ShangMi (SM) Cipher Suites for Transport Layer Security (TLS) Protocol Version 1.3
draft-yang-tls-tls13-sm-suites-04

Abstract

This document specifies a set of cipher suites for the Transport Layer Security (TLS) protocol version 1.3 to support ShangMi (SM) cryptographic algorithms.

The use of these cipher suites with TLSv1.3 is not endorsed by the IETF. The SM cipher suites are becoming mandatory in China, and so this document provides a description of how to use the SM cipher suites with TLSv1.3 so that implementers can produce interworking implementations.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on January 5, 2021.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. The SM Algorithms
• 1.2. Terminology
• 2. Supported Cipher Suites
• 3. Cipher Suites Definitions
• 3.1. TLS Versions
• 3.2. Authentication
• 3.2.1. SM2 Signature Scheme
• 3.3. Key Exchange
• 3.3.1. Hello Messages
• 3.3.2. CertificateRequest
• 3.3.3. Certificate
• 3.3.4. CertificateVerify
• 3.4. Key Scheduling
• 3.5. Cipher
• 3.5.1. AEAD_SM4_GCM
• 3.5.2. AEAD_SM4_CCM
• 3.6. Hash
• 4. IANA Considerations
• 5. Security Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Test Vectors
• A.1. SM4-GCM Test Vectors
• A.2. SM4-CCM Test Vectors
• Appendix B. Contributors
• Author's Address

1. Introduction

This document describes two new cipher suites for the Transport Layer Security (TLS) protocol version 1.3 (TLSv1.3, [RFC8446]). The new cipher suites are (see also Section 2):

   CipherSuite TLS_SM4_GCM_SM3 = { 0x00, 0xC6 };
   CipherSuite TLS_SM4_CCM_SM3 = { 0x00, 0xC7 };

These new cipher suites contain several ShangMi (SM) cryptographic algorithms that provide both authentication and confidentiality. For a more detailed introduction to SM cryptographic algorithms, please read Section 1.1. These cipher suites follow the TLSv1.3 requirements. Specifically, all the cipher suites mentioned in this document use ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) as the key exchange scheme and use SM4 in either GCM (Galois/Counter Mode) mode or CCM (Counter with CBC-MAC) mode to meet the needs of TLSv1.3 to have an AEAD (Authenticated Encryption with Associated Data) capable encryption algorithm.

For the details about how these new cipher suites negotiate shared encryption keys and protect the record structure, please read Section 3.

The cipher suites defined in this document are not recommended by the IETF. The SM cipher suites are becoming mandatory in China, and so this document provides a description of how to use the SM cipher suites with TLSv1.3 so that implementers can produce interworking implementations.

1.1. The SM Algorithms

The new cipher suites defined in this document use several different SM cryptographic algorithms including SM2 for authentication, SM4 for encryption and SM3 as the hash function.

SM2 is a set of elliptic curve based cryptographic algorithms including digital signature, public key encryption and key exchange scheme. In this document, only the SM2 digital signature algorithm is involved, which has already been added to ISO/IEC 14888-3:2018 [ISO-SM2] (as well as in [GBT.32918.2-2016]). SM4 is a block cipher defined in [GBT.32907-2016] and now is being standardized by ISO to ISO/IEC 18033-3:2010 [ISO-SM4]. SM3 is a hash function which produces an output of 256 bits. SM3 has already been accepted by ISO in ISO/IEC 10118-3:2018 [ISO-SM3], and also been described by [GBT.32905-2016].

1.2. Terminology

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, and indicate requirement levels for compliant TLSv1.3 implementations.

2. Supported Cipher Suites

The cipher suites defined here have the following identifiers:

   CipherSuite TLS_SM4_GCM_SM3 = { 0x00, 0xC6 };
   CipherSuite TLS_SM4_CCM_SM3 = { 0x00, 0xC7 };

To accomplish a TLSv1.3 handshake, additional objects have been introduced along with the cipher suites as follows.

The SM2 signature algorithm and SM3 hash function used in the Signature Algorithm extension defined in appendix-B.3.1.3 of [RFC8446]:

   SignatureScheme sm2sig_sm3 = { 0x0708 };

The SM2 elliptic curve ID used in the Supported Groups extension defined in appendix-B.3.1.4 of [RFC8446]:

   NamedGroup curveSM2 = { 41 };

3. Cipher Suites Definitions

3.1. TLS Versions

The new cpher suites defined in this document are only applicable to TLSv1.3. Implementations of this document MUST NOT apply these cipher suites to any older versions of TLS.

