RFC5958

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.

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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.

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.

ASN.1 Syntax Notation

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

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.

- 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.

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 ]],
   ...
 }

 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

 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.

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

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.

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.

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:

- 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-----

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.

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/.

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.

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 <[email protected]>

Restrictions on usage: none

Author:

 Sean Turner <[email protected]>

Intended usage: COMMON

Change controller:

 The IESG

References

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.

[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).

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.

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 --

}

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

}

-- 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

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: [email protected]