RFC 5958: Asymmetric Key Packages

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    Internet Engineering Task Force (IETF)                         S. Turner
    Request for Comments: 5958                                          IECA
    Obsoletes: 5208                                              August 2010
    Category: Standards Track
    ISSN: 2070-1721

                            Asymmetric Key Packages

    Abstract

       This document defines the syntax for private-key information and a
       content type for it.  Private-key information includes a private key
       for a specified public-key algorithm and a set of attributes.  The
       Cryptographic Message Syntax (CMS), as defined in RFC 5652, can be
       used to digitally sign, digest, authenticate, or encrypt the
       asymmetric key format content type.  This document obsoletes RFC
       5208.

    Status of This Memo

       This is an Internet Standards Track document.

       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).  Further
       information on Internet Standards is available in 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/rfc5958.

    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
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       publication of this document.  Please review these documents
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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.

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       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified
       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

    1.  Introduction

       This document defines the syntax for private-key information and a
       Cryptographic Message Syntax (CMS) [RFC5652] content type for it.
       Private-key information includes a private key for a specified
       public-key algorithm and a set of attributes.  The CMS can be used to
       digitally sign, digest, authenticate, or encrypt the asymmetric key
       format content type.  This document obsoletes PKCS #8 v1.2 [RFC5208].

    1.1.  Requirements 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].

    1.2.  ASN.1 Syntax Notation

       The key package is defined using ASN.1 [X.680], [X.681], [X.682], and
       [X.683].

    1.3.  Summary of Updates to RFC 5208

       The following summarizes the updates to [RFC5208]:

       - Changed the name "PrivateKeyInfo" to "OneAsymmetricKey".  This
         reflects the addition of the publicKey field to allow both parts of
         the asymmetric key to be conveyed separately.  Not all algorithms
         will use both fields; however, the publicKey field was added for
         completeness.

       - Defined Asymmetric Key Package CMS content type.

       - Removed redundant IMPLICIT from attributes.

       - Added publicKey to OneAsymmetricKey and updated the version number.

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       - Added that PKCS #9 attributes may be supported.

       - Added discussion of compatibility with other private-key formats.

       - Added requirements for encoding rule set.

       - Changed imports from PKCS #5 to [RFC5912] and [RFC5911].

       - Replaced ALGORITHM-IDENTIFIER with ALGORITHM from [RFC5912].

       - Registers application/pkcs8 media type and .p8 file extension.

    2.  Asymmetric Key Package CMS Content Type

       The asymmetric key package CMS content type is used to transfer one
       or more plaintext asymmetric keys from one party to another.  An
       asymmetric key package MAY be encapsulated in one or more CMS
       protecting content types (see Section 4).  Earlier versions of this
       specification [RFC5208] did not specify a particular encoding rule
       set, but generators SHOULD use DER [X.690] and receivers MUST support
       BER [X.690], which also includes DER [X.690].

       The asymmetric key package content type has the following syntax:

         ct-asymmetric-key-package CONTENT-TYPE ::=
           { AsymmetricKeyPackage IDENTIFIED BY id-ct-KP-aKeyPackage }

         id-ct-KP-aKeyPackage OBJECT IDENTIFIER ::=
           { joint-iso-itu-t(2) country(16) us(840) organization(1)
             gov(101) dod(2) infosec(1) formats(2)
             key-package-content-types(78) 5
           }

         AsymmetricKeyPackage ::= SEQUENCE SIZE (1..MAX) OF OneAsymmetricKey

         OneAsymmetricKey ::= SEQUENCE {
           version                   Version,
           privateKeyAlgorithm       PrivateKeyAlgorithmIdentifier,
           privateKey                PrivateKey,
           attributes            [0] Attributes OPTIONAL,
           ...,
           [[2: publicKey        [1] PublicKey OPTIONAL ]],
           ...
         }

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         PrivateKeyInfo ::= OneAsymmetricKey

         -- PrivateKeyInfo is used by [P12].  If any items tagged as version
         -- 2 are used, the version must be v2, else the version should be
         -- v1.  When v1, PrivateKeyInfo is the same as it was in [RFC5208].