3.2. Authentication

3.2.1. SM2 Signature Scheme

All cipher suites defined in this document MUST use the SM2 signature algorithm as the authentication method when doing a TLSv1.3 handshake.

The SM2 signature is defined in [ISO-SM2]. SM2 signature algorithm is based on elliptic curves. SM2 signature algorithm uses a fixed elliptic curve parameter set defined in [GBT.32918.5-2016]. This curve has the name curveSM2 and has been assigned the value 41 as shown in Section 4. Unlike other elliptic curve based public key algorithms like ECDSA, SM2 MUST NOT select other elliptic curves. But it is acceptable to write test cases that use other elliptic curve parameter sets for SM2, take Annex F.14 of [ISO-SM2] as a reference.

Implementations of the cipher suites defined in this document SHOULD conform to what [GBT.32918.5-2016] requires, that is to say, the only valid elliptic curve parameter for SM2 signature algorithm (a.k.a curveSM2) is defined as follows:

   curveSM2: a prime field of 256 bits

   y^2 = x^3 + ax + b

   p  = FFFFFFFE FFFFFFFF FFFFFFFF FFFFFFFF
        FFFFFFFF 00000000 FFFFFFFF FFFFFFFF
   a  = FFFFFFFE FFFFFFFF FFFFFFFF FFFFFFFF
        FFFFFFFF 00000000 FFFFFFFF FFFFFFFC
   b  = 28E9FA9E 9D9F5E34 4D5A9E4B CF6509A7
        F39789F5 15AB8F92 DDBCBD41 4D940E93
   n  = FFFFFFFE FFFFFFFF FFFFFFFF FFFFFFFF
        7203DF6B 21C6052B 53BBF409 39D54123
   Gx = 32C4AE2C 1F198119 5F990446 6A39C994
        8FE30BBF F2660BE1 715A4589 334C74C7
   Gy = BC3736A2 F4F6779C 59BDCEE3 6B692153
        D0A9877C C62A4740 02DF32E5 2139F0A0

The SM2 signature algorithm requests an identifier value when generating or verifying a signature. Implementations of this document MUST use the following ASCII string value as the SM2 identifier when doing a TLSv1.3 key exchange:

   TLSv1.3+GM+Cipher+Suite

Except if either a client or a server needs to verify the peer's SM2 certificate contained in the Certificate message, then the following ASCII string value SHOULD be used as the SM2 identifier according to [GMT.0009-2012]:

   1234567812345678

Expressed as octets, this is:

   0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38,
   0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38

In practice, the SM2 identifier used in a certificate signature depends on the CA who signs that certificate. CAs may choose values other than the ones mentioned above. Implementations of this document SHOULD confirm this information by themselves.

3.3. Key Exchange

3.3.1. Hello Messages

The new cipher suites defined in this document update the key exchange information in the Hello messages. Implementations of these new ciphers suites MUST conform to the new requirements.

3.3.1.1. ClientHello

A TLSv1.3 client MUST include the new cipher suites in its 'cipher_suites' array of the ClientHello structure defined in Section 4.1.2 of [RFC8446].

Other requirements on the extensions of ClientHello message are:

• For the supported_groups extension, 'curveSM2' MUST be included;
• For the signature_algorithms extension, 'sm2sig_sm3' MUST be included;
• For the signature_algorithms_cert extension (if present), 'sm2sig_sm3' MUST be included;
• For the key_share extension, a KeyShareEntry with SM2 related values MUST be added if the client wants to start a TLSv1.3 key negotiation using SM cipher suites.

3.3.1.2. ServerHello

If a TLSv1.3 server receives a ClientHello message containing the new cipher suites defined in this document, it MAY choose to use the new cipher suites. If so, then the server MUST put one of the new cipher suites defined in this document into its ServerHello's 'cipher_suites' array and eventually send it to the client side.

A TLSv1.3 server's choice of what cipher suite to use depends on the configuration of the server. For instance, a TLSv1.3 server may be configured to include the new cipher suites defined in this document, or it may not be. Typical TLSv1.3 server applications also provide a mechanism that configures the cipher suite preference at server side. If a server is not configured to use the cipher suites defined in this document, it SHOULD choose another cipher suite in the list that the TLSv1.3 client provides; otherwise the server MUST abort the handshake with an "illegal_parameter" alert.

