TLS Working Group V. Gupta
Internet-Draft Sun Labs
Expires: February 27, 2003 S. Blake-Wilson
BCI
B. Moeller
Technische Universitaet Darmstadt
C. Hawk
Independent Consultant
August 29, 2002
ECC Cipher Suites for TLS
<draft-ietf-tls-ecc-02.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes new key exchange algorithms based on Elliptic
Curve Cryptography (ECC) for the TLS (Transport Layer Security)
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.
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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 RFC 2119 [1].
Please send comments on this document to the TLS mailing list.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Key Exchange Algorithms . . . . . . . . . . . . . . . . . . . 5
2.1 ECDH_ECDSA . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 ECDHE_ECDSA . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 ECDH_RSA . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 ECDHE_RSA . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 ECDH_anon . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Client Authentication . . . . . . . . . . . . . . . . . . . . 9
3.1 ECDSA_sign . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 ECDSA_fixed_ECDH . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 RSA_fixed_ECDH . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Data Structures and Computations . . . . . . . . . . . . . . . 11
4.1 Server Certificate . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Server Key Exchange . . . . . . . . . . . . . . . . . . . . . 12
4.3 Certificate Request . . . . . . . . . . . . . . . . . . . . . 17
4.4 Client Certificate . . . . . . . . . . . . . . . . . . . . . . 18
4.5 Client Key Exchange . . . . . . . . . . . . . . . . . . . . . 19
4.6 Certificate Verify . . . . . . . . . . . . . . . . . . . . . . 20
4.7 Elliptic Curve Certificates . . . . . . . . . . . . . . . . . 22
4.8 ECDH, ECDSA and RSA Computations . . . . . . . . . . . . . . . 22
5. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 26
7. Intellectual Property Rights . . . . . . . . . . . . . . . . . 27
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
Elliptic Curve Cryptography (ECC) is emerging as an attractive
public-key cryptosystem for mobile/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 [2], 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
128 | 283 | 3072
192 | 409 | 7680
256 | 571 | 15360
Table 1: Comparable key sizes (in bits)
Smaller key sizes result in power, bandwidth and computational
savings that make ECC especially attractive for constrained
environments.
This document describes additions to TLS to support ECC. In
particular, it 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. Section 4 specifies various data structures needed
for an ECC-based handshake, their encoding in TLS messages and the
processing of those messages. Section 5 defines new ECC-based cipher
suites and identifies a small subset of these as recommended for all
implementations of this specification. Section 6, Section 7 and
Section 8 mention security considerations, intellectual property
rights, and acknowledgments, respectively. This is followed by a
list of references cited in this document and the authors' contact
information.
Implementation of this specification requires familiarity with both
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TLS [3] and ECC [5][6][7][8] .
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2. Key Exchange Algorithms
This document introduces five new ECC-based key exchange algorithms
for TLS. All of them use ECDH to compute the TLS premaster secret
and 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, DH_RSA, DHE_DSS, DHE_RSA and DH_anon (see [3]),
respectively.
Key
Exchange
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
Note that 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 X509.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.
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Client Server
------ ------
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*+
<-------- ServerHelloDone
Certificate*+
ClientKeyExchange
CertificateVerify*+
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Figure 1: Message flow in a full TLS handshake
* message is not sent under some conditions
+ message is not sent unless the client is
authenticated
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 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.
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 MUST generate an ECDH key pair on the same curve as the
server's long-term public key and send its public key in the
ClientKeyExchange message (except when using client authentication
algorithm ECDSA_fixed_ECDH or RSA_fixed_ECDH, in which case the
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modifications from section Section 3.2 or Section 3.3 apply).
Both client and server MUST perform an ECDH operation and use the
resultant shared secret as the premaster secret. All ECDH
calculations are performed as specified in Section 4.8
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 MUST send 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 MUST generate an ECDH key pair on the same curve as the
server's ephemeral ECDH key and send its public key in the
ClientKeyExchange message.
Both client and server MUST perform an ECDH operation (Section 4.8)
and use the resultant shared secret as the premaster secret.
