Using the Boneh-Franklin and Boneh-Boyen Identity-Based Encryption Algorithms with the Cryptographic Message Syntax (CMS)
RFC 5409

Network Working Group                                          L. Martin
Request for Comments: 5409                              Voltage Security
Category: Informational                                     M. Schertler
                                                                   Axway
                                                            January 2009

  Using the Boneh-Franklin and Boneh-Boyen Identity-Based Encryption
        Algorithms with the Cryptographic Message Syntax (CMS)

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.

Copyright Notice

   Copyright (c) 2009 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
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Abstract

   This document describes the conventions for using the Boneh-Franklin
   (BF) and Boneh-Boyen (BB1) identity-based encryption algorithms in
   the Cryptographic Message Syntax (CMS) to encrypt content-encryption
   keys.  Object identifiers and the convention for encoding a
   recipient's identity are also defined.

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Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
      1.2. IBE Overview ...............................................3
   2. Using Identity-Based Encryption .................................3
   3. Key Encryption Algorithm Identifiers ............................6
   4. Processing by the Sender ........................................7
   5. Processing by the Receiver ......................................7
   6. ASN.1 Module ....................................................8
   7. Security Considerations .........................................9
      7.1. Attacks outside the Scope of This Document .................9
      7.2. Attacks within the Scope of This Document .................10
      7.3. Attacks to Which the Protocols Defined in This Document
           Are Susceptible............................................11
   8. References .....................................................12
      8.1. Normative References ......................................12
      8.2. Informative References ....................................12

1.  Introduction

   This document defines the way to use the Boneh-Franklin [IBCS] and
   Boneh-Boyen [IBCS] identity-based encryption (IBE) public-key
   algorithms in the Cryptographic Message Syntax (CMS) [CMS].  IBE is a
   public-key technology for encrypting content-encryption keys (CEKs)
   that can be implemented within the framework of the CMS: the
   recipient's identity is incorporated into the EnvelopedData CMS
   content type using the OtherRecipientInfo CHOICE in the RecipientInfo
   field as defined in Section 6.2.5 of [CMS].  This document does not
   describe the implementation of the BF and BB1 algorithms, which are
   described in detail in [IBCS].

   IBE algorithms are a type of public-key cryptographic algorithm in
   which the public key is calculated directly from a user's identity
   instead of being generated randomly.  This requires a different set
   of steps for encryption and decryption than would be used with other
   public-key algorithms, and these steps are defined in Sections 4 and
   5 of this document, respectively.

   This document also defines the object identifiers and syntax of the
   object that is used to define the identity of a message recipient.

   CMS values and identity objects are defined using ASN.1 [ASN1].

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1.1.  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 RFC 2119 [KEYWORDS].

1.2.  IBE Overview

   In addition to the client components that are described in this
   document, the following additional components are required for a
   complete IBE messaging system.

      o  A Private-Key Generator (PKG).  The PKG contains the
         cryptographic material, known as a master secret, for
         generating an individual's IBE private key.  A PKG accepts an
         IBE user's private key request and, after successfully
         authenticating them in some way, returns their IBE private key.

      o  A Public Parameter Server (PPS).  IBE system parameters include
         publicly sharable cryptographic material, known as IBE public
         parameters, and policy information for the PKG.  A PPS provides
         a well-known location for distribution of IBE public parameters
         and policy information for the IBE PKG.

   The interactions of senders and receivers of IBE-encrypted messages
   are described in [IBE].  All communications between users of an IBE
   system and the PPS or PKG MUST be protected using TLS [TLS] as
   described in [IBE].  This provides confidentiality and integrity of
   all information that is delivered to users as well as authentication
   of the PPS and PKG.

2.  Using Identity-Based Encryption

   To use IBE, the ori field in RecipientInfo MUST be used.  The fields
   are set as follows: oriType is set to ibeORIType; oriValue is set to
   ibeORIValue.

