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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.

Copyright Notice

 Copyright (c) 2010 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
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 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.

Turner & Brown Informational [Page 1] RFC 5915 Elliptic Curve Private Key Structure June 2010

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
 }

Turner & Brown Informational [Page 2] RFC 5915 Elliptic Curve Private Key Structure June 2010

 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:

Turner & Brown Informational [Page 3] RFC 5915 Elliptic Curve Private Key Structure June 2010

—-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.

Turner & Brown Informational [Page 4] RFC 5915 Elliptic Curve Private Key Structure June 2010

 [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.

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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.

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 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

Turner & Brown Informational [Page 7]