Internet DRAFT - draft-nir-tls-rfc4492bis
draft-nir-tls-rfc4492bis
TLS Working Group Y. Nir
Internet-Draft Check Point
Intended status: Experimental August 4, 2014
Expires: February 5, 2015
Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier
draft-nir-tls-rfc4492bis-00
Abstract
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 Elliptic Curve
Diffie-Hellman (ECDH) key agreement in a TLS handshake and the use of
Elliptic Curve Digital Signature Algorithm (ECDSA) as a new
authentication mechanism.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 5, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Nir Expires February 5, 2015 [Page 1]
Internet-Draft ECC Cipher Suites for TLS August 2014
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 4
2. Key Exchange Algorithm . . . . . . . . . . . . . . . . . . . . 4
2.1. ECDH_ECDSA . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. ECDHE_ECDSA . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. ECDH_RSA . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. ECDHE_RSA . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5. ECDH_anon . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Client Authentication . . . . . . . . . . . . . . . . . . . . 8
3.1. ECDSA_sign . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. ECDSA_fixed_ECDH . . . . . . . . . . . . . . . . . . . . . 9
3.3. RSA_fixed_ECDH . . . . . . . . . . . . . . . . . . . . . . 9
4. TLS Extensions for ECC . . . . . . . . . . . . . . . . . . . . 9
5. Data Structures and Computations . . . . . . . . . . . . . . . 10
5.1. Client Hello Extensions . . . . . . . . . . . . . . . . . 11
5.1.1. Supported Elliptic Curves Extension . . . . . . . . . 12
5.1.2. Supported Point Formats Extension . . . . . . . . . . 13
5.2. Server Hello Extension . . . . . . . . . . . . . . . . . . 14
5.3. Server Certificate . . . . . . . . . . . . . . . . . . . . 15
5.4. Server Key Exchange . . . . . . . . . . . . . . . . . . . 16
5.5. Certificate Request . . . . . . . . . . . . . . . . . . . 20
5.6. Client Certificate . . . . . . . . . . . . . . . . . . . . 21
5.7. Client Key Exchange . . . . . . . . . . . . . . . . . . . 22
5.8. Certificate Verify . . . . . . . . . . . . . . . . . . . . 24
5.9. Elliptic Curve Certificates . . . . . . . . . . . . . . . 25
5.10. ECDH, ECDSA, and RSA Computations . . . . . . . . . . . . 25
6. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . . . 26
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
10. Version History for This Draft . . . . . . . . . . . . . . . . 28
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1. Normative References . . . . . . . . . . . . . . . . . . . 29
11.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Equivalent Curves (Informative) . . . . . . . . . . . 30
Appendix B. Differences from RFC 4492 . . . . . . . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31
Nir Expires February 5, 2015 [Page 2]
Internet-Draft ECC Cipher Suites for TLS August 2014
1. Introduction
Elliptic Curve Cryptography (ECC) is emerging as an attractive
public-key cryptosystem, in particular for mobile (i.e., wireless)
environments. Compared to currently prevalent cryptosystems such as
RSA, ECC offers equivalent security with smaller key sizes. This is
illustrated in the following table, based on [Lenstra+Verheul], which
gives approximate comparable key sizes for symmetric- and asymmetric-
key cryptosystems based on the best-known algorithms for attacking
them.
+-----------+-----+------------+
| Symmetric | ECC | DH/DSA/RSA |
+-----------+-----+------------+
| 80 | 163 | 1024 |
| 112 | 233 | 2048 |
| 128 | 283 | 3072 |
| 192 | 409 | 7680 |
| 256 | 571 | 15360 |
+-----------+-----+------------+
Table 1: Comparable Key Sizes (in bits)
Smaller key sizes result in savings for power, memory, bandwidth, and
computational cost that make ECC especially attractive for
constrained environments.
This document describes additions to TLS to support ECC, applicable
to TLS versions 1.0 [RFC2246], 1.1 [RFC4346], and 1.2 [RFC5246]. The
use of ECC in TLS 1.3 is defined in [I-D.ietf-tls-tls13], and is
explicitly out of scope for this document. In particular, this
document defines:
o the use of the Elliptic Curve Diffie-Hellman (ECDH) key agreement
scheme with long-term or ephemeral keys to establish the TLS
premaster secret, and
o the use of fixed-ECDH certificates and ECDSA for authentication of
TLS peers.
The remainder of this document is organized as follows. Section 2
provides an overview of ECC-based key exchange algorithms for TLS.
Section 3 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 Section 4.
Section 5 specifies various data structures needed for an ECC-based
handshake, their encoding in TLS messages, and the processing of
those messages. Section 6 defines ECC-based cipher suites and
identifies a small subset of these as recommended for all
implementations of this specification. Section 7 discusses security
Nir Expires February 5, 2015 [Page 3]
Internet-Draft ECC Cipher Suites for TLS August 2014
considerations. Section 8 describes IANA considerations for the name
spaces created by this document's predecessor. Section 9 gives
acknowledgements. Appendix B provides differences from [RFC4492],
the document that this one replaces.
Implementation of this specification requires familiarity with TLS,
TLS extensions [RFC4366], and ECC (TBD: reference Wikipedia here?).
1.1. Conventions Used in This Document
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 [RFC2119].
2. Key Exchange Algorithm
This document introduces five new ECC-based key exchange algorithms
for TLS. All of them use ECDH to compute the TLS premaster secret,
and they differ only in the lifetime of ECDH keys (long-term or
ephemeral) and 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.
