draft-ietf-tls-rfc4492bis.xml

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<rfc ipr="trust200902" docName="draft-ietf-tls-rfc4492bis-17" category="std" obsoletes="4492">
  <front>
    <title abbrev="ECC Cipher Suites for TLS">Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier</title>
    <author initials="Y." surname="Nir" fullname="Yoav Nir">
      <organization abbrev="Check Point">Check Point Software Technologies Ltd.</organization>
      <address>
        <postal>
          <street>5 Hasolelim st.</street>
          <city>Tel Aviv</city>
          <code>6789735</code>
          <country>Israel</country>
        </postal>
        <email>ynir.ietf@gmail.com</email>
      </address>
    </author>

    <author fullname="Simon Josefsson" initials="S." surname="Josefsson">
      <organization>SJD AB</organization>
      <address>
        <email>simon@josefsson.org</email>
      </address>
    </author>

    <author fullname="Manuel Pegourie-Gonnard"
            initials="M." surname="Pegourie-Gonnard">
      <organization>Independent / PolarSSL</organization>
      <address>
        <email>mpg@elzevir.fr</email>
      </address>
    </author>

    <date year="2017"/>
    <area>Security Area</area>
    <workgroup>TLS Working Group</workgroup>
    <keyword>Internet-Draft</keyword>
    <abstract>
      <t> This document describes key exchange algorithms based on Elliptic Curve Cryptography (ECC) for the Transport
        Layer Security (TLS) protocol.  In particular, it specifies the use of Ephemeral Elliptic Curve Diffie-Hellman
        (ECDHE) key agreement in a TLS handshake and the use of Elliptic Curve Digital Signature Algorithm (ECDSA) and
        Edwards Digital Signature Algorithm (EdDSA) as authentication mechanisms.</t>
      <t> This document obsoletes and replaces RFC 4492.</t>
    </abstract>
  </front>
 <!--
 Changes from RFC 4492:
  * Added mention of TLS 1.2
  * Removed "required reading" for ECC from the Introduction. ECC is not longer "exotic"
  * Moved the original authors to the Acknowledgement section.
  * Added EdDSA.
  * Added CFRG curves.
  -->
  <middle>
    <!-- ====================================================================== -->
    <section anchor="introduction" title="Introduction">
      <t> This document describes additions to TLS to support ECC, applicable to TLS versions 1.0
        <xref target="RFC2246"/>, 1.1 <xref target="RFC4346"/>, and 1.2 <xref target="RFC5246"/>.  The use of ECC in
        TLS 1.3 is defined in <xref target="I-D.ietf-tls-tls13" />, and is explicitly out of scope for this document.
        In particular, this document defines:<list style="symbols">
        <t> the use of the ECDHE key agreement scheme with ephemeral keys to establish the
          TLS premaster secret, and</t>
        <t>the use of ECDSA and EdDSA signatures for authentication of TLS peers.</t></list></t>
      <t> The remainder of this document is organized as follows. <xref target="ecdh"/> provides an overview of
        ECC-based key exchange algorithms for TLS. <xref target="clientauth"/> describes the use of ECC certificates
        for client authentication.  TLS extensions that allow a client to negotiate the use of specific curves and
        point formats are presented in <xref target="tlsext"/>. <xref target="structs"/> specifies various data
        structures needed for an ECC-based handshake, their encoding in TLS messages, and the processing of those
        messages.  <xref target="suites"/> defines ECC-based cipher suites and identifies a small subset of these
        as recommended for all implementations of this specification. <xref target="sec"/> discusses security
        considerations. <xref target="iana"/> describes IANA considerations for the name spaces created by this
        document's predecessor. <xref target="ack" /> gives acknowledgements. <xref target="rfc4492diff" />
        provides differences from <xref target="RFC4492" />, the document that this one replaces.</t>
      <t> Implementation of this specification requires familiarity with TLS, TLS extensions
        <xref target="RFC4366" />, and ECC.</t>
      <section anchor="mustshouldmay" title="Conventions Used in This Document">
        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
          "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described
          in <xref target="RFC2119"/>.</t>
      </section>
    </section>
    <section anchor="ecdh" title="Key Exchange Algorithm">
      <t> This document defines three new ECC-based key exchange algorithms for TLS.  All of them use Ephemeral
        ECDH (ECDHE) to compute the TLS premaster secret, and they differ only in the mechanism (if any) used to
        authenticate them.  The derivation of the TLS master secret from the premaster secret and the subsequent
        generation of bulk encryption/MAC keys and initialization vectors is independent of the key exchange
        algorithm and not impacted by the introduction of ECC.</t>
      <t> <xref target="tbl2"/> summarizes the new key exchange algorithms.  All of these key exchange algorithms
        provide forward secrecy if and only if fresh ephemeral keys are generated and used, and also destroyed
        after use.</t>
      <texttable anchor="tbl2" title="ECC Key Exchange Algorithms">
        <ttcol align="left">Algorithm</ttcol>
        <ttcol align="left">Description</ttcol>
        <c>ECDHE_ECDSA</c><c>Ephemeral ECDH with ECDSA or EdDSA signatures.</c>
        <c>ECDHE_RSA</c><c>Ephemeral ECDH with RSA signatures.</c>
        <c>ECDH_anon</c><c>Anonymous ephemeral ECDH, no signatures.</c>
      </texttable>
      <t> These key exchanges are analogous to DHE_DSS, DHE_RSA, and DH_anon, respectively.</t>
      <t> With ECDHE_RSA, a server
        can reuse its existing RSA certificate and easily comply with a constrained client's elliptic curve
        preferences (see <xref target="tlsext"/>).  However, the computational cost incurred by a server is higher
        for ECDHE_RSA than for the traditional RSA key exchange, which does not provide forward secrecy.</t>
      <t> The anonymous key exchange algorithm does not provide authentication of the server or the client.  Like
        other anonymous TLS key exchanges, it is subject to man-in-the-middle attacks. Applications using TLS with
        this algorithm SHOULD provide authentication by other means.</t>
      <t><figure>
        <artwork><![CDATA[
       Client                                        Server
       ------                                        ------
       ClientHello          -------->
                                                ServerHello
                                               Certificate*
                                         ServerKeyExchange*
                                       CertificateRequest*+
                            <--------       ServerHelloDone
       Certificate*+
       ClientKeyExchange
       CertificateVerify*+
       [ChangeCipherSpec]
       Finished             -------->
                                         [ChangeCipherSpec]
                            <--------              Finished
       Application Data     <------->      Application Data
            * message is not sent under some conditions
            + message is not sent unless client authentication
              is desired
]]></artwork><postamble>Figure 1: Message flow in a full TLS 1.2 handshake</postamble>
        </figure></t>
      <t> Figure 1 shows all messages involved in the TLS key establishment protocol (aka full handshake).  The
        addition of ECC has direct impact only on the ClientHello, the ServerHello, the server's Certificate
        message, the ServerKeyExchange, the ClientKeyExchange, the CertificateRequest, the client's Certificate
        message, and the CertificateVerify.  Next, we describe the ECC key exchange algorithm in greater detail
        in terms of the content and processing of these messages.  For ease of exposition, we defer discussion of
        client authentication and associated messages (identified with a + in Figure 1) until
        <xref target="clientauth" /> and of the optional ECC-specific extensions (which impact the Hello messages)
        until <xref target="tlsext" />.</t>
      <section anchor="ecdhe_ecdsa" title="ECDHE_ECDSA">
        <t> In ECDHE_ECDSA, the server's certificate MUST contain an ECDSA- or EdDSA-capable public key.</t>
        <t> The server sends its ephemeral ECDH public key and a specification of the corresponding curve in the
          ServerKeyExchange message.  These parameters MUST be signed with ECDSA or EdDSA using the private key
          corresponding to the public key in the server's Certificate.</t>
        <t> The client generates an ECDH key pair on the same curve as the server's ephemeral ECDH key and sends its
          public key in the ClientKeyExchange message.</t>
        <t> Both client and server perform an ECDH operation <xref target="alg_computes"/> and use the resultant
          shared secret as the premaster secret.</t>
      </section>
      <section anchor="ecdhe_rsa" title="ECDHE_RSA">
        <t> This key exchange algorithm is the same as ECDHE_ECDSA except that the server's certificate MUST contain
          an RSA public key authorized for signing, and that the signature in the ServerKeyExchange message must be
          computed with the corresponding RSA private key.</t>
      </section>
      <section anchor="ecdh_anon" title="ECDH_anon">
        <t> NOTE: Despite the name beginning with "ECDH_" (no E), the key used in ECDH_anon is ephemeral just like
          the key in ECDHE_RSA and ECDHE_ECDSA. The naming follows the example of DH_anon, where the key is also
          ephemeral but the name does not reflect it.</t>
        <t> In ECDH_anon, the server's Certificate, the CertificateRequest, the client's Certificate, and the
          CertificateVerify messages MUST NOT be sent.</t>
        <t> The server MUST send an ephemeral ECDH public key and a specification of the corresponding curve in
          the ServerKeyExchange message.  These parameters MUST NOT be signed.</t>
        <t> The client generates an ECDH key pair on the same curve as the server's ephemeral ECDH key and sends
          its public key in the ClientKeyExchange message.</t>
        <t> Both client and server perform an ECDH operation and use the resultant shared secret as the premaster
          secret.  All ECDH calculations are performed as specified in <xref target="alg_computes"/>.</t>
      </section>
      <section anchor="certchainalgs" title="Algorithms in Certificate Chains">    
        <t> This specification does not impose restrictions on signature schemes used anywhere in the certificate chain.
