting party at 1:00 GMT. The skew time is 30 seconds and the validity duration is 60 seconds, making the assertion validity interval between 12:59:30 GMT and 1:01:30 GMT. This interval begins 30 seconds before the time the assertion was generated and ends 90 seconds afterward.
At the relying party, the Policy Server performs the same calculations as it does at the asserting party to determine if the assertion it receives is valid.
Calculating Assertion Validity when CA SiteMinder® is at Both Sides of the Partnership
The total time the assertion is valid is the sum of the SSO validity duration plus two times the skew time. The equation is:
Assertion Validity = 2x Skew Time (asserting party) + SSO Validity Duration + 2x Skew Time (relying party)
The initial part of the equation (2 x Skew Time + SSO Validity Duration) represents the validity window at the asserting party. The second part of the equation (2 x Skew Time) represents the skew time of the system clock at the relying party. You multiply by 2 because you are accounting for the NotBefore and the NotOnOrAfter ends of the validity window.
Note: For the Policy Server, the SSO Validity Duration is only set at the asserting party.
Example
The values at the asserting party are as follows:
IssueInstant=5:00PM
SSO Validity Duration=60 seconds
Skew Time = 60 seconds
NotBefore = 4:59PM
NotOnOrAfter=5:02PM
The relying party takes the NotBefore and NotOnOrAfter values that is receives in the assertion then applies its skew time to calculate new values.
Skew Time = 180 seconds (3 minutes)
NotBefore = 4:56PM
NotOnOrAfter=5:05PM
Based on these values, the calculation for the total assertion validity window is:
120 seconds (2x60) + 60 seconds + 360 seconds (2x180) = 540 seconds (9 minutes).
|
Copyright © 2013 CA.
All rights reserved.
|
|