Internet-Draft R. Housley Intended status: Standards Track Vigil Security Expires: 8 March 2017 8 September 2016 Use of the Elliptic Curve Diffie-Hellamn Key Agreement Algorithm with X25519 and X448 in the Cryptographic Message Syntax (CMS) Abstract This document describes the conventions for using Elliptic Curve Diffie-Hellamn (ECDH) key agreement algorithm using curve25519 and curve448 in the Cryptographic Message Syntax (CMS). 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 http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on 8 March 2017. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Housley Using X25519 and X448 with CMS [Page 1] Internet-Draft September 2016 1. Introduction This document describes the conventions for using Elliptic Curve Diffie-Hellamn (ECDH) key agreement using curve25519 and curve448 [CURVE] in the Cryptographic Message Syntax (CMS) [CMS]. Key agreement is supported in three CMS content types: the enveloped-data content type [CMS], authenticated-data content type [CMS], and the authenticated-enveloped-data content type [AUTHENV]. The conventions for using some Elliptic Curve Cryptography (ECC) algorithms in CMS are described in [CMSECC]. These conventions cover the use of ECDH with some curves other than curve25519 and curve448 [CURVE]. Those other curves are not deprecated, but support for curve25519 and curve448 is encouraged. When these two curves are used with with Diffie-Hellman key agreement, they are referred to as X25519 and X448. 1.1. Terminology 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 [STDWORDS]. 1.2. ASN.1 CMS values are generated using ASN.1 [X680], which uses the Basic Encoding Rules (BER) and the Distinguished Encoding Rules (DER) [X690]. 2. Key Agreement In 1976, Diffie and Hellman describe a means for two parties to agree upon a shared secret value in manner that prevents eavesdroppers from learning the shared secret value [DH1976]. This secret may then be converted into pairwise symmetric keying material for use with other cryptographic algorithms. Over the years, many variants of this fundamental technique have been developed. This document describes the conventions for using Ephemeral-Static Elliptic Curve Diffie- Hellamn (ECDH) key agreement using X25519 and X448 [CURVE]. The originator uses an ephemeral public/private key pair that is generated on the same elliptic curve as the public key of the recipient. The ephemeral key pair is used for a single CMS protected content type, and then it is discarded. The originator obtains the recipient's static public key from the recipient's certificate [PROFILE]. Housley Using X25519 and X448 with CMS [Page 2] Internet-Draft September 2016 X25519 is described in Section 6.1 of [CURVE], and X448 is described in Section 6.2 of [CURVE]. Since curve25519 and curve448 have cofactors of 8 and 4, respectively, an input point of small order will eliminate any contribution from the other party's private key. As described in Section 7 of [CURVE], implementations MAY detect this situation by checking for the all-zero output. In [CURVE], the shared secret value that is produced by ECDH is called K. (In some other specifications, the shared secret value is called Z.) A key derivation function (KDF) is used to produce a pairwise key-encryption key from K, the length of the key-encryption key, and the DER-encoded ECC-CMS-SharedInfo structure [CMSECC]. The ECC-CMS-SharedInfo definition from [CMSECC] is repeated here for convenience. ECC-CMS-SharedInfo ::= SEQUENCE { keyInfo AlgorithmIdentifier, entityUInfo [0] EXPLICIT OCTET STRING OPTIONAL, suppPubInfo [2] EXPLICIT OCTET STRING } The ECC-CMS-SharedInfo keyInfo field contains the object identifier of the key-encryption algorithm and associated parameters. This algorithm will be used to wrap the content-encryption key. In this specification, the AES Key Wrap algorithm identifier has absent parameters. The ECC-CMS-SharedInfo entityUInfo field optionally contains additional keying material supplied by the sending agent. Note that [CMS] requires implementations to accept a KeyAgreeRecipientInfo SEQUENCE that includes the ukm field. If the ukm field is present, the ukm is placed in the entityUInfo field. The ukm value need not be longer than the key-encryption key that will be produced by the KDF. When present, the ukm ensures that a different key-encryption key is generated, even when the originator ephemeral private key is improperly used more than once. The ECC-CMS-SharedInfo suppPubInfo field contains the length of the generated key-encryption key, in bits, represented as a 32-bit number. For example, the key length for AES-256 would be 0x00000100. 