PKIX Working Group J.W. Park(KISA)
Internet Draft J.I. Lee(KISA)
Document: draft-ietf-pkix-sim-05.txt H.S. Lee(KISA)
Expires : December, 2005 S.J. Park(BCQRE)
Target category: Standard Track Polk, Tim(NIST)
June, 2005
Internet X.509 Public Key Infrastructure
Subject Identification Method (SIM)
<draft-ietf-pkix-sim-05.txt>
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines Privacy-Enhanced Protected Subject Identifier
(PEPSI) format for including a privacy sensitive identifier in the
subjectAltName extension of a certificate.
The PEPSI is an optional feature that may be used by a relying
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parties to determine whether the subject of a particular certificate
is also the person corresponding to a particular sensitive
identifier.
1. Introduction
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document (in uppercase,
as shown) are to be interpreted as described in [2119].
A Certification Authority (CA) issues X.509 public key certificates
to bind a public key to a subject. The subject is specified through
one or more subject names in the "subject" and/or "subjectAltName"
fields of a certificate. The "subject" field contains a
hierarchically structured distinguished name. The "subjectAltName
field" may contain an electronic mail address, IP address, or other
name forms that correspond to the subject.
For each particular CA, a subject name corresponds to a unique
person, device, group, or role. The CA will not knowingly issue
certificates to multiple entities under the same name. That is, for a
particular certificate issuer, all currently valid certificates
asserting the same subject name(s) are bound to the same entity.
Where the subject is a person, the name that is specified in the
subject field of the certificate typically reflects the legal name of
the individual and affiliated entities (e.g., their corporate
affiliation). In reality, however, there are individuals or
corporations that have the same or similar names. It may be difficult
for a relying party (e.g., a person or application) to associate the
certificate with a specific person. This ambiguity presents a problem
for many applications.
In some cases, applications need to ensure that the subject of
certificates issued by different CAs are in fact the same entity.
This requirement may be met by including a "permanent identifier" in
all certificates issued to the same subject, which is unique across
multiple CAs. By comparing the "permanent identifier", the relying
party may identify certificates from different CAs that are bound to
the same subject. This solution is defined in [RFC4043].
In many cases, such as a Social Security Number, a person or
corporation's identifier is regarded as a sensitive, private or
personal data. Such an identifier cannot simply be included as part
of the subject field, since its disclosure lead to misuse.
Therefore, privacy sensitive identifiers cannot be included in
certificates as plaintext.
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On the other hand, such an identifier is not actually a secret.
People choose to disclose these identifiers for certain classes of
transactions. For example, people may disclose their Social Security
Number to open a bank account or obtain a loan. This is typically
corroborated by presenting physical credentials (e.g., a driver
license) which confirms the person's name or address.
To support such applications in an online environment, relying
parties need to determine whether the subject of a particular
certificate is also the person corresponding to a particular
sensitive identifier. Ideally, applications would leverage the
applicants's electronic credential (e.g., the X.509 public key
certificate) to corroborate this identifier, but the subject field of
a certificate does not provide sufficient information.
To fulfill these demands, this document defines the Privacy-Enhanced
Protected Subject Identifier (PEPSI) format for including a privacy
sensitive identifier in a certificate. When the subject of the
certificate chooses to disclose the identifier and the relying party
may verify the identifier. A party that obtains the certificate
containing a PEPSI can not determine the privacy sensitive identifier
without performing a brute force attack, and each certificate must be
attacked separately. Furthermore, the subject can prove to the
relying party that they are associated with a valid identification
without disclosing the identifier. For example, a person could prove
they possessed a valid social security number without disclosing the
identifier itself.
In addition, this document defines an Encrypted PEPSI(EPEPSI) so that
sensitive identifier information can be exchanged without disclosing
the identifier.
This document is organized as follows:
- Section 2 establishes security and usability requirements;
- Section 3 provides an overview of the mechanism;
- Section 4 defines syntax and generation rules.
2. Requirements
2.1 Security Requirements
Given
- Alice, a certificate holder, with an a sensitive identifier SIIa
(such as her Social Security Number, SSN)
- Bob, a relying party who will rely on knowing Alice's true SIIa
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- Eve an attacker who has Alice's certificate
- An RA to whom Alice must divulge her SIIa; and
- A CA who will issue Alice's certificate.
