Re: [secdir] SecDir review of draft-ietf-jose-cookbook-06

[Richard Barnes <[e-mail]>]

On Tue, Dec 2, 2014 at 2:02 PM, ⌘ Matt Miller <[e-mail]> wrote:

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> Hello Yaron,
>
> Thank you very much for the review.  My other comments are inline.
>
> On 11/22/14, 12:39 PM, Yaron Sheffer 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 contains a large set of examples of JOSE objects:
> > JWK, JWS and JWE. The document is purely informational, though in
> > "real life", I would expect developers to use it as an
> > authoritative source.
> >
> > Summary
> >
> > The document is ready to be published, with some issues.
> >
> > By the way, I really appreciate the large effort that surely went
> > into creating this document.
> >
>
> Thank you!  It's always great to hear one's efforts are appreciated!
>
> > Details
> >
> > • Unless I missed it, the document does not mention a machine
> > readable repository of these examples, which I am sure the author
> > has created while writing the draft. Making such a repository
> > publicly available would result in a much more useful resource than
> > the current document, which essentially requires testers to scrape
> > the document when creating their test cases.
> >
>
> You did not miss it; I don't have a such a repository right now, but I
> can put one together.  Would something on github.com be acceptable, or
> is there a better suggestion?
>
> > • (Not a comment to the current document:) I wonder why there is
> > nothing explicit to distinguish a public key from a private key,
> > and they are only distinguished by the presence or absence of
> > several parameters, something that will not be natural to most
> > developers. PEM is doing it very well: "-----BEGIN RSA PRIVATE
> > KEY-----".
> >
> > • 3.4: the text is contradictory re: zero-padding of the value "d".
> > Is it using the minimum number of octets, or exactly 256 octets
> > (for a 2048-bit key)?
> >
>
> The intent is that "d" is not zero-padded, and I overqualified in my
> text.  Would the following be acceptable?
>
>    For a 2048-bit key, the field "n" is 256 octets in length when
>    decoded and the field "d" is not longer than 256 octets in
>    length when decoded.
>
>
> > • Why invent a new term "octet key", for a simple "symmetric key"?
>
> Very good point.  Will fix in the next revision.
>
> >
> > • 4.2: the first sentence discusses PS256, the actual example is
> > PS384.
> >
>
> Silly error that I thought I fixed long ago /-:
>
> Will fix in the next revision.
>
> > • 4.7: "only the JSON serialization" - please clarify which of the
> > three serializations is meant. Ditto top of 4.8.
> >
>
> Will fix in the next revision.
>
> > • 5.1.1: since this is a "cookbook", we should be using the public
> > key, not the private key. A private key would only be used in rare
> > cases. Similarly 5.2.1.
> >
>
> The private keys are included for both reproduction (which only needs
> the public key) and verification (which necessitates the private key).
>
> If I can put an online repository together, I can change the examples
> to just include the public keys; otherwise would the following in 5.1
> (and 5.2) be sufficient?
>
>    Note that only the RSA public key is necessary to perform the
>    encryption.  However, the example includes the RSA private key to
>    allow readers to validate the example's output.
>
>
> > • 5.3.1: the "plaintext content" is a list of keys, which is
> > either confusing to the reader, or an actual error in the
> > document.
> >
>
> It is not in error.  The most common usecase for password-based
> encryption was the import and export of key sets, and the Working
> Group desired a thorough example.
>
> Would it help if the following is added to 5.3?
>
>    A common use of password-based encryption is the import/export of
>    keys.  Therefore this example uses a JWK Set for the plaintext
>    content instead of the plaintext from figure 72.
>
>
> > • 5.3.5: In the General Serialization version, I don't understand
> > why only the encrypted key is per-recipient. I would expect the
> > PBES2 parameters too (e.g., the salt)  to be per-recipient.
> > Presumably each of them is using a different password, and there's
> > no reason to use a common salt. Similarly in 5.4.5.
> >
>
> For compatibility across serializations (compact, general JSON,
> flattened JSON), all of the parameters need to be in the JWE Protected
> Header.  In the general serialization, that means only the
> "encrypted_key" field is present for the (presumably) sole recipient.
>
> Would it be acceptable if the following were added to 5.3?
>
>    Note that if password-based encryption is used for multiple
>    recipients, it is expected that each recipient use different
>    values for the PBES2 parameters "p2s" and "p2c".
>
> > • 5.7: same as above, it makes sense for the per-recipient key to
> > have an ID, rather than the content encryption key (typically an
> > ephemeral key). And then that ID should be in the per-recipient
> > data in 5.7.5. And similarly for 5.8. The later Sec. 5.13 shows a
> > syntax for multiple recipients that's inconsistent with the
> > single-recipient case, which would make sense if we got rid of the
> > array.
> >
>
> For compatibility across serializations (compact, general JSON,
> flattened JSON), all of the parameters need to be in the JWE Protected
> Header.  Also, the mixing of "recipients" and "encrypted_key"/"header"
> in the top-level object is not permitted for the general serialization.
>
> > • 5.11: this example seems strange to me - why would anybody NOT
> > want to integrity-protect the key ID and algorithm? I would prefer
> > a more realistic example, or failing that, a recommendation to
> > developers to avoid this practice. Similarly 5.12, which is an even
> > worse idea.
> >
>
> Integrity protection was thoroughly discussed in the JOSE WG.  While
> there are some limited attacks possible when some parameters are
> unprotected, the WG felt there were enough use cases where these
> attacks are mitigated through other means that integrity protection of
> the part of all of the header is not always required.
>

It's also worth noting that of all of the CMS / S/MIME deployments that
have existed for many years, none of them apply integrity protection to the
CMS parameters that are equivalent to these JOSE parameters.

--Richard



>
> > Thanks, Yaron
> >
> > _______________________________________________ secdir mailing
> > list [e-mail] https://www.ietf.org/mailman/listinfo/secdir
> > wiki: http://tools.ietf.org/area/sec/trac/wiki/SecDirReview
>
> And thanks again for the thorough review.
>
> - --
> - - m&m
>
> Matt Miller < [e-mail] >
> Cisco Systems, Inc.
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