Elliptic Curve Private Key Structure

Internet Engineering Task Force (IETF)                         S. Turner
Request for Comments: 5915                                          IECA
Category: Informational                                         D. Brown
ISSN: 2070-1721                                                 Certicom
                                                               June 2010


                  Elliptic Curve Private Key Structure

Abstract

   This document specifies the syntax and semantics for conveying
   Elliptic Curve (EC) private key information.  The syntax and
   semantics defined herein are based on similar syntax and semantics
   defined by the Standards for Efficient Cryptography Group (SECG).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any
   errata, and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5915.

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1.  Introduction

   This document specifies a syntax and semantics for Elliptic Curve
   (EC) private key information.  EC private key information includes a
   private key and parameters.  Additionally, it may include the
   corresponding public key.  The syntax and semantics defined herein
   are based on similar syntax and semantics defined by the Standards
   for Efficient Cryptography Group (SECG) [SECG1].

   Most Public Key Infrastructures (PKIs) mandate local key generation;
   however, there are some PKIs that also support centralized key
   generation (e.g., the public-private key pair is generated by a
   Certification Authority).  The structure defined in this document
   allows the entity that generates the private and public keys to
   distribute the key pair and the associated domain parameters.

   This syntax is useful when distributing EC private keys using
   PrivateKeyInfo, as defined in PKCS #8 [RFC5208].  Distributing an EC
   private key with PKCS#8 [RFC5208] involves including:

   a) id-ecPublicKey, id-ecDH, or id-ecMQV (from [RFC5480]) with the
      namedCurve as the parameters in the privateKeyAlgorithm field; and
   b) ECPrivateKey in the PrivateKey field, which is an OCTET STRING.

   When an EC public key is included in the distributed PrivateKeyInfo,
   the publicKey field in ECPrivateKey is used.

2.  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 [RFC2119].

3.  Elliptic Curve Private Key Format

   This section gives the syntax for an EC private key.
   Computationally, an EC private key is an unsigned integer, but for
   representation, EC private key information SHALL have ASN.1 type
   ECPrivateKey:

   ECPrivateKey ::= SEQUENCE {
     version        INTEGER { ecPrivkeyVer1(1) } (ecPrivkeyVer1),
     privateKey     OCTET STRING,
     parameters [0] ECParameters {{ NamedCurve }} OPTIONAL,
     publicKey  [1] BIT STRING OPTIONAL
   }
   The fields of type ECPrivateKey have the following meanings:

   o  version specifies the syntax version number of the elliptic curve
      private key structure.  For this version of the document, it SHALL
      be set to ecPrivkeyVer1, which is of type INTEGER and whose value
      is one (1).

   o  privateKey is the private key.  It is an octet string of length
      ceiling (log2(n)/8) (where n is the order of the curve) obtained
      from the unsigned integer via the Integer-to-Octet-String-
      Primitive (I2OSP) defined in [RFC3447].

   o  parameters specifies the elliptic curve domain parameters
      associated to the private key.  The type ECParameters is discussed
      in [RFC5480].  As specified in [RFC5480], only the namedCurve
      CHOICE is permitted.  namedCurve is an object identifier that
      fully identifies the required values for a particular set of
      elliptic curve domain parameters.  Though the ASN.1 indicates that
      the parameters field is OPTIONAL, implementations that conform to
      this document MUST always include the parameters field.

   o  publicKey contains the elliptic curve public key associated with
      the private key in question.  The format of the public key is
      specified in Section 2.2 of [RFC5480].  Though the ASN.1 indicates
      publicKey is OPTIONAL, implementations that conform to this
      document SHOULD always include the publicKey field.  The publicKey
      field can be omitted when the public key has been distributed via
      another mechanism, which is beyond the scope of this document.
      Given the private key and the parameters, the public key can
      always be recomputed; this field exists as a convenience to the
      consumer.

4.  Other Considerations

   When generating a transfer encoding, generators SHOULD use
   Distinguished Encoding Rules (DER) [X.690] and receivers SHOULD be
   prepared to handle Basic Encoding Rules (BER) [X.690] and DER
   [X.690].

   Section 1 described a format for transporting EC private keys (i.e.,
   converting ECPrivateKey to PrivateKeyInfo [PKCS#8]); however, this
   format can also be used for local storage.

   Local storage of an unencrypted ECPrivateKey object is out of scope
   of this document.  However, one popular format uses the .pem file
   extension.  It is the PEM encoding, which is the Base64 encoding (see
   Section 4 of [RFC4648]), of the DER-encoded ECPrivateKey object that
   is sandwiched between:

   -----BEGIN EC PRIVATE KEY-----
   -----END EC PRIVATE KEY-----

   Another local storage format uses the .der file extension.  In this
   case, it is a DER [X.690] encoding of the ECPrivateKey object.