The table below summarizes the new key exchange algorithms, which
mimic DH_DSS, DHE_DSS, DH_RSA, DHE_RSA, and DH_anon, respectively.
+-------------+--------------------------------------------+
| Algorithm | Description |
+-------------+--------------------------------------------+
| ECDH_ECDSA | Fixed ECDH with ECDSA-signed certificates. |
| ECDHE_ECDSA | Ephemeral ECDH with ECDSA signatures. |
| ECDH_RSA | Fixed ECDH with RSA-signed certificates. |
| ECDHE_RSA | Ephemeral ECDH with RSA signatures. |
| ECDH_anon | Anonymous ECDH, no signatures. |
+-------------+--------------------------------------------+
Table 2: ECC Key Exchange Algorithms
The ECDHE_ECDSA and ECDHE_RSA key exchange mechanisms provide forward
secrecy. With ECDHE_RSA, a server can reuse its existing RSA
certificate and easily comply with a constrained client's elliptic
curve preferences (see Section 4). 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.
Nir Expires February 5, 2015 [Page 4]
Internet-Draft ECC Cipher Suites for TLS August 2014
The ECDH_RSA mechanism requires a server to acquire an ECC
certificate, but the certificate issuer can still use an existing RSA
key for signing. This eliminates the need to update the keys of
trusted certification authorities accepted by TLS clients. The
ECDH_ECDSA mechanism requires ECC keys for the server as well as the
certification authority and is best suited for constrained devices
unable to support RSA.
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. Implementations of this
algorithm SHOULD provide authentication by other means.
Note that there is no structural difference between ECDH and ECDSA
keys. A certificate issuer may use X.509 v3 keyUsage and
extendedKeyUsage extensions to restrict the use of an ECC public key
to certain computations. This document refers to an ECC key as ECDH-
capable if its use in ECDH is permitted. ECDSA-capable is defined
similarly.
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
Figure 1: Message flow in a full TLS handshake
Figure 1 shows all messages involved in the TLS key establishment
protocol (aka full handshake). The addition of ECC has direct impact
Nir Expires February 5, 2015 [Page 5]
Internet-Draft ECC Cipher Suites for TLS August 2014
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 each 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 Section 3 and of the optional ECC-specific extensions (which
impact the Hello messages) until Section 4.
2.1. ECDH_ECDSA
In ECDH_ECDSA, the server's certificate MUST contain an ECDH-capable
public key and be signed with ECDSA.
A ServerKeyExchange MUST NOT be sent (the server's certificate
contains all the necessary keying information required by the client
to arrive at the premaster secret).
The client generates an ECDH key pair on the same curve as the
server's long-term public key and sends its public key in the
ClientKeyExchange message (except when using client authentication
algorithm ECDSA_fixed_ECDH or RSA_fixed_ECDH, in which case the
modifications from Section 3.2 or Section 3.3.
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 Section 5.10.
2.2. ECDHE_ECDSA
In ECDHE_ECDSA, the server's certificate MUST contain an ECDSA-
capable public key and be signed with ECDSA.
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 using the private key
corresponding to the public key in the server's Certificate.
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.
Both client and server perform an ECDH operation Section 5.10 and use
the resultant shared secret as the premaster secret.
Nir Expires February 5, 2015 [Page 6]
Internet-Draft ECC Cipher Suites for TLS August 2014
2.3. ECDH_RSA
This key exchange algorithm is the same as ECDH_ECDSA except that the
server's certificate MUST be signed with RSA rather than ECDSA.
2.4. ECDHE_RSA
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. The server
certificate MUST be signed with RSA.
2.5. ECDH_anon
In ECDH_anon, the server's Certificate, the CertificateRequest, the
client's Certificate, and the CertificateVerify messages MUST NOT be
sent.
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.
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.
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 Section 5.10.
Note that while the ECDH_ECDSA, ECDHE_ECDSA, ECDH_RSA, and ECDHE_RSA
key exchange algorithms require the server's certificate to be signed
with a particular signature scheme, this specification (following the
similar cases of DH_DSS, DHE_DSS, DH_RSA, and DHE_RSA in the TLS base
documents) does not impose restrictions on signature schemes used
elsewhere in the certificate chain. (Often such restrictions will be
useful, and it is expected that this will be taken into account in
certification authorities' signing practices. However, such
restrictions are not strictly required in general: Even if it is
beyond the capabilities of a client to completely validate a given
chain, the client may be able to validate the server's certificate by
relying on a trusted certification authority whose certificate
appears as one of the intermediate certificates in the chain.)
Nir Expires February 5, 2015 [Page 7]
Internet-Draft ECC Cipher Suites for TLS August 2014
3. Client Authentication
This document defines three new client authentication mechanisms,
each named after the type of client certificate involved: ECDSA_sign,
ECDSA_fixed_ECDH, and RSA_fixed_ECDH. The ECDSA_sign mechanism is
usable with any of the non-anonymous ECC key exchange algorithms
described in Section 2 as well as other non-anonymous (non-ECC) key
exchange algorithms defined in TLS. The ECDSA_fixed_ECDH and
RSA_fixed_ECDH mechanisms are usable with ECDH_ECDSA and ECDH_RSA.
Their use with ECDHE_ECDSA and ECDHE_RSA is prohibited because the
use of a long-term ECDH client key would jeopardize the forward
secrecy property of these algorithms.