         Version ::= INTEGER { v1(0), v2(1) } (v1, ..., v2)

         PrivateKeyAlgorithmIdentifier ::= AlgorithmIdentifier
                                            { PUBLIC-KEY,
                                              { PrivateKeyAlgorithms } }

         PrivateKey ::= OCTET STRING
                            -- Content varies based on type of key.  The
                            -- algorithm identifier dictates the format of
                            -- the key.

         PublicKey ::= BIT STRING
                            -- Content varies based on type of key.  The
                            -- algorithm identifier dictates the format of
                            -- the key.

         Attributes ::= SET OF Attribute { { OneAsymmetricKeyAttributes } }

       The AsymmetricKeyPackage contains one or more OneAsymmetricKey
       elements.

       The syntax of OneAsymmetricKey accommodates a version number, an
       indication of the asymmetric algorithm to be used with the private
       key, a private key, optional keying material attributes (e.g.,
       userCertificate from [X.520]), and an optional public key.  In
       general, either the public key or the certificate will be present.
       In very rare cases will both the public key and the certificate be
       present as this includes two copies of the public key.
       OneAsymmetricKey renames the PrivateKeyInfo syntax defined in
       [RFC5208].  The new name better reflects the ability to carry both
       private- and public-key components.  Backwards compatibility with the
       original PrivateKeyInfo is preserved via version number.  The fields
       in OneAsymmetricKey are used as follows:

       - version identifies the version of OneAsymmetricKey.  If publicKey
         is present, then version is set to v2 else version is set to v1.

       - privateKeyAlgorithm identifies the private-key algorithm and
         optionally contains parameters associated with the asymmetric key
         pair.  The algorithm is identified by an object identifier (OID)
         and the format of the parameters depends on the OID, but the
         PrivateKeyAlgorithms information object set restricts the

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         permissible OIDs.  The value placed in
         privateKeyAlgorithmIdentifier is the value an originator would
         apply to indicate which algorithm is to be used with the private
         key.

       - privateKey is an OCTET STRING that contains the value of the
         private key.  The interpretation of the content is defined in the
         registration of the private-key algorithm.  For example, a DSA key
         is an INTEGER, an RSA key is represented as RSAPrivateKey as
         defined in [RFC3447], and an Elliptic Curve Cryptography (ECC) key
         is represented as ECPrivateKey as defined in [RFC5915].

       - attributes is OPTIONAL.  It contains information corresponding to
         the public key (e.g., certificates).  The attributes field uses the
         class ATTRIBUTE which is restricted by the
         OneAsymmetricKeyAttributes information object set.
         OneAsymmetricKeyAttributes is an open ended set in this document.
         Others documents can constrain these values.  Attributes from
         [RFC2985] MAY be supported.

       - publicKey is OPTIONAL.  When present, it contains the public key
         encoded in a BIT STRING.  The structure within the BIT STRING, if
         any, depends on the privateKeyAlgorithm.  For example, a DSA key is
         an INTEGER.  Note that RSA public keys are included in
         RSAPrivateKey (i.e., n and e are present), as per [RFC3447], and
         ECC public keys are included in ECPrivateKey (i.e., in the
         publicKey field), as per [RFC5915].

    3.  Encrypted Private Key Info

       This section gives the syntax for encrypted private-key information,
       which is used by [P12].

       Encrypted private-key information shall have ASN.1 type
       EncryptedPrivateKeyInfo:

         EncryptedPrivateKeyInfo ::= SEQUENCE {
           encryptionAlgorithm  EncryptionAlgorithmIdentifier,
           encryptedData        EncryptedData }

         EncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
                                            { CONTENT-ENCRYPTION,
                                              { KeyEncryptionAlgorithms } }

         EncryptedData ::= OCTET STRING

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       The fields in EncryptedPrivateKeyInfo are used as follows:

       - encryptionAlgorithm identifies the algorithm under which the
         private-key information is encrypted.

       - encryptedData is the result of encrypting the private-key
         information  (i.e., the PrivateKeyInfo).

