RFC 8422
Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier
Internet Engineering Task Force (IETF) Y. Nir Request for Comments: 8422 Check Point Obsoletes: 4492 S. Josefsson Category: Standards Track SJD AB ISSN: 2070-1721 M. Pegourie-Gonnard ARM August 2018 Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and EarlierAbstract
This document describes key exchange algorithms based on Elliptic Curve Cryptography (ECC) for the Transport Layer Security (TLS) protocol. In particular, it specifies the use of Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) key agreement in a TLS handshake and the use of the Elliptic Curve Digital Signature Algorithm (ECDSA) and Edwards-curve Digital Signature Algorithm (EdDSA) as authentication mechanisms. This document obsoletes RFC 4492. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8422.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Conventions Used in This Document . . . . . . . . . . . . 4 2. Key Exchange Algorithm . . . . . . . . . . . . . . . . . . . 4 2.1. ECDHE_ECDSA . . . . . . . . . . . . . . . . . . . . . . . 6 2.2. ECDHE_RSA . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3. ECDH_anon . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4. Algorithms in Certificate Chains . . . . . . . . . . . . 7 3. Client Authentication . . . . . . . . . . . . . . . . . . . . 8 3.1. ECDSA_sign . . . . . . . . . . . . . . . . . . . . . . . 8 4. TLS Extensions for ECC . . . . . . . . . . . . . . . . . . . 9 5. Data Structures and Computations . . . . . . . . . . . . . . 10 5.1. Client Hello Extensions . . . . . . . . . . . . . . . . . 10 5.1.1. Supported Elliptic Curves Extension . . . . . . . . . 11 5.1.2. Supported Point Formats Extension . . . . . . . . . . 13 5.1.3. The signature_algorithms Extension and EdDSA . . . . 13 5.2. Server Hello Extension . . . . . . . . . . . . . . . . . 14 5.3. Server Certificate . . . . . . . . . . . . . . . . . . . 15 5.4. Server Key Exchange . . . . . . . . . . . . . . . . . . . 16 5.4.1. Uncompressed Point Format for NIST Curves . . . . . . 19 5.5. Certificate Request . . . . . . . . . . . . . . . . . . . 20 5.6. Client Certificate . . . . . . . . . . . . . . . . . . . 21 5.7. Client Key Exchange . . . . . . . . . . . . . . . . . . . 22 5.8. Certificate Verify . . . . . . . . . . . . . . . . . . . 23 5.9. Elliptic Curve Certificates . . . . . . . . . . . . . . . 24 5.10. ECDH, ECDSA, and RSA Computations . . . . . . . . . . . . 24 5.11. Public Key Validation . . . . . . . . . . . . . . . . . . 26 6. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . . . 26 7. Implementation Status . . . . . . . . . . . . . . . . . . . . 27 8. Security Considerations . . . . . . . . . . . . . . . . . . . 27 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 10.1. Normative References . . . . . . . . . . . . . . . . . . 29 10.2. Informative References . . . . . . . . . . . . . . . . . 31 Appendix A. Equivalent Curves (Informative) . . . . . . . . . . 32 Appendix B. Differences from RFC 4492 . . . . . . . . . . . . . 33 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 34 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
This document describes additions to TLS to support ECC that are 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 [TLS1.3] and is explicitly out of scope for this document. In particular, this document defines: o the use of the ECDHE key agreement scheme with ephemeral keys to establish the TLS premaster secret, and o the use of ECDSA and EdDSA signatures 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 8 discusses security considerations. Section 9 describes IANA considerations for the name spaces created by this document's predecessor. 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.1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.2. Key Exchange Algorithm
This document defines three new ECC-based key exchange algorithms for TLS. All of them use Ephemeral ECDH (ECDHE) to compute the TLS premaster secret, and they differ only in the mechanism (if any) used to authenticate them. The derivation of the TLS master secret from the premaster secret and the subsequent generation of bulk encryption/MAC keys and initialization vectors is independent of the key exchange algorithm and not impacted by the introduction of ECC.
