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Thread-Index: AQHWhfPKZCd76N1m6EGBOv8Plhva3qlfDioAgAMq5hA= Date: Thu, 10 Sep 2020 18:51:59 +0000 Message-ID: References: In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: yes X-MS-TNEF-Correlator: authentication-results: dmarc.ietf.org; dkim=none (message not signed) header.d=none;dmarc.ietf.org; dmarc=none action=none header.from=evequefou.be; x-originating-ip: [72.49.212.17] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 4aab1d2d-1879-49df-8f7d-08d855ba9922 x-ms-traffictypediagnostic: CH2PR22MB1829: x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:10000; x-ms-exchange-senderadcheck: 1 x-microsoft-antispam: BCL:0; x-microsoft-antispam-message-info: 7HVB+C3DdOMhGxHaioXEq2vnQcAQumeGuqE5Qo6egw8o26ZIJHaxLV8bue83Ciur9J84NySOoSaRRaLKhTvp35ijxE2nwjM0XM+w4VkSQEY0OFIBnmnMleCodevDcG2qtVY5m36dDjEWioBo8fX59gRz11P9xpeUG5bnwy+MVvtjj4hMEKM/ilSMfA+zjsOulidmrNDwverX9UV7QgKci16mcsYPzsq8pCmv3dt/tKTu8yT0v7s8Pr9aNYguxvU46BgH6sE1Qwu8bY0vJc4w0t+Hn+pOi29FcPOxJ/CrkbOc+4X7JVyhWLUgjYKMrHAr x-forefront-antispam-report: CIP:255.255.255.255; CTRY:; LANG:en; SCL:1; SRV:; IPV:NLI; SFV:NSPM; H:CH2PR22MB2086.namprd22.prod.outlook.com; PTR:; CAT:NONE; SFS:(366004)(396003)(136003)(39830400003)(376002)(346002)(83380400001)(71200400001)(99936003)(33656002)(8676002)(2906002)(76116006)(26005)(7696005)(86362001)(9686003)(110136005)(5660300002)(478600001)(64756008)(6506007)(66946007)(52536014)(53546011)(316002)(55016002)(66556008)(66446008)(66476007)(186003)(4326008)(66616009)(8936002); DIR:OUT; SFP:1102; x-ms-exchange-antispam-messagedata: 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 x-ms-exchange-transport-forked: True Content-Type: multipart/signed; protocol="application/x-pkcs7-signature"; micalg=2.16.840.1.101.3.4.2.1; boundary="----=_NextPart_000_0104_01D68781.EE44DE30" MIME-Version: 1.0 X-OriginatorOrg: evequefou.be X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: CH2PR22MB2086.namprd22.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 4aab1d2d-1879-49df-8f7d-08d855ba9922 X-MS-Exchange-CrossTenant-originalarrivaltime: 10 Sep 2020 18:51:59.3277 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 41eaf50b-882d-47eb-8c4c-0b5b76a9da8f X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: 9FkfXLXsbUtvP0jO4HfGeAuOeOs0EmImPCzmkrNyQ/3PdEQCF8hJYnhnOJxK5/S/nZxA+x73TuMNY0H8KuzpQg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH2PR22MB1829 Archived-At: Subject: Re: [TLS] TLS ECH, how much can the hint stick out? X-BeenThere: tls@ietf.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 10 Sep 2020 18:52:04 -0000 ------=_NextPart_000_0104_01D68781.EE44DE30 Content-Type: multipart/alternative; boundary="----=_NextPart_001_0105_01D68781.EE44DE30" ------=_NextPart_001_0105_01D68781.EE44DE30 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable This is primarily an active attack, but not purely so. Clearly the MITM = is an active attacker, but there are situations in QUIC (and DTLS, I = presume) where a ClientHello gets retransmitted. Depending on server = infrastructure, the client might get two different responses. This = isn=E2=80=99t limited to cases where the observer/attacker is the one = performing the replay. =20 So I think we need to decide whether it=E2=80=99s a goal that, given = that relatively narrow situation, the observer shouldn=E2=80=99t be able = to identify ECH acceptance by comparing two ServerHellos that both = respond to the same ClientHello. =20 From: TLS On Behalf Of Christopher Patton Sent: Tuesday, September 8, 2020 2:23 PM To: Christian Huitema Cc: tls@ietf.org Subject: Re: [TLS] TLS ECH, how much can the hint stick out? =20 Hi Christian, Hi list,=20 =20 The "don't stick out" property is a steganographic security goal: we = want the "real" protocol, i.e. TLS with ECH acceptance, to be = indistinguishable from the "cover" protocol, i.e., the handshake pattern = in which the client sends a "dummy" ECH extension that is ignored or = rejected by the server. This is a property that TLS was never designed = to have, but it seems that we need some degree of it in order to deploy = ECH. The fundamental question that Christian raises is what is the right = threat model for this property. =20 The "status quo" threat model considers a distinguisher that is strictly = passive---it does not interfere with a handshake or probe the = server---and that does not know the ECH configuration. This seems (to = me, at least) sufficient to account for middleboxes that might ossify on = the ECH extension. It also seems achievable, both by the ECH as it is = and for the proposed change. =20 The distinguishing attacks described by Christian are much stronger, in = the sense that they involve an active attacker. I don't believe there is = any way of implementing ECH---either as it is or with the proposed = change---that defeats active attacks in general. In particular---and as = Christian points out---an active attacker can probe the server to learn = the ECH configuration (via the ECH retry logic), which it can use to = easily distinguish the real protocol from the cover protocol. This works = regardless of whether the change is adopted. =20 In my view, achieving don't-stick-out-security against active attackers = requires us to revisit the design of ECH altogether. The main difficulty = is that client-facing servers publish the ECH configuration in a way = that's easily accessible to an active attacker. Keeping the = configuration secret may provide a way to achieve security, and some = deployments might opt to do this; but the vast majority won't. We might = consider adding support for this deployment scenario, but this can (and = should, I think) wait for a later draft. =20 That said, it is worth considering mitigations against known attacks, in = particualr (1). I think the suggestion for mitigating (2) adds too much = complexity, and if it doesn't fully address the intended threat model (I = don't think it does), then we'll likely need to replace it in the = future. I think it's better to keep things simple until we fully address = the intended threat model. =20 Best, Chris P. ------=_NextPart_001_0105_01D68781.EE44DE30 Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: quoted-printable