2.1. ANSI-X9.63-KDF The ANSI-X9.63-KDF key derivation function is a simple construct based on a one-way hash function described in ANS X9.63 [X963]. This KDF is also described in Section 3.6.1 of [SEC1]. Housley Using X25519 and X448 with CMS [Page 3] Internet-Draft September 2016 Three values are concatenated to produce the input string to the KDF: 1. The shared secret value generated by ECDH, K. 2. The iteration counter, starting with one, as described below. 3. The DER-encoded ECC-CMS-SharedInfo structure. To generate a key-encryption key, generates one or more KM blocks, with the counter starting at 0x00000001, and incrementing the counter for each subsequent KM block until enough material has been generated. The KM blocks are concatenated left to right: KM(i) = Hash(K || INT32(counter=i) || DER(ECC-CMS-SharedInfo)) KEK = KM(counter=1) || KM(counter=2) ... KEK is the pairwise key-encryption key. 2.2. HKDF The HKDF key derivation function is a robust construct based on a one-way hash function described in RFC 5869 [HMAC]. HKDF is comprised of two steps: HKDF-Extract followed by HKDF-Expand. Three values are used as inputs to the HKDF: 1. The shared secret value generated by ECDH, K. 2. The length in octets of the keying data to be generated. 3. The DER-encoded ECC-CMS-SharedInfo structure. The ECC-CMS-SharedInfo structure includes the ukm. This field is optional, and if it is present, the ukm is also used as the HKDF salt. The length of the generated key-encryption key is used two places, once in bits, and once in octets. The ECC-CMS-SharedInfo structure includes the length of the generated key-encryption key in bits. The HKDF-Expand function takes an argument for the length of the generated key-encryption key in octets. In summary: if ukm is provided, then salt = ukm, else salt = zero PRK = HKDF-Extract(salt, K) KEK = HKDF-Expand(PRK, DER(ECC-CMS-SharedInfo), SizeInOctets(KEK)) KEK is the pairwise key-encryption key. Housley Using X25519 and X448 with CMS [Page 4] Internet-Draft September 2016 3. Enveloped-data Conventions The CMS enveloped-data content type [CMS] consists of an encrypted content and wrapped content-encryption keys for one or more recipients. The ECDH key agreement algorithm is used to generate a pairwise key-encryption key between the originator and a particular recipient. Then, the key-encryption key is used to wrap the content- encryption key for that recipient. When there more than one recipient, the same content-encryption key is wrapped for each of them. A compliant implementation MUST meet the requirements for constructing an enveloped-data content type in Section 6 of [CMS]. A content-encryption key MUST be randomly generated for each instance of an enveloped-data content type. The content-encryption key is used to encrypt the content. 3.1. EnvelopedData Fields The enveloped-data content type is ASN.1 encoded using the EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be populated as described in [CMS]; for the recipients that use X25519 or X448 the RecipientInfo kari choice MUST be used. 3.2. KeyAgreeRecipientInfo Fields The fields of the KeyAgreeRecipientInfo syntax MUST be populated as described in this section when X25519 or X448 is employed for one or more recipients. The KeyAgreeRecipientInfo version MUST be 3. The KeyAgreeRecipientInfo originator provides three alternatives for identifying the originator's public key, and the originatorKey alternative MUST be used. The originatorKey MUST contain an ephemeral key for the originator. The originatorKey algorithm field MUST contain the id-ecPublicKey object identifier along with ECParameters as specified in [PKIXECC]. The originator's ephemeral public key MUST be encoded using the type ECPoint as specified in [CMSECC]. As a convenience, the definitions are repeated here: id-ecPublicKey OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) ansi-X9-62(10045) keyType(2) 1 } ECPoint ::= OCTET STRING Housley Using X25519 and X448 with CMS [Page 5] Internet-Draft September 2016 ECParameters ::= CHOICE { namedCurve OBJECT IDENTIFIER -- implicitCurve NULL -- specifiedCurve SpecifiedECDomain -- } The object identifiers for X25519 and X448 have been assigned in [ID.curdle-pkix]. They are repeated below for convenience. When using X25519, the ECPoint contains exactly 32 octets, and the ECParameters namedCurve MUST contain the following object identifier: id-X25519 OBJECT IDENTIFIER ::= { 1.3.101.110 } When using X448, the ECPoint contains exactly 56 octets, and the ECParameters namedCurve MUST contain the following object identifier: id-X448 OBJECT IDENTIFIER ::= { 1.3.101.111 } KeyAgreeRecipientInfo ukm is optional. Note that [CMS] requires implementations to accept a KeyAgreeRecipientInfo SEQUENCE that includes the ukm field. If present, the ukm is placed in the entityUInfo field of the ECC-CMS-SharedInfo as input to the KDF. The ukm value need not be longer than the key-encryption key produced by the KDF. KeyAgreeRecipientInfo keyEncryptionAlgorithm MUST contain the object identifier of the key-encryption algorithm that will be used to wrap the content-encryption key. The conventions for using AES-128, AES-192, and AES-256 in the key wrap mode are specified in [CMSAES]. KeyAgreeRecipientInfo recipientEncryptedKeys includes a recipient identifier and encrypted key for one or more recipients. The RecipientEncryptedKey KeyAgreeRecipientIdentifier MUST contain either the issuerAndSerialNumber identifying the recipient's certificate or the RecipientKeyIdentifier containing the subject key identifier from the recipient's certificate. In both cases, the recipient's certificate contains the recipient's static X25519 or X448 public key. RecipientEncryptedKey EncryptedKey MUST contain the content- encryption key encrypted with the pairwise key-encryption key using the algorithm specified by the KeyWrapAlgorithm. 4. Authenticated-data Conventions The CMS authenticated-data content type [CMS] consists an authenticated content, a message authentication code (MAC), and encrypted authentication keys for one or more recipients. The ECDH key agreement algorithm is used to generate a pairwise key-encryption key between the originator and a particular recipient. Then, the Housley Using X25519 and X448 with CMS [Page 6] Internet-Draft September 2016 key-encryption key is used to wrap the authentication key for that recipient. When there more than one recipient, the same authentication key is wrapped for each of them. A compliant implementation MUST meet the requirements for constructing an authenticated-data content type in Section 9 of [CMS]. A authentication key MUST be randomly generated for each instance of an authenticated-data content type. The authentication key is used to compute the MAC over the content. 4.1. AuthenticatedData Fields The authenticated-data content type is ASN.1 encoded using the AuthenticatedData syntax. The fields of the AuthenticatedData syntax MUST be populated as described in [CMS]; for the recipients that use X25519 or X448 the RecipientInfo kari choice MUST be used. 4.2. KeyAgreeRecipientInfo Fields The fields of the KeyAgreeRecipientInfo syntax MUST be populated as described in Section 3.2 of this document. 5. Authenticated-Enveloped-data Conventions The CMS authenticated-enveloped-data content type content type [AUTHENV] consists of an authenticated and encrypted content and encrypted content-authenticated-encryption keys for one or more recipients. The ECDH key agreement algorithm is used to generate a pairwise key-encryption key between the originator and a particular recipient. Then, the key-encryption key is used to wrap the content- authenticated-encryption key for that recipient. When there more than one recipient, the same content-authenticated-encryption key is wrapped for each of them. A compliant implementation MUST meet the requirements for constructing an authenticated-data content type in Section 2 of [AUTHENV]. A content-authenticated-encryption key MUST be randomly generated for each instance of an authenticated-enveloped-data content type. The content-authenticated-encryption key key is used to authenticate and encrypt the content. 