Re: [TLS] OPTLS: Signature-less TLS 1.3

Hugo Krawczyk <> Tue, 24 March 2015 15:12 UTC

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From: Hugo Krawczyk <>
Date: Tue, 24 Mar 2015 11:11:43 -0400
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To: Rene Struik <>
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Subject: Re: [TLS] OPTLS: Signature-less TLS 1.3
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Hi Rene,

We are working on such a write up as  we speak. Exactly because the details
matter and we have been changing them to adapt to the WG requirements and
preferences then the writing has been delayed. We are now converging to a
protocol that the people in the interim meeting were quite happy with. It
is in the same spirit of OPTLS but it is not signature-less, i.e.,
semi-static keys for the servers are now signed with online signatures that
prove freshness by including the client's nonce under the signature.

We should have something decent to post in two weeks.
But let me note that there is nothing revolutionary in the protocol - it is
crypto engineering with well understood components, not inventing new
fundamental pieces.


On Tue, Mar 24, 2015 at 9:08 AM, Rene Struik <> wrote:

>  Hi Hugo:
> In your email of October 31, 2014 (see below), you introduced the OPTLS
> protocol to TLS. At the time, you omitted some details, since you simply
> wished to socialize the ideas. You did emphasize, though, that details
> matter and put forward that you were working on flashing out the full
> details of the protocol and security proof.
> After a few months of radio silence (to me), the protocol seems to be on
> the radar screen for discussion with TLS again. It would therefore be good
> to have a stable reference that describes this in full technical detail and
> makes this a stable and solid source for the cryptographic community to
> look at and reference.
> I do think that OPTLS has interesting features, perhaps also outside TLS,
> but would like to give this a thorough and critical look first (and invite
> others to do so as well). For this, it would help to have something beyond
> some emails on the TLS list and some "flashed out" draft spec Eric Rescorla
> worked on earlier this week (which, although useful, may change over
> time).  Having a stable document would make this more appealing to study
> and discuss in a wider audience, esp. for important potential use cases.
> If you have a paper (e.g., one you could post on the IACR ePrint server),
> that would be great. {Rest assured, this does not have to have the same
> size as your 2005 paper on HMQV for it to be useful.}
> Best regards, Rene
> ==
> "The presentation is sketchy and omits the exact procedure for key
> derivation. The latter is a crucial component for the security of the
> protocol, but before getting into these details we want to get a sense of
> whether the WG is interested in this approach. In the meantime, Hoeteck Wee
> and myself are working on the details of the protocol and the security
> proof."
> On 10/31/2014 8:54 PM, Hugo Krawczyk wrote:
>  During the TLS interim meeting of last week (Oct 22 2014) I suggested
> that TLS
> 1.3 should abandon signature-based authentication (other than for
> certificates)
> and be based solely on a combination of ephemeral Diffie-Hellman for PFS
> and
> static Diffie-Hellman for authentication. This has multiple benefits
> including
> major performance gain (by replacing the per-handshake RSA signature by the
> server with a much cheaper elliptic curve exponentiation), compatibility
> with
> the mechanisms required for forward secrecy, natural accommodation of a
> 0-RTT
> option, and a simple extension without signatures for client
> authentication.
> Below I present a schematic representation of the proposed protocol
> referred
> to as OPTLS where OPT stands for OPTimized and/or for One-Point-Three.
> The presentation is sketchy and omits the exact procedure for key
> derivation.
> The latter is a crucial component for the security of the protocol, but
> before getting into these details we want to get a sense of whether the WG
> is
> interested in this approach. In the meantime, Hoeteck Wee and myself are
> working on the details of the protocol and the security proof.
> We describe a setting with optional 0-RTT and server-only authentication.
> Client authentication can be added as a further option or as an extension
> (similar to the current 1.3 proposal) - see below.
> [K] symbols represent pointers to key material whose exact derivation is
> not
> included here except for specifying the basic DH values from which the key
> is
> derived (actual derivation will include further information similar to the
> extended hash mechanisms or SessionHash proposal considered for 1.3).
> Asterisks represent optional fields that the WG can decide to leave as
> optional,
> mandatory, or simply remove without changing the core cryptographic
> security of
> the protocol. All references to encryption mean "authenticated encryption"
> using the encrypt-then-mac paradigm (or any other secure AEAD mechanism).
> KeyShare's represent ephemeral Diffie-Hellman values exchanged by the
> parties.
> All the public key and Diffie-Hellman operations are assumed to happen
> over a
> cyclic group with generator g of order q (typically implemented by an
> elliptic
> curve group). We use multiplicative notation where ^ denotes
> exponentiation as
> in g^x, g^{xy} (here xy denotes multiplication of the scalars x and y),
> etc.
> Omitted from the current high level description is a mechanism for testing
> group
> membership of DH values or cofactor exponentiation (the specific mechanism
> depends on the group type and is typically very efficient for elliptic
> curves).
> The server has a long term private-public key pair denoted by (s,g^s)
> where
> s is uniform random in [0..q-1] (we refer to s and g^s as "static keys").
> We assume that the server has a certificate that includes the server's