The server can request ECC-based client authentication by including
one or more of these certificate types in its CertificateRequest
message. The server must not include any certificate types that are
prohibited for the negotiated key exchange algorithm. The client
must check if it possesses a certificate appropriate for any of the
methods suggested by the server and is willing to use it for
authentication.
If these conditions are not met, the client should send a client
Certificate message containing no certificates. In this case, the
ClientKeyExchange should be sent as described in Section 2, and the
CertificateVerify should not be sent. If the server requires client
authentication, it may respond with a fatal handshake failure alert.
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 Section 5.6) 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.
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.
3.1. ECDSA_sign
To use this authentication mechanism, the client MUST possess a
certificate containing an ECDSA-capable public key and signed with
ECDSA.
The client proves possession of the private key corresponding to the
certified key by including a signature in the CertificateVerify
message as described in Section 5.8.
Nir Expires February 5, 2015 [Page 8]
Internet-Draft ECC Cipher Suites for TLS August 2014
3.2. ECDSA_fixed_ECDH
To use this authentication mechanism, the client MUST possess a
certificate containing an ECDH-capable public key, and that
certificate MUST be signed with ECDSA. Furthermore, the client's
ECDH key MUST be on the same elliptic curve as the server's long-term
(certified) ECDH key. This might limit use of this mechanism to
closed environments. In situations where the client has an ECC key
on a different curve, it would have to authenticate using either
ECDSA_sign or a non-ECC mechanism (e.g., RSA). Using fixed ECDH for
both servers and clients is computationally more efficient than
mechanisms providing forward secrecy.
When using this authentication mechanism, the client MUST send an
empty ClientKeyExchange as described in Section 5.7 and MUST NOT send
the CertificateVerify message. The ClientKeyExchange is empty since
the client's ECDH public key required by the server to compute the
premaster secret is available inside the client's certificate. The
client's ability to arrive at the same premaster secret as the server
(demonstrated by a successful exchange of Finished messages) proves
possession of the private key corresponding to the certified public
key, and the CertificateVerify message is unnecessary.
3.3. RSA_fixed_ECDH
This authentication mechanism is identical to ECDSA_fixed_ECDH except
that the client's certificate MUST be signed with RSA.
Note that while the ECDSA_sign, ECDSA_fixed_ECDH, and RSA_fixed_ECDH
client authentication mechanisms require the client's certificate to
be signed with a particular signature scheme, this specification does
not impose restrictions on signature schemes used elsewhere in the
certificate chain. (Often such restrictions will be useful, and it
is expected that this will be taken into account in certification
authorities' signing practices. However, such restrictions are not
strictly required in general: Even if it is beyond the capabilities
of a server to completely validate a given chain, the server may be
able to validate the clients certificate by relying on a trust anchor
that appears as one of the intermediate certificates in the chain.)
4. TLS Extensions for ECC
Two new 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
Nir Expires February 5, 2015 [Page 9]
Internet-Draft ECC Cipher Suites for TLS August 2014
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 [RFC4366]; message details
are specified in Section 5. 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.
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 and compression techniques 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.
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 Section 5. That
section also describes the structure and processing of these
extensions in greater detail.
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.
5. Data Structures and Computations
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.
Nir Expires February 5, 2015 [Page 10]
Internet-Draft ECC Cipher Suites for TLS August 2014
5.1. Client Hello Extensions
This section specifies two TLS extensions that can be included with
the ClientHello message as described in [RFC4366], the Supported
Elliptic Curves Extension and the Supported Point Formats Extension.
When these extensions are sent:
The extensions SHOULD be sent along with any ClientHello message that
proposes ECC cipher suites.
Meaning of these extensions:
These extensions allow a client to enumerate the elliptic curves it
supports and/or the point formats it can parse.
Structure of these extensions:
The general structure of TLS extensions is described in [RFC4366],
and this specification adds two new types to ExtensionType.
enum { elliptic_curves(10), ec_point_formats(11) } ExtensionType;
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
EllipticCurveList. See Section 5.1.1 for details.
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 Section 5.1.2 for details.
Actions of the sender:
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.
Actions of the receiver:
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
Nir Expires February 5, 2015 [Page 11]
Internet-Draft ECC Cipher Suites for TLS August 2014
complete the handshake while using the curves and point formats
supported by the client (cf. Section 5.3 and Section 5.4).
NOTE: A server participating in an ECDHE-ECDSA key exchange may use
different curves for (i) the ECDSA key in its certificate, and (ii)
the ephemeral ECDH key in the ServerKeyExchange message. The server
must consider the extensions in both cases.
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.
5.1.1. Supported Elliptic Curves Extension
enum {
sect163k1 (1), sect163r1 (2), sect163r2 (3),
sect193r1 (4), sect193r2 (5), sect233k1 (6),
sect233r1 (7), sect239k1 (8), sect283k1 (9),
sect283r1 (10), sect409k1 (11), sect409r1 (12),
sect571k1 (13), sect571r1 (14), secp160k1 (15),
secp160r1 (16), secp160r2 (17), secp192k1 (18),
secp192r1 (19), secp224k1 (20), secp224r1 (21),
secp256k1 (22), secp256r1 (23), secp384r1 (24),
secp521r1 (25),
reserved (0xFE00..0xFEFF),
arbitrary_explicit_prime_curves(0xFF01),
arbitrary_explicit_char2_curves(0xFF02),
(0xFFFF)
} NamedCurve;
sect163k1, etc: Indicates support of the corresponding named curve or
class of explicitly defined curves. The named curves defined here
are those specified in SEC 2 [SECG-SEC2]. Note that many of these
curves are also recommended in ANSI X9.62 [ANSI.X9-62.2005] and FIPS
186-4 [FIPS.186-4]. Values 0xFE00 through 0xFEFF are reserved for
private use. Values 0xFF01 and 0xFF02 indicate that the client
supports arbitrary prime and characteristic-2 curves, respectively
(the curve parameters must be encoded explicitly in ECParameters).
