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Subject: SessionIndex and Privacy Text

Hi, some comments on Lines 977-981 in Core
1) its not clear to me whether this requirement is normative on all deployments or only on those for which privacy is a issue. The preface 'For privacy reasons' somewhat gives the impression that this is a general requirement. This contrasted to potential alternative text e.g. '*When* privacy is a concern ...'
2) when combined with the previous 'SHOULD be a small, positive integer', the 'MUST NOT be reused in subsequent assertions
given to different relying parties about the same principal' seems contradictory
    a) if small integers are used for SessionIndex, then for any reasonable number of simultaneously logged in users, the high cardinality of the individual SessionIndex values will prevent their use by rogue SPs for collusion, i.e. there will so be many users all with SessionIndex of '3' that no two SPs will be able to use this value to pinpoint a particular user. This is acknowledged by the last sentence of the para, 'MAY be a small integer value that is used in assertions issued on behalf of many different subjects at the same time'. Consequently, the restriction against reuse seems overconstraining.
I imagine that the MUST NOT reuse rule is motivated by the potential for the SessionIndex value to be something other than a small integer. We could differentiate these cases given their different aspects.
    b) if rogue SPs know that the IDP MUST NOT reuse SessionIndex values, this actually gives them more information than if the IDP were to simply insert random values (possibly within some defined range) for the SessionIndex. For instance, if two SPs are comparing logs for a bunch of users they know were simultaneously logged in at IDP, <AuthnStatement>s with the same SessionIndex can only be for different users and so can be discarded.  A bonus would be that the IDP would no longer  need to keep track of which SessionIndex's were used inter-SP (it would still need to intra-SP)
My proposed replacement text
When privacy is a concen, care must be taken to ensure that the SessionIndex element does not invalidate other privacy mechanisms - the value MUST NOT be a unique value identifying a principal's session at the authority. 
If the authority uses small positive integers (or reoccurring constants in a list) for the SessionIndex, then it SHOULD choose the range of values such that the cardinality of any one integer will be sufficiently high to prevent a particular user's actions from being correlated across multiple service providers. The authority SHOULD choose values for SessionIndex randomly from within this range (except when required to ensure unique values for subsequent AuthnStatements to the same service provider).
if the authority uses some other scheme for the SessionIndex attribute, it SHOULD choose values for SessionIndex randomly.

Paul Madsen
e:  p.madsen@entrust.com
p:  613-270-2632
c:  613-799-2632

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