Re: [secdir] Early SecDir Review of draft-ietf-p2psip-share-08

"Thomas C. Schmidt" <> Mon, 15 August 2016 19:20 UTC

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To: Russ Housley <>, "" <>
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From: "Thomas C. Schmidt" <>
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Date: Mon, 15 Aug 2016 21:20:40 +0200
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Cc: Alissa Cooper <>, IETF SecDir <>
Subject: Re: [secdir] Early SecDir Review of draft-ietf-p2psip-share-08
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I'm only now coming back to this long delayed issue - sorry for that.

Let me summarize the story about indexing (Section 3.1 in ShaRe):

  1. The application use case for shared resources is that of a small 
group of peers sharing a data structure, which can be an array. Array 
indexing according to Sec. 3.1 uses parts of the Node-ID (the 24 least 
significant bits of a SHA-1 hash) to generate isolation among peers, and 
at the same time generate an unambiguous, unique binding between a node 
and the array elements it is allowed to write.

  2. The threat of collisions is that this binding becomes ambiguous and 
- if not prevented - would cause an option for theft of 
resource/service. There is no threat of privilege escalation, here, as 
entries are signed.

  3. SHA-1 is collision-resistant and the probability of collisions is 
expected to be low [1], but the output is not perfectly random. So I 
agree that the ref to the RFC 3550 calculation is a bit too optimistic. 
However, neither me nor a crypto colleague could find a rigorous 
calculation of the SHA-1 collision probability ...

  4. The straight-forward counter measure against theft of resources in 
the case of a collision is a refusal of overwriting by the storing peer 
(see end of Section 6.1). This may exclude a node with colliding ID bits 
from participation in sharing a resource.

Now I see two choices:

  (1) We leave it that way, i.e., clarify the text in Section 3.1 and 
point out that a node under collision could re-join the overlay with a 
different node-ID.

  (2) We could advise a procedure to generate non-colliding ID bits by 
rehashing the node-ID. I.e., a node that experiences a collision could 
rehash its ID to obtain new ID bits and the storing peer could validate 
by also iterating the hashing.

(2) would complicate the whole process for considerably rare cases, why 
I'm in favor of (1).

What would you suggest?


[1] K. Chung, M. Mitzenmacher, and S. Vadhan: Why Simple Hash Functions 
Work: Exploiting the Entropy in a Data Stream, Theory of Computing , vol 
9, pp. 897-945.

On 01.04.2016 01:21, Russ Housley wrote:
> I reviewed this document for the Security Directorate after a request
> by the ART AD for an early review.
> Version reviewed: draft-ietf-p2psip-share-08
> Summary: Not Ready (from a Security Directorate perspective)
> Major Concerns:
> In Section 3.1, there is an algorithm for assigning index values, and
> the text says that the high-order 24 bits of the Node-ID serve as a
> pseudo-random identifier.  Since these 24 bits are obtained from the
> certificate that will be used to sign the stored data, the I think that
> the same bits will be used over and over.  If I got this correct, then
> they are not pseudo-random.
> In addition, Section 3.1 points to RFC 3550, Section 8.1 for a
> discussion of the probability of a collision.  The consequences of a
> collision seem to be different in the two documents.  The consequences
> of a collision in the index should be clearly described in this
> document.
> Minor Concerns:  None
> Nits:
> Please pick one spelling for Resource-IDs. (This is the spelling used
> in RFC 6940.)  However, this document sometimes uses "Resource Id".
> Section 4.1 includes several examples for array indices.  All of
> them are more than 32 bits: 0x123abc001, 0x123abc002, 0x123abc003,
> 0x123abc004, and 0x456def001.  The most straightforward solution is
> to drop one of the zero digits.
> To improve readability, I think the first sentence of Section 5.1
> should read: "In certain use cases, such as conferencing, it is
> desirable..."
> Section 5.1 says:
>    When defining the pattern, care must be taken to avoid conflicts
>    arising from two user names of witch one is a substring of the other.
> I think this paragraph would be improved with an acceptable example and
> a problematic example.
> In Section 5.3: s/AOR/Address of Record (AOR)/
> In Section 6.2: s/This allows to invalidate entire subtrees/
>                  /This allows the invalidation of entire subtrees/
> In Section 8, please provide a reference for RELOAD.


Prof. Dr. Thomas C. Schmidt
° Hamburg University of Applied Sciences                   Berliner Tor 7 °
° Dept. Informatik, Internet Technologies Group    20099 Hamburg, Germany °
°                   Fon: +49-40-42875-8452 °
°    Fax: +49-40-42875-8409 °