Internet Engineering Task Force (IETF) M. Pritikin
Request for Comments: 7894 Cisco Systems, Inc.
Category: Standards Track C. Wallace
ISSN: 2070-1721 Red Hound Software, Inc.
June 2016
Alternative Challenge Password Attributes
for Enrollment over Secure Transport
Abstract
This document defines a set of new Certificate Signing Request
attributes for use with the Enrollment over Secure Transport (EST)
protocol. These attributes provide disambiguation of the existing
overloaded uses for the challengePassword attribute defined in "PKCS
#9: Selected Object Classes and Attribute Types Version 2.0" (RFC
2985). Uses include the original certificate revocation password,
common authentication password uses, and EST-defined linking of
transport security identity.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7894.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Alternative Challenge Password Attributes . . . . . . . . . . 3
3.1. OTP Challenge Attribute . . . . . . . . . . . . . . . . . 4
3.2. Revocation Challenge Attribute . . . . . . . . . . . . . 4
3.3. EST Identity Linking Attribute . . . . . . . . . . . . . 5
4. Indicating Support for the Alternative Challenge Attributes . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
"PKCS #9: Selected Object Classes and Attribute Types Version 2.0"
[RFC2985] defined a challengePassword attribute that has been
overloaded by modern protocol usage with the appropriate
interpretation being provided by context rather than OID definition.
PKCS #9 defines the challengePassword attribute as "a password by
which an entity may request certificate revocation". The parsing and
embedding of this attribute within Certificate Signing Requests is
well supported by common PKI toolsets, but many workflows leverage
this supported field as a one-time password for authentication. For
example, this is codified in many Simple Certificate Enrollment
Protocol (SCEP) implementations as indicated by [SCEP]. Continuing
this trend, Enrollment over Secure Transport (EST) [RFC7030] defines
an additional semantic for the challengePassword attribute in
Section 3.5, in order to provide a linking of the Certificate Signing
Request (CSR) to the secure transport.
Where the context of the protocol operation fully defined the proper
semantic, and when only one use was required at a time, the
overloading of this field did not cause difficulties. Implementation
experience with EST has shown this to be a limitation though. There
are plausible use cases where it is valuable to use either of the
existing methods separately or in concert. For example, an EST
server might require the client to authenticate itself using the
existing client X.509 certificate as well as the user's username and
password, and to include a one-time password within the CSR, all
while maintaining identity linking to bind the CSR to the secure
transport. The overloading of a single attribute type should not be
the limiting factor for administrators attempting to meet their
security requirements.
This document defines the otpChallenge attribute for use when a one-
time password (OTP) value within the CSR is a requirement. The
revocationChallenge attribute is defined to allow disambiguated usage
of the original challenge password attribute semantics for
certificate revocation. The estIdentityLinking attribute is defined
to reference existing EST challenge password semantics with no
potential for confusion with legacy challenge password practices.
The attributes defined in this specification supplement existing EST
mechanisms and are not intended to displace current usage of any
existing EST authentication mechanisms. Conveying the authentication
value itself as an attribute may be preferable to using an HTTP or
Transport Layer Security (TLS) password or other TLS authentication
mechanism in environments where the certificate request processing
component is removed from the HTTP/TLS termination point, for
example, when a web application firewall is used.
2. 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 [RFC2119].
3. Alternative Challenge Password Attributes
The following sections describe three alternative challenge password
attributes for use with EST [RFC7030]. Appendix A provides an ASN.1
module containing the new definitions.
Each attribute described below is defined as a DirectoryString with a
maximum length of 255, which features several possible encoding
options. Attribute values generated in accordance this document
SHOULD use the PrintableString encoding whenever possible. If
internationalization issues make this impossible, the UTF8String
alternative SHOULD be used. Attribute processing systems MUST be
able to recognize and process the PrintableString and UTF8String
string types in DirectoryString values. Support for other string
types is OPTIONAL.
3.1. OTP Challenge Attribute
The otpChallenge attribute is defined as a DirectoryString with a
maximum length of 255. This is consistent with the challengePassword
attribute as originally defined in PKCS #9 [RFC2985]. The
otpChallenge attribute is identified by the id-aa-otpChallenge object
identifier. This facilitates reuse of the existing challengePassword
code by associating the new object identifiers with the existing
parsing and generation code. This attribute provides a means of
conveying a one-time password value as part of a CSR request.
Generation, verification, storage, etc., of the value is not
addressed by this specification. [RFC4226] and [RFC6238] define one-
time password mechanisms that MAY be used with this attribute.
ub-aa-otpChallenge INTEGER ::= 255
id-aa-otpChallenge OBJECT IDENTIFIER ::= {
id-smime 56
}
otpChallenge ATTRIBUTE ::= {
WITH SYNTAX DirectoryString {ub-aa-otpChallenge}
EQUALITY MATCHING RULE caseExactMatch
SINGLE VALUE TRUE
ID id-aa-otpChallenge
}
3.2. Revocation Challenge Attribute
The original PKCS #9 challengePassword field has been overloaded, and
the common use is unclear. The revocationChallenge attribute defined
here provides an unambiguous method of indicating the original PKCS
#9 intent for this attribute type. The revocationChallenge attribute
is identified by the id-aa-revocationChallenge object identifier.
