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Subject: RE: Contradictory requirements?
Tim, an attempt has been made to address some of the issues in the bindings document. Briefly, support for intermediaries is a property of a specific profile or protocol binding. The web browser profile, for example, does not permit any intermediaries: users MUST transit directly from an AP to a RP and the token semantics are specific to (AP, RP) pairs. Those profiles/bindings that permit the use of (untrusted) intermediaries must explain how the assertions are protected against misuse through tampering or impersonation. In some cases, confidentiality may also be required and this generates another level of requirement on the profile/binding. In the SOAP profile, for example, intermediaries are permitted. The concept of attachment integrity of assertions is introduced precisely to support an end-to-end security model (only the original "holder" of the assertions could have attached them to the document). This still leaves the question of confidentiality open: until we have the XML-encryption specification in hand, we cant mandate the use of encryption just for assertion elements. I am not sure we need to worry about "trusted" intermediaries. My thinking would be that all "interesting" intermediaries are untrusted. Maybe the language of [R-Confidentiality] needs to be revised. - prateek -----Original Message----- From: Tim Moses [mailto:tim.moses@entrust.com] Sent: Tuesday, June 19, 2001 9:00 AM To: 'OASIS Security Services group' Subject: Contradictory requirements? Colleagues - Here are some excerpts from our requirements documents, with commentary ... "[R-Bindings] ... SAML should define bindings ... to ... the following protocols: standard commercial browsers ..." - This suggests a "token-style" authentication scheme as described in draft-sstc-bindings-model-04.doc, Section 3.1. "[R-Confidentiality] SAML data should be protected from observation by ... untrusted intermediaries." - This suggests that intermediaries may be of two types: trusted and untrusted. "[R-Intermediaries] SAML data ... will be structured in a way that they can be passed from an asserting party through one or more intermediaries to a relying party. The validity of the message or assertion can be established without requiring a direct connection between the asserting and relying parties." - This prompts a number of questions: "Are the intermediaries referred to of the trusted or untrusted type?" I assume that (in order to satisfy the "single-sign-on", "third-party security service" and "application chain" scenarios) the intermediaries referred to must have access to the contents of messages and assertions. But, must the ultimate relying party trust them not to deliberately, or accidentally, impersonate the principal, or cause another browser to impersonate the principal? The scenarios referred to in the last paragraph are written in terms of chains with a single intermediary. But, the requirement includes the phrase "one or more". So, should we think of the scenarios as just a special case of the more general requirement in which chains may contain many intermediaries? If intermediaries have to be of the trusted type, then how will a relying party tell what intermediaries have handled the token and whether or not they should be trusted? If intermediaries may be of the untrusted type, then it seems unlikely that we will find a solution that can satisfy both the [R-Intermediaries] and the [R-Bindings] requirements. We may have to remove the second sentence of the [R-Intermediaries] requirement. Thoughts anyone? Best regards. Tim. ---------------------------------------------------------------------------- ----------- Tim Moses Tel: 613.270.3183
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