[Search] [pdf|bibtex] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02 03 04 05 06 07 rfc3560                               
S/MIME Working Group                                         R. Housley
Internet Draft                                                   SPYRUS
expires in six months                                     February 2000

                       CMS RSAES-OAEP Conventions


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   This document describes the use of the RSAES-OAEP key transport
   method of key management within the Cryptographic Message Syntax

   This draft is being discussed on the "ietf-smime" mailing list.  To
   join the list, send a message to <ietf-smime-request@imc.org> with
   the single word "subscribe" in the body of the message.  Also, there
   is a Web site for the mailing list at <http://www.imc.org/ietf-

Housley                                                         [Page 1]

INTERNET DRAFT                                             February 2000

1  Introduction

   When the variant of the RSA algorithm specified in PKCS #1 Version
   1.5 [PKCS#1v1.5] is used for key management, it is vulnerable to
   adaptive chosen ciphertext attacks.  The use of PKCS #1 Version 1.5
   key transport in interactive applications is especially vulnerable.
   Exploitation of this identified vulnerability, revealing the result
   of a particular RSA decryption, requires access to an oracle which
   will respond to hundreds of thousands of ciphertexts, which are
   constructed adaptively in response to previously-received replies
   providing information on the successes or failures of attempted
   decryption operations.  As a result, the attack appears significantly
   less feasible in store-and-forward environments, such as S/MIME.
   When PKCS #1 Version 1.5 key transport is applied as an intermediate
   encryption layer within an interactive request-response
   communications environment, exploitation could be more feasible.

   An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
   [PKCS#1v2.0].  This new document supersedes RFC 2313.  PKCS #1
   Version 2.0 preserves support for the encryption padding format
   defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also defines a
   new alternative.  To resolve the adaptive chosen ciphertext
   vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
   of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
   is used to provide confidentiality.

   This document specifies the use of RSAES-OAEP key transport algorithm
   in the Cryptographic Message Syntax (CMS) [CMS].

   CMS supports variety of architectures for certificate-based key
   management, particularly the one defined by the PKIX working group

   CMS values are generated using ASN.1 [X.208-88], using the Basic
   Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
   (DER) [X.509-88].

   Throughout this document, when the terms MUST, MUST NOT, SHOULD and
   MAY are used in capital letters, their use conforms to the
   definitions in [MUSTSHOULD].  [MUSTSHOULD] defines these key words to
   help make the intent of standards track documents as clear as
   possible.  The same key words are used in this document to help
   implementers achieve interoperability.  Implementations that claims
   compliance with this document MUST provide the capabilities as
   indicated by the MUST, MUST NOT, SHOULD and MAY terms.

Housley                                                         [Page 2]

INTERNET DRAFT                                             February 2000

2  Enveloped-data Conventions

The CMS enveloped-data content type consists of encrypted content and
wrapped content-encryption keys for one or more recipients.  The RSAES-
OAEP key transport algorithm is used to wrap the content-encryption key
for one recipient.

Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 6, "Enveloped-data
Content Type".  [CMS] Section 6 should be studied before reading this
section, because this section does not repeat the [CMS] text.

A content-encryption key MUST be randomly generated for each instance of
an enveloped-data content type.  The content-encryption key is used to
encipher the content.

2.1  EnvelopedData Fields

   The enveloped-data content type is ASN.1 encoded using the
   EnvelopedData syntax.  The fields of the EnvelopedData syntax must be
   populated as follows:

   The EnvelopedData version MUST be either 0 or 2.

   The EnvelopedData originatorInfo field MUST be absent.

   The EnvelopedData recipientInfos CHOICE MUST be
   KeyTransRecipientInfo.  See section 2.2 for further discussion of

   The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
   field MUST be specify a symmetric encryption algorithm.
   Implementations MUST support the encryption of Triple-DES content-
   encryption keys, but implementations MAY support other algorithms as

   The EnvelopedData unprotectedAttrs MAY be present.