2.3 ECDH_RSA
This key exchange algorithm is the same as ECDH_ECDSA except 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 the
server's certificate MUST contain an RSA public key authorized for
signing and 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 MUST generate an ECDH key pair on the same curve as the
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server's ephemeral ECDH key and send its public key in the
ClientKeyExchange message.
Both client and server MUST perform an ECDH operation and use the
resultant shared secret as the premaster secret. All ECDH
calculations are performed as specified in Section 4.8
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3. Client Authentication
This document defines three new client authentication mechanisms
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 [3]. 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 4.4) 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 MUST prove possession of the private key corresponding to
the certified key by including a signature in the CertificateVerify
message as described in Section 4.6.
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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.
When using this authentication mechanism, the client MUST send an
empty ClientKeyExchange as described in Section 4.5 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
the client's certificate MUST be signed with RSA.
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4. Data Structures and Computations
This section specifies the data structures and computations used by
ECC-based key mechanisms specified in Section 2 and Section 3. The
presentation language used here is the same as that used in TLS [3].
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.
4.1 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
4.7.
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).
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Key Exchange 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.
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.
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.
4.2 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
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(and the corresponding elliptic curve domain parameters) to the
client.
Structure of this message:
enum { explicit_prime (1), explicit_char2 (2),
named_curve (3), (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.
struct {
opaque a <1..2^8-1>;
opaque b <1..2^8-1>;
opaque seed <0..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 [7].
seed: This is an optional parameter used to derive the coefficients
of a randomly generated elliptic curve.
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 [7]. Note that this byte string may represent an elliptic
curve point in compressed or uncompressed form. Implementations
of this specification MUST support the uncompressed form and MAY
support the compressed form.
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enum { ec_basis_trinomial, ec_basis_pentanomial } ECBasisType;
ec_basis_trinomial: Indicates representation of a characteristic two
field using a trinomial basis.
ec_basis_pentanomial: Indicates representation of a characteristic
two field using a pentanomial basis.
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 (240..247), (255)
} NamedCurve;
sect163k1, etc: Indicates use of the corresponding named curve
specified in SEC 2 [12]. Note that many of these curves are also
recommended in ANSI X9.62 [7], and FIPS 186-2 [9]. Values 240
through 247 are reserved for private use.
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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 (and optional seed) 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.
m: This is the degree of the characteristic-two field F2^m.
k: The exponent k for the trinomial basis representation x^m + x^k
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+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.
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;
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select (SignatureAlgorithm) {
case ecdsa:
digitally-signed struct {
opaque sha_hash[20];
};
} Signature;
NOTE: SignatureAlgorithm is 'rsa' for the ECDHE_RSA key exchange
algorithm and 'anonymous' for ECDH_anon. These cases are defined in
TLS [3]. SignatureAlgorithm is 'ecdsa' for ECDHE_ECDSA. ECDSA
signatures are generated and verified as described in Section 4.8.
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 [6]. It conveys this information to
the client in the ServerKeyExchange message using the format defined
above.
Actions of the recipient:
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.
4.3 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(5), rsa_fixed_ecdh(6),
ecdsa_fixed_ecdh(7), (255)
} ClientCertificateType;
ecdsa_sign, etc Indicates that the server would like to use the
corresponding client authentication method specified in Section 3.
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NOTE: SSL 3.0 [4] assigns values 5 and 6 differently
(rsa_ephemeral_dh and dss_ephemeral_dh); these
ClientCertificateType values are not used by TLS.
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 an appropriate certificate for
use with any of the requested methods, and decides whether or not to
proceed with client authentication.
4.4 Client Certificate
When this message is sent:
This message is sent in response to a CertificateRequest when a
client has a suitable certificate.
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 4.7.
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).
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Client
Authentication Method Client Certificate Type
--------------------- -----------------------
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.
4.5 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:
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This message is used to convey ephemeral data relating to the key
exchange belonging to the client (such as its ephemeral ECDH public
key).