We wish to design a PEPSI, using a password that Alice chooses, that
has the following properties:
- Alice can prove her true SII, SIIa to Bob;
- Eve has a large work factor to determine the Alice's SIIa from
her certificate, even if Alice chooses a weak password, and a
very large work factor if Alice chooses a good password;
- Even if Eve can determine SIIa, she has an equally hard problem
to find any other SII'a from any other PEPSI; that is, there is
nothing she can pre-compute that helps every attack against a
PEPSI, and nothing she learns from a successful attack
that helps in any other attack;
- The CA does not learn Alice's SIIa except the case where the CA
needs to validate the SII passed by the RA.
- The CA can treat the PEPSI as an additional name form in the
"subjectAltName" extesnion with no special processing; and
- Alice cannot find another SII'a, password combination that will
allow her to use the certificate to prove a false SII to Bob.
2.2 Usability Requirements
In addition to the security properties stated above, we have the
following usability requirements:
- When the PEPSI is used, any custom processing occurs at the
relying party. Alice can use commercial off the shelf software
(e.g., a standard browser) without modifications.
2.3 Solution
The solution: Let SIM = R || PEPSI where
PEPSI = H(H( P || R || SIItype || SII))
Then:
1. Alice picks a password P, and gives P, SIItype, and SII
to the RA.
2. The RA validates SIItype and SII.
3. The RA generates a random value R.
4. The RA generates the SIM = R, PEPSI where PEPSI =
H(H(P || R || SIItype || SII)).
5. The RA sends the PEPSI to Alice by out-of-band and passes
the SIM to the CA.
6. Alice sends a certRequest form to CA which generates
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Alice's certificate including the SIM as a form of
otherName from the GeneralName structure in SubjectAltName
extension.
7. Alice sends Bob her Cert, as well as P, SIItype, and SII.
8. Bob can compute PEPSI' = H(H(P || R || SIItype || SII)) and
compare SIM' = R || PEPSI' to the value in the SIM, thereby
verifying SII.
If Alice wishes to prove she has a valid identifier, without
disclosing it, then steps 7 and 8 are as follows:
7. Alice sends intermediate value H(P || R || SIItype ||
SII)) and her certificate to Bob.
8. Bob can get R form the SIM in the certificate, then
compute H(intermediate value) and compare it to the
value in SIM, thereby verifying Alice's knowledge of
P and SII.
Eve has to exhaustively search the H(P || R || SIItype || SII) space
to find Alice's SII. This is a fairly hard problem even if Alice uses
a poor password, and a really hard problem if Alice uses a fairly
good password.
Even if Eve finds Alice's P, SIItype, SII pairs or constructs a
massive dictionary of P, SIItype, SII pairs, it doesn't help find any
other SII, because a new R is used for each PEPSI.
3. Procedures
3.1 Symbols
The following cryptography symbols are defined in this document.
H() Cryptographically secure hash algorithm.
SHA-1[FIPS 180-1] or more secure hash function is
specification required.
SII Sensitive Identification Information.
(e.g, Social Security Number or Maryland Driver
License)
SIItype Object Identifier that identifies the type of SII.
P A user chosen password.
R The random number value generated by RA.
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PEPSI Privacy-Enhnaced Protected Subject Information.
Calculated from the input value P, R, SIItype, SII
using two iteration of H().
E() The public key encryption algorithm used by RA.
Used for encrypting the PEPSI.
EPEPSI Encrypted PEPSI.
D() The public key decryption algorithm used by CA.
Used for decrypting the EPEPSI.
3.2 SII and SIItype
The user presents evidence that a particular SII has been assigned to
them. The SII is composed of an alphanumeric string. The SIItype is
an OID which defines the format and scope of the SII value. For
example, in Korea, SIItype is as blow.
KISA specific OIDs id-KISA OBJECT IDENTIFIER ::=
{iso(1) member-body(2) korea(410) kisa(200004)}
KISA arcs
id-npki OBJECT IDENTIFIER ::= {id-KISA 10}
id-attribute OBJECT IDENTIFIER ::= {id-npki 1}
id-kisa-identifyData OBJECT IDENTIFIER ::= {id-attribute 1}
id-VID OBJECT IDENTIFIER ::= {id-kisa-identifyData 10}
id-SII OBJECT IDENTIFIER ::= {id-VID 1}
For closed communities, the SIItype value may be assigned by the CA
itself.