   These fields have the following meanings:

   ibeORIType defines the object identifier (OID) that indicates that
   the subsequent ibeORIValue is the information necessary to decrypt
   the message using IBE.  This field MUST be set to the following:

   ibeORIType OBJECT IDENTIFIER ::= {
     joint-iso-itu(2) country(16) us(840)
     organization(1) identicrypt(114334)
     ibcs(1) cms(4) ori-oid(1) version(1)
   }

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   ibeORIValue defines the identity that was used in the IBE algorithm
   to encrypt the CEK.  This is an IBERecipientInfo type, which is
   defined as follows:

   IBERecipientInfo ::= SEQUENCE {
     cmsVersion         INTEGER { v3(3) },
     keyFetchMethod     OBJECT IDENTIFIER,
     recipientIdentity  IBEIdentityInfo,
     serverInfo         SEQUENCE SIZE (1..MAX) OF
       OIDValuePairs OPTIONAL,
     encryptedKey       EncryptedKey
   }

   The fields of IBERecipientInfo MUST be set as follows.

   The cmsVersion MUST be set to 3.

   The keyFetchMethod is the OID that defines the method of retrieving
   the private key that the recipient MUST use.  This SHOULD be set to
   uriPPSOID [IBE], which is defined to be the following:

   uriPPSOID OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     pps-schemas(3) ic-schemas(1) pps-uri(1) version(1)
   }

   The recipientIdentity is the data that the sender used to calculate
   the IBE public key that the sender used to encrypt the content-
   encryption key.  This recipientIdentity is used to calculate IBE
   public and private keys as described in [IBCS].  This MUST be a DER-
   encoded [DER] IBEIdentityInfo type [IBE], which is defined as
   follows:

   IBEIdentityInfo ::= SEQUENCE {
     district        IA5String,
     serial          INTEGER,
     identityType    OBJECT IDENTIFIER,
     identityData    OCTET STRING
   }

   The identityType defines the format that is used to encode the
   information that defines the identity of the recipient.  This MUST be
   set to cmsIdentityOID to indicate that identityData contains an
   EmailIdentityData type.  The value of cmsIdentityOID is the
   following:

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   cmsIdentityOID OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     keyschemas(2) icschemas(1) email(1) version(1)
   }

   The identityData MUST be an EmailIdentityData type, which is defined
   as follows:

   EmailIdentityData ::= SEQUENCE {
     rfc822Name   IA5String,
     time         GeneralizedTime
   }

   The rfc822Name field is the email address of the recipient in the
   format defined in Section 4.2.1.6 of [PKIX] for the rfc822Name
   subjectAltName variant.  Rules for encoding Internet mail addresses
   that include internationalized domain names are specified in Section
   7.5 of [PKIX].

   The value of the time field is the UTC time after which the sender
   wants to let the recipient decrypt the message, so it may be called
   the "not-before" time.  This is usually set to the time when the
   message is encrypted, but MAY be set to a future time.  The value of
   "time" MUST be expressed in Greenwich Mean Time (Zulu), MUST include
   seconds (i.e., times are always YYYYMMDDHHMMSSZ), even where the
   number of seconds is equal to zero, and MUST be expressed to the
   nearest second.

   The sender of an IBE-encrypted message may want to express this time
   rounded to a time interval to create a key lifetime.  A key lifetime
   reduces the number of IBE private keys that a recipient needs to
   retrieve, but still forces the IBE user to periodically re-
   authenticate.  Based on the time interval chosen a recipient would
   only have to retrieve a new IBE key once during the interval.  To do
   this, follow the steps below.  Let "time-interval" be the number of
   seconds in this larger time interval.

      1. Find the GeneralizedTime for the not-before value.
      2. Convert this GeneralizedTime into the number of seconds since
         January 1, 1970.  Call this "total-time".
      3. Calculate reduced-time =
         (floor (total-time / time-interval)) * time-interval.
      4. Convert reduced-time to a GeneralizedTime to get the not-before
         "time" value.

