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From: Alan Ford <alan.ford@gmail.com>
In-Reply-To: <20200520164023.GH45434@MacBook-Pro-64.local>
Date: Wed, 20 May 2020 21:38:18 +0100
Cc: MultiPath WG <multipathtcp@ietf.org>,
 mptcp Upstreaming <mptcp@lists.01.org>,
 Todd Malsbary <todd.malsbary@linux.intel.com>
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Message-Id: <882ACBE0-07FC-4BB8-B0BC-60A884DA1DD7@gmail.com>
References: <20200520164023.GH45434@MacBook-Pro-64.local>
To: Christoph Paasch <cpaasch=40apple.com@dmarc.ietf.org>
Archived-At: <https://mailarchive.ietf.org/arch/msg/multipathtcp/9LN5GLZFC_LHiTNOpdMNwJVWxbg>
Subject: Re: [multipathtcp] ADD_ADDR HMAC ambiguity
Precedence: list

Hi Christoph,

Your interpretation is the intended one.

3.4.1 says "rightmost 64 bits of an HMAC=E2=80=9D =E2=80=A6 the HMAC is =
256 bits in both 3.2 and 3.4.1. The truncation is the separate from the =
definition of the HMAC algorithm used.

Best regards,
Alan

> On 20 May 2020, at 17:40, Christoph Paasch =
<cpaasch=3D40apple.com@dmarc.ietf.org> wrote:
>=20
> Hello,
>=20
> Todd Malsbary from Intel is looking at the HMAC-support for ADD_ADDR =
in
> RFC8684. And it appears that there can be an ambiguity on how to =
interpret
> the truncation.
>=20
> We want to clarify this and make sure all implementations,... are on =
the
> same page with the intention of the RFC:
>=20
> RFC8684, Section 3.4.1 says:
> " The Truncated HMAC parameter present in this option is the rightmost
>  64 bits of an HMAC, negotiated and calculated in the same way as for
>  MP_JOIN as described in Section 3.2. "
>=20
> Now, in Section 3.2 it mentions:
> " This specification defines that HMAC as defined in [RFC2104] is =
used,
>  along with the SHA-256 hash algorithm [RFC6234], and that the output
>  is truncated to the leftmost 160 bits (20 octets). "
>=20
>=20
> One can read these in two different ways:
>=20
> In Section 3.4.1, the truncation to the rightmost 64 bits is based on =
the
> full 256-bit HMAC-output or whether it is rather the 64 rightmost bits =
of
> the leftmost 160bits (the ones mentioned in Section 3.2).
>=20
>=20
> Can we clarify which of the two options is the correct one?
>=20
>=20
>=20
> I personally interpret it as "right most 64 bits of the 256-bit =
HMAC-output".
>=20
>=20
>=20
> Thanks,
> Christoph
>=20
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