TLS Working Group V. Gupta Internet-Draft Sun Labs Expires: December 2003 S. Blake-Wilson BCI B. Moeller Technische Universitaet Darmstadt C. Hawk Independent Consultant June 2003 ECC Cipher Suites for TLS Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 30, 2003. Copyright Notice Copyright (C) The Internet Society (2003). 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. Gupta, et al. Expires December 2003 [Page 1] Internet-Draft ECC Cipher Suites for TLS June 2003 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 . . . . . . . . . . . . . . . . . . . . . . . . 23 6. Security Considerations . . . . . . . . . . . . . . . . . . . 25 7. Intellectual Property Rights . . . . . . . . . . . . . . . . . 26 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27 References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 29 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 30 Gupta, et al. Expires December 2003 [Page 2] Internet-Draft ECC Cipher Suites for TLS June 2003 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 Gupta, et al. Expires December 2003 [Page 3] Internet-Draft ECC Cipher Suites for TLS June 2003 TLS [3] and ECC [5][6][7][9] . Gupta, et al. Expires December 2003 [Page 4] Internet-Draft ECC Cipher Suites for TLS June 2003 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. Gupta, et al. Expires December 2003 [Page 5] Internet-Draft ECC Cipher Suites for TLS June 2003 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 Gupta, et al. Expires December 2003 [Page 6] Internet-Draft ECC Cipher Suites for TLS June 2003 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 Gupta, et al. Expires December 2003 [Page 7] Internet-Draft ECC Cipher Suites for TLS June 2003 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 Gupta, et al. Expires December 2003 [Page 8] Internet-Draft ECC Cipher Suites for TLS June 2003 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. Gupta, et al. Expires December 2003 [Page 9] Internet-Draft ECC Cipher Suites for TLS June 2003 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. Gupta, et al. Expires December 2003 [Page 10] Internet-Draft ECC Cipher Suites for TLS June 2003 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). Gupta, et al. Expires December 2003 [Page 11] Internet-Draft ECC Cipher Suites for TLS June 2003 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 Gupta, et al. Expires December 2003 [Page 12] Internet-Draft ECC Cipher Suites for TLS June 2003 (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. Gupta, et al. Expires December 2003 [Page 13] Internet-Draft ECC Cipher Suites for TLS June 2003 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. Gupta, et al. Expires December 2003 [Page 14] Internet-Draft ECC Cipher Suites for TLS June 2003 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 Gupta, et al. Expires December 2003 [Page 15] Internet-Draft ECC Cipher Suites for TLS June 2003 +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; Gupta, et al. Expires December 2003 [Page 16] Internet-Draft ECC Cipher Suites for TLS June 2003 select (SignatureAlgorithm) { case ecdsa: digitally-signed struct { opaque sha_hash[sha_size]; }; } 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. 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 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. Gupta, et al. Expires December 2003 [Page 17] Internet-Draft ECC Cipher Suites for TLS June 2003 enum { ecdsa_sign(?), rsa_fixed_ecdh(?), ecdsa_fixed_ecdh(?), (255) } ClientCertificateType; ecdsa_sign, etc Indicates that the server would like to use the corresponding client authentication method specified in Section 3. EDITOR: The values used for ecdsa_sign, rsa_fixed_ecdh, and ecdsa_fixed_ecdh have been left as ?. These values will be assigned when this draft progresses to RFC. Earlier versions of this draft used the values 5, 6, and 7 - however these values have been removed since they are used differently by SSL 3.0 and their use by TLS is being deprecated. 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). Gupta, et al. Expires December 2003 [Page 18] Internet-Draft ECC Cipher Suites for TLS June 2003 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: Gupta, et al. Expires December 2003 [Page 19] Internet-Draft ECC Cipher Suites for TLS June 2003 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: Gupta, et al. Expires December 2003 [Page 20] Internet-Draft ECC Cipher Suites for TLS June 2003 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[sha_size]; }; } 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. Gupta, et al. Expires December 2003 [Page 21] Internet-Draft ECC Cipher Suites for TLS June 2003 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 o the ECKAS-DH1 scheme with the ECSVDP-DH secret value derivation primitive, the KDF1 key derivation function using SHA-1 [8], and null key derivation parameters "P" for elliptic curve parameters where field elements are represented as octet strings of length 24 or less (using the IEEE 1363 