The NamedCurve name space is maintained by IANA. See Section 8 for
information on how new value assignments are added.
Nir Expires February 5, 2015 [Page 12]
Internet-Draft ECC Cipher Suites for TLS August 2014
struct {
NamedCurve elliptic_curve_list<1..2^16-1>
} EllipticCurveList;
Items in elliptic_curve_list are ordered according to the client's
preferences (favorite choice first).
As an example, a client that only supports secp192r1 (aka NIST P-192;
value 19 = 0x0013) and secp224r1 (aka NIST P-224; value 21 = 0x0015)
and prefers to use secp192r1 would include a TLS extension consisting
of the following octets. Note that the first two octets indicate the
extension type (Supported Elliptic Curves Extension):
00 0A 00 06 00 04 00 13 00 15
A client that supports arbitrary explicit characteristic-2 curves
(value 0xFF02) would include an extension consisting of the following
octets:
00 0A 00 04 00 02 FF 02
5.1.2. Supported Point Formats Extension
enum { uncompressed (0), ansiX962_compressed_prime (1),
ansiX962_compressed_char2 (2), reserved (248..255)
} ECPointFormat;
struct {
ECPointFormat ec_point_format_list<1..2^8-1>
} ECPointFormatList;
Three point formats are included in the definition of ECPointFormat
above. The uncompressed point format is the default format in that
implementations of this document MUST support it for all of their
supported curves. Compressed point formats reduce bandwidth by
including only the x-coordinate and a single bit of the y-coordinate
of the point. Implementations of this document MAY support the
ansiX962_compressed_prime and ansiX962_compressed_char2 formats,
where the former applies only to prime curves and the latter applies
only to characteristic-2 curves. (These formats are specified in
[ANSI.X9-62.2005].) Values 248 through 255 are reserved for private
use.
The ECPointFormat name space is maintained by IANA. See Section 8
for information on how new value assignments are added.
Items in ec_point_format_list are ordered according to the client's
preferences (favorite choice first).
Nir Expires February 5, 2015 [Page 13]
Internet-Draft ECC Cipher Suites for TLS August 2014
A client that can parse only the uncompressed point format (value 0)
includes an extension consisting of the following octets; note that
the first two octets indicate the extension type (Supported Point
Formats Extension):
00 0B 00 02 01 00
A client that in the case of prime fields prefers the compressed
format (ansiX962_compressed_prime, value 1) over the uncompressed
format (value 0), but in the case of characteristic-2 fields prefers
the uncompressed format (value 0) over the compressed format
(ansiX962_compressed_char2, value 2), may indicate these preferences
by including an extension consisting of the following octets:
00 0B 00 04 03 01 00 02
5.2. Server Hello Extension
This section specifies a TLS extension that can be included with the
ServerHello message as described in [RFC4366], the Supported Point
Formats Extension.
When this extension is sent:
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.
Meaning of this extension:
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, or for the curve that is used in the server's
public key that will appear in its Certificate message when using the
ECDH_ECDSA or ECDH_RSA key exchange algorithm).
Structure of this extension:
The server's Supported Point Formats Extension has the same structure
as the client's Supported Point Formats Extension (see
Section 5.1.2). Items in elliptic_curve_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 (e.g., the server may prefer to receive points in compressed
format even when a client cannot parse this format: the same client
may nevertheless be capable of outputting points in compressed
format).
Nir Expires February 5, 2015 [Page 14]
Internet-Draft ECC Cipher Suites for TLS August 2014
Actions of the sender:
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.
Actions of the receiver:
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. Section 5.6 and Section 5.7). If
no Supported Point Formats Extension is received with the
ServerHello, this is equivalent to an extension allowing only the
uncompressed point format.
5.3. Server Certificate
When this message is sent:
This message is sent in all non-anonymous ECC-based key exchange
algorithms.
Meaning of this message:
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
Section 5.9.
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).
+-------------+-----------------------------------------------------+
| Algorithm | Server Certificate Type |
+-------------+-----------------------------------------------------+
| ECDH_ECDSA | Certificate MUST contain an ECDH-capable public |
| | key. It MUST be signed with ECDSA. |
| ECDHE_ECDSA | Certificate MUST contain an ECDSA-capable public |
| | key. It MUST be signed with ECDSA. |
| ECDH_RSA | Certificate MUST contain an ECDH-capable public |
| | key. It MUST be signed with RSA. |
Nir Expires February 5, 2015 [Page 15]
Internet-Draft ECC Cipher Suites for TLS August 2014
| ECDHE_RSA | Certificate MUST contain an RSA public key |
| | authorized for use in digital signatures. It MUST |
| | be signed with RSA. |
+-------------+-----------------------------------------------------+
Table 3: Server Certificate Types
Structure of this message:
Identical to the TLS Certificate format.
