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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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to this document. Code Components extracted from this document must
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 Standards Track [Page 1]RFC 5958 Asymmetric Key Packages August 2010
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.
Turner Standards Track [Page 2]RFC 5958 Asymmetric Key Packages August 2010
- 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 ]],
...
}
Turner Standards Track [Page 3]RFC 5958 Asymmetric Key Packages August 2010
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
Turner Standards Track [Page 4]RFC 5958 Asymmetric Key Packages August 2010
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
Turner Standards Track [Page 5]RFC 5958 Asymmetric Key Packages August 2010
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:
Turner Standards Track [Page 6]RFC 5958 Asymmetric Key Packages August 2010
- 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-----
Turner Standards Track [Page 7]RFC 5958 Asymmetric Key Packages August 2010
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.
Turner Standards Track [Page 8]RFC 5958 Asymmetric Key Packages August 2010
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.
Turner Standards Track [Page 9]RFC 5958 Asymmetric Key Packages August 2010
[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.
Turner Standards Track [Page 10]RFC 5958 Asymmetric Key Packages August 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 --
}
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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
}
Turner Standards Track [Page 12]RFC 5958 Asymmetric Key Packages August 2010
-- 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
Turner Standards Track [Page 13]RFC 5958 Asymmetric Key Packages August 2010
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
Turner Standards Track [Page 14]