3.3 User Chosen Password
The user selects a password as one of the input values for the SIM.
The strength of the password is critical to protection of the user's
SII. User should be encouraged to select passwords that will be
difficult to guess and long enough to protect against bute force
attacks.
Implementations of this specification MUST permit a user to select
passwords of up to 28 characters. RAs SHOULD implement password
selection rules to prevent user selection of trivial passwords. See
[FIPS 112] and [FIPS 180-1] for selecting security criteria for
password and an automated password generator algorithm that randomly
creates simple pronounceable syllables as passwords.
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3.4 Random Number Generation
A The RA generates a random number, R. A new R MUST be generated for
each SIM in a certificate. The length of R MUST be the same as the
length of the output of the hash algorithm H. For example, where H
is SHA-1 the random number MUST be 160 bits.
Whenever a certificate or key needs to be updated, a new R SHOULD be
generated and the SIM SHOULD be recomputed. Replicating the value of
the previous SIM from a previous certificate permits an attacker to
identify certificates associated with the same physical person, which
may be undesirable.
A Random Number Generator(RNG) meets the requirements defined in
[FIPS 140-2] is strongly recommended.
3.5 Generation of PEPSI
The SIM in the subjectAltName extension within the certificate
identifies an entity even if it has multiple names in its
certificates. RAs MUST calculate the unique value of the SIM with
the designated inputs according to the following algorithm:
SIM = R || PEPSI
PEPSI = H(H(P || R || SIItype || SII))
The SII is known to a RA at user enrollment. One of SHA-1 or SHA-256
MUST be used for ensuring the integrity of PEPSI.
Note that the P, SIItype, and SII passing from the end entity to the
RA MUST be protected.
The syntax and the associated OID for SIM are also provided in the
ASN.1 modules in Section 4.1. Also, Section 4.2 describes the syntax
for PEPSI in the ASN.1 modules.
3.6 Encryption of PEPSI
This section describes procedures for encrypting the SIItype, SII and
PEPSI.
It may be required where the CA itself verifies SII before issuing
the certificate. To meet this requirement, RAs SHOULD encrypt the
SIItype, SII, and PEPSI with the CA's public key and send it to CA in
addition to SIM. Therefore, RA MUST acquire the CA's public
encryption certificate although the way of obtaining it is not
covered in this document.
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EPEPSI = E(SIItype || SII || PEPSI)
The syntax and the associated OID for EPEPSI is provided in the ASN.1
modules in Section 4.3.
3.7 Certification Request
As described above, the certificate request message MAY contain the
PEPSI. [RFC2314] and [RFC2511] are widely used message syntaxes for
certification requests.
Basically, PKCS#10 is consists of a distinguished name, a public key
and a optionally a set of attributes, collectively signed by the end
entity. The PEPSI SHOULD be included in the attribute field of
In case of using the RFC2511(CRMF) for certificate request message,
an PEPSI SHOULD be included in the 'regInfo' field defined in Section
3 of [RFC2511].
In case that CA itself verifies SII before issuing the certificate,
the value of PEPSI in certification request form SHOULD be changed to
EPEPSI which the requestor has made by himself or herself. The
requestor MUST acquire the CA's public encryption certificate
although the way of obtaining it is not covered in this document.
3.8 Certification
A CA issue certificate containing the SIM as a form of otherName from
the GeneralName structure in "SubjectAltName" extension.
In an environment in which a CA verifies SII before issuing the
certificate, a CA decrypts the EPEPSI with its own private key
according to the following algorithm. It then validates that the SII
value is correct.
SIItype, SII, PEPSI = D(EPEPSI)
4. Definition
4.1 SIM Naming Syntax
This section specifies the syntax for SIM name form included in
subjectAltName extension. The SIM is composed of the three fields,
the hash algorithm identifier, the authority chosen random value, and
the value of the PEPSI itself.
