Network Working Group                                 Sean Turner, IECA
Internet Draft                                          30                                          20 October 2008 2009
Intended Status: Standard Track
Obsoletes:
Updates: RFC 5208 (once approved)
Expires: 30 20 April 2009 2010

                          Asymmetric Key Packages
                  draft-turner-asymmetrickeyformat-01.txt
                  draft-turner-asymmetrickeyformat-02.txt

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Abstract

   This document defines the syntax for private key information and a
   content type for it.  Private-key information includes a private key
   for some a specified public-key algorithm and a set of attributes. The document
   also describes a syntax for encrypted private keys.  The
   Cryptographic Message Syntax, Syntax (CMS), as defined in RFC 3852, can be
   used to digitally sign, digest, authenticate, or encrypt the
   asymmetric key format content type.  This document obsoletes updates RFC 5208.

Table of Contents

   1. Introduction...................................................2
      1.1. Requirements Terminology..................................2
      1.2. ASN.1 Syntax Notation.....................................2
      1.3. Changes since RFC 5208....................................2
   2. Asymmetric Key Package Content Type............................3
   3. Encrypted Private Key Info.....................................5
   4. Protecting the AsymmetricKeyPackage............................5
   5. Other Considerations...........................................6
   6. Security Considerations........................................6
   7. IANA Considerations............................................7
   8. References.....................................................7
      8.1. Normative References......................................7
      8.2. Non-Normative References..................................7
   APPENDIX A: ASN.1 Module..........................................9

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 some a specified
   public-key algorithm and a set of attributes. The document
   also describes a syntax for encrypted private keys.  The
   Cryptographic Message Syntax [RFC3852] CMS can be used to
   digitally sign, digest, authenticate, or encrypt the asymmetric key
   format content type.  This document obsoletes updates PKCS#8 v1.2 [RFC5208]. [RFC5208]
   sections 5 and 7, and it adds two new sections; the first covers
   protecting the Asymmetric Key Content Type, and the second discusses
   compatibility with other private-key formats.

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. Changes since Summary of Updates to RFC 5208

   The following are summarizes the changes since updates to [RFC5208]:

   - Changed the name "PrivateKeyInfo" to "OneAsymmetricKey".  This
     reflects the addition of the public key 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 aKeyAttrs to align text with module. attributes.

   - Added public key publicKey to OneAsymmetricKey and added new updated the version number.

   - Added that PKCS#9 attributes MAY may be supported.

   - Added Other Considerations section. discussion of compatibility with other private-key formats.

   - Added requirements for encoding rule set.

   - Changed imports from PKCS#5 to [RFCTBD3].

2. Asymmetric Key Package CMS Content Type

   This section updates section 5 of [RFC5208].

   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).  This content type MUST  Earlier versions of this
   specification [RFC5208] did not specify a particular encoding rule
   set, but generators SHOULD use DER [X.690] and receivers SHOULD be
   prepared to handle BER [X.690] and DER encoded [X.690].

   The asymmetric key package content type has the following syntax:

     PKCS7-CONTENT-TYPE ::= TYPE-IDENTIFIER

     asymmetric-key-package PKCS7-CONTENT-TYPE ::=
       { AsymmetricKeyPackage IDENTIFIED BY id-ct-KP-aKeyPackage }

     id-ct-KP-aKeyPackage OBJECT IDENTIFIER ::= |
       { TBD }

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

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

     PrivateKeyInfo ::= OneAsymmetricKey

     -- Used PrivateKeyInfo is used by [P12].  If version is set to 1,
     -- publicKey MUST be absent. When v1, PrivateKeyInfo is the same
     -- as it was in [P12] [RFC5208].

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

     PrivateKeyAlgorithmIdentifier ::= AlgorithmIdentifier
                                        { { PrivateKeyAlgorithms } }

     PrivateKey ::= OCTET STRING
                        -- Content varies based on type of key. The
                        -- algorithm identifier dictates the format of
                        -- the key. DSA's is an INTEGER ECDSA's is an
                        -- INTEGER, and RSA is as per [RFC3447].

     PublicKey ::= OCTET BIT STRING
                       -- Content varies based on type of key. The
                       -- algorithm identifier dictates the format of
                       -- the key. DSA is an INTEGER, ECDSA is an OCTET
                       -- STRING, and RSA is a sequence of two INTEGERs
                       -- [PKI-ALG].

     Attributes ::= Set of Attribute

   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, and optionally optional keying material attributes (e.g.,
   certificates)
   userCertificate from [X.520]), and a 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 is a renamed extension of 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 version of the asymmetric key package content
     structure.  For this version of the specification, version MUST be
     v1 if the OneAsymmetricKey.  If publicKey field
     is absent and it MUST be present, then version is set to v2 if the
     publicKey field 2 else version is present. set to 1.

