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For single sign-on, the values of the Skew Time and the Validity Duration determine how SiteMinder FSS calculates the total time that an assertion is valid. FSS applies the skew time to the generation and consumption of assertions.
Note: In this description, the asserting party is the SAML 1.x Producer, SAML 2.0 Identity Provider, or WS-Federation Account Partner. The relying party is the SAML 1.x Consumer, the SAML 2.0 Service Provider, or the WS-Federation Resource Partner.
In the assertion document, the the NotBefore and NotOnOrAfter values represent the beginning and end of the validity interval.
At the asserting party, FSS sets the assertion validity. FSS determines the beginning of the validity interval by taking the system time when the assertion is generated. It sets the IssueInstant value in the assertion based on this time. FSS 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, FSS 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, FSS performs the same calculations as it does at the asserting party to determine if the assertion it receives is valid.
Calculating Assertion Validity if FSS is at Both Sides of the Partnership
If FSS is at both sides of a partnership, the total time the assertion is valid is the sum of the validity duration plus two times the skew time. The equation is:
Assertion Validity = 2 x Skew Time (asserting party) + Validity Duration+ 2 x Skew Time (relying party)
The initial part of the equation (2 x Skew Time + Validity Duration) represents the beginning and end of 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 FSS, the Validity Duration is only set at the asserting party.
Example
Asserting Party
The values at the asserting party are as follows:
IssueInstant=5:00PM
Validity Duration=60 seconds
Skew Time = 60 seconds
NotBefore = 4:59PM
NotOnOrAfter=5:02PM
Relying Party
The relying party takes the NotBefore and NotOnOrAfter values from the assertion and applies its skew time to those values to calculate new NotBefore and NotOnOrAfter 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).
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