Actions of the sender:
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; in
particular, the public key MUST employ a named curve (not the same
curve as an explicit curve) unless the client has indicated support
for explicit curves of the appropriate type. If the client has used
a Supported Point Formats Extension, both the server's public key
point and (in the case of an explicit curve) the curve's base point
MUST respect the client's choice of point formats. (A server that
cannot satisfy these requirements MUST NOT choose an ECC cipher suite
in its ServerHello message.)
Actions of the receiver:
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. Section 5.1.)
5.4. Server Key Exchange
When this message is sent:
This message is sent when using the ECDHE_ECDSA, ECDHE_RSA, and
ECDH_anon key exchange algorithms.
Meaning of this message:
This message is used to convey the server's ephemeral ECDH public key
(and the corresponding elliptic curve domain parameters) to the
client.
Structure of this message:
Nir Expires February 5, 2015 [Page 16]
Internet-Draft ECC Cipher Suites for TLS August 2014
enum { explicit_prime (1), explicit_char2 (2),
named_curve (3), reserved(248..255) } ECCurveType;
explicit_prime: Indicates the elliptic curve domain parameters are
conveyed verbosely, and the underlying finite field is a prime
field.
explicit_char2: Indicates the elliptic curve domain parameters are
conveyed verbosely, and the underlying finite field is a
characteristic-2 field.
named_curve: Indicates that a named curve is used. This option
SHOULD be used when applicable.
Values 248 through 255 are reserved for private use.
The ECCurveType name space is maintained by IANA. See Section 8 for
information on how new value assignments are added.
struct {
opaque a <1..2^8-1>;
opaque b <1..2^8-1>;
} ECCurve;
a, b: These parameters specify the coefficients of the elliptic
curve. Each value contains the byte string representation of a
field element following the conversion routine in Section 4.3.3 of
[ANSI.X9-62.2005].
struct {
opaque point <1..2^8-1>;
} ECPoint;
point: This is the byte string representation of an elliptic curve
point following the conversion routine in Section 4.3.6 of
[ANSI.X9-62.2005]. This byte string may represent an elliptic
curve point in uncompressed or compressed format; it MUST conform
to what the client has requested through a Supported Point Formats
Extension if this extension was used.
enum { ec_basis_trinomial, ec_basis_pentanomial } ECBasisType;
ec_basis_trinomial: Indicates representation of a characteristic-2
field using a trinomial basis.
ec_basis_pentanomial: Indicates representation of a characteristic-2
field using a pentanomial basis.
Nir Expires February 5, 2015 [Page 17]
Internet-Draft ECC Cipher Suites for TLS August 2014
struct {
ECCurveType curve_type;
select (curve_type) {
case explicit_prime:
opaque prime_p <1..2^8-1>;
ECCurve curve;
ECPoint base;
opaque order <1..2^8-1>;
opaque cofactor <1..2^8-1>;
case explicit_char2:
uint16 m;
ECBasisType basis;
select (basis) {
case ec_trinomial:
opaque k <1..2^8-1>;
case ec_pentanomial:
opaque k1 <1..2^8-1>;
opaque k2 <1..2^8-1>;
opaque k3 <1..2^8-1>;
};
ECCurve curve;
ECPoint base;
opaque order <1..2^8-1>;
opaque cofactor <1..2^8-1>;
case named_curve:
NamedCurve namedcurve;
};
} ECParameters;
curve_type: This identifies the type of the elliptic curve domain
parameters.
prime_p: This is the odd prime defining the field Fp.
curve: Specifies the coefficients a and b of the elliptic curve E.
base: Specifies the base point G on the elliptic curve.
order: Specifies the order n of the base point.
cofactor: Specifies the cofactor h = #E(Fq)/n, where #E(Fq)
represents the number of points on the elliptic curve E defined
over the field Fq (either Fp or F2^m).
m: This is the degree of the characteristic-2 field F2^m.
k: The exponent k for the trinomial basis representation x^m + x^k+1.
k1, k2, k3: The exponents for the pentanomial representation x^m +
x^k3 + x^k2 + x^k1 + 1 (such that k3 > k2 > k1).
namedcurve: Specifies a recommended set of elliptic curve domain
parameters. All those values of NamedCurve are allowed that refer
to a specific curve. Values of NamedCurve that indicate support
for a class of explicitly defined curves are not allowed here
(they are only permissible in the ClientHello extension); this
applies to arbitrary_explicit_prime_curves(0xFF01) and
Nir Expires February 5, 2015 [Page 18]
Internet-Draft ECC Cipher Suites for TLS August 2014
arbitrary_explicit_char2_curves(0xFF02).
struct {
ECParameters curve_params;
ECPoint public;
} ServerECDHParams;
curve_params: Specifies the elliptic curve domain parameters
associated with the ECDH public key.
public: The ephemeral ECDH public key.
The ServerKeyExchange message is extended as follows.
enum { ec_diffie_hellman } KeyExchangeAlgorithm;
ec_diffie_hellman: Indicates the ServerKeyExchange message contains
an ECDH public key.
select (KeyExchangeAlgorithm) {
case ec_diffie_hellman:
ServerECDHParams params;
Signature signed_params;
} ServerKeyExchange;
params: Specifies the ECDH public key and associated domain
parameters.
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.
enum { ecdsa } SignatureAlgorithm;
select (SignatureAlgorithm) {
case ecdsa:
digitally-signed struct {
opaque sha_hash[sha_size];
};
} Signature;
ServerKeyExchange.signed_params.sha_hash
SHA(ClientHello.random + ServerHello.random +
ServerKeyExchange.params);
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" for ECDHE_ECDSA. ECDSA
signatures are generated and verified as described in Section 5.10,
Nir Expires February 5, 2015 [Page 19]
Internet-Draft ECC Cipher Suites for TLS August 2014
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 <0..2^16-1>, the
contents of which are the DER encoding corresponding to the following
ASN.1 notation.
