[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00 draft-ietf-curdle-cms-ecdh-new-curves

Internet-Draft                                                R. Housley
Intended status: Standards Track                          Vigil Security
Expires: 15 October 2016                                   15 April 2016


 Use of the Elliptic Curve Diffie-Hellamn Key Agreement Algorithm with
  Curve 25519 and Curve 448 in the Cryptographic Message Syntax (CMS)

               <draft-housley-cms-ecdh-new-curves-00.txt>


Abstract

   This document describes the conventions for using Elliptic Curve
   Diffie-Hellamn (ECDH) key agreement algorithm using Curve 25519 and
   Curve 448 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 15 October 2016.

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 ECDH and New Curves with CMS           [Page 1]


Internet-Draft                                             15 April 2016


1.  Introduction

   This document describes the conventions for using Elliptic Curve
   Diffie-Hellamn (ECDH) key agreement using Curve 25519 and Curve 448
   [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 Curve 25519 and Curve 448
   [CURVE].  Those other curves are not deprecated, but support for
   Curve 25519 and Curve 448 is encouraged.

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 Curve 25519 and Curve 448.

   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].

   ECDH with Curve 25519 is described in Section 6.1 of [CURVE], and
   ECDH with Curve 448 is described in Section 6.2 of [CURVE].  Since
   Curve 25519 and Curve 448 have cofactors of 8 and 4, respectively, an



Housley            Using ECDH and New Curves with CMS           [Page 2]


Internet-Draft                                             15 April 2016


   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.  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].

   Three values are concatenated to produce the input string to the KDF:
      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.



Housley            Using ECDH and New Curves with CMS           [Page 3]


Internet-Draft                                             15 April 2016


   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:

      KEK = KM(counter=1) || KM(counter=2) ...

      KM(i) = Hash(K || INT32(counter=i) || DER(ECC-CMS-SharedInfo))

   The output of the KDF is the pairwise key-encryption key.

2.2.  HKDF

   {{{ Should we specify a way to use HKDF from RFC 5869? }}}

      if ukm is provided, then salt = ukm, else salt = zero
      PRK = HKDF-Extract(salt, K)

      KEK = HKDF-Expand(PRK, DER(ECC-CMS-SharedInfo), SizeOf(KEK))

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 stated 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 encipher 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 ECDH
   with Curve 25519 or Curve 448 the RecipientInfo kari choice MUST be
   used.





Housley            Using ECDH and New Curves with CMS           [Page 4]


Internet-Draft                                             15 April 2016


3.2. KeyAgreeRecipientInfo Fields

   The fields of the KeyAgreeRecipientInfo syntax MUST be populated as
   described in this section when ECDH with Curve 25519 or Curve 448 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 courtesy, 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

      ECParameters ::= CHOICE {
        namedCurve         OBJECT IDENTIFIER
        -- implicitCurve   NULL
        -- specifiedCurve  SpecifiedECDomain -- }

   The object identifiers for Curve 25519 and Curve 448 have been
   assigned in [ID.josefsson-pkix-newcurves].  They are repeated below
   for convenience.

   When using Curve 25519, the ECPoint contains exactly 32 octets, and
   the ECParameters namedCurve MUST contain the following object
   identifier:

      id-Curve25519 OBJECT IDENTIFIER ::= { 1 3 6 1 4 1 11591 15 1 }

   When using Curve 448, the ECPoint contains exactly 56 octets, and the
   ECParameters namedCurve MUST contain the following object identifier:

      id-Curve448 OBJECT IDENTIFIER ::= { 1 3 6 1 4 1 11591 15 2 }

   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.



Housley            Using ECDH and New Curves with CMS           [Page 5]


Internet-Draft                                             15 April 2016


   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 ECDH public key with
   Curve 25519 or Curve 448 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
   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 stated 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
   ECDH with Curve 25519 or Curve 448 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.



Housley            Using ECDH and New Curves with CMS           [Page 6]


Internet-Draft                                             15 April 2016


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 stated 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
   the AuthEnvelopedData syntax.  The fields of the AuthEnvelopedData
   syntax MUST be populated as described in [AUTHENV]; for the
   recipients that use ECDH with Curve 25519 or Curve 448 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 ECDH with Curve 25519 or Curve 448, and the originator obtains
   that public key from the recipient's certificate.  The conventions in
   this section augment RFC 5280.

   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 Curve 25519 and Curve 448.  The
   EcpkParameters parameters structure is repeated here for convenience:



Housley            Using ECDH and New Curves with CMS           [Page 7]


Internet-Draft                                             15 April 2016


      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.  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 Curve 25519 and
   Curve 448:

      ECDH with SHA-256 as the KDF; 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 SHA-384 as the KDF; 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 SHA-512 as the KDF; 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 SHA-256 as the KDF; 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 SHA-384 as the KDF; 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






Housley            Using ECDH and New Curves with CMS           [Page 8]


Internet-Draft                                             15 April 2016


      ECDH with SHA-512 as the KDF; 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

   {{{ Should we specify a way to use HKDF from RFC 5869 with ECDH? }}}


8.  Security Considerations

   Please consult the security considerations of [CMS] and [AUTHENV] for
   security considerations related to the enveloped-data content type
   and the authenticated-enveloped-data content type, respectively.

   Please consult the security considerations of [CURVES] for security
   considerations related to the use of ECDH with Curve 25519 and Curve
   448.

   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,
   even when the originator ephemeral private key is improperly used
   more than once.

9.  IANA Considerations

   No IANA registrations are requested in this document.

10.  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.





Housley            Using ECDH and New Curves with CMS           [Page 9]


Internet-Draft                                             15 April 2016


   [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.

   [ID.josefsson-pkix-newcurves]
              Josefsson, S., "Using Curve25519 and Curve448 in PKIX",
              12 October 2015, 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.

   [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,
              <http://www.secg.org/sec1-v2.pdf>.

   [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, 2002.

   [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, 2002.








Housley            Using ECDH and New Curves with CMS          [Page 10]


Internet-Draft                                             15 April 2016


11.  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.

12.  Acknowledgements

   Thanks to Jim Schaad and Stefan Santesson for their insightful
   suggestions.

Author Address

   Russ Housley
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA
   housley@vigilsec.com





















Housley            Using ECDH and New Curves with CMS          [Page 11]


Html markup produced by rfcmarkup 1.122, available from https://tools.ietf.org/tools/rfcmarkup/