Subject: [eap] keying appendix E vs. EMSK guidelines

From: Jari Arkko <[email-masked]>
Date: Wed, 21 Jan 2004 08:53:48 +0200

Description of issue: keying appendix E vs. EMSK guidelines
Submitter name: Jari Arkko
Submitter email address: [email-masked]
Date first submitted: Jan 21, 2004
Reference:
Document: Keying Framework
Comment type: T
Priority: 1
Section: Appendix E
Rationale/Explanation of issue:

Appendix E of keying-01 talks about the derivation of
suitable AAA Keys for handoff situations. It gives the
following formulae:

    AAA-Key-B      = PRF(EMSK(0,63),AAA-Key-A,
                     B-Called-Station-Id,Calling-Station-Id)

    AAA-Key-E      = PRF(EMSK(0,63),AAA-Key-A,
                     E-Called-Station-Id,Calling-Station-Id)

But draft-salowey-eap-keying-02 discusses EMSK usage, and
I think we have agreed at least about this:

      o The application MUST NOT use the EMSK in any other way except to
         derive Application Master Session Keys (AMSK) using the key
         derivation specified in section 3 this document.  They MUST NOT
         use the EMSK directly.

      o Applications MUST define distinct key labels and application
         specific data used in the key derivation described in section 3.

Appendix E appears to break the second requirement. Joe's
draft gives the following construction for AMSKs:

       AMSK = KDF(EMSK, key label, optional application data, length)

Perhaps appendix E could be corrected to be inline with this?
Here's the suggested text change:

    AAA-Key-B      = PRF(EMSK(0,63),"EAP AAA-Key derivation for multiple attachments",
                     AAA-Key-A,B-Called-Station-Id,Calling-Station-Id)

    AAA-Key-E      = PRF(EMSK(0,63),"EAP AAA-Key derivation for multiple attachments",
                     AAA-Key-A,E-Called-Station-Id,Calling-Station-Id)

Also, the rules about the calling and called station ids could perhaps
be made less link layer specific:

     Calling-Station-Id  = STA MAC address
     B-Called-Station-Id = AP B MAC address
     E-Called-Station-Id = AP E MAC address

=>

     Calling-Station-Id  = peer identity
     B-Called-Station-Id = second attachment point identity
     E-Called-Station-Id = third attachment point identity

--Jari

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