2.2  KeyTransRecipientInfo Fields

   The enveloped-data content type is ASN.1 encoded using the
   EnvelopedData syntax.  The fields of the EnvelopedData syntax must be
   populated as follows:

   The KeyTransRecipientInfo version MUST be either 0 or 2.  If the
   RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the
   version MUST be 0.  If the RecipientIdentifier is

Housley                                                         [Page 3]

INTERNET DRAFT                                             February 2000

   subjectKeyIdentifier, then the version MUST be 2.

   The KeyTransRecipientInfo RecipientIdentifier provides two
   alternatives for specifying the recipient's certificate, and thereby
   the recipient's public key.  The recipient's certificate must contain
   a RSA public key.  The content-encryption key is encrypted with the
   recipient's RSA public key.  The issuerAndSerialNumber alternative
   identifies the recipient's certificate by the issuer's distinguished
   name and the certificate serial number; the subjectKeyIdentifier
   identifies the recipient's certificate by the X.509
   subjectKeyIdentifier extension value.

   The KeyTransRecipientInfo keyEncryptionAlgorithm specifies that the
   RSAES-OAEP algorithm, and its associated parameters, was used to
   encrypt the content-encryption key for the recipient.  The key-
   encryption process is described in [PKCS#1v2.0].  See section 3 of
   this document for the algorithm identifier and the parameter syntax.

   The KeyTransRecipientInfo encryptedKey is the result of encrypting
   the content-encryption key in the recipient's RSA public key using
   the RSAES-OAEP algorithm. When using a Triple-DES content-encryption
   key, implementations MUST adjust the parity bits for each DES key
   comprising the Triple-DES key prior to RSAES-OAEP encryption.

3  RSAES-OAEP Algorithm Identifiers and Parameters

   The RSAES-OAEP key transport algorithm is the RSA encryption scheme
   defined in RFC 2347 [PKCS#1v2.0], where the message to be encrypted
   is the content-encryption key.  The algorithm identifier for RSAES-
   OAEP is:

        iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 7  }

   The AlgorithmIdentifier parameters field must be present, and the
   parameters field must contain RSAES-OAEP-params.  RSAES-OAEP-params
   have the following syntax:

      RSAES-OAEP-params  ::=  SEQUENCE  {
        hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
        maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
        pSourceFunc [2] AlgorithmIdentifier DEFAULT pSpecifiedEmptyIdentifier  }

      sha1Identifier  ::=  AlgorithmIdentifier {
        id-sha1, NULL  }

      mgf1SHA1Identifier  ::=  AlgorithmIdentifier  {
        id-mgf1, sha1Identifier  }

Housley                                                         [Page 4]

INTERNET DRAFT                                             February 2000

      pSpecifiedEmptyIdentifier  ::=  AlgorithmIdentifier  {
        id-pSpecified, OCTET STRING SIZE (0)  }

      id-sha1  OBJECT IDENTIFIER ::=  {
        iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26  }

      id-mgf1  OBJECT IDENTIFIER  ::=  {
        iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 8  }

      id-pSpecified  OBJECT IDENTIFIER  ::=  {
        iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 9  }

   The fields of type RSAES-OAEP-params have the following meanings:

      hashFunc identifies the one-way hash function.  Implementations
      MUST support SHA-1 [SHA1].  The SHA-1 algorithm identifier is
      comprised of the id-sha1 object identifier and a parameter of

      maskGenFunc identifies the mask generation function.
      Implementations MUST support MFG1 [PKCS#1v2.0].  MFG1 requires a
      one-way hash function, and it is identified in the parameter field
      of the algorithm identifier.  Implementations MUST support SHA-1
      [SHA1].  The MFG1 algorithm identifier is comprised of the id-mgf1
      object identifier and a parameter of the SHA-1 algorithm
      identifier.  Again, the SHA-1 algorithm identifier is comprised of
      the id-sha1 object identifier and a parameter of NULL.

      pSourceFunc identifies the source (and possibly the value) of the
      encoding parameters, commonly called P.  Implementations MUST
      represent P by an algorithm identifier, id-pSpecified, indicating
      that P is explicitly provided as an OCTET STRING in the
      parameters.   The default value for P is an empty string.  In this
      case, pHash in EME-OAEP contains the hash of a zero length string.