> public
> key g^s and a CA-signed binding of this key to the server's identity.
> We discuss the implementation of such certificates below.
> ClientHello
> ClientKeyShare
> ClientData* [K0]       -------->
>                                            ServerHello
>                                            ServerKeyShare
>                                            ServerCertificate* [K1]
>                                            ServerFinished [K2]
>                        <--------           ServerData* [K2]
> ClientFinished* [K2]   -------->
>             Protected Application Data [K3]
>                        <------->
> The basic 1-RTT case omits the ClientData* field. It includes a
> ClientKeyShare
> g^x and a ServerKeyShare g^y and an optional (encrypted) server
> certificate.
> If the certificate is sent (it can be omitted if the client has indicated
> that
> it knows the server key as in the case in the 0-RTT scenario) and is
> encrypted,
> the encryption key K1 is derived from g^{xy}.
> Key K2 is an encryption key derived from both g^{xs} and g^{xy}. It is
> used to
> authenticate-encrypt the ServerFinished and ClientFinished messages (which
> include a hash of the previous traffic) and to encrypt data from the
> server if
> such data is piggybacked to the second message.
> Key K3 is the "application key" used to derive key material for protection
> of
> application data.  This key material will include (directional)
> Authenticated
> Encryption keys and, possibly, keys for derivation of further re-key
> material.
> K3 is computed from  both g^{xs} and g^{xy} similarly to K2, but its
> derivation
> will be different than K2, e.g., using a separating key expansion
> technique.
> The above protocol is compatible with a 0-RTT protocol such as QUIC. In
> this
> case, the client is assumed to have information about the server's public
> key
> and other security parameters. The server is assumed to have some
> mechanism in
> place for detecting replay (e.g., via timestamps, stored client nonces,
> etc.).
> The resulting protocol is as described above where the ClientData field is
> sent
> encrypted under key K0 derived from g^{xs}.
> The rest of the protocol is identical to the above.
> Note: In this case, ServerCertificate is not sent as the client had to know
> the server's public key before the first message (one can imagine a setting
> where the server may send a different certificate in the second message -
> if
> desired, this can be accommodated too as an option or extension).
> Client authentication can be supported via an option or extension. It would
> include a client certificate for a static DH key g^c sent in the third
> message
> (the certificate can be encrypted under key K2 to provide client's identity
> protection). In this case, the key for computing the ClientFinished
> message and
> the application key K3 would be derived from a combination of the values
> g^xy,
> g^xs, g^yc (and possibly also g^{cs}).
> Note on Finished messages: The above spec sets ServerFinished as mandatory
> and
> ClientFinished as optional. The latter option is needed for a 1-RTT
> protocol.
> In principle, both Finished messages could be omitted and still obtain
> security
> via implicit authentication (assuming the inputs to key derivation are
> chosen
> appropriately). But given the advantages of ServerFinished for providing
> explicit authentication, key confirmation, and active forward secrecy (see
> below), it seems advisable to always include it. Including ClientFinished
> provides key confirmation from client and also explicit client
> authentication
> when client certificate is included. ClientFinished also provides Universal
> Composable (UC) security (this is a result of the Canetti-Krawczyk proof
> that
> CK security implies UC security when a client confirmation step is
> included).
> Note on certificates: Since in current practice servers hold certificates
> for
> RSA signature keys rather than for static DH keys, the certificate field
> in the
> above protocol will be implemented by a pair consisting of (i) the
> server's RSA
> signature certificate and (ii) the server's signature using this RSA key
> on the
> server's static public DH key g^s. The latter signature by the server is
> performed only when a new static DH key is created (how often this happens
> and
> how many such keys are created is completely up to the server - it has the
> advantage that these keys can be changed often to increase security against
> leaked keys). This use of RSA also enjoys the high efficiency of RSA
> verification for the client.
> The handling of Client certificates would be similar.
> Note on forward security (a.k.a. as PFS for Perfect Forward Security):
> PFS is provided by the (mandatory) use of ephemeral Diffie-Hellman keys.
> The meaning of PFS is that an attacker that finds the (long-term) static
> private keys of the parties cannot compromise past session keys. Without
> the
> ServerFinished message the above protocol ensures forward secrecy against
> passive attackers (i.e., for sessions where the attacker did not choose
> g^y).
> With ServerFinished, PFS holds also against active attackers.  A similar
> consideration applies to ClientFinished.
> Client certificates in the first message: We note that in cases where
> client
> certificates can be sent in the clear in the first message of the
> protocol, one
> can provide PFS and mutual authentication in a 1-RTT at essentially the
> same
> cost of an unauthenticated DH exchange (i.e., a cost of little more than
> two
> exponentiations). In such a setting one can also obtain mutual
> authentication in
> a 0-RTT protocol (with forward secrecy with respect to the client's key).
> These options, however, require HMQV-like mechanisms and may raise IP
> issues (this
> can be investigated further if the WG is interested).
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