          The previous version of this document required the signatures to match, but this restriction, originating
          in previous TLS versions is lifted here as it had been in RFC 5246.</t>
      </section>
    </section>
    <section anchor="clientauth" title="Client Authentication">
      <t> This document defines a client authentication mechanism, named after the type of client certificate
        involved: ECDSA_sign. The ECDSA_sign mechanism is usable with any of the non-anonymous ECC key exchange
        algorithms described in <xref target="ecdh"/> as well as other non-anonymous (non-ECC) key exchange
        algorithms defined in TLS.</t>
      <t> Note that client certificates with EdDSA public keys also use this mechanism. </t>
      <t> The server can request ECC-based client authentication by including this certificate type in its
        CertificateRequest message. The client must check if it possesses a certificate appropriate for the method
        suggested by the server and is willing to use it for authentication.</t>
      <t> If these conditions are not met, the client SHOULD send a client Certificate message containing no
        certificates.  In this case, the ClientKeyExchange MUST be sent as described in <xref target="ecdh"/>, and
        the CertificateVerify MUST NOT be sent.  If the server requires client authentication, it may respond with
        a fatal handshake failure alert.</t>
      <t> If the client has an appropriate certificate and is willing to use it for authentication, it must send
        that certificate in the client's Certificate message (as per <xref target="client_cert"/>) and prove
        possession of the private key corresponding to the certified key.  The process of determining an
        appropriate certificate and proving possession is different for each authentication mechanism and
        described below.</t>
      <t> NOTE: It is permissible for a server to request (and the client to send) a client certificate of a
        different type than the server certificate.</t>
      <section anchor="ecdsa_sign" title="ECDSA_sign">
        <t> To use this authentication mechanism, the client MUST possess a certificate containing an ECDSA-
          or EdDSA-capable public key.</t>
        <t> The client proves possession of the private key corresponding to the certified key by including a
          signature in the CertificateVerify message as described in <xref target="cert_verify"/>.</t>
      </section>
    </section>
    <section anchor="tlsext" title="TLS Extensions for ECC">
      <t> Two TLS extensions are defined in this specification: (i) the Supported Elliptic Curves Extension,
        and (ii) the Supported Point Formats Extension.  These allow negotiating the use of specific curves and
        point formats (e.g., compressed vs. uncompressed, respectively) during a handshake starting a new session.
        These extensions are especially relevant for constrained clients that may only support a limited number of
        curves or point formats.  They follow the general approach outlined in <xref target="RFC4366"/>; message
        details are specified in <xref target="structs"/>.  The client enumerates the curves it supports and the
        point formats it can parse by including the appropriate extensions in its ClientHello message.  The server
        similarly enumerates the point formats it can parse by including an extension in its ServerHello message.</t>
      <t> A TLS client that proposes ECC cipher suites in its ClientHello message SHOULD include these extensions.
        Servers implementing ECC cipher suites MUST support these extensions, and when a client uses these
        extensions, servers MUST NOT negotiate the use of an ECC cipher suite unless they can complete the
        handshake while respecting the choice of curves specified by the client.
        This eliminates the possibility that a negotiated ECC handshake will be subsequently aborted due to a
        client's inability to deal with the server's EC key.</t>
      <t> The client MUST NOT include these extensions in the ClientHello message if it does not propose any ECC
        cipher suites.  A client that proposes ECC cipher suites may choose not to include these extensions.  In
        this case, the server is free to choose any one of the elliptic curves or point formats listed in
        <xref target="structs"/>.  That section also describes the structure and processing of these extensions
        in greater detail.</t>
      <t> In the case of session resumption, the server simply ignores the Supported Elliptic Curves Extension and
        the Supported Point Formats Extension appearing in the current ClientHello message. These extensions only
        play a role during handshakes negotiating a new session.</t>
    </section>
    <section anchor="structs" title="Data Structures and Computations">
      <t> This section specifies the data structures and computations used by ECC-based key mechanisms specified
        in the previous three sections.  The presentation language used here is the same as that used in TLS.
        Since this specification extends TLS, these descriptions should be merged with those in the TLS specification
        and any others that extend TLS.  This means that enum types may not specify all possible values, and
        structures with multiple formats chosen with a select() clause may not indicate all possible cases.</t>
      <section anchor="ch_extensions" title="Client Hello Extensions">
        <t> This section specifies two TLS extensions that can be included with the ClientHello message as described
          in <xref target="RFC4366"/>, the Supported Elliptic Curves Extension and the Supported Point Formats
          Extension.</t>
        <t> When these extensions are sent:</t>
        <t> The extensions SHOULD be sent along with any ClientHello message that proposes ECC cipher suites.</t>
        <t> Meaning of these extensions:</t>
        <t> These extensions allow a client to enumerate the elliptic curves it supports and/or the point formats
          it can parse.</t>
        <t> Structure of these extensions:</t>
        <t> The general structure of TLS extensions is described in <xref target="RFC4366"/>, and this specification
          adds two types to ExtensionType.<figure><artwork><![CDATA[
   enum {
       elliptic_curves(10),
       ec_point_formats(11)
   } ExtensionType;
]]></artwork></figure><list style="symbols">
          <t> elliptic_curves (Supported Elliptic Curves Extension): Indicates the set of elliptic
            curves supported by the client.  For this extension, the opaque extension_data field contains
            NamedCurveList.  See <xref target="supp_ec_ext"/> for details.</t>
          <t> ec_point_formats (Supported Point Formats Extension): Indicates the set of point formats
            that the client can parse.  For this extension, the opaque extension_data field contains ECPointFormatList.
            See <xref target="supp_pf_ext"/> for details.</t></list></t>
        <t> Actions of the sender:</t>
        <t> A client that proposes ECC cipher suites in its ClientHello message appends these extensions (along
          with any others), enumerating the curves it supports and the point formats it can parse.  Clients SHOULD
          send both the Supported Elliptic Curves Extension and the Supported Point Formats Extension.  If the
          Supported Point Formats Extension is indeed sent, it MUST contain the value 0 (uncompressed) as one of the
          items in the list of point formats.</t>
        <t> Actions of the receiver:</t>
        <t> A server that receives a ClientHello containing one or both of these extensions MUST use the client's
          enumerated capabilities to guide its selection of an appropriate cipher suite.  One of the proposed ECC
          cipher suites must be negotiated only if the server can successfully complete the handshake while using
          the curves and point formats supported by the client (cf. <xref target="server_cert"/> and
          <xref target="ske"/>).</t>
        <t> NOTE: A server participating in an ECDHE_ECDSA key exchange may use different curves for the ECDSA or
          EdDSA key in its certificate, and for the ephemeral ECDH key in the ServerKeyExchange message.  The server
          MUST consider the extensions in both cases.</t>
        <t> If a server does not understand the Supported Elliptic Curves Extension, does not understand the
          Supported Point Formats Extension, or is unable to complete the ECC handshake while restricting itself
          to the enumerated curves and point formats, it MUST NOT negotiate the use of an ECC cipher suite.
