Re: [TLS] secdir review of draft-ietf-tls-ecdhe-psk-aead-03

[Daniel Migault <daniel.migault@xxxxxxxxxxxx>]

Hi Benjamin,

Thank you for the review. Please find my comments inline and let me know if you agree with the proposed text. I believe the only point not addressed concerns the addition of CCM-256 which has been remove after discussion during the WGLC.

Thanks you for the review,

Yours,

Daniel

On Fri, May 19, 2017 at 12:38 AM, Benjamin Kaduk <kaduk@xxxxxxx> wrote:

Hi all,

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 is ready with nits.

Essentially, we are filling in a gap in the TLS (< 1.3) ciphersuite
space, thought of as a cross product of key exchange and
cipher+mac/AEAD -- we have some of the combinations (PSK with ECDHE
but no AES-[GC]CM, PSK with AES-GCM but only non-EC DH, etc.) but
not quite this one.

That said, it seems a little silly to only partially fill the gap
(by omitting the AES_256_CCM* cipher suites), even though there is
not currently demand for them.

MGLT: TLS_ECDHE_PSK_WITH_AES_256_CCM_8_SHA256 and TLS_ECDHE_PSK_WITH_AES_256_CCM_SHA384  were mentioned in the 01 and removed after  the WGLC. [0] The issue was whether or not introducing new ciphers not supported by TLS1.3. In 01, we though of adding the code point for TLS1.2 and then specify that TLS1.3 was only implementing a **subset** of the cipher suites proposed.In case of needs these could still be supported later by TLS1.3. The consensus seems to not introduce ciphers that would not be handled by TLS1.3. If the WG decides otherwise, these could still be added.   

[0] https://mailarchive.ietf.org/arch/msg/tls/M442CwmUMxrYJR8FjCh3h-a69o4/?qid=6e4713be1d71bae6718c6e6e6c4b8007

 

This document is just assembling pieces that were already specified
elsewhere, so it need not contain much detail itself, which is fine.
That said, I think section 3 should probably state explicitly which
pieces it uses, instead of a vague reference of being "based on RFC
4279".  So, "The ServerKeyExchange and ClientKeyExchange messages
from RFC 5489 for ECDHE_PSK are used, and the premaster secret is
computed in the same manner as for ECDHE_PSK key exchange in RFC
5489."  (I am not sure why RFC 4279 is cited in the current text; it
does not cover ECDHE_PSK.)

MGLT: I agree we coudl be more explicit, here are the changes I propose. I believe it address your purpose.

OLD:

Messages and pre-master secret construction in this 
document are based on [RFC4279].

NEW:

Messages and pre-master secret construction in this 
document are defined in [RFC5489]. The ServerKeyExchange
and ClientKeyExchange messages and premaster secret is 
computed as for  the ECDHE_PSK key exchange.  

The premaster_secret structure so used basically ends up putting the
ECDH output first followed by the static PSK; with the pre-TLS 1.2
PRF, that would give the ECDH shared secret to md5 and the PSK to
sha1, which is perhaps another reason to not use these with pre-1.2
worth mentioning (in addition to the AEAD availability).

MGLT: I believe the following text address your concern, however I am wondering if we are not going beyond the scope of expected considerations. 

In addition, it is worth noting that TLS1.0 <xref target="RFC2246"/> and TL1.2 <xref target="RFC4346"/> splits the premaster in two parts. The PRF results of mixing the two pseudorandom streams with distinct hash functions (MD5 and SHA-1) by exclusive-ORing them together. In the case of ECDHE_PSK authentication, the PSK and pre-master are treated by distinct hash function with distinct properties. This may introduce vulnerabilities over the expected security provided by the constructed pre-master. As such TLS1.0 and TLS1.1 SHOULD NOT be used with ECDHE_PSK.

 

The security considerations largely refer to those of the other
documents providing the pieces that are combined together here;
those referenced security considerations sections are more than
adequate here, as this document itself does not really do anything
particularly novel.  That said, if we are going to reiterate the
entropy requirement for PSKs inline, we probably ought to also
reiterate the nonce-reuse considerations for GCM and CCM.  The
relevant constructions help, but there are still ways to mess up and
reuse a nonce when doing crypto in parallel, if I remember the
GCM/TLS document's security considerations correctly.

MGLT: I believe the following text address your comments.

 <t>GCM or CCM encryption - even of different clear text - re-using a
nonce with a same key undermines the security of GCM and CCM. As a
result, GCM and CCM MUST only be used with system guaranteeing nonce
uniqueness <xref target="RFC5116"/>.</t>

Some other editorial nits follow.

I see we're already discussing "perfect forward secrecy" vs.
"forward secrecy"; my preference is for just "forward secrecy" (and
I may have been one of the more active voices in causing TLS 1.3 to
switch), but in the grand scheme of things it doesn't really matter.

MGLT: perfect forward secrecy has been removed and replaced by forward secrecy.
 

In section 2, the RFC 7525 reference that "AEAD algorithms [...] are
strongly recommended" is scoped to just (D)TLS, so the text here
should probably also list that qualification.

 

MGLT: I believe the following text address your concern:

OLD text:
<t>AEAD algorithms that combine encryption and integrity protection are
strongly recommended for <xref target="RFC7525" /> and non-AEAD algorithms
are forbidden to use in TLS 1.3 <xref target="I-D.ietf-tls-tls13"/>.

NEW text:
<t>AEAD algorithms that combine encryption and integrity protection are
strongly recommended for (D)TLS <xref target="RFC7525" /> and non-AEAD algorithms
are forbidden to use in TLS 1.3 <xref target="I-D.ietf-tls-tls13"/>.

In section 4, "... make use of the authenticated encryption with
additional data (AEAD) defined in TLS 1.2" seems to make AEAD into
something of a unique object, as opposed to a class of things with
multiple possible instantiations.  I would consider something like
"... (AEAD) concept".

In section 4, "these cipher suites MUST NOT be negotiated in TLS
versions prior to 1.2" should probably clarify that "these" cipher
suites are the new ones specified by this document.

MGLT: I believe the following text address your comment:

OLD:
these cipher suites MUST NOT be negotiated in TLS  versions prior to 1.2.

NEW:
the cipher suites defined in this document MUST NOT be negotiated in TLS
versions prior to 1.2.

 OLD:
Clients MUST NOT offer these cipher suites

NEW:
Clients MUST NOT offer these cipher suites defined in this document

 

s/Servers,\nwhich/Servers\nthat/

MGLT: Done
 

Does the last paragraph of section 5 want a note "RFC Editor please
remove this paragraph"?

MGLT: Done
 

Section 6, third paragraph, s/by using short PSK/by using a short
PSK/.  Also, s/by a human and thus/by a human which thus/, maybe?

MGLT: Probably ;-) done. thanks.
 

Last paragraph page 4, comma after "i.e.".

MGLT: Added

-Ben

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