       The encryption process involves the following two steps:

       1. The private-key information is encoded, yielding an octet string.
          Generators SHOULD use DER [X.690] and receivers MUST support BER
          [X.690], which also includes DER [X.690].

       2. The result of step 1 is encrypted with the secret key to give an
          octet string, the result of the encryption process.

    4.  Protecting the AsymmetricKeyPackage

       CMS protecting content types, [RFC5652] and [RFC5083], can be used to
       provide security to the AsymmetricKeyPackage:

       - SignedData can be used to apply a digital signature to the
         AsymmetricKeyPackage.

       - EncryptedData can be used to encrypt the AsymmetricKeyPackage with
         symmetric encryption, where the sender and the receiver already
         share the necessary encryption key.

       - EnvelopedData can be used to encrypt the AsymmetricKeyPackage with
         symmetric encryption, where the sender and the receiver do not
         share the necessary encryption key.

       - AuthenticatedData can be used to protect the AsymmetricKeyPackage
         with message authentication codes, where key management information
         is handled in a manner similar to EnvelopedData.

       - AuthEnvelopedData can be used to protect the AsymmetricKeyPackage
         with algorithms that support authenticated encryption, where key
         management information is handled in a manner similar to
         EnvelopedData.

    5.  Other Private-Key Format Considerations

       This document defines the syntax and the semantics for a content type
       that exchanges asymmetric private keys.  There are two other formats
       that have been used for the transport of asymmetric private keys:

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       - Personal Information Exchange (PFX) Syntax Standard [P12], which is
         more commonly referred to as PKCS #12 or simply P12, is a transfer
         syntax for personal identity information, including private keys,
         certificates, miscellaneous secrets, and extensions.
         OneAsymmetricKey, PrivateKeyInfo, and EncryptedPrivateKeyInfo can
         be carried in a P12 message.  The private key information,
         OneAsymmetricKey and PrivateKeyInfo, are carried in the P12 keyBag
         BAG-TYPE.  EncryptedPrivateKeyInfo is carried in the P12
         pkcs8ShroudedKeyBag BAG-TYPE.  In current implementations, the file
         extensions .pfx and .p12 can be used interchangeably.

       - Microsoft's private-key proprietary transfer syntax.  The .pvk file
         extension is used for local storage.

       The .pvk and .p12/.pfx formats are not interchangeable; however,
       conversion tools exist to convert from one format to another.

       To extract the private-key information from the AsymmetricKeyPackage,
       the encapsulating layers need to be removed.  At a minimum, the outer
       ContentInfo [RFC5652] layer needs to be removed.  If the
       AsymmetricKeyPackage is encapsulated in a SignedData [RFC5652], then
       the SignedData and EncapsulatedContentInfo layers [RFC5652] also need
       to be removed.  The same is true for EnvelopedData, EncryptedData,
       and AuthenticatedData all from [RFC5652] as well as AuthEnvelopedData
       from [RFC5083].  Once all the outer layers are removed, there are as
       many sets of private-key information as there are OneAsymmetricKey
       structures.  OneAsymmetricKey and PrivateKeyInfo are the same
       structure; therefore, either can be saved as a .p8 file or copied in
       to the P12 KeyBag BAG-TYPE.  Removing encapsulating security layers
       will invalidate any signature and may expose the key to unauthorized
       disclosure.

       .p8 files are sometimes PEM-encoded.  When .p8 files are PEM encoded
       they use the .pem file extension.  PEM encoding is either the Base64
       encoding, from Section 4 of [RFC4648], of the DER-encoded
       EncryptedPrivateKeyInfo sandwiched between:

       -----BEGIN ENCRYPTED PRIVATE KEY-----
       -----END ENCRYPTED PRIVATE KEY-----

       or the Base64 encoding, see Section 4 of [RFC4648], of the DER-
       encoded PrivateKeyInfo sandwiched between:

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

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    6.  Security Considerations

       Protection of the private-key information is vital to public-key
       cryptography.  Disclosure of the private-key material to another
       entity can lead to masquerades.  The encryption algorithm used in the
       encryption process must be as 'strong' as the key it is protecting.