          Depending on what other cipher suites are proposed by the client and supported by the server, this may
          result in a fatal handshake failure alert due to the lack of common cipher suites.</t>
        <section anchor="supp_ec_ext" title="Supported Elliptic Curves Extension">
          <t> RFC 4492 defined 25 different curves in the NamedCurve registry (now renamed the "Supported Groups" registry,
            although the enumeration below is still named NamedCurve) for use in TLS. Only three have seen
            much use. This specification is deprecating the rest (with numbers 1-22). This specification also
            deprecates the explicit curves with identifiers 0xFF01 and 0xFF02. It also adds the new curves
            defined in <xref target="RFC7748"/>. The end result is as follows:</t>
          <figure><artwork><![CDATA[
        enum {
            deprecated(1..22),
            secp256r1 (23), secp384r1 (24), secp521r1 (25),
            x25519(29), x448(30),
            reserved (0xFE00..0xFEFF),
            deprecated(0xFF01..0xFF02),
            (0xFFFF)
        } NamedCurve;
]]></artwork></figure>
          <t> Note that other specifications have since added other values to this enumeration. Some of those values
            are not curves at all, but finite field groups. See <xref target="RFC7919"/>.</t>
          <t> secp256r1, etc: Indicates support of the corresponding named curve or groups.  
            The named curves secp256r1, secp384r1, and secp521r1 are specified in SEC 2
            <xref target="SECG-SEC2"/>. These curves are also recommended in ANSI X9.62
            <xref target="ANSI.X9-62.2005"/> and FIPS 186-4 <xref target="FIPS.186-4"/>. The rest of this document
            refers to these three curves as the "NIST curves" because they were originally standardized by the National
            Institute of Standards and Technology. The curves x25519 and x448
            are defined in <xref target="RFC7748"/>.  Values 0xFE00 through 0xFEFF are reserved for private use.</t>
          <t> The predecessor of this document also supported explicitly defined prime and char2 curves, but these are
            deprecated by this specification.</t>
          <t> The NamedCurve name space is maintained by IANA.  See <xref target="iana"/> for information on how new
            value assignments are added.</t>
          <figure><artwork><![CDATA[
        struct {
            NamedCurve named_curve_list<2..2^16-1>
        } NamedCurveList;
]]></artwork></figure>
          <t> Items in named_curve_list are ordered according to the client's preferences (favorite choice
            first).</t>
          <t> As an example, a client that only supports secp256r1 (aka NIST P-256; value 23 = 0x0017) and secp384r1
            (aka NIST P-384; value 24 = 0x0018) and prefers to use secp256r1 would include a TLS extension consisting
            of the following octets.  Note that the first two octets indicate the extension type (Supported Elliptic
            Curves Extension):</t>
          <figure><artwork><![CDATA[
        00 0A 00 06 00 04 00 17 00 18
]]></artwork></figure>
        </section>
        <section anchor="supp_pf_ext" title="Supported Point Formats Extension">
          <figure><artwork><![CDATA[
        enum {
            uncompressed (0),
            deprecated (1..2),
            reserved (248..255)
        } ECPointFormat;
        struct {
            ECPointFormat ec_point_format_list<1..2^8-1>
        } ECPointFormatList;
]]></artwork></figure>
          <t> Three point formats were included in the definition of ECPointFormat above. This specification
            deprecates all but the uncompressed point format. Implementations of this document MUST support the
            uncompressed format for all of their supported curves, and MUST NOT support other formats for curves
            defined in this specification. For backwards compatibility purposes, the point format list extension
            MAY still be included, and contain exactly one value: the uncompressed point format (0). RFC 4492 
            specified that if this extension is missing, it means that only the uncompressed point format is 
            supported, so interoperability with implementations that support the uncompressed format should work
            with or without the extension.</t>
          <t> If the client sends the extension and the extension does not contain the uncompressed point format,
            and the client has used the Supported Groups extension to indicate support for any of the curves defined
            in this specification then the server MUST abort the handshake and return an illegal_parameter alert.</t>
          <t> The ECPointFormat name space is maintained by IANA.  See <xref target="iana"/> for information on how
            new value assignments are added.</t>
          <t> A client compliant with this specification that supports no other curves MUST send the following octets;
            note that the first two octets indicate the extension type (Supported Point Formats Extension):<figure><artwork><![CDATA[
        00 0B 00 02 01 00
]]></artwork></figure></t>
        </section>
        <section anchor="sighash_ext" title="The signature_algorithms Extension and EdDSA">
          <t> The signature_algorithms extension, defined in section 7.4.1.4.1 of <xref target="RFC5246"/>, advertises
            the combinations of signature algorithm and hash function that the client supports. The pure (non pre-hashed)
            forms of EdDSA do not hash the data before signing it. For this reason it does not make sense to combine them
            with a signature algorithm in the extension.</t>
          <t> For bits-on-the-wire compatibility with TLS 1.3, we define a new dummy value in the HashAlgorithm registry
            which we will call "Intrinsic" (value TBD5) meaning that hashing is intrinsic to the signature algorithm.</t>
          <t> To represent ed25519 and ed448 in the signature_algorithms extension, the value shall be (TBD5,TBD3) and 
            (TBD5,TBD4) respectively.</t>  
        </section>
      </section>
      <section anchor="serverhello_ext" title="Server Hello Extension">
        <t> This section specifies a TLS extension that can be included with the ServerHello message as described
          in <xref target="RFC4366"/>, the Supported Point Formats Extension.</t>
        <t> When this extension is sent:</t>
        <t> The Supported Point Formats Extension is included in a ServerHello message in response to a ClientHello
          message containing the Supported Point Formats Extension when negotiating an ECC cipher suite.</t>
        <t> Meaning of this extension:</t>
        <t> This extension allows a server to enumerate the point formats it can parse (for the curve that will appear
          in its ServerKeyExchange message when using the ECDHE_ECDSA, ECDHE_RSA, or ECDH_anon key exchange algorithm.</t>
        <t> Structure of this extension:</t>
        <t> The server's Supported Point Formats Extension has the same structure as the client's Supported Point
          Formats Extension (see <xref target="supp_pf_ext"/>).  Items in ec_point_format_list here are ordered
          according to the server's preference (favorite choice first).  Note that the server MAY include items that
          were not found in the client's list. However, without extensions this specification allows exactly one point
          format, so there is not really any opportunity for mismatches.</t>
        <t> Actions of the sender:</t>
        <t> A server that selects an ECC cipher suite in response to a ClientHello message including a Supported
          Point Formats Extension appends this extension (along with others) to its ServerHello message, enumerating
          the point formats it can parse.  The Supported Point Formats Extension, when used, MUST contain the value 0
          (uncompressed) as one of the items in the list of point formats.</t>
        <t> Actions of the receiver:</t>
        <t> A client that receives a ServerHello message containing a Supported Point Formats Extension MUST respect
          the server's choice of point formats during the handshake (cf. <xref target="client_cert"/> and
          <xref target="cke"/>).  If no Supported Point Formats Extension is received with the ServerHello, this is
          equivalent to an extension allowing only the uncompressed point format.</t>
      </section>
      <section anchor="server_cert" title="Server Certificate">
        <t> When this message is sent:</t>
        <t> This message is sent in all non-anonymous ECC-based key exchange algorithms.</t>
        <t> Meaning of this message:</t>
        <t> This message is used to authentically convey the server's static public key to the client.  The following
          table shows the server certificate type appropriate for each key exchange algorithm.  ECC public keys MUST