   - privateKeyAlgorithm identifies the private key private-key algorithm and
     optionally contains parameters associated with the asymmetric key.
     The algorithm is identified by an OID object identifier (OID) and the parameters
     format of the parameters depends on the OID. The value placed in
     privateKeyAlgorithmIdentifier is the value an originator would
     apply to indicate which algorithm was used. is to be used with the private
     key.

   - privateKey is an OCTET STRING whose contents is are the value of the DER encoded
     private key.  The interpretation of the contents 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 ECC key is represented as ECPrivateKey
     as defined in [RFCTBD2].

   - 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 SupportedAttributes
     parameterized type.  SupportedAttributes 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 as an OCTET STRING.  The structure within the octet string,
     if any, depends on the privateKeyAlgorithm.

3. Encrypted Private Key Info

   This section gives the syntax for encrypted private-key information,
   which For example, a DSA key
     is used with [P12].

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

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

     EncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
                                        { { KeyEncryptionAlgorithms } }

     EncryptedData ::= OCTET STRING

   The EncAsymmetricKeyPackage contains one or more
   EncryptedPrivateKeyInfo elements.  The fields an INTEGER.  Other documents may define additional private key
     formats.  Note that RSA public keys are included in
   EncryptedPrivateKeyInfo RSAPrivateKey
     (i.e., n and e are used present), as follows:

   - encryptionAlgorithm identifies the algorithm under which the
     private-key information is encrypted.  Implementations MUST support
     the TBD algorithm.

   - encryptedData is the result of encrypting the private-key
     information per [RFC3447], and ECC public keys
     are included in ECPrivateKey (i.e., in the PrivateKeyInfo).

   The encryption process involves the following two steps:

   1. The private-key information is BER encoded, yielding an octet
      string.

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

4. publicKey field), as per
     [RFCTBD2].

3. Protecting the AsymmetricKeyPackage

   CMS [RFC3852] and [RFC5083] protecting content types 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 to
     provide confidentiality but does not distribute the content
     encryption keys.

   - EnvelopedData can be used to encrypt the AsymmetricKeyPackage with
     simple symmetric encryption, where the sender and the receiver
     already 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 AsymmetricKeyPackage
     with algorithms that support authenticated encryption, where key
     management information is handled in a manner similar to
     EnvelopedData.

5.

4. Other Private-Key Format Considerations

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

   - Personal Information Exchange (PFX) or Syntax Standard [P12], which is
     more commonly referred to as
     P12 [P12], PKCS #12 or simply P12, is a transfer
     syntax for personal identity information, including private keys,
     certificates, miscellaneous secrets, and extensions.  Both PrivateKeyInfo
     OneAsymmetricKey, PrivateKeyInfo, and EncryptedPrivateKeyInfo
     [RFC5208] can be carried in a P12 message.

   - Microsoft's Exchange Security format, which is a proprietary
     format.

   When locally storing  The private keys, key
     information, OneAsymmetricKey and PrivateKeyInfo, are carried in
     the file format P12 keyBag BAG-TYPE.  EncryptedPrivateKeyInfo is either a DER
   encoded file with carried in the
     P12 pkcs8ShroudedKeyBag BAG-TYPE. In current implementations, the
     file extension extensions .pfx and .p12 or a PEM encoded can be used interchangeably.

   - Microsoft's private key proprietary transfer syntax.  The .pvk file with
     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.

   [RFCTBD3] defines the appication/pkcs8 media type and .p8 file extension .pem.

   When
   extension.

   To extract the private key is a character string, information from the OCTET STRING contains
   an embedded UTF8String.

6. Security Considerations

   Protection of AsymmetricKeyPackage,
   the private-key information is vital encapsulating layers need to public-key
   cryptography.  Disclosure of be removed.  At a minimum, the private-key material to another
   entity can lead outer
   ContentInfo [RFC5652] layer needs to masquerades.  The encryption algorithm used be removed.  If the
   AsymmetricKeyPackage is encapsulated in a SignedData [RFC5652], then
   the
   encryption process must SignedData and EncapsulatedContentInfo layers [RFC5652] also need
   to be removed. The same is true for EnvelopedData, EncryptedData, and
   AuthenticatedData all from [RFC5652] as 'strong' well as AuthEnvelopedData
   from [RFC5083].  Once all the outer layers are removed, there are as
   many sets of private key it 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 KeyBack 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 protecting. the Base64
   encoding [RFC2045] of either the DER encoded EncryptedPrivateKeyInfo
   sandwiched between:

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

   or the PrivateKeyInfo or the OneAsymmetricKey sandwiched between:

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

5. Security Considerations

   The security considerations in [RFC5208] also apply to this document.