   Local storage of an encrypted ECPrivateKey object is out of scope of
   this document.  However, ECPrivateKey should be the format for the
   plaintext key being encrypted.  DER [X.690] encoding the ECPrivateKey
   will promote interoperability if the key is encrypted for transport
   to another party.  PEM encoding the DER-encoded ECPrivateKey is
   common; "Proc-Type:" and "DEK-INFO:" fields [RFC1421] followed by the
   DER-encoded ECPrivateKey are sandwiched between:

   -----BEGIN EC PRIVATE KEY-----
   -----END EC PRIVATE KEY-----

5.  Security Considerations

   This structure does not protect the EC private key information in any
   way.  This structure should be combined with a security protocol to
   protect it.

   Protection of the private key information is vital to public key
   cryptography.  The consequences of disclosure depend on the purpose
   of the private key.  If a private key is used for signature, then the
   disclosure allows unauthorized signing.  If a private key is used for
   key management, then disclosure allows unauthorized parties to access
   the managed keying material.  The encryption algorithm used in the
   encryption process must be as 'strong' as the key it is protecting.

6.  References

6.1.  Normative References

   [RFC1421]  Linn, J., "Privacy Enhancement for Internet Electronic
              Mail: Part I: Message Encryption and Authentication
              Procedures", RFC 1421, February 1993.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, March 2009.

   [RFC5912]  Schaad, J. and P. Hoffman, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)" RFC 5912,
              June 2010.

   [SECG1]    Standards for Efficient Cryptography Group (SECG), "SEC 1:
              Elliptic Curve Cryptography", Version 2.0, May 2009.

   [X.680]    ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
              Information Technology - Abstract Syntax Notation One.

   [X.681]    ITU-T Recommendation X.681 (2002) | ISO/IEC 8824-2:2002,
              Information Technology - Abstract Syntax Notation One:
              Information Object Specification.

   [X.682]    ITU-T Recommendation X.682 (2002) | ISO/IEC 8824-3:2002,
              Information Technology - Abstract Syntax Notation One:
              Constraint Specification.

   [X.683]    ITU-T Recommendation X.683 (2002) | ISO/IEC 8824-4:2002,
              Information Technology - Abstract Syntax Notation One:
              Parameterization of ASN.1 Specifications, 2002.

   [X.690]    ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
              Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER).

7.2.  Informative References

   [RFC5208]  Kaliski, B., "Public-Key Cryptography Standards (PKCS) #8:
              Private-Key Information Syntax Specification Version 1.2",
              RFC 5208, May 2008.

Appendix A.  ASN.1 Module

   This appendix provides ASN.1 definitions for the structures described
   in this specification using ASN.1 as defined in [X.680], [X.681],
   [X.682], and [X.683] for compilers that support the 2002 ASN.1.

   ECPrivateKey { iso(1) identified-organization(3) dod(6)
     internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-ecprivateKey(65) }

   DEFINITIONS EXPLICIT TAGS ::=

   BEGIN

   -- EXPORTS ALL;

   IMPORTS

   -- FROM New PKIX ASN.1 [RFC5912]

   ECParameters, NamedCurve
     FROM PKIXAlgs-2009
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-algorithms2008-02(56) }

   ;

   ECPrivateKey ::= SEQUENCE {
     version        INTEGER { ecPrivkeyVer1(1) } (ecPrivkeyVer1),
     privateKey     OCTET STRING,
     parameters [0] ECParameters {{ NamedCurve }} OPTIONAL,
     publicKey  [1] BIT STRING OPTIONAL
   }

   END

Appendix B.  Differences with SECG1

   This appendix lists the differences between this document and
   [SECG1]:

   1.  This document uses the I2OSP routine defined in [RFC3447] while
       SECG1 defines its own routine.  The two routines result in the
       same output.

   2.  SECG1 constrains its parameters (i.e., the curves) to
       SECGCurveNames.  This document constrains the parameters to
       NamedCurve from [RFC5480].

   3.  This document requires parameters be present while SECG1 does
       not.

   4.  This document specifies requirements for encoding rules while
       SECG1 did not.

Acknowledgements

   The authors would like to thank Simon Blake-Wilson and John O. Goyo
   for their work on defining the structure in [SECG1].  The authors
   would also like to thank Pasi Eronen, Alfred Hoenes, Joel Jaegglie,
   Avshalom Houri, Russ Housley, Jim Schaad, and Carl Wallace for their
   comments.

Authors' Addresses

   Sean Turner
   IECA, Inc.
   3057 Nutley Street, Suite 106
   Fairfax, VA 22031
   USA

   EMail: turners@ieca.com


   Daniel R. L. Brown
   Certicom Corp
   5520 Explorer Drive #400
   Mississauga, ON L4W 5L1
   Canada

   EMail: dbrown@certicom.com