4  SMIMECapabilities Attribute Conventions

   RFC 2633, Section 2.5.2 defines the SMIMECapabilities signed
   attribute (defined as a SEQUENCE of SMIMECapability SEQUNCEs) to be
   used to specify a partial list of algorithms that the software
   announcing the SMIMECapabilities can support.  When constructing a
   signedData object, compliant software MAY include the
   SMIMECapabilities signed attribute announcing that it supports the
   RSAES-OAEP algorithm.

   The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the
   id-RSAES-OAEP object identifier in the capabilityID field and MUST
   include a RSAES-OAEP-Default-Identifier SEQUENCE in the parameters

Housley                                                         [Page 5]

INTERNET DRAFT                                             February 2000


      RSAES-OAEP-Default-Identifier  ::=  AlgorithmIdentifier  {
        id-RSAES-OAEP,  {
          sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier  }  }

   The SMIMECapability SEQUENCE representing RSAES-OAEP MUST be DER-
   encoded as follows: {{{TBD}}}.


   CMS         Housley, R.  Cryptographic Message Syntax.  RFC 2630.
               June 1999.

   MUSTSHOULD  Bradner, S.   Key Words for Use in RFCs to Indicate
               Requirement Levels.  BCP 14, RFC 2119.  March 1997.

   PKCS#1v1.5  Kaliski, B.  PKCS #1: RSA Encryption, Version 1.5.
               RFC 2313.  March 1998.

   PKCS#1v2.0  Kaliski, B.  PKCS #1: RSA Encryption, Version 2.0.
               RFC 2347.  October 1998.

   PROFILE     Housley, R., W. Ford, W. Polk, and D. Solo.  Internet
               X.509 Public Key Infrastructure: Certificate and CRL
               Profile.  RFC 2459.  January 1999.

   RANDOM      Eastlake, D., S. Crocker, and J. Schiller.  Randomness
               Recommendations for Security.  RFC 1750.  December 1994.

   SHA1        National Institute of Standards and Technology.
               FIPS Pub 180-1: Secure Hash Standard.  17 April 1995.

   X.208-88    CCITT.  Recommendation X.208: Specification of Abstract
               Syntax Notation One (ASN.1).  1988.

   X.209-88    CCITT.  Recommendation X.209: Specification of Basic Encoding
               Rules for Abstract Syntax Notation One (ASN.1).  1988.

   X.509-88    CCITT.  Recommendation X.509: The Directory - Authentication
               Framework.  1988.

Security Considerations

   Implementations must protect the RSA private key and the content-
   encryption key.  Compromise of the RSA private key may result in the
   disclosure of all messages protected with that key.  Compromise of

Housley                                                         [Page 6]

INTERNET DRAFT                                             February 2000

   the content-encryption key may result in disclosure of the associated
   encrypted content.

   Implementations must protect the key management private key and the
   message-authentication key.  Compromise of the key management private
   key permits masquerade of authenticated data.  Compromise of the
   message-authentication key may result in undetectable modification of
   the authenticated content.

   The generation of RSA public/private key pairs relies on a random
   numbers.  The use of inadequate pseudo-random number generators
   (PRNGs) to generate cryptographic keys can result in little or no
   security.  An attacker may find it much easier to reproduce the PRNG
   environment that produced the keys, searching the resulting small set
   of possibilities, rather than brute force searching the whole key
   space.  The generation of quality random numbers is difficult.  RFC
   1750 [RANDOM] offers important guidance in this area.


   This document is the result of contributions from many professionals.
   I appreciate the hard work of all members of the IETF S/MIME Working
   Group.  I wish to extend a special thanks to Burt Kaliski.

Author Address

   Russell Housley
   381 Elden Street
   Suite 1120
   Herndon, VA 20170


Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  In addition, the
   ASN.1 module presented in Appendix A may be used in whole or in part

Housley                                                         [Page 7]

INTERNET DRAFT                                             February 2000

   without inclusion of the copyright notice.  However, this document
   itself may not be modified in any way, such as by removing the
   copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process shall be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.  This
   document and the information contained herein is provided on an "AS

Housley                                                         [Page 8]