5.1. AuthEnvelopedData Fields The authenticated-enveloped-data content type is ASN.1 encoded using Housley Using X25519 and X448 with CMS [Page 7] Internet-Draft September 2016 the AuthEnvelopedData syntax. The fields of the AuthEnvelopedData syntax MUST be populated as described in [AUTHENV]; for the recipients that use X25519 or X448 the RecipientInfo kari choice MUST be used. 5.2. KeyAgreeRecipientInfo Fields The fields of the KeyAgreeRecipientInfo syntax MUST be populated as described in Section 3.2 of this document. 6. Certificate Conventions RFC 5280 [PROFILE] specifies the profile for using X.509 Certificates in Internet applications. A recipient static public key is needed for X25519 or X448, and the originator obtains that public key from the recipient's certificate. The conventions in this section augment RFC 5280 [PROFILE]. The id-ecPublicKey object identifier continues to identify the static ECDH public key for the recipient. The associated EcpkParameters parameters structure is specified in [PKIXALG], and the namedCurve alternative MUST be used. The object identifiers from Section 3.2 of this document are used for X25519 and X448. The EcpkParameters parameters structure is repeated here for convenience: EcpkParameters ::= CHOICE { ecParameters ECParameters, namedCurve OBJECT IDENTIFIER, implicitlyCA NULL } The certificate issuer MAY use indicate the intended usage for the certified public key by including the key usage certificate extension as specified in Section 4.2.1.3 of [PROFILE]. If the keyUsage extension is present in a certificate that conveys an ECDH static public key, then the key usage extension MUST set the keyAgreement bit. 7. Key Agreement Algorithm Identifiers The following object identifiers are assigned to indicate ECDH with HKDF using various one-way hash functions. These are expected to be used as AlgorithmIdentifiers with a parameter that specifies the key- encryption algorithm. Housley Using X25519 and X448 with CMS [Page 8] Internet-Draft September 2016 dhSinglePass-stdDH-hkdf-sha256-scheme OBJECT IDENTIFIER ::= { TBD0 } dhSinglePass-stdDH-hkdf-sha384-scheme OBJECT IDENTIFIER ::= { TBD1 } dhSinglePass-stdDH-hkdf-sha512-scheme OBJECT IDENTIFIER ::= { TBD2 } 8. SMIMECapabilities Attribute Conventions A sending agent MAY announce to other agents that it supports ECDH key agreement using the SMIMECapabilities signed attribute in a signed message [SMIME] or a certificate [CERTCAP]. Following the pattern established in [CMSECC], the SMIMECapabilities associated with ECDH carries a DER-encoded object identifier that identifies support for ECDH in conjunction with a particular KDF, and it includes a parameter that names the key wrap algorithm. The following SMIMECapabilities values (in hexidecimal) from [CMSECC] might be of interest to implementations that support X25519 and X448: ECDH with ANSI-X9.63-KDF using SHA-256; uses AES-128 key wrap: 30 15 06 06 2B 81 04 01 0B 01 30 0B 06 09 60 86 48 01 65 03 04 01 05 ECDH with ANSI-X9.63-KDF using SHA-384; uses AES-128 key wrap: 30 15 06 06 2B 81 04 01 0B 02 30 0B 06 09 60 86 48 01 65 03 04 01 05 ECDH with ANSI-X9.63-KDF using SHA-512; uses AES-128 key wrap: 30 15 06 06 2B 81 04 01 0B 03 30 0B 06 09 60 86 48 01 65 03 04 01 05 ECDH with ANSI-X9.63-KDF using SHA-256; uses AES-256 key wrap: 30 15 06 06 2B 81 04 01 0B 01 30 0B 06 09 60 86 48 01 65 03 04 01 2D ECDH with ANSI-X9.63-KDF using SHA-384; uses AES-256 key wrap: 30 15 06 06 2B 81 04 01 0B 02 30 0B 06 09 60 86 48 01 65 03 04 01 2D ECDH with ANSI-X9.63-KDF using SHA-512; uses AES-256 key wrap: 30 15 06 06 2B 81 04 01 0B 03 30 0B 06 09 60 86 48 01 65 03 04 01 2D Housley Using X25519 and X448 with CMS [Page 9] Internet-Draft September 2016 The following SMIMECapabilities values (in hexidecimal) based on the algorithm identifiers in Section 7 of this document might be of interest to implementations that support X25519 and X448: ECDH with HKDF using SHA-256; uses AES-128 key wrap: TBD ECDH with HKDF using SHA-384; uses AES-128 key wrap: TBD ECDH with HKDF using SHA-512; uses AES-128 key wrap: TBD ECDH with HKDF using SHA-256; uses AES-256 key wrap: TBD ECDH with HKDF using SHA-384; uses