       The asymmetric key package contents are not protected.  This content
       type can be combined with a security protocol to protect the contents
       of the package.

    7.  IANA Considerations

       This document makes use of object identifiers to identify a CMS
       content type and the ASN.1 module found in Appendix A.  The CMS
       content type OID is registered in a DoD arc.  The ASN.1 module OID is
       registered in an arc delegated by RSADSI to the SMIME Working Group.
       No further action by IANA is necessary for this document or any
       anticipated updates.

       This specification also defines a new media subtype that IANA has
       registered at http://www.iana.org/.

    7.1.  Registration of media subtype application/pkcs8

       Type name: application

       Subtype name: pkcs8

       Required parameters: None

       Optional parameters: None

       Encoding considerations: binary

       Security considerations: Carries a cryptographic private key.
                                See section 6.

       Interoperability considerations:

             The PKCS #8 object inside this media type MUST be DER-encoded
             PrivateKeyInfo.

       Published specification: RFC 5958

       Applications which use this media type:

         Any MIME-compliant transport that processes asymmetric keys.

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       Additional information:

         Magic number(s): None
         File extension(s): .p8
         Macintosh File Type Code(s):

       Person & email address to contact for further information:

         Sean Turner <turners@ieca.com>

       Restrictions on usage: none

       Author:

         Sean Turner <turners@ieca.com>

       Intended usage: COMMON

       Change controller:

         The IESG

    8.  References

    8.1.  Normative References

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

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

       [RFC5652]   Housley, R., "Cryptographic Message Syntax (CMS)", STD
                   70, RFC 5652, September 2009.

       [RFC5911]   Hoffman, P. and J. Schaad, "New ASN.1 Modules for
                   Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
                   June 2010.

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

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

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       [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.

       [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).

    8.2.  Informative References

       [P12]       RSA Laboratories, "PKCS #12 v1.0: Personal Information
                   Exchange Syntax", June 1999.

       [RFC2985]   Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
                   Classes and Attribute Types Version 2.0", RFC 2985,
                   November 2000.

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

       [RFC5083]   Housley, R., "Cryptographic Message Syntax (CMS)
                   Authenticated-Enveloped-Data Content Type", RFC 5083,
                   November 2007.

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

       [X.520]     ITU-T Recommendation X.520 (2005) | ISO/IEC 9594-6:2005,
                   Information technology - Open Systems Interconnection -
                   The Directory: Selected attribute types.

       [RFC5915]   Turner, S. and D. Brown, "Elliptic Curve Private Key
                   Structure", RFC 5915, June 2010.

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    Appendix A.  ASN.1 Module

       This annex provides the normative ASN.1 definitions for the
       structures described in this specification using ASN.1 as defined in
       [X.680] through [X.683].

       AsymmetricKeyPackageModuleV1
         { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
           smime(16) modules(0) id-mod-asymmetricKeyPkgV1(50) }

       DEFINITIONS IMPLICIT TAGS ::=

       BEGIN

       -- EXPORTS ALL

       IMPORTS

       -- FROM New SMIME ASN.1 [RFC5911]

       Attribute{}, CONTENT-TYPE
        FROM CryptographicMessageSyntax-2009
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
            smime(16) modules(0) id-mod-cms-2004-02(41) }

       -- From New PKIX ASN.1 [RFC5912]
       ATTRIBUTE
        FROM PKIX-CommonTypes-2009
          { iso(1) identified-organization(3) dod(6) internet(1)
            security(5) mechanisms(5) pkix(7) id-mod(0)
            id-mod-pkixCommon-02(57) }

       -- From New PKIX ASN.1 [RFC5912]

       AlgorithmIdentifier{}, ALGORITHM, PUBLIC-KEY, CONTENT-ENCRYPTION
         FROM AlgorithmInformation-2009
           { iso(1) identified-organization(3) dod(6) internet(1)
             security(5) mechanisms(5) pkix(7) id-mod(0)
             id-mod-algorithmInformation-02(58) }

       ;

       ContentSet CONTENT-TYPE ::= {
        ct-asymmetric-key-package,
        ... -- Expect additional content types --
       }

