Network Working Group                                         B. Kaliski
Request for Comments: 5208                                           EMC
Category: Informational                                         May 2008

Public-Key Cryptography Standards (PKCS) #8:
Private-Key Information Syntax Specification Version 1.2

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

IESG Note

   The IESG thanks RSA Laboratories for transferring change control to
   the IETF.  Enhancements to this specification that preserve backward
   compatibility are expected in an upcoming IETF standards track
   document.

Abstract

   This document represents a republication of PKCS #8 v1.2 from RSA
   Laboratories' Public Key Cryptography Standard (PKCS) series.  Change
   control is transferred to the IETF.  The body of this document,
   except for the security considerations section, is taken directly
   from the PKCS #8 v1.2 specification.

   This document describes a syntax for private-key information.

   1. Introduction ....................................................2
   2. Definitions .....................................................2
   3. Symbols and Abbreviations .......................................2
   4. General Overview ................................................2
   5. Private-Key Information Syntax ..................................3
   6. Encrypted Private-Key Information Syntax ........................4
   7. Security Considerations .........................................4
   Appendix A. ASN.1 Syntax ...........................................5
   Informative References .............................................6

1. Introduction

   This document describes a syntax for private-key information.
   Private-key information includes a private key for some public-key
   algorithm and a set of attributes.  The document also describes a
   syntax for encrypted private keys.  A password-based encryption
   algorithm (e.g., one of those described in [PKCS#5]) could be used to
   encrypt the private-key information.

   The intention of including a set of attributes is to provide a simple
   way for a user to establish trust in information such as a
   distinguished name or a top-level certification authority's public
   key.  While such trust could also be established with a digital
   signature, encryption with a secret key known only to the user is
   just as effective and possibly easier to implement.  A non-exhaustive
   list of attributes is given in [PKCS#9].

2. Definitions

   For the purposes of this document, the following definitions apply.

      AlgorithmIdentifier: A type that identifies an algorithm (by
      object identifier) and any associated parameters.  This type is
      defined in [X.509].

      ASN.1: Abstract Syntax Notation One, as defined in [X.208].

      Attribute: A type that contains an attribute type (specified by
      object identifier) and one or more attribute values.  This type is
      defined in [X.501].

      BER: Basic Encoding Rules, as defined in [X.209].

3. Symbols and Abbreviations

No symbols or abbreviations are defined in this document.

4. General Overview

   The next two sections specify private-key information syntax and
   encrypted private-key information syntax.

   This document exports two types: PrivateKeyInfo (Section 6) and
   EncryptedPrivateKeyInfo (Section 7).

5. Private-Key Information Syntax

   This section gives the syntax for private-key information.

   Private-key information shall have ASN.1 type PrivateKeyInfo:

      PrivateKeyInfo ::= SEQUENCE {
        version                   Version,
        privateKeyAlgorithm       PrivateKeyAlgorithmIdentifier,
        privateKey                PrivateKey,
        attributes           [0]  IMPLICIT Attributes OPTIONAL }

      Version ::= INTEGER

      PrivateKeyAlgorithmIdentifier ::= AlgorithmIdentifier

      PrivateKey ::= OCTET STRING

      Attributes ::= SET OF Attribute

   The fields of type PrivateKeyInfo have the following meanings:

      version is the syntax version number, for compatibility with
      future revisions of this document.  It shall be 0 for this version
      of the document.

      privateKeyAlgorithm identifies the private-key algorithm.  One
      example of a private-key algorithm is PKCS #1's rsaEncryption
      [PKCS#1].

      privateKey is an octet string whose contents are the value of the
      private key.  The interpretation of the contents is defined in the
      registration of the private-key algorithm.  For an RSA private
      key, for example, the contents are a BER encoding of a value of
      type RSAPrivateKey.

      attributes is a set of attributes.  These are the extended
      information that is encrypted along with the private-key
      information.

6. Encrypted Private-Key Information Syntax

   This section gives the syntax for encrypted private-key information.

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

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

      EncryptionAlgorithmIdentifier ::= AlgorithmIdentifier

      EncryptedData ::= OCTET STRING

   The fields of type EncryptedPrivateKeyInfo have the following
   meanings:

      encryptionAlgorithm identifies the algorithm under which the
      private-key information is encrypted.  Two examples are PKCS #5's
      pbeWithMD2AndDES-CBC and pbeWithMD5AndDES-CBC [PKCS#5].

      encryptedData is the result of encrypting the private-key
      information.

   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.

7. 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.

Appendix A. ASN.1 Syntax

PKCS-8 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-8(8)
         modules(1) pkcs-8(1)}

-- $Revision: 1.5 $

-- This module has been checked for conformance with the ASN.1
-- standard by the OSS ASN.1 Tools

DEFINITIONS IMPLICIT TAGS ::=

BEGIN

-- EXPORTS All --
-- All types and values defined in this module is exported for use in
-- other ASN.1 modules.

IMPORTS

informationFramework
         FROM UsefulDefinitions {joint-iso-itu-t(2) ds(5) module(1)
                                 usefulDefinitions(0) 3}

Attribute
         FROM InformationFramework informationFramework

AlgorithmIdentifier, ALGORITHM-IDENTIFIER
         FROM PKCS-5 {iso(1) member-body(2) us(840) rsadsi(113549)
         pkcs(1) pkcs-5(5) modules(16) pkcs-5(1)};

-- Private-key information syntax

PrivateKeyInfo ::= SEQUENCE {
   version Version,
   privateKeyAlgorithm AlgorithmIdentifier {{PrivateKeyAlgorithms}},
   privateKey PrivateKey,
   attributes [0] Attributes OPTIONAL }

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

PrivateKey ::= OCTET STRING

Attributes ::= SET OF Attribute

-- Encrypted private-key information syntax

EncryptedPrivateKeyInfo ::= SEQUENCE {
    encryptionAlgorithm AlgorithmIdentifier {{KeyEncryptionAlgorithms}},
    encryptedData EncryptedData
}

EncryptedData ::= OCTET STRING

PrivateKeyAlgorithms ALGORITHM-IDENTIFIER ::= {
     ... -- For local profiles
}

KeyEncryptionAlgorithms ALGORITHM-IDENTIFIER ::= {
     ... -- For local profiles
}

END

Informative References

   [PKCS#1]  RSA Laboratories. PKCS #1: RSA Encryption Standard. Version
             1.5, November 1993.

   [PKCS#5]  RSA Laboratories. PKCS #5: Password-Based Encryption
             Standard. Version 1.5, November 1993.

   [PKCS#9]  RSA Laboratories. PKCS #9: Selected Attribute Types.
             Version 1.1, November 1993.

   [X.208]   CCITT. Recommendation X.208: Specification of Abstract
             Syntax Notation One (ASN.1). 1988.

   [X.209]   CCITT. Recommendation X.209: Specification of Basic
             Encoding Rules for Abstract Syntax Notation One (ASN.1).
             1988.

   [X.501]   CCITT. Recommendation X.501: The Directory - Models. 1988.

   [X.509]   CCITT. Recommendation X.509: The Directory - Authentication
             Framework. 1988.

Author's Addresses

   Burt Kaliski
   EMC Corporation
   176 South Street
   Hopkinton, MA  01748
   USA

   EMail: kaliski_burt@emc.com

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