          be encoded in certificates as described in <xref target="eccerts"/>.</t>
        <t> NOTE: The server's Certificate message is capable of carrying a chain of certificates.  The restrictions
          mentioned in Table 3 apply only to the server's certificate (first in the chain).</t>
        <texttable anchor="tbl3" title="Server Certificate Types">
        <ttcol align="left">Algorithm</ttcol>
        <ttcol align="left">Server Certificate Type</ttcol>
        <c>ECDHE_ECDSA</c><c>Certificate MUST contain an ECDSA- or EdDSA-capable public key.</c>
        <c>ECDHE_RSA</c><c>Certificate MUST contain an RSA public key.</c>
      </texttable>
      <t> Structure of this message:</t>
      <t> Identical to the TLS Certificate format.</t>
      <t> Actions of the sender:</t>
      <t> The server constructs an appropriate certificate chain and conveys it to the client in the Certificate
        message.  If the client has used a Supported Elliptic Curves Extension, the public key in the server's
        certificate MUST respect the client's choice of elliptic curves. A server that cannot satisfy this requirement 
        MUST NOT choose an ECC cipher suite in its ServerHello message.)</t>
      <t> Actions of the receiver:</t>
      <t> The client validates the certificate chain, extracts the server's public key, and checks that the key
        type is appropriate for the negotiated key exchange algorithm.  (A possible reason for a fatal handshake
        failure is that the client's capabilities for handling elliptic curves and point formats are exceeded;
        cf. <xref target="ch_extensions"/>.)</t>
      </section>
      <section anchor="ske" title="Server Key Exchange">
        <t> When this message is sent:</t>
        <t> This message is sent when using the ECDHE_ECDSA, ECDHE_RSA, and ECDH_anon key exchange algorithms.</t>
        <t> Meaning of this message:</t>
        <t> This message is used to convey the server's ephemeral ECDH public key (and the corresponding elliptic
          curve domain parameters) to the client.</t>
        <t> The ECCCurveType enum used to have values for explicit prime and for explicit char2 curves. Those
          values are now deprecated, so only one value remains:</t>
        <t> Structure of this message:<figure><artwork><![CDATA[
        enum {
            deprecated (1..2),
            named_curve (3),
            reserved(248..255)
        } ECCurveType;
]]></artwork></figure></t>
        <t> The value named_curve indicates that a named curve is used. This option is now the only remaining format.</t>
        <t> Values 248 through 255 are reserved for private use.</t>
        <t> The ECCurveType name space is maintained by IANA.  See <xref target="iana"/> for information on how new
          value assignments are added.</t>
        <t> RFC 4492 had a specification for an ECCurve structure and an ECBasisType structure. Both of these are
          omitted now because they were only used with the now deprecated explicit curves.</t>
        <t><figure><artwork><![CDATA[
        struct {
            opaque point <1..2^8-1>;
        } ECPoint;
]]></artwork></figure></t>
          <t> point: This is the byte string representation of an elliptic curve point following the
            conversion routine in Section 4.3.6 of <xref target="ANSI.X9-62.2005"/>.  This byte string may
            represent an elliptic curve point in uncompressed, compressed, or hybrid format, but this specification
            deprecates all but the uncompressed format. For the NIST curves, the format is repeated in <xref 
            target="UncompressedFormat"/> for convenience. For the X25519 and X448 curves, the only valid 
            representation is the one specified in <xref target="RFC7748"/> - a 32- or 56-octet representation of 
            the u value of the point. This structure MUST NOT be used with Ed25519 and Ed448 public keys.</t>
          <t> <figure><artwork><![CDATA[
        struct {
            ECCurveType    curve_type;
            select (curve_type) {
                case named_curve:
                    NamedCurve namedcurve;
            };
        } ECParameters;
]]></artwork></figure></t>
          <t> curve_type: This identifies the type of the elliptic curve domain parameters.</t>
          <t> namedCurve: Specifies a recommended set of elliptic curve domain parameters.  All those
            values of NamedCurve are allowed that refer to a curve capable of Diffie-Hellman.  With the deprecation
            of the explicit curves, this now includes all of the NamedCurve values.</t>
          <t><figure><artwork><![CDATA[
        struct {
            ECParameters    curve_params;
            ECPoint         public;
        } ServerECDHParams;
]]></artwork></figure></t>
          <t> curve_params: Specifies the elliptic curve domain parameters associated with the ECDH
            public key.</t>
          <t> public: The ephemeral ECDH public key.</t>
        <t> The ServerKeyExchange message is extended as follows.<figure><artwork><![CDATA[
        enum {
            ec_diffie_hellman
        } KeyExchangeAlgorithm;
]]></artwork></figure><list style="symbols">
          <t> ec_diffie_hellman: Indicates the ServerKeyExchange message contains an ECDH public key.</t></list><figure><artwork><![CDATA[
   select (KeyExchangeAlgorithm) {
       case ec_diffie_hellman:
           ServerECDHParams    params;
           Signature           signed_params;
   } ServerKeyExchange;
]]></artwork></figure><list style="symbols">
          <t> params: Specifies the ECDH public key and associated domain parameters.</t>
          <t> signed_params: A hash of the params, with the signature appropriate to that hash applied.
            The private key corresponding to the certified public key in the server's Certificate message is used
            for signing.<figure><artwork><![CDATA[
     enum {
         ecdsa(3),
         ed25519(TBD3)
         ed448(TBD4)
     } SignatureAlgorithm;
     select (SignatureAlgorithm) {
        case ecdsa:
             digitally-signed struct {
                 opaque sha_hash[sha_size];
             };
        case ed25519,ed448:
             digitally-signed struct {
                 opaque rawdata[rawdata_size];
             };
     } Signature;
   ServerKeyExchange.signed_params.sha_hash
       SHA(ClientHello.random + ServerHello.random +
                              ServerKeyExchange.params);
   ServerKeyExchange.signed_params.rawdata
       ClientHello.random + ServerHello.random +
                              ServerKeyExchange.params;
]]></artwork></figure></t></list></t>
        <t> NOTE: SignatureAlgorithm is "rsa" for the ECDHE_RSA key exchange algorithm and "anonymous" for ECDH_anon.
          These cases are defined in TLS.  SignatureAlgorithm is "ecdsa" or "eddsa" for ECDHE_ECDSA.  ECDSA signatures
          are generated and verified as described in <xref target="alg_computes"/>, and SHA in the above template for
          sha_hash accordingly may denote a hash algorithm other than SHA-1.  As per ANSI X9.62, an ECDSA signature
          consists of a pair of integers, r and s.  The digitally-signed element is encoded as an opaque vector
          &lt;0..2^16-1&gt;, the contents of which are the DER encoding corresponding to the following ASN.1
          notation.<figure><artwork><![CDATA[
           Ecdsa-Sig-Value ::= SEQUENCE {
               r       INTEGER,
               s       INTEGER
           }
]]></artwork></figure></t>
        <t>EdDSA signatures in both the protocol and in certificates that conform to <xref target="PKIX-EdDSA"/> are 
          generated and verified according to <xref target="RFC8032"/>. The digitally-signed
          element is encoded as an opaque vector&lt;0..2^16-1&gt;, the contents of which is the octet string output of
          the EdDSA signing algorithm.</t>
        <t> Actions of the sender:</t>
        <t> The server selects elliptic curve domain parameters and an ephemeral ECDH public key corresponding to
          these parameters according to the ECKAS-DH1 scheme from IEEE 1363 <xref target="IEEE.P1363.1998"/>.
          It conveys this information to the client in the ServerKeyExchange message using the format defined above.</t>
        <t> Actions of the receiver:</t>
        <t> The client verifies the signature (when present) and retrieves the server's elliptic curve domain
          parameters and ephemeral ECDH public key from the ServerKeyExchange message.  (A possible reason for a
          fatal handshake failure is that the client's capabilities for handling elliptic curves and point formats
          are exceeded; cf. <xref target="ch_extensions"/>.)</t>
        <section anchor="UncompressedFormat" title="Uncompressed Point Format for NIST curves">
          <t> The following represents the wire format for representing ECPoint in ServerKeyExchange records.