Ecdsa-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
Actions of the sender:
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 [IEEE.P1363.1998]. It conveys this
information to the client in the ServerKeyExchange message using the
format defined above.
Actions of the receiver:
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.
Section 5.1.)
5.5. Certificate Request
When this message is sent:
This message is sent when requesting client authentication.
Meaning of this message:
The server uses this message to suggest acceptable client
authentication methods.
Structure of this message:
The TLS CertificateRequest message is extended as follows.
enum {
ecdsa_sign(64), rsa_fixed_ecdh(65),
ecdsa_fixed_ecdh(66), (255)
} ClientCertificateType;
Nir Expires February 5, 2015 [Page 20]
Internet-Draft ECC Cipher Suites for TLS August 2014
ecdsa_sign, etc. Indicates that the server would like to use the
corresponding client authentication method specified in Section 3.
Actions of the sender:
The server decides which client authentication methods it would like
to use, and conveys this information to the client using the format
defined above.
Actions of the receiver:
The client determines whether it has a suitable certificate for use
with any of the requested methods and whether to proceed with client
authentication.
5.6. Client Certificate
When this message is sent:
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, RSA_fixed_ECDH, and
ECDSA_fixed_ECDH authentication methods if the public key point
specified in it respects the server's choice of point formats. If no
Supported Point Formats Extension has been used, a certificate can
only be considered suitable for use with these authentication methods
if the point is represented in uncompressed point format.)
Meaning of this message:
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 Section 3. ECC public keys
must be encoded in certificates as described in Section 5.9.
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).
Nir Expires February 5, 2015 [Page 21]
Internet-Draft ECC Cipher Suites for TLS August 2014
+------------------+------------------------------------------------+
| Client | Client Certificate Type |
| Authentication | |
| Method | |
+------------------+------------------------------------------------+
| ECDSA_sign | Certificate MUST contain an ECDSA-capable |
| | public key and be signed with ECDSA. |
| ECDSA_fixed_ECDH | Certificate MUST contain an ECDH-capable |
| | public key on the same elliptic curve as the |
| | server's long-term ECDH key. This certificate |
| | MUST be signed with ECDSA. |
| RSA_fixed_ECDH | Certificate MUST contain an ECDH-capable |
| | public key on the same elliptic curve as the |
| | server's long-term ECDH key. This certificate |
| | MUST be signed with RSA. |
+------------------+------------------------------------------------+
Table 4: Client Certificate Types
Structure of this message:
Identical to the TLS client Certificate format.
Actions of the sender:
The client constructs an appropriate certificate chain, and conveys
it to the server in the Certificate message.
Actions of the receiver:
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.
5.7. Client Key Exchange
When this message is sent:
This message is sent in all key exchange algorithms. If client
authentication with ECDSA_fixed_ECDH or RSA_fixed_ECDH is used, this
message is empty. Otherwise, it contains the client's ephemeral ECDH
public key.
Meaning of the message:
This message is used to convey ephemeral data relating to the key
exchange belonging to the client (such as its ephemeral ECDH public
key).
Nir Expires February 5, 2015 [Page 22]
Internet-Draft ECC Cipher Suites for TLS August 2014
Structure of this message:
The TLS ClientKeyExchange message is extended as follows.
enum { implicit, explicit } PublicValueEncoding;
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"). (This is "explicit"
in ECC cipher suites except when the client uses the
ECDSA_fixed_ECDH or RSA_fixed_ECDH client authentication
mechanism.)
struct {
select (PublicValueEncoding) {
case implicit: struct { };
case explicit: ECPoint ecdh_Yc;
} ecdh_public;
} ClientECDiffieHellmanPublic;
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 or compressed format. Here, the format MUST
conform to what the server has requested through a Supported Point
Formats Extension if this extension was used, and MUST be
uncompressed if this extension was not used.
struct {
select (KeyExchangeAlgorithm) {
case ec_diffie_hellman: ClientECDiffieHellmanPublic;
} exchange_keys;
} ClientKeyExchange;
Actions of the sender:
The client selects an ephemeral ECDH public key corresponding to the
parameters it received from the server according to the ECKAS-DH1
scheme from IEEE 1363. It conveys this information to the client in
the ClientKeyExchange message using the format defined above.
Actions of the receiver:
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.
Nir Expires February 5, 2015 [Page 23]
Internet-Draft ECC Cipher Suites for TLS August 2014
5.8. Certificate Verify
When this message is sent:
This message is sent when the client sends a client certificate
containing a public key usable for digital signatures, e.g., when the
client is authenticated using the ECDSA_sign mechanism.
Meaning of the message:
This message contains a signature that proves possession of the
private key corresponding to the public key in the client's
Certificate message.
Structure of this message:
The TLS CertificateVerify message and the underlying Signature type
are defined in the TLS base specifications, and the latter is
extended here in Section 5.4. For the ecdsa case, the signature
field in the CertificateVerify message contains an ECDSA signature
computed over handshake messages exchanged so far, exactly similar to
CertificateVerify with other signing algorithms:
CertificateVerify.signature.sha_hash
SHA(handshake_messages);
ECDSA signatures are computed as described in Section 5.10, 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 <0..2^16-1>, the contents of
which are the DER encoding [CCITT.X690] corresponding to the
following ASN.1 notation [CCITT.X680].