FE2OSP); in this case, the premaster secret is the output of the ECKAS-DH1 scheme, i.e. the 20-byte SHA-1 output from the KDF. o the ECKAS-DH1 scheme with the identity map as key derivation function for elliptic curve parameters where field elements are represented as octet strings of length more than 24; in this case, the premaster secret is the x-coordinate of the ECDH shared secret elliptic curve point, i.e. the octet string Z in IEEE 1363 terminology. Note that a new extension may be introduced in the future to allow the use of a different KDF during computation of the premaster secret. In this event, the new KDF would be used in place of the process detailed above. This may be desirable, for example, to support compatibility with the planned NIST key agreement standard. All ECDSA computations MUST be performed according to ANSI X9.62 [7] 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 [8] and sha_size (see Section 4.2 and Section 4.6) is 20. However, an alternative hash function, such as one of the new SHA hash functions specified in FIPS 180-2 [8], may be used instead if the certificate containing the EC public key explicitly requires use of another hash function. All RSA signatures must be generated and verified according to PKCS#1 [10]. Gupta, et al. Expires December 2003 [Page 22] Internet-Draft ECC Cipher Suites for TLS June 2003 5. Cipher Suites The table below defines new ECC cipher suites that use the key exchange algorithms specified in Section 2. EDITOR: Most of the cipher suites below have been left as ??. The values 47-4C correspond to cipher suites which are known to have been implemented and are therefore proposed here. The final determination of cipher suite numbers will occur when this draft progresses to RFC. Implementers using the values 47-4C should therefore be wary that these values may change. 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 Gupta, et al. Expires December 2003 [Page 23] Internet-Draft ECC Cipher Suites for TLS June 2003 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, TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, and TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA. Gupta, et al. Expires December 2003 [Page 24] Internet-Draft ECC Cipher Suites for TLS June 2003 6. Security Considerations This document is based on [3], [6], [7] and [11]. The appropriate security considerations of those documents apply. For ECDH (Section 4.8), this document specifies two different ways to compute the premaster secret. The choice of the method is determined by the elliptic curve. Earlier versions of this specification used the KDF1 key derivation function with SHA-1 in all cases; the current version keeps this key derivation function only for curves where field elements are represented as octet strings of length 24 or less (i.e. up to 192 bits), but omits it for larger curves. Rationale: Using KDF1 with SHA-1 limits the security to at most 160 bits, independently of the elliptic curve used for ECDH. For large curves, this would result in worse security than expected. Using a specific key derivation function for ECDH is not really necessary as TLS always uses its PRF to derive the master secret from the premaster secret. For large curves, the current specification handles ECDH like the basic TLS specification [11] handles standard DH. For smaller curves where the extra KDF1 step does not weaken security, the current specification keeps the KDF1 step to obtain compatibility with existing implementations of earlier versions of this specification. Note that the threshold for switching between the two ECDH calculation methods is necessarily somewhat arbitrary; 192-bit ECC corresponds to approximately 96 bits of security in the light of square root attacks, so the 160 bits provided by SHA-1 are comfortable at this limit. Gupta, et al. Expires December 2003 [Page 25] Internet-Draft ECC Cipher Suites for TLS June 2003 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. Gupta, et al. Expires December 2003 [Page 26] Internet-Draft ECC Cipher Suites for TLS June 2003 8. Acknowledgments The authors wish to thank Bill Anderson and Tim Dierks. Gupta, et al. Expires December 2003 [Page 27] Internet-Draft ECC Cipher Suites for TLS June 2003 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, . [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, . [5] SECG, "Elliptic Curve Cryptography", SEC 1, 2000, . [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, "Secure Hash Standard", FIPS 180-2, 2002. [9] NIST, "Digital Signature Standard", FIPS 186-2, 2000. [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, . [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. Gupta, et al. Expires December 2003 [Page 28] Internet-Draft ECC Cipher Suites for TLS June 2003 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 Gupta, et al. Expires December 2003 [Page 29] Internet-Draft ECC Cipher Suites for TLS June 2003 Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Gupta, et al. Expires December 2003 [Page 30]