Elliptic Curve Cryptography (ECC) Brainpool curves were an option for authentication and key exchange in the Transport Layer Security (TLS) protocol version 1.2 but were deprecated by the IETF for use with TLS version 1.3 because they had little usage. However, these curves have not been shown to have significant cryptographical weaknesses, and there is some interest in using several of these curves in TLS 1.3. This document provides the necessary protocol mechanisms for using ECC Brainpool curves in TLS 1.3. This approach is not endorsed by the IETF.

This document is not an Internet Standards Track specification; it is published for informational purposes. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not candidates for any level of Internet Standard; see 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/rfc8734.

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[RFC 5639] specifies a new set of elliptic curve groups over finite prime fields for use in cryptographic applications. These groups, denoted as ECC Brainpool curves, were generated in a verifiably pseudorandom way and comply with the security requirements of relevant standards from ISO [ISO1][ISO2], ANSI [ANSI1], NIST [FIPS], and SECG [SEC2]. [RFC 8422] defines the usage of elliptic curves for authentication and key agreement in TLS 1.2 and earlier versions, and [RFC 7027] defines the usage of the ECC Brainpool curves for authentication and key exchange in TLS. The latter is applicable to TLS 1.2 and earlier versions but not to TLS 1.3, which deprecates the ECC Brainpool curve IDs defined in [RFC 7027] due to the lack of widespread deployment. However, there is some interest in using these curves in TLS 1.3. The negotiation of ECC Brainpool curves for key exchange in TLS 1.3, according to [RFC 8446], requires the definition and assignment of additional NamedGroup IDs. This document provides the necessary definition and assignment of additional SignatureScheme IDs for using three ECC Brainpool curves from [RFC 5639]. This approach is not endorsed by the IETF. Implementers and deployers need to be aware of the strengths and weaknesses of all security mechanisms that they use.

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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

According to [RFC 8446], the "supported_groups" extension is used for the negotiation of Diffie-Hellman groups and elliptic curve groups for key exchange during a handshake starting a new TLS session. This document adds new named groups for three elliptic curves defined in [RFC 5639] to the "supported_groups" extension, as follows. The encoding of Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) parameters for sec256r1, secp384r1, and secp521r1, as defined in Section 4.2.8.2 of RFC 8446, also applies to this document. Test vectors for a Diffie-Hellman key exchange using these elliptic curves are provided in Appendix A.

According to [RFC 8446], the name space SignatureScheme is used for the negotiation of elliptic curve groups for authentication via the "signature_algorithms" extension. Besides, it is required to specify the hash function that is used to hash the message before signing. This document adds new SignatureScheme types for three elliptic curves defined in [RFC 5639], as follows.

IANA has updated the references for the ECC Brainpool curves listed in the "TLS Supported Groups" [IANA-TLS] subregistry of the "Transport Layer Security (TLS) Parameters" registry to refer to this document. Table 1 IANA has updated the references for the ECC Brainpool curves in the "TLS SignatureScheme" subregistry [IANA-TLS] of the "Transport Layer Security (TLS) Parameters" registry to refer to this document. Table 2

The security considerations of [RFC 8446] apply accordingly. The confidentiality, authenticity, and integrity of the TLS communication is limited by the weakest cryptographic primitive applied. In order to achieve a maximum security level when using one of the elliptic curves from Table 1 for key exchange and/or one of the signature algorithms from Table 2 for authentication in TLS, parameters of other deployed cryptographic schemes should be chosen at commensurate strengths, for example, according to the recommendations of [NIST800-57] and [RFC 5639]. In particular, this applies to (a) the key derivation function, (b) the algorithms and key length of symmetric encryption and message authentication, and (c) the algorithm, bit length, and hash function for signature generation. Furthermore, the private Diffie-Hellman keys should be generated from a random keystream with a length equal to the length of the order of the group E(GF(p)) defined in [RFC 5639]. The value of the private Diffie-Hellman keys should be less than the order of the group E(GF(p)). When using ECDHE key agreement with the curves brainpoolP256r1tls13, brainpoolP384r1tls13, or brainpoolP512r1tls13, the peers MUST validate each other's public value Q by ensuring that the point is a valid point on the elliptic curve. If this check is not conducted, an attacker can force the key exchange into a small subgroup, and the resulting shared secret can be guessed with significantly less effort. Implementations of elliptic curve cryptography for TLS may be susceptible to side-channel attacks. Particular care should be taken for implementations that internally transform curve points to points on the corresponding "twisted curve", using the map (x',y') = (x*Z^2, y*Z^3) with the coefficient Z specified for that curve in [RFC 5639], in order to take advantage of an efficient arithmetic based on the twisted curve's special parameters (A = -3). Although the twisted curve itself offers the same level of security as the corresponding random curve (through mathematical equivalence), arithmetic based on small curve parameters may be harder to protect against side-channel attacks. General guidance on resistance of elliptic curve cryptography implementations against side-channel attacks is given in [BSI1] and [HMV].