This is primarily an active attack, but not purely = so.=C2=A0 Clearly the MITM is an active attacker, but there are = situations in QUIC (and DTLS, I presume) where a ClientHello gets = retransmitted.=C2=A0 Depending on server infrastructure, the client = might get two different responses.=C2=A0 This isn=E2=80=99t limited to = cases where the observer/attacker is the one performing the = replay.

 

So I think we need to decide whether it=E2=80=99s a = goal that, given that relatively narrow situation, the observer = shouldn=E2=80=99t be able to identify ECH acceptance by comparing two = ServerHellos that both respond to the same ClientHello.

 

From: TLS = <tls-bounces@ietf.org> On Behalf Of Christopher = Patton
Sent: Tuesday, September 8, 2020 2:23 PM
To: = Christian Huitema <huitema@huitema.net>
Cc: = tls@ietf.org
Subject: Re: [TLS] TLS ECH, how much can the hint = stick out?

 

Hi Christian, Hi list,

 

The "don't stick out" property is a = steganographic security goal: we want the "real" protocol, = i.e. TLS with ECH acceptance, to be indistinguishable from the = "cover" protocol, i.e., the handshake pattern in which the = client sends a "dummy" ECH extension that is ignored or = rejected by the server. This is a property that TLS was never designed = to have, but it seems that we need some degree of it in order to deploy = ECH. The fundamental question that Christian raises is what is the right = threat model for this property.

 

The "status quo" threat model considers a = distinguisher that is strictly passive---it does not interfere with a = handshake or probe the server---and that does not know the ECH = configuration. This seems (to me, at least) sufficient to account for = middleboxes that might ossify on the ECH extension. It also seems = achievable, both by the ECH as it is and for the proposed = change.

 

The distinguishing attacks described by Christian are = much stronger, in the sense that they involve an active attacker. I = don't believe there is any way of implementing ECH---either as it is or = with the proposed change---that defeats active attacks in general. In = particular---and as Christian points out---an active attacker can probe = the server to learn the ECH configuration (via the ECH retry logic), = which it can use to easily distinguish the real protocol from the cover = protocol. This works regardless of whether the change is = adopted.

 

In my view, achieving don't-stick-out-security against = active attackers requires us to revisit the design of ECH altogether. = The main difficulty is that client-facing servers publish the ECH = configuration in a way that's easily accessible to an active attacker. = Keeping the configuration secret may provide a way to achieve security, = and some deployments might opt to do this; but the vast majority won't. = We might consider adding support for this deployment scenario, but this = can (and should, I think) wait for a later = draft.

 

That said, it is worth considering mitigations against = known attacks, in particualr (1). I think the suggestion for mitigating = (2) adds too much complexity, and if it doesn't fully address the = intended threat model (I don't think it does), then we'll likely need to = replace it in the future. I think it's better to keep things simple = until we fully address the intended threat = model.

 

Best,

Chris = P.

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