The following extensions MUST conform to the new requirements:

• For the key_share extension, a KeyShareEntry with SM2 related values MUST be added if the server wants to start a TLSv1.3 key negotiation using SM cipher suites.

3.3.2. CertificateRequest

If a CertificateRequest message is sent by the server to require the client to send its certificate for authentication purposes, the following requirements MUST be fulfilled:

• The only valid signature algorithm present in 'signature_algorithms' extension MUST be 'sm2sig_sm3'. That is to say, if server chooses to use an SM cipher suite, the signature algorithm for client's certificate SHOULD only be SM2 and SM3 capable ones.

3.3.3. Certificate

When a server sends the Certificate message containing the server certificate to the client side, several new rules are added that will affect the certificate selection:

• The public key in the certificate MUST be a valid SM2 public key.
• The signature algorithm used by the CA to sign current certificate MUST be 'sm2sig_sm3'.
• The certificate MUST be capable of signing, e.g., the digitalSignature bit of X.509's Key Usage extension is set.

3.3.4. CertificateVerify

In the certificateVerify message, the signature algorithm MUST be 'sm2sig_sm3', indicating that the hash function MUST be SM3 and the signature algorithm MUST be SM2.

3.4. Key Scheduling

As described in Section 1.1, SM2 is actually a set of cryptographic algorithms including one key exchange protocol which defines methods such as key derivation function, etc. In this document, SM2 key exchange protocol is not introduced and SHALL NOT be used in the key exchange steps defined in Section 3.3. Implementations of this document MUST always conform to what TLSv1.3 [RFC8446] and its successors require about the key derivation and related methods.

3.5. Cipher

The new cipher suites introduced in this document add two new AEAD encryption algorithms, AEAD_SM4_GCM and AEAD_SM4_CCM, which stand for SM4 cipher in Galois/Counter mode and SM4 cipher [GBT.32907-2016] in Counter with CBC-MAC mode, respectively.

This section defines the AEAD_SM4_GCM and AEAD_SM4_CCM AEAD algorithms in a style similar to what [RFC5116] used to define AEAD ciphers based on AES cipher.

3.5.1. AEAD_SM4_GCM

The AEAD_SM4_GCM authenticated encryption algorithm works as specified in [GCM], using SM4 as the block cipher, by providing the key, nonce, plaintext, and associated data to that mode of operation. An authentication tag conforming to the requirements of Section 5.2 of TLSv1.3 [RFC8446] MUST be constructed by the details in the TLS record header. The additional data input that forms the authentication tag MUST be the TLS record header. The AEAD_SM4_GCM ciphertext is formed by appending the authentication tag provided as an output to the GCM encryption operation to the ciphertext that is output by that operation. AEAD_SM4_GCM has four inputs: an SM4 key, an initialization vector (IV), a plaintext content, and optional additional authenticated data (AAD). AEAD_SM4_GCM generates two outputs: a ciphertext and message authentication code (also called an authentication tag). To have a common set of terms for AEAD_SM4_GCM and AEAD_SM4_CCM, the AEAD_SM4_GCM IV is referred to as a nonce in the remainder of this document. A simple test vector of AEAD_SM4_GCM and AEAD_SM4_CCM is given in Appendix A of this document.

The nonce is generated by the party performing the authenticated encryption operation.
Within the scope of any authenticated-encryption key, the nonce value MUST be unique. That is, the set of nonce values used with any given key MUST NOT contain any duplicates. Using the same nonce for two different messages encrypted with the same key destroys the security properties of GCM mode. To generate the nonce, implementations of this document MUST conform to TLSv1.3 (See [RFC8446], Section 5.3).

The input and output lengths are as follows:

   the SM4 key length is 16 octets,

   the max plaintext length is 2^36 - 31 octets,

   the max AAD length is 2^61 - 1 octets,

   the nonce length is 12 octets,

   the authentication tag length is 16 octets, and

   the max ciphertext length is 2^36 - 15 octets.

A security analysis of GCM is available in [MV04].

3.5.2. AEAD_SM4_CCM

The AEAD_SM4_CCM authenticated encryption algorithm works as specified in [CCM], using SM4 as the block cipher. AEAD_SM4_CCM has four inputs: an SM4 key, a nonce, a plaintext, and optional additional authenticated data (AAD). AEAD_SM4_CCM generates two outputs: a ciphertext and a message authentication code (also called an authentication tag). The formatting and counter generation functions are as specified in Appendix A of [CCM], and the values of the parameters identified in that appendix are as follows:

   the nonce length n is 12,

   the tag length t is 16, and

   the value of q is 3.

