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From: John Bradley <ve7jtb@ve7jtb.com>
In-Reply-To: <54E372C1.8040204@gmx.net>
Date: Tue, 17 Feb 2015 20:42:06 -0800
Message-Id: <F6E16FC2-1044-4E3B-A7F3-118CE0408E8D@ve7jtb.com>
References: <54E372C1.8040204@gmx.net>
To: Hannes Tschofenig <hannes.tschofenig@gmx.net>
Archived-At: <http://mailarchive.ietf.org/arch/msg/oauth/hpTxKP5822-d4PRPavjqLlaLsNQ>
Cc: "oauth@ietf.org" <oauth@ietf.org>, Naveen Agarwal <naa@google.com>
Subject: Re: [OAUTH-WG] draft-ietf-oauth-spop-10
Precedence: list


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We have discussed on the list making allowing the entropy for plain to =
be smaller.

32bytes was selected as a good value for S256. =20

The reason for S256 vs plain is that you are assuming an attacker is =
going to intercept the request, and thus you want to have enough entropy =
to prevent offline brute force.

For plain you are assuming that the attacker can't intercept the =
request, and if they do more entropy won't help you.

In the plain case you are only protecting against a on line guess for a =
single use token.   That could safely be much lower entropy as it is not =
protecting against off-line brute force.
128bits would be more than enough in that case.   I left it as 256bits =
for both to not introduce more options and not confuse people about why =
plain needs less entropy than S256.

What is the WG feeling should we recommend different minimum values for =
each of the transforms?

John B.

> On Feb 17, 2015, at 8:56 AM, Hannes Tschofenig =
<hannes.tschofenig@gmx.net> wrote:
>=20
> Hi Nat, John, Naveen,
>=20
> thanks a lot for your work on the document.
>=20
> I still need responses to this mail to complete the shepherd writeup:
> http://www.ietf.org/mail-archive/web/oauth/current/msg14100.html
>=20
> I definitely need the IPR confirmation.
>=20
> It would also be helpful to have someone who implemented the
> specification as it currently is. I asked Brian and Thorsten for
> clarification regarding their statements that they implemented earlier
> versions of the spec.
>=20
> As a final remark I still believe that the text regarding the =
randomness
> is still a bit inconsistent. Here are two examples:
>=20
> 1) In the Security Consideration you write that "The security model
> relies on the fact that the code verifier is not learned or guessed by
> the attacker.  It is vitally important to adhere to this principle. "
>=20
> 2) In Section 4.1 you, however, write: "NOTE: code verifier SHOULD =
have
> enough entropy to make it impractical to guess the value.  It is
> RECOMMENDED that the output of a suitable random number generator be
> used to create a 32-octet sequence."
>=20
> There is clearly a long way from a SHOULD have enough entropy to the
> text in the security consideration section where you ask for 32 bytes
> entropy.
>=20
> It is also not clear why you ask for 32 bytes of entropy in =
particular.
>=20
> Ciao
> Hannes
>=20


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