Re: [secdir] Secdir review of draft-ietf-perc-double-10

Eric Rescorla <> Thu, 01 November 2018 21:25 UTC

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From: Eric Rescorla <>
Date: Thu, 1 Nov 2018 14:25:14 -0700
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To: Radia Perlman <>
Cc: Richard Barnes <>,, IESG <>,
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Subject: Re: [secdir] Secdir review of draft-ietf-perc-double-10
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On Wed, Oct 31, 2018 at 6:45 PM Radia Perlman <>

> On Wed, Oct 31, 2018 at 12:30 PM Richard Barnes <> wrote:
>> Hi Radia,
>> Thanks for the review.  Responses to the substantive comments inline.
>> Will take a look at the editorial ones in a bit.
>> On Wed, Oct 31, 2018 at 2:25 PM Radia Perlman <>
>> wrote:
>>> I have reviewed this document as part of the security directorate's
>>> ongoing effort to review all IETF documents being processed by the IESG.
>>> These comments were written primarily for the benefit of the security area
>>> directors.  Document editors and WG chairs should treat these comments just
>>> like any other last call comments.
>>> This document specifies a syntax for doubly encrypting Secure Real Time
>>> Protocol (SRTP) packets so that the inner (media) content is encrypted with
>>> a key known only to the endpoints while some header content is encrypted
>>> with a separate key known by both the endpoints and network relay points
>>> called Media Distributors (with the outer encryption key (usually) changing
>>> on each hop).
>>> Below are details for consideration by the authors and other potential
>>> security reviewers.
>>> Substantive comments:
>>> Page 2 First paragraph:
>>> AES-GCM is the only specified cryptographic mode, but may not be the
>>> appropriate mode to use in distributing this content because there may be
>>> cases where we want to guarantee the integrity of the data end-to-end while
>>> allowing intermediaries to see the content. To support this case, it might
>>> make sense to allow the encryption and integrity protection keys to be
>>> separate so that an intermediary might be given the encryption key but not
>>> the integrity protection key.
>> I think you mean we would give the intermediary the integrity key, not
>> the encryption key :)  Since we want to enable the intermediary to modify
>> things, but not read the content.
> No, I really mean an intermediary might need the encryption key, so it can
> read the contents, but not the integrity key, because it shouldn't modify
> the contents.  An example is an intermediary scanning for viruses.  It
> needs to see the contents, but is not going to modify the contents. That
> way the integrity check really means that the source is vouching for the
> authenticity of the content ("end-to-end integrity")

This is a common thing that people want to do, but it's not a primary
design objective of PERC.

I can't think of any cases of an intermediary needing to modify encrypted
> contents, and therefore needing the integrity key but not the encryption
> key.

You don't generally want to modify encrypted contents but rather have some
of the contents (the meta-data) only encrypted HBH


> I'll look at the rest of the comments later..
> Radia