Table 1 summarizes the new key exchange algorithms. All of these key exchange algorithms provide forward secrecy if and only if fresh ephemeral keys are generated and used, and also destroyed after use. +-------------+------------------------------------------------+ | Algorithm | Description | +-------------+------------------------------------------------+ | ECDHE_ECDSA | Ephemeral ECDH with ECDSA or EdDSA signatures. | | ECDHE_RSA | Ephemeral ECDH with RSA signatures. | | ECDH_anon | Anonymous ephemeral ECDH, no signatures. | +-------------+------------------------------------------------+ Table 1: ECC Key Exchange Algorithms These key exchanges are analogous to DHE_DSS, DHE_RSA, and DH_anon, respectively. 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. The anonymous key exchange algorithm does not provide authentication of the server or the client. Like other anonymous TLS key exchanges, it is subject to man-in-the-middle attacks. Applications using TLS with this algorithm SHOULD provide authentication by other means.
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 1.2 Handshake Figure 1 shows all messages involved in the TLS key establishment protocol (aka full handshake). The addition of ECC has direct impact only on the ClientHello, the ServerHello, the server's Certificate message, the ServerKeyExchange, the ClientKeyExchange, the CertificateRequest, the client's Certificate message, and the CertificateVerify. Next, we describe the ECC key exchange algorithm in greater detail in terms of the content and processing of these messages. For ease of exposition, we defer discussion of client authentication and associated messages (identified with a '+' in Figure 1) until Section 3 and of the optional ECC-specific extensions (which impact the Hello messages) until Section 4.2.1. ECDHE_ECDSA
In ECDHE_ECDSA, the server's certificate MUST contain an ECDSA- or EdDSA-capable public key. The server sends its ephemeral ECDH public key and a specification of the corresponding curve in the ServerKeyExchange message. These parameters MUST be signed with ECDSA or EdDSA using the private key corresponding to the public key in the server's Certificate. 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 (see Section 5.10) and use the resultant shared secret as the premaster secret.2.2. 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 the signature in the ServerKeyExchange message must be computed with the corresponding RSA private key.2.3. ECDH_anon
NOTE: Despite the name beginning with "ECDH_" (no E), the key used in ECDH_anon is ephemeral just like the key in ECDHE_RSA and ECDHE_ECDSA. The naming follows the example of DH_anon, where the key is also ephemeral but the name does not reflect it. 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 5.10.2.4. Algorithms in Certificate Chains
This specification does not impose restrictions on signature schemes used anywhere in the certificate chain. The previous version of this document required the signatures to match, but this restriction, originating in previous TLS versions, is lifted here as it had been in RFC 5246.
3. Client Authentication
This document defines a client authentication mechanism named after the type of client certificate involved: ECDSA_sign. The ECDSA_sign mechanism is usable with any of the non-anonymous ECC key exchange algorithms described in Section 2 as well as other non-anonymous (non-ECC) key exchange algorithms defined in TLS. Note that client certificates with EdDSA public keys also use this mechanism. The server can request ECC-based client authentication by including this certificate type in its CertificateRequest message. The client must check if it possesses a certificate appropriate for the method suggested by the server and is willing to use it for authentication. If these conditions are not met, the client SHOULD send a client Certificate message containing no certificates. In this case, the ClientKeyExchange MUST be sent as described in Section 2, and the CertificateVerify MUST 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 is 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- or EdDSA-capable public key. 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.
4. TLS Extensions for ECC
Two TLS extensions are defined in this specification: (i) the Supported Elliptic Curves Extension and (ii) the Supported Point Formats Extension. These allow negotiating the use of specific curves and point formats (e.g., compressed vs. uncompressed, respectively) during a handshake starting a new session. These extensions are especially relevant for constrained clients that may only support a limited number of curves or point formats. They follow the general approach outlined in [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 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.
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