   An example of this algorithm for computing a one-week time interval
   is as follows.

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      1. Suppose that the GeneralizedTime is 20020401000000Z.
      2. Then the total-time is 1017612000.
      3. A time-interval of 1 week is 604800 seconds.
         So the reduced-time = (floor (1017612000 / 604800)) * 604800 =
         1017273600.
      4. This gives the GeneralizedTime form of the reduced-time of
         20020328000000Z.

   When issuing IBE private keys, a PKG SHOULD NOT issue them too far
   into the future.  This restriction is to prevent an adversary who
   obtains an IBE user's authentication credentials from requesting
   private keys far into the future and therefore negating the periodic
   IBE user re-authentication that key lifetime provides.  For example,
   if a one-week period is chosen for the key lifetime, then IBE private
   keys should not be issued more than one week in advance.  Otherwise,
   once an adversary gains access to the PKG via the stolen IBE user
   credentials, they can request all future keys and negate the IBE user
   authentication restraints in place.

   The serverInfo is an optional sequence of OID-value pairs that are
   defined to be the following:

   OIDValuePairs ::= SEQUENCE {
     fieldID     OBJECT IDENTIFIER,
     fieldData   OCTET STRING
   }

   These can be used to convey any other information that might be used
   by a PKG.  Examples of such information could include the user
   interface that the recipient will experience.  Differences in the
   user interface could include localization information or commercial
   branding information.  A client MUST ignore any part of serverInfo
   that it is unable to process.

   The encryptedKey is the result of encrypting the CEK with an IBE
   algorithm using recipientIdentity as the IBE public key.

3.  Key Encryption Algorithm Identifiers

   The BF and BB1 algorithms as defined in [IBCS] have the following
   object identifiers.  These object identifiers are also defined in the
   ASN.1 module in [IBCS].

   bf OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     ibcs(1) ibcs1(1) ibe-algorithms(2) bf(1)
   }

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   This is the object identifier that MUST be inserted in the
   keyEncryptionAlgorithm field in the CMS when the BF algorithm is used
   to encrypt the CEK.

   bb1 OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     ibcs(1) ibcs1(1) ibe-algorithms(2) bb1(2)
   }

   This is the object identifier that MUST be inserted in the
   keyEncryptionAlgorithm field in the CMS when the BB1
   algorithm is used to encrypt the CEK.

4.  Processing by the Sender

   The sender of a message that uses IBE to encrypt content-
   encryption keys performs the following steps:

      1. Selects a set of IBE public parameters to use in the
         subsequent steps in accordance with his local security
         policy.  He then determines the URI where the public
         parameters can be obtained using the process described in
         [IBE].  This information MUST be encoded in the
         IBEIdentityInfo as described in Section 2.

      2. Sets the fields of an OtherRecipientInfo object to
         their appropriate values as described in Section 2.

      3. Calculates an IBE public key as defined in [IBCS]
         using this IBEIdentityInfo as the identity information.

      4. This IBE public key is then used to encrypt the
         content-encryption key (CEK), using the algorithms that are
         defined in [IBCS].

      5. Sets encryptedKey to the IBE-encrypted CEK.

      6. Within the CMS, keyEncryptionAlgorithm MUST then be
         set to the appropriate OID for the IBE algorithm that was
         used (see Section 3).

5.  Processing by the Receiver

   Upon receiving a message that has a CEK encrypted with IBE,
   the recipient performs the following steps to decrypt the
   CEK:

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      1. Determines that the CEK is IBE-encrypted by noting that
         the oriType of the OtherRecipientInfo type is set to
         ibeORIType.

      2. Determines that the recipientIdentity was used as the
         identity in IBE encryption of the CEK.

      3. Determines the location of the IBE public parameters
         and the IBE Private Key Generator as described in
         [IBE].