Suggest an OID in the id-on arc as follows:
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id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) }
id-on-SIM OBJECT IDENTIFIER ::= { id-on 6 }
SIM ::= SEQUENCE {
hashAlg AlgorithmIdentifier,
authorityRandom OCTET STRING, -- RA chosen random number
-- used in computation of
-- pEPSI
pEPSI OCTET STRING -- hash of HashContent
-- with algorithm hashAlg
}
4.2 PEPSI
This section specifies the syntax for PEPSI. The PEPSI is generated
by performing the same hash function twice. The PEPSI is generated
over the ASN.1 structure HashContent. HashContent has four values:
the user selected password, the authority chosen random number, the
identifer type, and the identifier itself.
HashContent ::= SEQUENCE {
userPassword UTF8String,
-- user supplied password
authorityRandom OCTET STRING,
-- RA chosen random number
identifierType OBJECT IDENTIFIER, -- SIItype
identifier UTF8String -- SII
}
Before calculating a PEPSI, conforming implementations MUST process
the userPassword with the six step [LDAPBIS STRPREP] string
preparation algorithm as follows:
* In step 2, Map, the mapping shall include procedssing of
characters commonly mapped to nothing, as specified in Appendix
B.1 of [RFC 3454].
* Omit step 6, Insignificant Character Removal.
4.3 Encrypted PEPSI
This section describes the syntax for Encrypted PEPSI. The Encrypted
PEPSI has three fields: identifierType, identifier, and PEPSI.
EncryptedPEPSI ::= SEQUENCE {
identifierType OBJECT IDENTIFIER, -- SIItype
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identifier UTF8String, -- SII
pEPSI OCTET STRING -- PEPSI
}
When it is used for certificate request, the OID in 'regInfo' of
RFC2511 and PKCS#10 is as follows:
id-regEPEPSI OBJECT IDENTIFIER ::= { id-pkip 3 }
5. Example Usage of SIM
Depending on a different security environments, there are three
possible use cases with SIM.
1. When a relying party doesn't have any information about
the certificate user.
2. When a relying party already have the SII of certificate
user.
3. When a certificate user want to disclose his SII.
For the use case 1, SII and a user chosen password P which only a
user know must be sent to a relying party via secure commnuication.
Together with the password, the certificate including the SIM must be
transmitted. Then the relying party can get R from the certificate.
The relying party can verify that the subject of certificate issued
by a CA is in fact the same entity who sends the password and the
certificate.
If comparing first two use cases, their overall procedure is almost
same except that the SII isn't required by the relying party.
Threfore, a certificate user transmit simply the password, P, and the
certificate. The rest of detailed procedure is the same as that of
the use case 1.
For the last speical use case that a certificate user want to
disclose his SII, the only information sent by a ceritificate user is
the intermediate value of the PEPSI, H(R || P || SIItype || SII).
Upon receiving of it, the relying party can verify the user by
rehashing the value and comparing it with SIM in the certificate.
6. Security Considerations
A cryptographically secure hash function such as SHA-1 and SHA-256
must be used for maintaining the integrity of the PEPSI since the
security of the PEPSI by composing hash value of R, SIItype, and SII
is totally depending on hash function. Specifically, it is
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impossible for an attacker to find user P, authority random number,
and SII which generate the same value of the SIM since SHA-1 requires
2^80 work factor where 160 is the bit-length of the hash value of
SHA-1.
In addition, a fairly good password is needed to protect against the
brute force attacks on H(P || R || SIItype || SII) space. Due to the
short length of the SII, it is likely that an attacker can reasonably
guess it with partial information about gender, age, and date of
birth. Therefore it is important for users to select a fairly good
password in order that an attacker is unable to verify whether the
guessed SII is accurate or not unless he collects all input values
for SIM.
7. IANA Considerations
In the future, IANA may be asked to establish a registry of object
identifiers to promote interoperability in the specification of SII
types.
8. References
8.1 Normative Reference
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2510] Adams, C. and S. Farrell, "Internet X.509 Public Key
Infrastructure, Certificate Management Protocols", RFC
2510, March 1999.
[RFC2511] Myers, M., Adams, C., Solo, D. and D. Kemp, "Internet
X.509 Certificate Request Message Format", RFC 2511,
March 1999.
[RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax
Version 1.5", RFC 2314, March 1998.
