L. Martin
S/MIME Working Group Voltage Security
Internet Draft Mark Schertler
Expires: February 2008 Tumbleweed Communications
August 2007
Using the Boneh-Franklin and Boneh-Boyen identity-based encryption
algorithms with the Cryptographic Message Syntax (CMS)
<draft-ietf-smime-bfibecms-05.txt>
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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...................................................9
7. Security considerations.......................................10
7.1. Attacks that are outside the scope of this document......10
7.2. Attacks that are within the scope of this document.......11
7.3. Attacks to which the protocols defined in this document are
susceptible...................................................11
8. IANA considerations...........................................12
9. References....................................................13
9.1. Normative references.....................................13
9.2. Informative references...................................13
Authors' Addresses...............................................14
Intellectual property statement..................................14
Disclaimer of validity...........................................15
Copyright statement..............................................15
Acknowledgment...................................................15
1. Introduction
This document defines the way to use the Boneh-Franklin [BF] and
Boneh-Boyen [BB1] 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.
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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].
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 secure distribution
of IBE public parameters and policy information for the IBE
PKG.
The interaction of senders and receivers of IBE-encrypted messages
are described in [IBE].
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
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ibeORIType OBJECT IDENTIFIER ::= { joint-iso-itu(2) country(16)
us(840) organization(1) identicrypt(114334) ibcs(1)
cms(4) ori-oid(1) }
ibeORIValue defines the identity that was used in the IBE algorithm
to encrypt the CEK. This is an IBERecipientInfo type.
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 have the following meanings:
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. How to retrieve an IBE
private key using the steps defined in [IBE] is defined by the
keyFetchMethod OID. The method for retrieving private keys that is
specified in [IBE] is defined by cmsPPSOID.
cmsPPSOID OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
us(840) organization(1) identicrypt(114334) pps-schemas(3)
ic-schemas(1) pps-uri(1)
}
The recipientIdentity is the data that was used to calculate the IBE
public key that was used to encrypt the content-encryption key. This
MUST be an IBEIdentityInfo type. This recipientIdentity is used to
calculate IBE public and private keys as described in [IBCS].
IBEIdentityInfo ::= SEQUENCE {
district IA5String,
serial INTEGER,
identitySchema OBJECT IDENTIFIER,
identityData OCTET STRING
}
The fields of IBEIdentityInfo have the following meanings.
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The district and serial are unique identifiers that are used to
construct the URI for the location of the necessary IBE public
parameters. The construction and use of this URI is defined in [IBE].
The identitySchema 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
EmailIdentitySchema type.
cmsIdentityOID OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
us(840) organization(1) identicrypt(114334) keyschemas(2)
icschemas(1) rfc822email(1)
}
The identityData field contains the identify information for the
recipient. If the contents of the field is an ASN.1 structure, the
structure MUST be DER encoded [DER] before placing it in the OCTET
STRING.
EmailIdentitySchema ::= SEQUENCE {
rfc822Email IA5String,
time GeneralizedTime
}
The rfc822Email is the e-mail address of the recipient in the format
defined by [RFC822]. E-mail addresses that contain non-ASCII
characters MUST be encoded using punycode [RFC3492].
The value of "time" 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. UTC time values are
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 following steps. Let "time-interval" be the number
of seconds in this larger time interval.
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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.
1. Say 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 reduced-time 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 1 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 series of OID-value pairs that 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.
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].
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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) }
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].
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6. ASN.1 module
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
IBEOtherRecipientInfo ::= SEQUENCE {
oriType OBJECT IDENTIFIER,
oriValue IBERecipientInfo
}
ibeORIType OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
us(840) organization(1) identicrypt(114334) ibcs(1)
cms(4) ori-oid(1)
}
IBERecipientInfo ::= SEQUENCE {
cmsVersion INTEGER { v3(3) },
keyFetchMethod OBJECT IDENTIFIER,
recipientIdentity IBEIdentityInfo,
serverInfo SEQUENCE SIZE (1..MAX) OF
OIDValuePairs OPTIONAL,
encryptedKey EncryptedKey
}
IBEIdentityInfo ::= SEQUENCE {
district IA5String,
serial INTEGER,
identitySchema OBJECT IDENTIFIER,
identityData OCTET STRING
}
OIDValuePairs ::= SEQUENCE {
fieldID OBJECT IDENTIFIER,
fieldData OCTET STRING
}
EncryptedKey ::= OCTET STRING
EmailIdentitySchema ::= SEQUENCE {
rfc822Email IA5String,
time GeneralizedTime
}
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cmsIdentityOID OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
us(840) organization(1) identicrypt(114334) keyschemas(2)
icschemas(1) rfc822email(1)
}
cmsPPSOID OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
us(840) organization(1) identicrypt(114334) pps-schemas(3)
ic-schemas(1) pps-uri(1)
}
END
7. Security considerations
This document is based on [CMS] and [IBCS], and the relevant security
considerations of those documents apply.
7.1. Attacks that are 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, 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 PPS and PKG
is infeasible for attackers to obtain.
Attacks that require administrative 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
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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 that are 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
o masquerade as a PPS or PKG
o perform a 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
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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 [RFC2827, RFC3882] 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.
8. IANA considerations
All of the object identifiers used in this document were assigned by
the National Institute of Standards and Technology (NIST), so no
further action by the IANA is necessary for this document.
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9. References
9.1. Normative references
[ASN1] CCITT, "Recommendation X.209: Specification of Basic Encoding
Rules for Abstract Syntax Notation One (ASN.1)," 1998.
[CMS] R. Housley, "Cryptographic Message Syntax," RFC 3369, August
2002.
[DER] ITU-T Recommendation X.680: Information Technology - Abstract
Syntax Notation One, 1997.
[IBCS] X. Boyen, L. Martin, "Identity-based Cryptography Standard
(IBCS) #1: Supersingular Curve Implementations of the BF
and BB1 Cryptosystems," draft-ieft-martin-ibcs-03.txt.
[IBE] G. Appenzeller, L. Martin and M. Schertler, "Identity-based
Encryption Architecture," draft-ietf-ibearch-01.txt.
[KEYWORDS] S. Brander, "Key Words for Use in RFCs to Indicate
Requirement Levels," BCP 14, RFC 2119, March 1997.
[RFC822] D. Crocker, "Standard for the format of ARPA internet text
messages," RFC 822, August 1982.
[RFC2827] P. Ferguson and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing," RFC 2827, BCP 38, May 2000.
[RFC3492] A. Costello, "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications (IDNA),"
RFC 3492, March 2003.
[RFC3882] D. Turk, "Configuring BGP to Block Denial-of-Service
Attacks," RFC 3882, September 2004.
[TLS] T. Dierks and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.1," RFC 4346, April 2006.
9.2. Informative references
[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
Phone: +1 650 543 1280
Email: martin@voltage.com
Mark Schertler
Tumbleweed Communications
700 Saginaw Dr
Redwood City CA 94063
Phone: +1 650 216 2039
Email: mark.schertler@tumbleweed.com
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Disclaimer of validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Copyright statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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