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       ct-asymmetric-key-package CONTENT-TYPE ::=
        { AsymmetricKeyPackage IDENTIFIED BY id-ct-KP-aKeyPackage }

       id-ct-KP-aKeyPackage OBJECT IDENTIFIER ::=
         { joint-iso-itu-t(2) country(16) us(840) organization(1)
             gov(101) dod(2) infosec(1) formats(2)
             key-package-content-types(78) 5
         }

       AsymmetricKeyPackage ::= SEQUENCE SIZE (1..MAX) OF OneAsymmetricKey

       OneAsymmetricKey ::= SEQUENCE {
         version                   Version,
         privateKeyAlgorithm       PrivateKeyAlgorithmIdentifier,
         privateKey                PrivateKey,
         attributes            [0] Attributes OPTIONAL,
         ...,
         [[2: publicKey        [1] PublicKey OPTIONAL ]],
         ...
       }

       PrivateKeyInfo ::= OneAsymmetricKey

       -- PrivateKeyInfo is used by [P12]. If any items tagged as version
       -- 2 are used, the version must be v2, else the version should be
       -- v1. When v1, PrivateKeyInfo is the same as it was in [RFC5208].

       Version ::= INTEGER { v1(0), v2(1) } (v1, ..., v2)

       PrivateKeyAlgorithmIdentifier ::= AlgorithmIdentifier
                                          { PUBLIC-KEY,
                                            { PrivateKeyAlgorithms } }

       PrivateKey ::= OCTET STRING
                         -- Content varies based on type of key. The
                         -- algorithm identifier dictates the format of
                         -- the key.

       PublicKey ::= BIT STRING
                         -- Content varies based on type of key. The
                         -- algorithm identifier dictates the format of
                         -- the key.

       Attributes ::= SET OF Attribute { { OneAsymmetricKeyAttributes } }

       OneAsymmetricKeyAttributes ATTRIBUTE ::= {
         ... -- For local profiles
       }

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       -- An alternate representation that makes full use of ASN.1
       -- constraints follows.  Also note that PUBLIC-KEY needs to be
       -- imported from the new PKIX ASN.1 Algorithm Information module
       -- and PrivateKeyAlgorithms needs to be commented out.

       -- OneAsymmetricKey ::= SEQUENCE {
       --  version                  Version,
       --  privateKeyAlgorithm      SEQUENCE {
       --   algorithm                 PUBLIC-KEY.&id({PublicKeySet}),
       --    parameters               PUBLIC-KEY.&Params({PublicKeySet}
       --                               {@privateKeyAlgorithm.algorithm})
       --                               OPTIONAL}
       --  privateKey               OCTET STRING (CONTAINING
       --                             PUBLIC-KEY.&PrivateKey({PublicKeySet}
       --                             {@privateKeyAlgorithm.algorithm})),
       --  attributes           [0] Attributes OPTIONAL,
       --  ...,
       --  [[2: publicKey       [1] BIT STRING (CONTAINING
       --                             PUBLIC-KEY.&Params({PublicKeySet}
       --                             {@privateKeyAlgorithm.algorithm})
       --                             OPTIONAL,
       --  ...
       --    }

       EncryptedPrivateKeyInfo ::= SEQUENCE {
         encryptionAlgorithm  EncryptionAlgorithmIdentifier,
         encryptedData        EncryptedData }

       EncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
                                           { CONTENT-ENCRYPTION,
                                             { KeyEncryptionAlgorithms } }

       EncryptedData ::= OCTET STRING -- Encrypted PrivateKeyInfo

       PrivateKeyAlgorithms ALGORITHM ::= {
         ... -- Extensible
       }

       KeyEncryptionAlgorithms ALGORITHM ::= {
         ... -- Extensible
       }

       END

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    Acknowledgements

       Many thanks go out to the Burt Kaliski and Jim Randall at RSA.
       Without the prior version of the document, this one wouldn't exist.

       I'd also like to thank Pasi Eronen, Roni Even, Alfred Hoenes, Russ
       Housley, Jim Schaad, and Carl Wallace.

    Author's Address

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

       EMail: turners@ieca.com

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