Ecdsa-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
Actions of the sender:
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.
Actions of the receiver:
Nir Expires February 5, 2015 [Page 24]
Internet-Draft ECC Cipher Suites for TLS August 2014
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.
5.9. Elliptic Curve Certificates
X.509 certificates containing ECC public keys or signed using ECDSA
MUST comply with [RFC3279] or another RFC that replaces or extends
it. Clients SHOULD use the elliptic curve domain parameters
recommended in ANSI X9.62, FIPS 186-4, and SEC 2 [SECG-SEC2].
5.10. ECDH, ECDSA, and RSA Computations
All ECDH calculations (including parameter and key generation as well
as the shared secret calculation) are performed according to
[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.
(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.)
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.
The default hash function is SHA-1 [FIPS.180-2], and sha_size (see
Section 5.4 and Section 5.8) is 20. However, an alternative hash
function, such as one of the new SHA hash functions specified in FIPS
180-2 [FIPS.180-2], may be used instead if the certificate containing
the EC public key explicitly requires use of another hash function.
(The mechanism for specifying the required hash function has not been
standardized, but this provision anticipates such standardization and
obviates the need to update this document in response. Future PKIX
RFCs may choose, for example, to specify the hash function to be used
with a public key in the parameters field of subjectPublicKeyInfo.)
All RSA signatures must be generated and verified according to
Nir Expires February 5, 2015 [Page 25]
Internet-Draft ECC Cipher Suites for TLS August 2014
[PKCS#1] block type 1.
6. Cipher Suites
The table below defines new ECC cipher suites that use the key
exchange algorithms specified in Section 2.
+---------------------------------------+----------------+
| CipherSuite | Identifier |
+---------------------------------------+----------------+
| TLS_ECDH_ECDSA_WITH_NULL_SHA | { 0xC0, 0x01 } |
| TLS_ECDH_ECDSA_WITH_RC4_128_SHA | { 0xC0, 0x02 } |
| TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA | { 0xC0, 0x03 } |
| TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA | { 0xC0, 0x04 } |
| TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA | { 0xC0, 0x05 } |
| TLS_ECDHE_ECDSA_WITH_NULL_SHA | { 0xC0, 0x06 } |
| TLS_ECDHE_ECDSA_WITH_RC4_128_SHA | { 0xC0, 0x07 } |
| TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA | { 0xC0, 0x08 } |
| TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA | { 0xC0, 0x09 } |
| TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA | { 0xC0, 0x0A } |
| TLS_ECDH_RSA_WITH_NULL_SHA | { 0xC0, 0x0B } |
| TLS_ECDH_RSA_WITH_RC4_128_SHA | { 0xC0, 0x0C } |
| TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA | { 0xC0, 0x0D } |
| TLS_ECDH_RSA_WITH_AES_128_CBC_SHA | { 0xC0, 0x0E } |
| TLS_ECDH_RSA_WITH_AES_256_CBC_SHA | { 0xC0, 0x0F } |
| TLS_ECDHE_RSA_WITH_NULL_SHA | { 0xC0, 0x10 } |
| TLS_ECDHE_RSA_WITH_RC4_128_SHA | { 0xC0, 0x11 } |
| TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA | { 0xC0, 0x12 } |
| TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA | { 0xC0, 0x13 } |
| TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA | { 0xC0, 0x14 } |
| TLS_ECDH_anon_WITH_NULL_SHA | { 0xC0, 0x15 } |
| TLS_ECDH_anon_WITH_RC4_128_SHA | { 0xC0, 0x16 } |
| TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA | { 0xC0, 0x17 } |
| TLS_ECDH_anon_WITH_AES_128_CBC_SHA | { 0xC0, 0x18 } |
| TLS_ECDH_anon_WITH_AES_256_CBC_SHA | { 0xC0, 0x19 } |
+---------------------------------------+----------------+
Table 5: TLS ECC cipher suites
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 [RFC2246]
and [RFC4346]. AES ciphers are defined in [RFC5246].
Server implementations SHOULD support all of the following cipher
suites, and client implementations SHOULD support at least one of
them:
Nir Expires February 5, 2015 [Page 26]
Internet-Draft ECC Cipher Suites for TLS August 2014
o TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA
o TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA
o TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
o TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA.
7. Security Considerations
Security issues are discussed throughout this memo.
For TLS handshakes using ECC cipher suites, the security
considerations in appendices D of all three TLS base documemts apply
accordingly.
Security discussions specific to ECC can be found in
[IEEE.P1363.1998] and [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.
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 might be considered more conservative
than curves over F_2^m as there is no choice between multiple fields
of similar size for characteristic 2). Note, however, that algebraic
structure can also lead to implementation efficiencies, and
implementers and users may, therefore, need to balance conservatism
against a need for efficiency. Concrete attacks are known against
only very few special classes of curves, such as supersingular
curves, and these classes are excluded from the ECC standards that
this document references [IEEE.P1363.1998], [ANSI.X9-62.2005].
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 [IEEE.P1363.1998], [ANSI.X9-62.2005], and [FIPS.186-4].
Implementers and users must also consider whether they need forward
secrecy. Forward secrecy refers to the property that session keys
are not compromised if the static, certified keys belonging to the
server and client are compromised. The ECDHE_ECDSA and ECDHE_RSA key
Nir Expires February 5, 2015 [Page 27]
Internet-Draft ECC Cipher Suites for TLS August 2014
exchange algorithms provide forward secrecy protection in the event
of server key compromise, while ECDH_ECDSA and ECDH_RSA do not.