[RFC3280] Housley, R., Polk, T, Ford, W. and Solo, D., "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3454] P. Hoffman, M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
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[RFC4043] D. Pinkas, T. Gindin, "Internet X.509 Public Key
Infrastructure Permanent Identifier", RFC4043,
May 2005.
8.2 Informative Reference
[LDAPBIS STRPREP] Zeilenga, K., "LDAP: Internationalized String
Preparation", draft-ietf-ldapbis-strprep-xx.txt, a work
in progress.
[CMC] J. Schaad, M. Myers, X. Liu, J. Weinstein, "Certificate
Management Messages over CMS",
draft-ietf-pkix-cmc-base-03.txt.
[FIPS 112] Fedreal Information Processing Standards Publication
(FIPS PUB) 112, "Password Usage", 30 May 1985.
[FIPS 180-1] Federal Information Processing Standards Publication
(FIPS PUB) 180-1, "Secure Hash Standard", 17 April 1995.
[FIPS 181] Federal Information Processing Standards Publication
(FIPS PUB) 181, "Automated Password Generator(APG)",
5 October 1993.
[FIPS 140-2] Federal Information Processing Standards Publication
(FIPS PUB) 140-2, "Security Requirements for
Cryptographic Modules", 25 May 2001.
9. Acknowledgments
Jim Schaad(Soaring Hawk Consulting), Seungjoo Kim, Jaeho Yoon, Baehyo
Park(KISA), Bill Burr, Morrie Dworkin(NIST), and ISTF(Internet
Security Technical Forum, http://www.istf.org) have contributed
significantly to work on the PEPSI concept and identified a potential
security attack. Also their comments on the set of desirable
properties for the PEPSI and enhancements to the PEPSI were most
illumination. Also, thanks for Russell Housely, Stephen Kent for
their contribution for this document.
10. Authors' Address
Jongwook Park
Korea Information Security Agency
78, Garak-Dong, Songpa-Gu, Seoul, 138-803
REPUBLIC OF KOREA
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Phone: 2-405-5432
Email: khopri@kisa.or.kr
Jaeil Lee
Korea Information Security Agency
Phone: 2-405-5300
Email: jilee@kisa.or.kr
Hongsub, Lee
Korea Information Security Agency
78, Garak-Dong, Songpa-Gu, Seoul, 138-803
REPUBLIC OF KOREA
Phone: 2-405-5100
EMail: hslee@kisa.or.kr
Sungjun Park
BCQRE Co.,Ltd
Yuil Bldg. Dogok-dong 411-14, Kangnam-ku, Seoul, 135-270
REPUBLIC OF KOREA
Email: sjpark@bcqre.com
Tim Polk
National Institute of Standards and Technology
100 Bureau Drive, MS 8930
Gaithersburg, MD 20899
Email: tim.polk@nist.gov
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Appendix A. "Compilable" ASN.1 Module, 1988 Syntax
PKIXSIM {iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-sim2005(38) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL
IMPORTS
AlgorithmIdentifier, AttributeTypeAndValue FROM PKIX1Explicit88
{iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-pkix1-explicit(18)}
-- SIM
-- SIM certificate OID
id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) }
id-on-SIM OBJECT IDENTIFIER ::= { id-on 6 }
-- Certificate Syntax
SIM ::= SEQUENCE {
hashAlg AlgorithmIdentifier,
authorityRandom OCTET STRING, -- RA chosen radom number
-- used in computation of
-- pEPSI
pEPSI OCTET STRING -- hash of HashContent
-- with algorithm hashAlg
}
-- PEPSI
UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
-- The content of this type conforms to RFC 2279
HashContent ::= SEQUENCE {
userPassword UTF8String,
-- user supplied password
authorityRandom OCTET STRING,
-- RA chosen random number
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identifierType OBJECT IDENTIFIER, -- SIItype
identifier UTF8String -- SII
}
-- Encrypted PEPSI
-- OID for encapsulated content type
id-regEPEPSI OBJECT IDENTIFIER ::= { id-pkip 3 }
EncryptedPEPSI ::= SEQUENCE {
identifierType OBJECT IDENTIFIER, -- SIItype
identifier UTF8String, -- SII
pEPSI OCTET STRING -- PEPSI
}
END
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