AES-256 key wrap: TBD ECDH with HKDF using SHA-512; uses AES-256 key wrap: TBD 9. Security Considerations Please consult the security considerations of [CMS] for security considerations related to the enveloped-data content type and the authenticated-data content type. Please consult the security considerations of [AUTHENV] for security considerations related to the authenticated-enveloped-data content type. Please consult the security considerations of [CURVES] for security considerations related to the use of X25519 and X448. The originator uses an ephemeral public/private key pair that is generated on the same elliptic curve as the public key of the recipient. The ephemeral key pair is used for a single CMS protected content type, and then it is discarded. If the originator wants to be able to decrypt the content (for enveloped-data and authenticated- enveloped-data) or check the authentication (for authenticated-data), then the originator needs to treat themselves as a recipient. As specified in [CMS], implementations MUST support processing of the KeyAgreeRecipientInfo ukm field, so interoperability is not a concern if the ukm is present or absent. The ukm is placed in the entityUInfo field of the ECC-CMS-SharedInfo structure. When present, the ukm ensures that a different key-encryption key is generated, Housley Using X25519 and X448 with CMS [Page 10] Internet-Draft September 2016 even when the originator ephemeral private key is improperly used more than once. 10. IANA Considerations Three object identifiers for the Key Agreement Algorithm Identifiers in Sections 7 are needed. Are they going to come from an IANA registry or from the registry that assigned the object identifiers in [ID.curdle-pkix]? 11. Normative References [AUTHENV] Housley, R., "Cryptographic Message Syntax (CMS) Authenticated-Enveloped-Data Content Type", RFC 5083, November 2007. [CERTCAP] Santesson, S., "X.509 Certificate Extension for Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities", RFC 4262, December 2005. [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 5652, September 2009. [CURVES] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, January 2016. [HKDF] Krawczyk, H., and P. Eronen, "HMAC-based Extract-and- Expand Key Derivation Function (HKDF)", RFC 5869, May 2010. [ID.curdle-pkix] Josefsson, S., and J. Schaad, "Algorithm Identifiers for Ed25519, Ed25519ph, Ed448, Ed448ph, X25519 and X448 for use in the Internet X.509 Public Key Infrastructure", 15 August 2016, Work-in-progress. [PKIXALG] Bassham, L., Polk, W., and R. Housley, "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279, April 2002. [PKIXECC] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, March 2009. Housley Using X25519 and X448 with CMS [Page 11] Internet-Draft September 2016 [PROFILE] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. [SEC1] Standards for Efficient Cryptography Group, "SEC 1: Elliptic Curve Cryptography", version 2.0, May 2009, . [SMIME] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, January 2010. [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [X680] ITU-T, "Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation", ITU-T Recommendation X.680, 2015. [X690] ITU-T, "Information technology -- ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, 2015. 12. Informative References [CMSECC] Turner, S., and D. Brown, "Use of Elliptic Curve Cryptography (ECC) Algorithms in Cryptographic Message Syntax (CMS)", RFC 5753, January 2010. [CMSAES] Schaad, J., "Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS)", RFC 3565, July 2003. [DH1976] Diffie, W., and M. E. Hellman, "New Directions in Cryptography", IEEE Trans. on Info. Theory, Vol. IT-22, Nov. 1976, pp. 644-654. [X963] "Public-Key Cryptography for the Financial Services Industry: Key Agreement and Key Transport Using Elliptic Curve Cryptography", American National Standard X9.63-2001, 2001. 13. Acknowledgements Thanks to Jim Schaad, Stefan Santesson, Sean Turner for their review and insightful suggestions. Housley Using X25519 and X448 with CMS [Page 12] Internet-Draft September 2016 Author Address Russ Housley 918 Spring Knoll Drive Herndon, VA 20170 USA housley@vigilsec.com Housley Using X25519 and X448 with CMS [Page 13]