Similarly, if the client is providing a static, certified key,
ECDSA_sign client authentication provides forward secrecy protection
in the event of client key compromise, while ECDSA_fixed_ECDH and
RSA_fixed_ECDH do not. Thus, to obtain complete forward secrecy
protection, ECDHE_ECDSA or ECDHE_RSA must be used for key exchange,
with ECDSA_sign used for client authentication if necessary. Here
again the security benefits of forward secrecy may need to be
balanced against the improved efficiency offered by other options.
8. IANA Considerations
[RFC4492], the predecessor of this document has already defined the
IANA registries for the following:
o NamedCurve Section 5.1
o ECPointFormat Section 5.1
o ECCurveType Section 5.4
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. Any
additional assignments require IETF Consensus action.
9. Acknowledgements
Most of the text is this document is taken from [RFC4492], the
predecessor of this document. The authors of that document were:
o Simon Blake-Wilson
o Nelson Bolyard
o Vipul Gupta
o Chris Hawk
o Bodo Moeller
In the predecessor document, the authors acknowledged the
contributions of Bill Anderson and Tim Dierks.
10. Version History for This Draft
NOTE TO RFC EDITOR: PLEASE REMOVE THIS SECTION
Changes from RFC 4492 to draft-nir-tls-rfc4492bis-00:
o Added TLS 1.2 to references.
Nir Expires February 5, 2015 [Page 28]
Internet-Draft ECC Cipher Suites for TLS August 2014
o Moved RFC 4492 authors to acknowledgements.
o Removed list of required reading for ECC.
11. References
11.1. Normative References
[ANSI.X9-62.2005]
American National Standards Institute, "Public Key
Cryptography for the Financial Services Industry, The
Elliptic Curve Digital Signature Algorithm (ECDSA)",
ANSI X9.62, 2005.
[CCITT.X680]
International Telephone and Telegraph Consultative
Committee, "Abstract Syntax Notation One (ASN.1):
Specification of basic notation", CCITT Recommendation
X.680, July 2002.
[CCITT.X690]
International Telephone and Telegraph Consultative
Committee, "ASN.1 encoding rules: Specification of basic
encoding Rules (BER), Canonical encoding rules (CER) and
Distinguished encoding rules (DER)", CCITT Recommendation
X.690, July 2002.
[FIPS.186-4]
National Institute of Standards and Technology, "Digital
Signature Standard", FIPS PUB 186-4, 2013, <http://
nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf>.
[PKCS#1] RSA Laboratories, "RSA Encryption Standard, Version 1.5",
PKCS 1, November 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
Nir Expires February 5, 2015 [Page 29]
Internet-Draft ECC Cipher Suites for TLS August 2014
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[SECG-SEC2]
CECG, "Recommended Elliptic Curve Domain Parameters",
SEC 2, 2000.
11.2. Informative References
[FIPS.180-2]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002, <http://
csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.
[I-D.ietf-tls-tls13]
Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.3", draft-ietf-tls-tls13-02 (work
in progress), July 2014.
[IEEE.P1363.1998]
Institute of Electrical and Electronics Engineers,
"Standard Specifications for Public Key Cryptography",
IEEE Draft P1363, 1998.
[Lenstra+Verheul]
Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
Sizes", Journal of Cryptology 14 (2001) 255-293, 2001.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
Appendix A. Equivalent Curves (Informative)
All of the NIST curves [FIPS.186-4] and several of the ANSI curves
[ANSI.X9-62.2005] are equivalent to curves listed in Section 5.1.1.
In the following table, multiple names in one row represent aliases
for the same curve.
Nir Expires February 5, 2015 [Page 30]
Internet-Draft ECC Cipher Suites for TLS August 2014
Curve names chosen by different standards organizations
+-----------+------------+------------+
| SECG | ANSI X9.62 | NIST |
+-----------+------------+------------+
| sect163k1 | | NIST K-163 |
| sect163r1 | | |
| sect163r2 | | NIST B-163 |
| sect193r1 | | |
| sect193r2 | | |
| sect233k1 | | NIST K-233 |
| sect233r1 | | NIST B-233 |
| sect239k1 | | |
| sect283k1 | | NIST K-283 |
| sect283r1 | | NIST B-283 |
| sect409k1 | | NIST K-409 |
| sect409r1 | | NIST B-409 |
| sect571k1 | | NIST K-571 |
| sect571r1 | | NIST B-571 |
| secp160k1 | | |
| secp160r1 | | |
| secp160r2 | | |
| secp192k1 | | |
| secp192r1 | prime192v1 | NIST P-192 |
| secp224k1 | | |
| secp224r1 | | NIST P-224 |
| secp256k1 | | |
| secp256r1 | prime256v1 | NIST P-256 |
| secp384r1 | | NIST P-384 |
| secp521r1 | | NIST P-521 |
+-----------+------------+------------+
Table 6: Equivalent curves defined by SECG, ANSI, and NIST
Appendix B. Differences from RFC 4492
o Added TLS 1.2
Nir Expires February 5, 2015 [Page 31]
Internet-Draft ECC Cipher Suites for TLS August 2014
Author's Address
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 6789735
Israel
Email: ynir.ietf@gmail.com
Nir Expires February 5, 2015 [Page 32]