Structure of this message:
The TLS ClientKeyExchange message is extended as follows.
enum { yes, no } EphemeralPublicKey;
yes, no: Indicates whether or not the client is providing an
ephemeral ECDH public key. (In ECC ciphersuites, this is "yes"
except when the client uses the ECDSA_fixed_ECDH or RSA_fixed_ECDH
client authentication mechanism.)
struct {
select (EphemeralPublicKey) {
case yes: ECPoint ecdh_Yc;
case no: struct { };
} ecdh_public;
} ClientECDiffieHellmanPublic;
ecdh_Yc: Contains the client's ephemeral ECDH public key.
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 [6]. It conveys this information to the client
in the ClientKeyExchange message using the format defined above.
Actions of the recipient:
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.
4.6 Certificate Verify
When this message is sent:
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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 is extended as follows.
enum { ecdsa } SignatureAlgorithm;
select (SignatureAlgorithm) {
case ecdsa:
digitally-signed struct {
opaque sha_hash[20];
};
} Signature;
For the ecdsa case, the signature field in the CertificateVerify
message contains an ECDSA signature computed over handshake messages
exchanged so far. ECDSA signatures are computed as described in
Section 4.8. As per ANSI X9.62, an ECDSA signature consists of a
pair of integers r and s. These integers are both converted into
byte strings of the same length as the curve order n using the
conversion routine specified in Section 4.3.1 of [7]. The two byte
strings are concatenated, and the result is placed in the signature
field.
Actions of the sender:
The client computes its signature over all handshake messages sent or
received starting at client hello 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:
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.
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4.7 Elliptic Curve Certificates
X509 certificates containing ECC public keys or signed using ECDSA
MUST comply with [14]. Clients SHOULD use the elliptic curve domain
parameters recommended in ANSI X9.62 [7], FIPS 186-2 [9], and SEC 2
[12].
4.8 ECDH, ECDSA and RSA Computations
All ECDH calculations (including parameter and key generation as well
as the shared secret calculation) MUST be performed according to [6]
using the ECKAS-DH1 scheme with the ECSVDP-DH secret value derivation
primitive, and the KDF1 key derivation function using SHA-1 [9]. The
output of this scheme, i.e. the 20-byte SHA-1 output from the KDF,
is the premaster secret.
DISCUSSION POINT:
Using KDF1 with SHA-1 limits the security to at most 160 bits,
independently of the elliptic curve used for ECDH. An alternative
way to define the protocol would be to employ the identity map as key
derivation function (in other words, omit the SHA-1 step and directly
use the octet string representation of the x coordinate of the
elliptic curve point resulting from the ECDH computation as premaster
secret). This is similar to conventional DH in TLS [3], and it is
appropriate for TLS given that TLS already defines a PRF for
determining the actual symmetric keys.
The TLS PRF (which is used to derive the master secret from the
premaster secret and the symmetric keys from the master secret) has
an internal security limit of at most 288 bits (128 + 160 for MD5 and
SHA-1), so the use of KDF1 with SHA-1 can be seen to actually weaken
the theoretical security of the protocol.
Options to solve this problem include the following:
o Omit the SHA-1 step as describe above. (BM)
o Continue to use KDF1 with SHA-1 for curves up to, say, 288 bits
(more precisely: for curves where FE2OSP returns an octet string
up to 36 octets) and omit the SHA-1 step only for larger curves.
(SBW/BM)
o Continue to use KDF1 with SHA-1 for curves up to, say, 288 bits
and use KDF1 with SHA-256 or SHA-384 or SHA-512 for larger curves.
(SBW)
The first solution will break compatibility with existing
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implementations based on draft-ietf-tls-ecc-01.txt. The second and
third solutions are somewhat of a kludge, but maintain compatibility
(in a large range).
OPEN QUESTION: We invite comments on which of these solutions would
be preferred.
END OF DISCUSSION POINT.
All ECDSA computations MUST be performed according to ANSI X9.62 [7]
using the SHA-1 [9] hash function. The 20 bytes of the SHA-1 are run
directly through the ECDSA algorithm with no additional hashing.
