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 

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Paul 
Madsen

e:  

p:  613-270-2632

c:  613-799-2632

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