   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.

   The encrypted asymmetric key package contents are protected; as noted
   above the encryption algorithm must be as 'strong' as the key it is
   protecting.

7.

6. IANA Considerations

   None: All identifiers are already registered.  Please remove this
   section prior to publication as an RFC.

8.

7. References

8.1.

7.1. Normative References

   [RFC2045]   Freed, .N, and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, November 1996.

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

   [RFC3852]

   [RFC5652]   Housley, R., "Cryptographic Message Syntax (CMS)", RFC3852,
   July 2004. RFC
               5652, September 2009.

   [RFC5208]   Kaliski, B., "PKCS #8: Private Key Information Syntax
               Standard Version 1.2", RFC 5208, May 2008.

   [RFCTBD1]   Schaad, J., and P. Hoffman, "New ASN.1 Modules for PKIX",
               draft-ietf-pkix-new-asn1-07.txt, work-in-progress.

   [RFCTBD3]   Jennings C., and J. Fischl "Certificate Management
               Service for The Session Initiation Protocol (SIP)",
               draft-ietf-sip-certs-09.txt, work-in-progress.

   [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.

   [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.

7.2. Non-Normative 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 "Public-Key Cryptography
               Standards (PKCS) #1: RSA Cryptography Specifications
               Version 2.1", RFC 3447, February 2003.

   [RFC5208] Kaliski, B., "PKCS #8: Private Key Information Syntax
   Standard Version 1.2", RFC 5208, May 2008.

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

   [PKI-ALG]

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

   [RFCTBD2]   Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
   "Elliptic Curve Cryptography Subject Public D. Brown, "EC Private Key Information", draft-
   ietf-pkix-ecc-subpubkeyinfo, work-in-progress. Info
               Structure", draft-turner-ecprivatekey-00.txt, work-in-
               progress.

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  { tbd }

   DEFINITIONS IMPLICIT TAGS ::=

   BEGIN

   -- EXPORTS ALL
   IMPORTS NOTHING

   Attribute{}, ATTRIBUTE, AlgorithmIdentifier{} ATTRIBUTE
     FROM PKIX-CommonTypes-2009  -- FROM PKIX-CommonTypes [RFCTBD1]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkixCommon(43)
         id-mod-pkixCommon-02(57) }

     id-aes128-wrap, id-aes192-wrap, id-aes1256-wrap

   AlgorithmIdentifier{}
     FROM CMSAesRsaesOaep AlgorithmInformation-2009  -- FROM [RFCTBD1]
       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) modules(0) id-mod-cms-aes(19) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-algorithmInformation-02(58) }

   ;

   PKCS7-CONTENT-TYPE ::= TYPE-IDENTIFIER

   KeyPackageContentTypes PKCS7-CONTENT-TYPE ::= {
     asymmetric-key-package |
     asymmetric-key-package,
     ... -- Expect additional content types --
   }

   asymmetric-key-package PKCS7-CONTENT-TYPE ::=
     { AsymmetricKeyPackage IDENTIFIED BY id-ct-KP-aKeyPackage }

   id-ct-KP-aKeyPackage OBJECT IDENTIFIER ::=
     { TBD }

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

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

   PrivateKeyInfo ::= OneAsymmetricKey

   -- PrivateKeyInfo is used by [P12].  If version is set to 1,
   -- publicKey MUST be absent. When v1, PrivateKeyInfo is the same
   -- as it was in [RFC5208].

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

   PrivateKeyAlgorithmIdentifier ::= AlgorithmIdentifier
                                      { { PrivateKeyAlgorithms } }
   PrivateKey ::= OCTET STRING
                     -- Content varies based on type of key key. The
                     -- DSA is INTEGER, ECDSA is ECPublicKey algorithm identifier dictates the format of
                     -- the key.

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

   Attributes ::= Set of Attribute { { SupportAttributes SupportedAttributes } }

   SupportedAttributes ATTRIBUTE :: {
     ... -- For local profiles
   }

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

   EncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
                                       { { KeyEncryptionAlgorithms } }

   EncryptedData ::= OCTET STRING -- Encrypted PrivateKeyInfo

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

   KeyEncryptionAlgorithms ALGORITHM-IDENTIFIER ::= {
     id-aes128-wrap |
     id-aes192-wrap |
     id-aes256-wrap,
     ... -- Extensible
   }

   END

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.

   We'd also like to thank Pasi Eronen and Eronen, Russ Housley. Housley, 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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