            The first octet of the representation indicates the form, which may be compressed, uncompressed, or
            hybrid. This specification supports only the uncompressed format for these curves. This is followed by
            the binary representation of the X value in "big-endian" or "network" format, followed by the binary
            representation of the Y value in "big-endian" or "network" format. There are no internal length markers, 
            so each number representation occupies as many octets as implied by the curve parameters. For P-256 this
            means that each of X and Y use 32 octets, padded on the left by zeros if necessary. For P-384 they take 48 
            octets each, and for P-521 they take 66 octets each.</t>
          <t> Here's a more formal representation:<figure><artwork><![CDATA[
          enum {
              uncompressed(4),
              (255)
            } PointConversionForm;
            
          struct {
              PointConversionForm  form;
              opaque               X[coordinate_length];
              opaque               Y[coordinate_length];
          } UncompressedPointRepresentation;
          ]]></artwork></figure></t>
        </section>
      </section>
      <section anchor="certreq" title="Certificate Request">
        <t> When this message is sent:</t>
        <t> This message is sent when requesting client authentication.</t>
        <t> Meaning of this message:</t>
        <t> The server uses this message to suggest acceptable client authentication methods.</t>
        <t> Structure of this message:</t>
        <t> The TLS CertificateRequest message is extended as follows.<figure><artwork><![CDATA[
        enum {
            ecdsa_sign(64),
            deprecated1(65),  /* was rsa_fixed_ecdh */
            deprecated2(66),  /* was ecdsa_fixed_ecdh */
            (255)
        } ClientCertificateType;
]]></artwork></figure><list style="symbols">
          <t> ecdsa_sign: Indicates that the server would like to use the corresponding client
            authentication method specified in <xref target="clientauth"/>.</t></list></t>
        <t> Note that RFC 4492 also defined RSA and ECDSA certificates that included a fixed ECDH
          public key. These mechanisms saw very little implementation so this specification is deprecating them.</t>    
        <t> Actions of the sender:</t>
        <t> The server decides which client authentication methods it would like to use, and conveys this information
          to the client using the format defined above.</t>
        <t> Actions of the receiver:</t>
        <t> The client determines whether it has a suitable certificate for use with any of the requested methods
          and whether to proceed with client authentication.</t>
      </section>
      <section anchor="client_cert" title="Client Certificate">
        <t> When this message is sent:</t>
        <t> This message is sent in response to a CertificateRequest when a client has a suitable certificate and
          has decided to proceed with client authentication.  (Note that if the server has used a Supported Point
          Formats Extension, a certificate can only be considered suitable for use with the ECDSA_sign
          authentication method if the public key point specified in it is uncompressed, as that is the only point
          format still supported.</t>
        <t> Meaning of this message:</t>
        <t> This message is used to authentically convey the client's static public key to the server.  The
          following table summarizes what client certificate types are appropriate for the ECC-based client
          authentication mechanisms described in <xref target="clientauth"/>.  ECC public keys must be encoded in
          certificates as described in <xref target="eccerts"/>.</t>
        <t> NOTE: The client's Certificate message is capable of carrying a chain of certificates.  The
          restrictions mentioned in Table 4 apply only to the client's certificate (first in the chain).</t>
        <t> The certificate MUST contain an ECDSA- or EdDSA-capable public key.</t>
       <t> Structure of this message:</t>
      <t> Identical to the TLS client Certificate format.</t>
      <t> Actions of the sender:</t>
      <t> The client constructs an appropriate certificate chain, and conveys it to the server in the Certificate message.</t>
      <t> Actions of the receiver:</t>
      <t> The TLS server validates the certificate chain, extracts the client's public key, and checks that the key
        type is appropriate for the client authentication method.</t>
      </section>
      <section anchor="cke" title="Client Key Exchange">
        <t> When this message is sent:</t>
        <t> This message is sent in all key exchange algorithms.  It contains the client's ephemeral ECDH
          public key.</t>
        <t> Meaning of the message:</t>
        <t> This message is used to convey ephemeral data relating to the key exchange belonging to the client (such
          as its ephemeral ECDH public key).</t>
        <t> Structure of this message:</t>
        <t> The TLS ClientKeyExchange message is extended as follows.</t>
        <t><figure><artwork><![CDATA[
        enum {
            implicit,
            explicit
        } PublicValueEncoding;
]]></artwork></figure></t>
        <t><list style="symbols">
          <t> implicit, explicit: For ECC cipher suites, this indicates whether the client's ECDH public
            key is in the client's certificate ("implicit") or is provided, as an ephemeral ECDH public key, in the
            ClientKeyExchange message ("explicit").  The implicit encoding is deprecated and is retained here for 
            backward compatibility only.</t></list></t>
        <t><figure><artwork><![CDATA[
        struct {
            ECPoint ecdh_Yc;
        } ClientECDiffieHellmanPublic;
]]></artwork></figure></t>
          <t> ecdh_Yc: Contains the client's ephemeral ECDH public key as a byte string ECPoint.point,
            which may represent an elliptic curve point in uncompressed format.</t>
          <t><figure><artwork><![CDATA[
        struct {
            select (KeyExchangeAlgorithm) {
                case ec_diffie_hellman: ClientECDiffieHellmanPublic;
            } exchange_keys;
        } ClientKeyExchange;
]]></artwork></figure></t>

        <t> Actions of the sender:</t>
        <t> The client selects an ephemeral ECDH public key corresponding to the parameters it received from the
          server. The format is the same as in <xref target="ske"/>.</t>
        <t> Actions of the receiver:</t>
        <t> The server retrieves the client's ephemeral ECDH public key from the ClientKeyExchange message and checks
          that it is on the same elliptic curve as the server's ECDH key.</t>
      </section>
      <section anchor="cert_verify" title="Certificate Verify">
        <t> When this message is sent:</t>
        <t> This message is sent when the client sends a client certificate containing a public key usable for
          digital signatures.</t>
        <t> Meaning of the message:</t>
        <t> This message contains a signature that proves possession of the private key corresponding to the public
          key in the client's Certificate message.</t>
        <t> Structure of this message:</t>
        <t> The TLS CertificateVerify message and the underlying Signature type are defined in the TLS base
          specifications, and the latter is extended here in <xref target="ske"/>.  For the ecdsa and eddsa cases, the
          signature field in the CertificateVerify message contains an ECDSA or EdDSA (respectively) signature computed
          over handshake messages exchanged so far, exactly similar to CertificateVerify with other signing
          algorithms:<figure><artwork><![CDATA[
        CertificateVerify.signature.sha_hash
            SHA(handshake_messages);
        CertificateVerify.signature.rawdata
            handshake_messages;
]]></artwork></figure></t>
        <t> ECDSA signatures are computed as described in <xref target="alg_computes"/>, and SHA in the above
          template for sha_hash accordingly may denote a hash algorithm other than SHA-1.  As per ANSI X9.62, an
          ECDSA signature consists of a pair of integers, r and s.  The digitally-signed element is encoded as an
          opaque vector &lt;0..2^16-1&gt;, the contents of which are the DER encoding <xref target="CCITT.X690"/>
          corresponding to the following ASN.1 notation <xref target="CCITT.X680"/>.<figure><artwork><![CDATA[
        Ecdsa-Sig-Value ::= SEQUENCE {
            r       INTEGER,
            s       INTEGER
        }
]]></artwork></figure></t>
        <t>EdDSA signatures are generated and verified according to <xref target="RFC8032"/>. The digitally-signed
          element is encoded as an opaque vector&lt;0..2^16-1&gt;, the contents of which is the octet string output of
          the EdDSA signing algorithm.</t>
        <t> Actions of the sender:</t>
        <t> The client computes its signature over all handshake messages sent or received starting at client hello
          and up to but not including this message.  It uses the private key corresponding to its certified public
          key to compute the signature, which is conveyed in the format defined above.</t>
        <t> Actions of the receiver:</t>
        <t> The server extracts the client's signature from the CertificateVerify message, and verifies the
          signature using the public key it received in the client's Certificate message.</t>
      </section>
      <section anchor="eccerts" title="Elliptic Curve Certificates">
        <t> X.509 certificates containing ECC public keys or signed using ECDSA MUST comply with
          <xref target="RFC3279"/> or another RFC that replaces or extends it. X.509 certificates containing ECC
