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

Table of Contents

   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.
  1. 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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