[IANA-TLS]

IANA, "Transport Layer Security (TLS) Parameters",
<https://www.iana.org/assignments/tls-parameters>.

[RFC2119]

S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC5639]

M. Lochter, and J. Merkle, "Elliptic Curve Cryptography (ECC) Brainpool Standard Curves and Curve Generation", RFC 5639, DOI 10.17487/RFC5639, March 2010,
<https://www.rfc-editor.org/info/rfc5639>.

[RFC7027]

J. Merkle, and M. Lochter, "Elliptic Curve Cryptography (ECC) Brainpool Curves for Transport Layer Security (TLS)", RFC 7027, DOI 10.17487/RFC7027, October 2013,
<https://www.rfc-editor.org/info/rfc7027>.

[RFC8174]

B. Leiba, "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.

[RFC8422]

Y. Nir, S. Josefsson, and M. Pegourie-Gonnard, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier", RFC 8422, DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.

[RFC8446]

E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[ANSI1]

American National Standards Institute, "Public Key Cryptography For The Financial Services Industry: the Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI X9.62, November 2005.

[BSI1]

Bundesamt fuer Sicherheit in der Informationstechnik, "Minimum Requirements for Evaluating Side-Channel Attack Resistance of Elliptic Curve Implementations", July 2011.

[FIPS]

National Institute of Standards and Technology, "Digital Signature Standard (DSS)", FIPS PUB 186-4, DOI 10.6028/NIST.FIPS.186-4, July 2013,
<https://doi.org/10.6028/NIST.FIPS.186-4>.

[HMV]

D Hankerson, A Menezes, and S Vanstone, "Guide to Elliptic Curve Cryptography", Springer Verlag, 2004.

[ISO1]

International Organization for Standardization, "Information Technology - Security Techniques - Digital Signatures with Appendix - Part 3: Discrete Logarithm Based Mechanisms", ISO/IEC 14888-3, November 2018.

[ISO2]

International Organization for Standardization, "Information Technology - Security techniques - Cryptographic techniques based on elliptic curves - Part 2: Digital signatures", ISO/IEC 15946-2:2002, December 2002.

[NIST800-57]

National Institute of Standards and Technology, "Recommendation for Key Management - Part 1: General (Revised)", NIST Special Publication 800-57, DOI 10.6028/NIST.SP.800-57ptlr4, January 2016,
<https://doi.org/10.6028/NIST.SP.800-57ptlr4>.

[SEC1]

Standards for Efficient Cryptography Group, "SEC1: Elliptic Curve Cryptography", May 2019.

[SEC2]

Standards for Efficient Cryptography Group, "SEC 2: Recommended Elliptic Curve Domain Parameters", January 2010.

This non-normative Appendix provides some test vectors (for example, Diffie-Hellman key exchanges using each of the curves defined in Table 1). The following notation is used in all of the subsequent sections:

d_A:

the secret key of party A

x_qA:

the x-coordinate of the public key of party A

y_qA:

the y-coordinate of the public key of party A

d_B:

the secret key of party B

x_qB:

the x-coordinate of the public key of party B

y_qB:

the y-coordinate of the public key of party B

x_Z:

the x-coordinate of the shared secret that results from completion of the Diffie-Hellman computation, i.e., the hex representation of the premaster secret

y_Z:

the y-coordinate of the shared secret that results from completion of the Diffie-Hellman computation

The field elements x_qA, y_qA, x_qB, y_qB, x_Z, and y_Z are represented as hexadecimal values using the FieldElement-to-OctetString conversion method specified in [SEC1].

Curve brainpoolP256r1

Curve brainpoolP384r1

Curve brainpoolP512r1

Leonie Bruckert

Johannes Merkle

Manfred Lochter