          public keys or signed using EdDSA MUST comply with <xref target="PKIX-EdDSA"/>. Clients SHOULD use the
          elliptic curve domain parameters recommended in ANSI X9.62, FIPS 186-4, and SEC 2
          <xref target="SECG-SEC2"/> or in <xref target="RFC8032"/>.</t>
        <t>EdDSA keys using the Ed25519 algorithm MUST use the ed25519 signature algorithm, and Ed448
          keys MUST use the ed448 signature algorithm. This document does not define use of Ed25519ph and Ed448ph keys
          with TLS. Ed25519, Ed25519ph, Ed448, and Ed448ph keys MUST NOT be used with ECDSA.</t>
      </section>
      <section anchor="alg_computes" title="ECDH, ECDSA, and RSA Computations">
        <t> All ECDH calculations for the NIST curves (including parameter and key generation as well as the shared
          secret calculation) are performed according to <xref target="IEEE.P1363.1998"/> using the ECKAS-DH1 scheme
          with the identity map as key derivation function (KDF), so that the premaster secret is the x-coordinate
          of the ECDH shared secret elliptic curve point represented as an octet string.  Note that this octet string
          (Z in IEEE 1363 terminology) as output by FE2OSP, the Field Element to Octet String Conversion Primitive,
          has constant length for any given field; leading zeros found in this octet string MUST NOT be truncated.</t>
        <t> (Note that this use of the identity KDF is a technicality.  The complete picture is that ECDH is
          employed with a non-trivial KDF because TLS does not directly use the premaster secret for anything other
          than for computing the master secret.  In TLS 1.0 and 1.1, this means that the MD5- and SHA-1-based TLS
          PRF serves as a KDF; in TLS 1.2 the KDF is determined by ciphersuite; it is conceivable that future TLS
          versions or new TLS extensions introduced in the future may vary this computation.)</t>
        <t> An ECDHE key exchange using X25519 (curve x25519) goes as follows: Each party picks a secret key d
          uniformly at random and computes the corresponding public key x = X25519(d, G). Parties exchange their
          public keys, and compute a shared secret as x_S = X25519(d, x_peer). If either party obtains all-zeroes x_S,
          it MUST abort the handshake (as required by definition of X25519 and X448). ECDHE for X448 works similarily,
          replacing X25519 with X448, and x25519 with x448.  The derived shared secret is used directly as
          the premaster secret, which is always exactly 32 bytes when ECDHE with X25519 is used and 56 bytes when
          ECDHE with X448 is used.</t>
        <t> All ECDSA computations MUST be performed according to ANSI X9.62 or its successors.  Data to be 
          signed/verified is hashed, and the result run directly through the ECDSA algorithm with no additional 
          hashing. A secure hash function such as SHA-256, SHA-384, or SHA-512 from <xref target="FIPS.180-4"/>
          MUST be used.</t>
        <t> All EdDSA computations MUST be performed according to <xref target="RFC8032"/> or its succesors. Data
          to be signed/verified is run through the EdDSA algorithm wih no hashing (EdDSA will internally run the data
          through the PH function). The context parameter for Ed448 MUST be set to the empty string.</t>
        <t> RFC 4492 anticipated the standardization of a mechanism for specifying the required hash function in the certificate,
        perhaps in the parameters field of the subjectPublicKeyInfo. Such standardization never took place, and as a result,
        SHA-1 is used in TLS 1.1 and earlier (except for EdDSA, which uses identity function). TLS 1.2 added a
        SignatureAndHashAlgorithm parameter to the DigitallySigned struct, thus allowing agility in choosing the
        signature hash. EdDSA signatures MUST have HashAlgorithm of TBD5 (Intrinsic).</t>
        <t> All RSA signatures must be generated and verified according to <xref target="PKCS1"/> block type 1.</t>
      </section>
      <section anchor="valid25519" title="Public Key Validation">
        <t> With the NIST curves, each party MUST validate the public key sent by its peer in the ClientKeyExchange
          and ServerKeyExchange messages. A receiving party MUST 
          check that the x and y parameters from the peer's public value satisfy the curve equation, y^2 = x^3 + ax + b 
          mod p. See section 2.3 of <xref target="Menezes"/> for details. Failing to do so allows attackers to gain 
          information about the private key, to the point that they may recover the entire private key in a few requests, 
          if that key is not really ephemeral.</t>
        <t>With X25519 and X448, a receiving party MUST check whether the computed premaster secret is the all-zero
          value and abort the handshake if so, as described in section 6 of <xref target="RFC7748"/>.</t>
        <t>Ed25519 and Ed448 internally do public key validation as part of signature verification.</t>
      </section>
    </section>
    <section anchor="suites" title="Cipher Suites">
      <t> The table below defines ECC cipher suites that use the key exchange algorithms specified in <xref target="ecdh"/>.</t>
      <texttable anchor="tbl5" title="TLS ECC cipher suites">
          <ttcol align="left">CipherSuite</ttcol>
          <ttcol align="left">Identifier</ttcol>
          <c>TLS_ECDHE_ECDSA_WITH_NULL_SHA</c><c>{ 0xC0, 0x06 }</c>
          <c>TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA</c><c>{ 0xC0, 0x08 }</c>
          <c>TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA</c><c>{ 0xC0, 0x09 }</c>
          <c>TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA</c><c>{ 0xC0, 0x0A }</c>
          <c>TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256</c><c>{ 0xC0, 0x2B }</c>
          <c>TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384</c><c>{ 0xC0, 0x2C }</c>
                    <c> </c><c> </c>
          <c>TLS_ECDHE_RSA_WITH_NULL_SHA</c><c>{ 0xC0, 0x10 }</c>
          <c>TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA</c><c>{ 0xC0, 0x12 }</c>
          <c>TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA</c><c>{ 0xC0, 0x13 }</c>
          <c>TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA</c><c>{ 0xC0, 0x14 }</c>
          <c>TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256</c><c>{ 0xC0, 0x2F }</c>
          <c>TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384</c><c>{ 0xC0, 0x30 }</c>
                    <c> </c><c> </c>
          <c>TLS_ECDH_anon_WITH_NULL_SHA</c><c>{ 0xC0, 0x15 }</c>
          <c>TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA</c><c>{ 0xC0, 0x17 }</c>
          <c>TLS_ECDH_anon_WITH_AES_128_CBC_SHA</c><c>{ 0xC0, 0x18 }</c>
          <c>TLS_ECDH_anon_WITH_AES_256_CBC_SHA</c><c>{ 0xC0, 0x19 }</c>
      </texttable>
      <t> The key exchange method, cipher, and hash algorithm for each of these cipher suites are easily determined
        by examining the name.  Ciphers (other than AES ciphers) and hash algorithms are defined in
        <xref target="RFC2246"/> and <xref target="RFC4346"/>. AES ciphers are defined in <xref target="RFC5246"/>, 
        and AES-GCM ciphersuites are in <xref target="RFC5289"/>.</t>
      <t> Server implementations SHOULD support all of the following cipher suites, and client implementations
        SHOULD support at least one of them:<list style="symbols">
        <t> TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256</t>
        <t> TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA</t>
        <t> TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256</t>
        <t> TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256</t></list></t>
    </section>
    <section anchor="impstat" title="Implementation Status">
      <t> Both ECDHE and ECDSA with the NIST curves are widely implemented, supported in all major browsers and all 
        widely used TLS libraries. ECDHE with Curve25519 is by now implemented in several browsers and several TLS 
        libraries including OpenSSL. Curve448 and EdDSA have working, interoperable implementations, but are not yet
        as widely deployed.</t>
    </section>
    <section anchor="sec" title="Security Considerations">
      <t> Security issues are discussed throughout this memo.</t>
      <t> For TLS handshakes using ECC cipher suites, the security considerations in appendices D of all three TLS
        base documemts apply accordingly.</t>
      <t> Security discussions specific to ECC can be found in <xref target="IEEE.P1363.1998"/> and
        <xref target="ANSI.X9-62.2005"/>. One important issue that implementers and users must consider is elliptic
        curve selection.  Guidance on selecting an appropriate elliptic curve size is given in Table 1. Security
        considerations specific to X25519 and X448 are discussed in section 7 of <xref target="RFC7748"/>.</t>
      <t> Beyond elliptic curve size, the main issue is elliptic curve structure.  As a general principle, it is
        more conservative to use elliptic curves with as little algebraic structure as possible. Thus, random curves
        are more conservative than special curves such as Koblitz curves, and curves over F_p with p random are more
        conservative than curves over F_p with p of a special form, and curves over F_p with p random are 
        considered more conservative than curves over F_2^m as there is no choice between multiple fields of similar
        size for characteristic 2.</t>
      <t> Another issue is the potential for catastrophic failures when a single elliptic curve is widely used.
        In this case, an attack on the elliptic curve might result in the compromise of a large number of keys.
