Re: [TLS] I-D Action: draft-ietf-tls-ecdhe-psk-aead-00.txt

[John Mattsson <[e-mail]>]

Hi Dan, Sean, Nikos,

First, let me state that I think requiring 128-bit key management for
AES-128 is quite reasonable.

HTTP/2 [RFC7540] also has some related text in Section 9.2.1 that applies
when TLS is used for HTTP/2. "Endpoints MAY treat negotiation of key sizes
smaller than the lower limits as a connection error (Section 5.4.1
<https://tools.ietf.org/html/rfc7540#section-5.4.1>) of type
INADEQUATE_SECURITY."


I think this is a question for TLS in general,
draft-ietf-tls-ecdhe-psk-aead should just follow what the TLS WG decides
as the general way forward. Wether that is MAY/SHOULD/SHALL treat groups
with insufficient security as an error.

I think the real problem here are TLS libraries supporting 1024 MODP and
160 ECC at all, support for these should have been removed before 2010.
Nikos [0] recommends a RCF forbidding weak elliptic curves from TLS 1.2
(i.e. WeakDH DieDieDie!!!). I think this is a great idea and much better
than bundling it into a code-point assignment RFC. (My current plans for
the next update of 3GPP’s TLS and DTLS profiles is simply to forbid
support of anything weaker than 2048 MODP and 255-bit ECC).

[0]. https://mailarchive.ietf.org/arch/msg/tls/4PZsc_Dy-aT299BYrlBKvZs0BOQ



Cheers,
John


On 11/07/16 22:26, "TLS on behalf of Dan Harkins" <[e-mail] on
behalf of [e-mail]> wrote:

>
>  I'm glad I have to opportunity to make you happy Sean :-)
>
>On Mon, July 11, 2016 7:40 am, Sean Turner wrote:
>> I think I can take this bit:
>>
>> On Jul 10, 2016, at 06:51, Peter Dettman
>><[e-mail]>
>> wrote:
>>>
>>> I'm also curious whether there is a precedent in other RFCs for an
>>> explicit minimum curve bits, or perhaps a de facto implementer's rule?
>>
>> I'd be happy to be wrong here. but to my knowledge no there's not been
>> an explicit minimum for curve bits.  There have however been similar (at
>> least in my non-cryptographer mind) for RSA key sizes so if we wanted to
>> define an explicit minimum curve bits then we could.
>
>  draft-ietf-tls-pwd-07 includes a RECOMMENDED practice of ensuring
>the curves used provide commensurate strength with the ciphersuite
>negotiated. Section 10, "Implementation Considerations", says:
>
>   It is RECOMMENDED that implementations take note of the strength
>   estimates of particular groups and to select a ciphersuite providing
>   commensurate security with its hash and encryption algorithms.  A
>   ciphersuite whose encryption algorithm has a keylength less than the
>   strength estimate, or whose hash algorithm has a blocksize that is
>   less than twice the strength estimate SHOULD NOT be used.
>
>  And I would like to take this opportunity to remind everyone that
>the only difference between TLS_ECDHE_PSK_WITH_AES_128_GCM_SHA256
>and TLS_ECCPWD_WITH_AES_128_GCM_SHA256 is that the latter is resistant
>to dictionary attack and the former is not.
>
>  regards,
>
>  Dan.
>
>
>
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