All RSA signatures must be generated and verified according to PKCS#1
[10].
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5. Cipher Suites
The table below defines new ECC cipher suites that use the key
exchange algorithms specified in Section 2.
CipherSuite TLS_ECDH_ECDSA_WITH_NULL_SHA = { 0x00, 0x47 }
CipherSuite TLS_ECDH_ECDSA_WITH_RC4_128_SHA = { 0x00, 0x48 }
CipherSuite TLS_ECDH_ECDSA_WITH_DES_CBC_SHA = { 0x00, 0x49 }
CipherSuite TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x4A }
CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA = { 0x00, 0x4B }
CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA = { 0x00, 0x4C }
CipherSuite TLS_ECDHE_ECDSA_WITH_NULL_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_ECDSA_WITH_RC4_128_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_RSA_WITH_NULL_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_RSA_WITH_RC4_128_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_RSA_WITH_NULL_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_RSA_WITH_RC4_128_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_anon_NULL_WITH_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_anon_WITH_RC4_128_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x?? }
CipherSuite TLS_ECDH_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x?? }
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 [3]. AES
ciphers are defined in [11].
Server implementations SHOULD support all of the following cipher
suites, and client implementations SHOULD support at least one of
them: TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA,
TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA,
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TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, and
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA.
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6. Security Considerations
This document is entirely concerned with security mechanisms.
This document is based on [3], [6], [7] and [11]. The appropriate
security considerations of those documents apply.
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7. Intellectual Property Rights
The IETF has been notified of intellectual property rights claimed in
regard to the specification contained in this document. For more
information, consult the online list of claimed rights (http://
www.ietf.org/ipr.html).
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in [13]. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat.
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8. Acknowledgments
The authors wish to thank Bill Anderson and Tim Dierks.
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References
[1] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
Sizes", Journal of Cryptology 14 (2001) 255-293, <http://
www.cryptosavvy.com/>.
[3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[4] Freier, A., Karlton, P. and P. Kocher, "The SSL Protocol
Version 3.0", November 1996, <http://wp.netscape.com/eng/ssl3/
draft302.txt>.
[5] SECG, "Elliptic Curve Cryptography", SEC 1, 2000, <http://
www.secg.org/>.
[6] IEEE, "Standard Specifications for Public Key Cryptography",
IEEE 1363, 2000.
[7] ANSI, "Public Key Cryptography For The Financial Services
Industry: The Elliptic Curve Digital Signature Algorithm
(ECDSA)", ANSI X9.62, 1998.
[8] NIST, "Digital Signature Standard", FIPS 180-1, 2000.
[9] NIST, "Secure Hash Standard", FIPS 186-2, 1995.
[10] RSA Laboratories, "PKCS#1: RSA Encryption Standard version
1.5", PKCS 1, November 1993.
[11] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[12] SECG, "Recommended Elliptic Curve Domain Parameters", SEC 2,
2000, <http://www.secg.org/>.
[13] Hovey, R. and S. Bradner, "The Organizations Involved in the
IETF Standards Process", RFC 2028, BCP 11, October 1996.
[14] Polk, T., Housley, R. and L. Bassham, "Algorithms and
Identifiers for the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile", RFC
3279, April 2002.
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Authors' Addresses
Vipul Gupta
Sun Microsystems Laboratories
2600 Casey Avenue
MS UMTV29-235
Mountain View, CA 94303
USA
Phone: +1 650 336 1681
EMail: vipul.gupta@sun.com
Simon Blake-Wilson
Basic Commerce & Industries, Inc.
96 Spandia Ave
Unit 606
Toronto, ON M6G 2T6
Canada
Phone: +1 416 214 5961
EMail: sblakewilson@bcisse.com
Bodo Moeller
Technische Universitaet Darmstadt
Alexanderstr. 10
64283 Darmstadt
Germany
Phone: +49 6151 16 6628
EMail: moeller@cdc.informatik.tu-darmstadt.de
Chris Hawk
Independent Consultant
EMail: chris@socialeng.com
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