        Again, this concern may need to be balanced against efficiency and interoperability improvements associated
        with widely-used curves. Substantial additional information on elliptic curve choice can be found in
        <xref target="IEEE.P1363.1998"/>, <xref target="ANSI.X9-62.2005"/>, and <xref target="FIPS.186-4"/>.</t>
      <t> The Introduction of <xref target="RFC8032"/> lists the security, performance, and operational advantages of 
        EdDSA signatures over ECDSA signatures using the NIST curves. </t>
      <t> All of the key exchange algorithms defined in this document provide forward secrecy. Some of the
        deprecated key exchange algorithms do not.</t>
    </section>
    <section anchor="iana" title="IANA Considerations">
      <t> <xref target="RFC4492" />, the predecessor of this document has already defined the IANA registries for
        the following:<list style="symbols">
        <t> Supported Groups <xref target="ch_extensions"/></t>
        <t> ECPointFormat <xref target="ch_extensions"/></t>
        <t> ECCurveType <xref target="ske"/></t></list></t>
      <t> IANA is requested to prepend "TLS" to the names of the previous three registries.</t>
      <t> For each name space, this document defines the initial value assignments and defines a range of 256 values
        (NamedCurve) or eight values (ECPointFormat and ECCurveType) reserved for Private Use.  The policy for any additional
        assignments is "Specification Required". The previous version of this document required IETF review.</t>
      <t> NOTE: IANA, please update the registries to reflect the new policy.</t>
      <t> NOTE: RFC editor please delete these two notes prior to publication.</t>
      <t> IANA, please update these two registries to refer to this document.</t>
      <t> IANA is requested to assigned the value 29 to x25519, and the value
        30 to x448 in the TLS Supported Groups Registry. This replaces the
        temporary registrations ecdh_x25519(29) and ecdh_x448(30).</t>
      <t> IANA is requested to assign two values from the TLS SignatureAlgorithm Registry with names ed25519(TBD3) 
        and ed448(TBD4) with this document as reference. To keep compatibility with TLS 1.3, TBD3 should be 7, 
        and TBD4 should be 8.</t>
      <t> IANA is requested to assign one value from the "TLS HashAlgorithm Registry" with name Intrinsic(TBD5)
        and this document as reference. To keep compatibility with TLS 1.3, TBD5 should be 8 and DTLS-OK should
        be set to true (Y).</t>
    </section>
    <section anchor="ack" title="Acknowledgements">
      <t> Most of the text is this document is taken from <xref target="RFC4492"/>, the predecessor of this
        document. The authors of that document were:<list style="symbols">
        <t> Simon Blake-Wilson</t>
        <t> Nelson Bolyard</t>
        <t> Vipul Gupta</t>
        <t> Chris Hawk</t>
        <t> Bodo Moeller</t></list></t>
      <t> In the predecessor document, the authors acknowledged the contributions of Bill Anderson and Tim Dierks.</t>
      <t> The author would like to thank Nikos Mavrogiannopoulos, Martin Thomson, and Tanja Lange for contributions
        to this document.</t>
    </section>
    <section anchor="history" title="Version History for This Draft">
      <t> NOTE TO RFC EDITOR: PLEASE REMOVE THIS SECTION</t>
      <t> Changes from draft-ietf-tls-rfc4492bis-03 to draft-nir-tls-rfc4492bis-05:<list style="symbols">
        <t> Add support for CFRG curves and signatures work.</t>
        </list></t>
      <t> Changes from draft-ietf-tls-rfc4492bis-01 to draft-nir-tls-rfc4492bis-03:<list style="symbols">
        <t> Removed unused curves.</t>
        <t> Removed unused point formats (all but uncompressed)</t>
        </list></t>
      <t> Changes from draft-nir-tls-rfc4492bis-00 and draft-ietf-tls-rfc4492bis-00 to
        draft-nir-tls-rfc4492bis-01:<list style="symbols">
        <t> Merged errata</t>
        <t> Removed ECDH_RSA and ECDH_ECDSA</t>
        </list></t>
      <t> Changes from RFC 4492 to draft-nir-tls-rfc4492bis-00:<list style="symbols">
        <t> Added TLS 1.2 to references.</t>
        <t> Moved RFC 4492 authors to acknowledgements.</t>
        <t> Removed list of required reading for ECC.</t>
        <t> Prepended "TLS" to the names of the three registries defined in the IANA Considerations section.</t></list></t>
    </section>

    <!-- ====================================================================== -->
  </middle>
  <!-- ====================================================================== -->
  <back>
    <references title="Normative References">
      <reference anchor="RFC7748">
        <front>
          <title>Elliptic Curves for Security</title>
            <author initials="A" surname="Langley" fullname="Adam Langley"></author>
            <author initials="M" surname="Hamburg" fullname="Mike Hamburg"></author>
            <author initials="S" surname="Turner" fullname="Sean Turner"></author>
            <date month="January" year="2016"/>
          </front>
        <seriesInfo name='RFC' value='7748' />
        <format type='HTML' target='https://tools.ietf.org/html/rfc7748' />
      </reference>
      <reference anchor="RFC8032">
        <front>
          <title>Edwards-Curve Digital Signature Algorithm (EdDSA)</title>
            <author initials="S" surname="Josefsson" fullname="Simon Josefsson"></author>
            <author initials="I" surname="Liusvaara" fullname="Ilari Liusvaara"></author>
            <date month="January" year="2017"/>
          </front>
        <seriesInfo name="RFC" value="8032"/>
        <format type="HTML" target="https://tools.ietf.org/html/rfc8032"/>
      </reference>
      <reference anchor="PKIX-EdDSA" target="https://tools.ietf.org/html/draft-ietf-curdle-pkix-01">
        <front>
          <title>Algorithm Identifiers for Ed25519, Ed25519ph, Ed448, Ed448ph, X25519 and X448 for use in the Internet X.509 Public Key Infrastructure</title>
          <author initials="S." surname="Josefsson" fullname="Simon Josefsson">
            <organization />
          </author>
          <author initials="J." surname="Schaad" fullname="Jim Schaad">
            <organization />
          </author>
          <date month="August" year="2016" />
        </front>
      </reference>      
      <reference anchor='RFC2246'>
        <front>
          <title>The TLS Protocol Version 1.0</title>
          <author initials='T.' surname='Dierks' fullname='Tim Dierks'/>
          <author initials='C.' surname='Allen' fullname='Christopher Allen' />
          <date year='1999' month='January' />
        </front>
        <seriesInfo name='RFC' value='2246' />
        <format type='HTML' target='http://tools.ietf.org/html/rfc2246' />
      </reference>
      <reference anchor='RFC4346'>
        <front>
          <title>The Transport Layer Security (TLS) Protocol Version 1.1</title>
          <author initials='T.' surname='Dierks' fullname='T. Dierks' />
          <author initials='E.' surname='Rescorla' fullname='E. Rescorla' />
          <date year='2006' month='April' />
        </front>
        <seriesInfo name='RFC' value='4346' />
        <format type='HTML' target='http://tools.ietf.org/html/rfc4346' />
      </reference>
      <reference anchor='RFC5289'>
        <front>
          <title>TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)</title>
          <author initials='E.' surname='Rescorla' fullname='E. Rescorla' />
          <date year='2008' month='August' />
        </front>
        <seriesInfo name='RFC' value='5289' />
        <format type='HTML' target='http://tools.ietf.org/html/rfc5289' />
      </reference>
      <reference anchor='RFC5246'>
        <front>
          <title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
          <author initials='T.' surname='Dierks' fullname='T. Dierks' />
          <author initials='E.' surname='Rescorla' fullname='E. Rescorla' />
          <date year='2008' month='August' />
        </front>
        <seriesInfo name='RFC' value='5246' />
        <format type='HTML' target='http://tools.ietf.org/html/rfc5246' />
      </reference>
      <reference anchor='SECG-SEC2'>
        <front>
          <title>Recommended Elliptic Curve Domain Parameters</title>
          <author>
            <organization>CECG</organization>
          </author>
          <date year='2000'/>
        </front>
        <seriesInfo name='SEC' value='2' />
        <format type='PDF' target='http://www.secg.org/download/aid-784/sec2-v2.pdf' />
      </reference>
      <reference anchor='RFC4366'>
        <front>
          <title>Transport Layer Security (TLS) Extensions</title>
          <author initials='S.' surname='Blake-Wilson' fullname='S. Blake-Wilson'>
            <organization /></author>
          <author initials='M.' surname='Nystrom' fullname='M. Nystrom'>
            <organization /></author>
          <author initials='D.' surname='Hopwood' fullname='D. Hopwood'>
            <organization /></author>
          <author initials='J.' surname='Mikkelsen' fullname='J. Mikkelsen'>
            <organization /></author>
          <author initials='T.' surname='Wright' fullname='T. Wright'>
            <organization /></author>
          <date year='2006' month='April' />
        </front>
        <seriesInfo name='RFC' value='4366' />
        <format type='TXT' octets='66344' target='http://www.ietf.org/rfc/rfc4366.txt' />
      </reference>
      <reference anchor='RFC3279'>
        <front>
          <title>Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile</title>
          <author initials='L.' surname='Bassham' fullname='L. Bassham'><organization /></author>
          <author initials='W.' surname='Polk' fullname='W. Polk'><organization /></author>