      4. Obtains the IBE public parameters from the location
         determined in Step 3 using the process defined in
         [IBE].

      5. Obtains the IBE private key needed to decrypt the
         encrypted CEK using the process defined in [IBE].

      6. Decrypts the CEK using the IBE private key obtained in
         Step 4 using the algorithms described in [IBCS].

6.  ASN.1 Module

   The following ASN.1 module summarizes the ASN.1 definitions
   defined by this document.

   IBECMS-module {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     ibcs(1) cms(4) module(5) version(1)
   }

   DEFINITIONS IMPLICIT TAGS ::= BEGIN

   IMPORTS IBEIdentityInfo, uriPPSOID FROM

      IBEARCH-module { joint-iso-itu-t(2) country(16)
        us(840) organization(1) identicrypt(114334) ibcs(1)
        ibearch(5) module(5) version(1)
      };

   IBEOtherRecipientInfo ::= SEQUENCE {
     oriType   OBJECT IDENTIFIER,
     oriValue  IBERecipientInfo
   }

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   ibeORIType OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     ibcs(1) cms(4) ori-oid(1) version(1)
   }

   IBERecipientInfo ::= SEQUENCE {
     cmsVersion         INTEGER { v3(3) },
     keyFetchMethod     OBJECT IDENTIFIER,
     recipientIdentity  IBEIdentityInfo,
     serverInfo         SEQUENCE SIZE (1..MAX) OF
       OIDValuePairs OPTIONAL,
     encryptedKey       EncryptedKey
   }

   OIDValuePairs ::= SEQUENCE {
     fieldID     OBJECT IDENTIFIER,
     fieldData   OCTET STRING
   }

   EncryptedKey ::= OCTET STRING

   EmailIdentityData ::= SEQUENCE {
     rfc822Name   IA5String,
     time         GeneralizedTime
   }

   cmsIdentityOID OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     keyschemas(2) icschemas(1) email(1) version(1)
   }

   END

7.  Security Considerations

   This document is based on [CMS], [IBCS], and [IBE], and the relevant
   security considerations of those documents apply.

7.1.  Attacks outside the Scope of This Document

   Attacks on the cryptographic algorithms that are used to implement
   IBE are outside the scope of this document.  Such attacks are
   detailed in [IBCS], which defines parameters that give 80-bit,

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   112-bit, 128-bit, and 256-bit encryption strength.  We assume that
   capable administrators of an IBE system will select parameters that
   provide a sufficient resistance to cryptanalytic attacks by
   adversaries.

   Attacks that give an adversary the ability to access or change the
   information on a PPS or PKG, especially the cryptographic material
   (referred to in this document as the master secret), will defeat the
   security of an IBE system.  In particular, if the cryptographic
   material is compromised, the adversary will have the ability to
   recreate any user's private key and therefore decrypt all messages
   protected with the corresponding public key.  To address this
   concern, it is highly RECOMMENDED that best practices for physical
   and operational security for PPS and PKG servers be followed and that
   these servers be configured (sometimes known as hardened) in
   accordance with best current practices [NIST].  An IBE system SHOULD
   be operated in an environment where illicit access to the PKG or the
   ability to modify the information distributed by the PPS is
   infeasible for attackers to obtain.

   Attacks that require administrative access or IBE-user-equivalent
   access to machines used by either the client or the server components
   defined in this document are also outside the scope of this document.

   We also assume that all administrators of a system implementing the
   protocols that are defined in this document are trustworthy and will
   not abuse their authority to bypass the security provided by an IBE
   system.  This is of particular importance with an IBE system, for an
   administrator of a PKG could potentially abuse his authority and
   configure the PKG to grant him any IBE private key that the PKG is
   capable of calculating.  To minimize the possibility of
   administrators doing this, a system implementing IBE SHOULD implement
   n-out-of-m control for critical administrative functions and SHOULD
   maintain auditable logs of all security-critical events that occur in
   an operating IBE system.