An authentication tag is also used in AEAD_SM4_CCM. The generation of the authentication tag MUST conform to TLSv1.3 (See [RFC8446], Section 5.2). The AEAD_SM4_CCM ciphertext is formed by appending the authentication tag provided as an output to the CCM encryption operation to the ciphertext that is output by that operation. The input and output lengths are as follows:

   the SM4 key length is 16 octets,

   the max plaintext length is 2^24 - 1 octets,

   the max AAD length is 2^64 - 1 octets, and

   the max ciphertext length is 2^24 + 15 octets.

To generate the nonce, implementations of this document MUST conform to TLSv1.3 (See [RFC8446], Section 5.3).

A security analysis of CCM is available in [J02].

3.6. Hash

SM3 is defined by ISO in [ISO-SM3]. During a TLSv1.3 handshake with SM cipher suites, the hash function is REQUIRED to be SM3. Implementations MUST use SM3 for digest, key derivation, Transcript-Hash and other purposes during a TLSv1.3 key exchange process.

4. IANA Considerations

IANA has assigned the values {0x00, 0xC6} and {0x00, 0xC7} with the names TLS_SM4_GCM_SM3, TLS_SM4_CCM_SM3, to the "TLS Cipher Suite" registry with this document as reference:

IANA has assigned the value 0x0708 with the name 'sm2sig_sm3', to the "TLS SignatureScheme" registry:

IANA has assigned the value 41 with the name 'curveSM2', to the "TLS Supported Groups" registry:

5. Security Considerations

At the time of writing, there are no known weak keys for SM cryptographic algorithms: SM2, SM3 and SM4, and no security issues have been found for these algorithms.

6. References

6.1. Normative References

6.2. Informative References

Appendix A. Test Vectors

All values are in hexadecimal and are in network byte order (big endian).

A.1. SM4-GCM Test Vectors

Initialization Vector:   00001234567800000000ABCD
Key:                     0123456789ABCDEFFEDCBA9876543210
Plaintext:               AAAAAAAAAAAAAAAABBBBBBBBBBBBBBBB
                         CCCCCCCCCCCCCCCCDDDDDDDDDDDDDDDD
                         EEEEEEEEEEEEEEEEFFFFFFFFFFFFFFFF
                         EEEEEEEEEEEEEEEEAAAAAAAAAAAAAAAA
Associated Data:         FEEDFACEDEADBEEFFEEDFACEDEADBEEFABADDAD2
CipherText:              17F399F08C67D5EE19D0DC9969C4BB7D
                         5FD46FD3756489069157B282BB200735
                         D82710CA5C22F0CCFA7CBF93D496AC15
                         A56834CBCF98C397B4024A2691233B8D
Authentication Tag:      83DE3541E4C2B58177E065A9BF7B62EC

A.2. SM4-CCM Test Vectors

Initialization Vector:   00001234567800000000ABCD
Key:                     0123456789ABCDEFFEDCBA9876543210
Plaintext:               AAAAAAAAAAAAAAAABBBBBBBBBBBBBBBB
                         CCCCCCCCCCCCCCCCDDDDDDDDDDDDDDDD
                         EEEEEEEEEEEEEEEEFFFFFFFFFFFFFFFF
                         EEEEEEEEEEEEEEEEAAAAAAAAAAAAAAAA
Associated Data:         FEEDFACEDEADBEEFFEEDFACEDEADBEEFABADDAD2
CipherText:              48AF93501FA62ADBCD414CCE6034D895
                         DDA1BF8F132F042098661572E7483094
                         FD12E518CE062C98ACEE28D95DF4416B
                         ED31A2F04476C18BB40C84A74B97DC5B
Authentication Tag:      16842D4FA186F56AB33256971FA110F4

Appendix B. Contributors

Wuqiong Pan
Ant Technology
wuqiong.pwq@antfin.com

Qin Long
Ant Technology
zhuolong.lq@antfin.com

Kepeng Li
Ant Technology
kepeng.lkp@antfin.com

Ke Zeng
Ant Technology
william.zk@antfin.com

Han Xiao
Ant Technology
han.xiao@antfin.com

Zhi Guan
Peking University
guan@pku.edu.cn

Author's Address