          <author initials='R.' surname='Housley' fullname='R. Housley'><organization /></author>
          <date year='2002' month='April' />
        </front>
        <seriesInfo name='RFC' value='3279' />
        <format type='TXT' target='http://www.ietf.org/rfc/rfc3279.txt' />
      </reference>
      <reference anchor="CCITT.X680">
        <front>
          <title>Abstract Syntax Notation One (ASN.1): Specification of basic notation</title>
          <author>
            <organization>International Telephone and Telegraph Consultative Committee</organization>
          </author>
          <date month="July" year="2002" />
        </front>
        <seriesInfo name="CCITT" value="Recommendation X.680" />
      </reference>
      <reference anchor="CCITT.X690">
        <front>
          <title>ASN.1 encoding rules: Specification of basic encoding Rules (BER), Canonical encoding rules (CER) and Distinguished encoding rules (DER)</title>
          <author>
            <organization>International Telephone and Telegraph Consultative Committee</organization>
          </author>
          <date month="July" year="2002" />
        </front>
        <seriesInfo name="CCITT" value="Recommendation X.690" />
      </reference>
      <reference anchor="FIPS.186-4" target="http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf">
        <front>
          <title>Digital Signature Standard</title>
          <author>
            <organization>National Institute of Standards and Technology</organization>
          </author>
          <date year="2013" />
        </front>
        <seriesInfo name="FIPS" value="PUB 186-4" />
      </reference>
      <reference anchor="ANSI.X9-62.2005">
        <front>
          <title>Public Key Cryptography for the Financial Services Industry, The Elliptic Curve Digital Signature Algorithm (ECDSA)</title>
          <author>
            <organization>American National Standards Institute</organization>
          </author>
          <date year="2005" />
        </front>
        <seriesInfo name="ANSI" value="X9.62" />
      </reference>
      <reference anchor='RFC2119'>
        <front>
          <title abbrev='RFC Key Words'>Key words for use in RFCs to Indicate Requirement Levels</title>
          <author initials='S.' surname='Bradner' fullname='Scott Bradner'>
            <organization>Harvard University</organization>
            <address>
              <postal>
                <street>1350 Mass. Ave.</street>
                <street>Cambridge</street>
                <street>MA 02138</street>
              </postal>
              <phone>- +1 617 495 3864</phone>
              <email>sob@harvard.edu</email>
            </address>
          </author>
          <date year='1997' month='March' />
          <area>General</area>
          <keyword>keyword</keyword>
        </front>
        <seriesInfo name='BCP' value='14' />
        <seriesInfo name='RFC' value='2119' />
        <format type='TXT' target='ftp://ftp.isi.edu/in-notes/rfc2119.txt' />
        <format type='HTML' octets='16553' target='http://xml.resource.org/public/rfc/html/rfc2119.html' />
        <format type='XML' octets='5703' target='http://xml.resource.org/public/rfc/xml/rfc2119.xml' />
      </reference>
      <reference anchor="PKCS1">
        <front>
          <title>RSA Encryption Standard, Version 1.5</title>
          <author>
            <organization>RSA Laboratories</organization>
          </author>
          <date month="November" year="1993" />
        </front>
        <seriesInfo name="PKCS" value="1" />
      </reference>
    </references>
    <references title="Informative References">
      <reference anchor="Menezes">
        <front>
          <title>On Reusing Ephemeral Keys In Diffie-Hellman Key Agreement Protocols</title>
          <author initials="A" surname="Menezes" fullname="A. Menezes" />
          <author initials="B" surname="Ustaoglu" fullname="B. UstaOglu" />
          <date month="December" year="2008" />
        </front>
        <seriesInfo name="IACR" value="Menezes2008" />
        <format type="PDF" target="http://www.cacr.math.uwaterloo.ca/techreports/2008/acr2008-24.pdf" />
      </reference>
      <reference anchor='I-D.ietf-tls-tls13'>
        <front>
          <title>The Transport Layer Security (TLS) Protocol Version 1.3</title>
          <author initials='E' surname='Rescorla' fullname='Eric Rescorla' />
          <date month='October' day='26' year='2016' />
        </front>
        <seriesInfo name='Internet-Draft' value='draft-ietf-tls-tls13-18' />
        <format type='HTML' target='http://tools.ietf.org/html/draft-ietf-tls-tls13-18' />
      </reference>
      <reference anchor="IEEE.P1363.1998">
        <front>
          <title>Standard Specifications for Public Key Cryptography</title>
          <author>
            <organization>Institute of Electrical and Electronics Engineers</organization>
          </author>
          <date month="" year="1998" />
        </front>
        <seriesInfo name="IEEE" value="Draft P1363" />
      </reference>
      <reference anchor='RFC4492'>
        <front>
          <title>Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)</title>
          <author initials='S.' surname='Blake-Wilson' fullname='S. Blake-Wilson' />
          <author initials='N.' surname='Bolyard' fullname='N. Bolyard' />
          <author initials='V.' surname='Gupta' fullname='V. Gupta' />
          <author initials='C.' surname='Hawk' fullname='C. Hawk' />
          <author initials='B.' surname='Moeller' fullname='B. Moeller' />
          <date year='2006' month='May' />
        </front>
        <seriesInfo name='RFC' value='4492' />
        <format type='HTML' target='http://tools.ietf.org/html/rfc4492' />
      </reference>
      <reference anchor="FIPS.180-4" target="http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf">
        <front>
          <title>Secure Hash Standard (SHS)</title>
          <author>
            <organization>National Institute of Standards and Technology</organization>
          </author>
          <date month="August" year="2015" />
        </front>
        <seriesInfo name="FIPS" value="PUB 180-4" />
      </reference>
      <reference  anchor='RFC7919' target='http://www.rfc-editor.org/info/rfc7919'>
        <front>
          <title>Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)</title>
          <author initials='D.' surname='Gillmor' fullname='D. Gillmor'><organization /></author>
          <date year='2016' month='August' />
        </front>
        <seriesInfo name='RFC' value='7919'/>
        <seriesInfo name='DOI' value='10.17487/RFC7919'/>
      </reference>
    </references>
    <!-- ====================================================================== -->
    <section anchor="equiv" title="Equivalent Curves (Informative)">
      <t> All of the NIST curves <xref target="FIPS.186-4"/> and several of the ANSI curves <xref target="ANSI.X9-62.2005"/> are equivalent to curves listed in <xref target="supp_ec_ext"/>.  In the following table, multiple names in one row represent aliases for the same curve.</t>
      <texttable anchor="tbl6" title="Equivalent curves defined by SECG, ANSI, and NIST">
        <preamble>Curve names chosen by different standards organizations</preamble>
        <ttcol align="left">SECG</ttcol>
        <ttcol align="left">ANSI X9.62</ttcol>
        <ttcol align="left">NIST</ttcol>
        <c>sect163k1</c><c></c><c>NIST K-163</c>
        <c>sect163r1</c><c></c><c></c>
        <c>sect163r2</c><c></c><c>NIST B-163</c>
        <c>sect193r1</c><c></c><c></c>
        <c>sect193r2</c><c></c><c></c>
        <c>sect233k1</c><c></c><c>NIST K-233</c>
        <c>sect233r1</c><c></c><c>NIST B-233</c>
        <c>sect239k1</c><c></c><c></c>
        <c>sect283k1</c><c></c><c>NIST K-283</c>
        <c>sect283r1</c><c></c><c>NIST B-283</c>
        <c>sect409k1</c><c></c><c>NIST K-409</c>
        <c>sect409r1</c><c></c><c>NIST B-409</c>
        <c>sect571k1</c><c></c><c>NIST K-571</c>
        <c>sect571r1</c><c></c><c>NIST B-571</c>
        <c>secp160k1</c><c></c><c></c>
        <c>secp160r1</c><c></c><c></c>
        <c>secp160r2</c><c></c><c></c>
        <c>secp192k1</c><c></c><c></c>
        <c>secp192r1</c><c>prime192v1</c><c>NIST P-192</c>
        <c>secp224k1</c><c></c><c></c>
        <c>secp224r1</c><c></c><c>NIST P-224</c>
        <c>secp256k1</c><c></c><c></c>
        <c>secp256r1</c><c>prime256v1</c><c>NIST P-256</c>
        <c>secp384r1</c><c></c><c>NIST P-384</c>
        <c>secp521r1</c><c></c><c>NIST P-521</c>
      </texttable>
    </section>
    <section anchor="rfc4492diff" title="Differences from RFC 4492">
      <t><list style="symbols">
        <t> Added TLS 1.2</t>
        <t> Merged Errata</t>
        <t> Removed the ECDH key exchange algorithms: ECDH_RSA and ECDH_ECDSA</t>
        <t> Deprecated a bunch of ciphersuites:<list style="synmbols">
          <t>TLS_ECDH_ECDSA_WITH_NULL_SHA</t>
          <t>TLS_ECDH_ECDSA_WITH_RC4_128_SHA</t>
          <t>TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA</t>
          <t>TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA</t>
          <t>TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA</t>
          <t>TLS_ECDH_RSA_WITH_NULL_SHA</t>
          <t>TLS_ECDH_RSA_WITH_RC4_128_SHA</t>
          <t>TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA</t>
          <t>TLS_ECDH_RSA_WITH_AES_128_CBC_SHA</t>
          <t>TLS_ECDH_RSA_WITH_AES_256_CBC_SHA</t>
          <t>All the other RC4 ciphersuites</t></list></t>
      </list></t>
        <t> Removed unused curves and all but the uncompressed point format.</t>
        <t> Added X25519 and X448.</t>
        <t> Deprecated explicit curves.</t>
        <t> Removed restriction on signature algorithm in certificate.</t>
    </section>
  </back>
</rfc>