   Similarly, we assume that users of an IBE system will behave
   responsibly, not sharing their authentication credentials with
   others.  Thus, attacks that require such assumptions are outside the
   scope of this document.

7.2.  Attacks within the Scope of This Document

   Attacks within the scope of this document are those that allow an
   adversary to:

      o  passively monitor information transmitted between users of an
         IBE system and the PPS and PKG

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      o  masquerade as a PPS or PKG

      o  perform a denial-of-service (DoS) attack on a PPS or PKG

      o  easily guess an IBE user's authentication credential

7.3.  Attacks to Which the Protocols Defined in This Document
      Are Susceptible

   All communications between users of an IBE system and the PPS or PKG
   are protected using TLS [TLS].  The IBE system defined in this
   document provides no additional security for the communications
   between IBE users and the PPS or PKG.  Therefore, the described IBE
   system is completely dependent on the TLS security mechanisms for
   authentication of the PKG or PPS server and for confidentiality and
   integrity of the communications.  Should there be a compromise of the
   TLS security mechanisms, the integrity of all communications between
   an IBE user and the PPS or PKG will be suspect.

   The protocols defined in this document do not explicitly defend
   against an attacker masquerading as a legitimate IBE PPS or PKG.  The
   protocols rely on the server authentication mechanism of TLS [TLS].
   In addition to the TLS server authentication mechanism, IBE client
   software can provide protection against this possibility by providing
   user interface capabilities that allows users to visually determine
   that a connection to PPS and PKG servers is legitimate.  This
   additional capability can help ensure that users cannot easily be
   tricked into providing valid authorization credentials to an
   attacker.

   The protocols defined in this document are also vulnerable to attacks
   against an IBE PPS or PKG.  Denial-of-service attacks against either
   component can result in users unable to encrypt or decrypt using IBE,
   and users of an IBE system SHOULD take the appropriate
   countermeasures [DOS, BGPDOS] that their use of IBE requires.

   The IBE user authentication method used by an IBE PKG SHOULD be of
   sufficient strength to prevent attackers from easily guessing the IBE
   user's authentication credentials through trial and error.

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8.  References

8.1.  Normative References

   [ASN1]     ITU-T Recommendation X.680: Information Technology -
              "Abstract Syntax Notation One (ASN.1): Specification of
              Basic Notation", July 2002.

   [CMS]      Housley, R., "Cryptographic Message Syntax (CMS)", RFC
              3852, July 2004.

   [DER]      ITU-T Recommendation X.690: OSI Networking and System
              Aspects: Abstract Syntax Notation One (ASN.1), July 2002.

   [DOS]      Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [IBCS]     Boyen, X. and L. Martin, "Identity-Based Cryptography
              Standard (IBCS) #1: Supersingular Curve Implementations of
              the BF and BB1 Cryptosystems", RFC 5091, December 2007.

   [IBE]      Appenzeller, G., Martin, L., and M. Schertler, "Identity-
              Based Encryption Architecture and Supporting Data
              Structures", RFC 5408, January 2009.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [PKIX]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

8.2.  Informative References

   [BGPDOS]   Turk, D., "Configuring BGP to Block Denial-of-Service
              Attacks", RFC 3882, September 2004.

   [NIST]     M. Souppaya, J. Wack and K. Kent, "Security Configuration
              Checklist Program for IT Products - Guidance for Checklist
              Users and Developers", NIST Special Publication SP 800-70,
              May 2005.

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Authors' Addresses

   Luther Martin
   Voltage Security
   1070 Arastradero Rd, Suite 100
   Palo Alto, CA 94304
   USA

   Phone: +1 650 543 1280
   EMail: martin@voltage.com

   Mark Schertler
   Axway
   1600 Seaport Blvd, Suite 400
   Redwood City, CA 94063
   USA

   Phone: +1 650 216 2039
   EMail: mschertler@us.axway.com

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