[RFC2985] discusses the original semantics for the PKCS #9 challenge
password attribute.
ub-aa-revocationChallenge INTEGER ::= 255
id-aa-revocationChallenge OBJECT IDENTIFIER ::= {
id-smime 57
}
revocationChallenge ATTRIBUTE ::= {
WITH SYNTAX DirectoryString {ub-aa-revocationChallenge}
EQUALITY MATCHING RULE caseExactMatch
SINGLE VALUE TRUE
ID id-aa-revocationChallenge
}
3.3. EST Identity Linking Attribute
EST defines a mechanism for associating identity information from an
authenticated TLS session with proof-of-possession information in a
certificate request. The mechanism was labeled using the pkcs-9-at-
challengePassword identifier from [RFC2985]. To avoid any confusion
with the semantics described in [RFC2985] or any other specifications
that similarly defined use of the PKCS #9 challenge password
attribute for their own purposes, a new object identifier is defined
here and associated with the semantics described in Section 3.5 of
[RFC7030].
ub-aa-est-identity-linking INTEGER ::= 255
id-aa-estIdentityLinking OBJECT IDENTIFIER ::= {
id-smime 58
}
estIdentityLinking ATTRIBUTE ::= {
WITH SYNTAX DirectoryString {ub-aa-est-identity-linking}
EQUALITY MATCHING RULE caseExactMatch
SINGLE VALUE TRUE
ID id-aa-estIdentityLinking
}
4. Indicating Support for the Alternative Challenge Attributes
The EST server MUST indicate these attributes, as the particular use
case requires, in every CSR Attributes Response. An EST server MAY
send both the estIdentityLinking attribute and the challengePassword
attribute [RFC7030] in a CSR Attributes Response to ensure support
for legacy [RFC7030] clients.
The client MUST include every indicated attribute for which it has
values in the subsequent CSR. If a client sees an estIdentityLinking
attribute in a CSR Attributes Response, it SHOULD prefer that and not
include a challengePassword attribute [RFC7030] in the resulting CSR.
EST clients that include an unsolicited estIdentityLinking attribute
MAY also include the challengePassword attribute [RFC7030] to ensure
support for legacy [RFC7030] servers.
EST servers MUST evaluate each challenge attribute independently.
All challenge attributes included by an EST client MUST be
successfully processed by an EST server for a request to be
considered valid. The EST server MAY ignore challenge attributes
according to local policy, for example, if the EST client is an
authenticated Registration Authority, the EST server ignores may ignore the
estIdentityLinking attribute within a CSR (see Section 3.7 of
[RFC7030]). The EST server MAY refuse enrollment requests that are
not encoded according to the policy of the Certification Authority
(CA).
5. Security Considerations
In addition to the security considerations expressed in the EST
specification [RFC7030], additional security considerations may be
associated with the mechanism used to generate and verify the
otpChallenge value. Where a one-time password is used, the security
considerations expressed in "HOTP: An HMAC-Based One-Time Password
Algorithm" [RFC4226] or "TOTP: Time-Based One-Time Password
Algorithm" [RFC6238] may be relevant. Similarly, the security
considerations from [RFC2985] that apply to the challenge attribute
are relevant as well.
6. IANA Considerations
Section 3 defines three attributes that have been assigned object
identifiers in the "SMI Security for S/MIME Attributes
(1.2.840.113549.1.9.16.2)" registry [RFC7107]:
Value Description Reference
-------- --------------------------------- ----------
56 id-aa-otpChallenge RFC 7894
57 id-aa-revocationChallenge RFC 7894
58 id-aa-estIdentityLinking RFC 7894
Appendix A contains an ASN.1 module. A module identifier has been
assigned in the "SMI Security for PKIX Module Identifier" registry
[RFC7299].
Value Description Reference
-------- --------------------------------- ----------
87 id-mod-EST-Alt-Challenge RFC 7894
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
DOI 10.17487/RFC2985, November 2000,
<http://www.rfc-editor.org/info/rfc2985>.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
<http://www.rfc-editor.org/info/rfc5272>.
[RFC5280] 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, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
DOI 10.17487/RFC5912, June 2010,
<http://www.rfc-editor.org/info/rfc5912>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<http://www.rfc-editor.org/info/rfc7030>.
7.2. Informative References
[RFC4226] M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and
O. Ranen, "HOTP: An HMAC-Based One-Time Password
Algorithm", RFC 4226, DOI 10.17487/RFC4226, December 2005,
<http://www.rfc-editor.org/info/rfc4226>.
[RFC6238] M'Raihi, D., Machani, S., Pei, M., and J. Rydell, "TOTP:
Time-Based One-Time Password Algorithm", RFC 6238,
DOI 10.17487/RFC6238, May 2011,
<http://www.rfc-editor.org/info/rfc6238>.
[RFC7107] Housley, R., "Object Identifier Registry for the S/MIME
Mail Security Working Group", RFC 7107,
DOI 10.17487/RFC7107, January 2014,
<http://www.rfc-editor.org/info/rfc7107>.
[RFC7299] Housley, R., "Object Identifier Registry for the PKIX
Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
<http://www.rfc-editor.org/info/rfc7299>.
[SCEP] Gutmann, P. and M. Pritikin, "Simple Certificate Enrolment
Protocol", Work in Progress, draft-gutmann-scep-02, March
2016.
Appendix A. ASN.1 Module
The following ASN.1 module includes the definitions to support usage
of the attributes defined in this specification. Modules from
[RFC5912] are imported (the original Standards Track source for the
imported structures is [RFC5280] and [RFC5272]).
Mod-EST-Alt-Challenge {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) 87
}
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
DirectoryString{}
FROM PKIX1Explicit-2009 {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51)
}
ATTRIBUTE
FROM PKIX-CommonTypes-2009 {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57)
};
ub-aa-otpChallenge INTEGER ::= 255
id-aa-otpChallenge OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 56
}
otpChallenge ATTRIBUTE ::= {
TYPE DirectoryString {ub-aa-otpChallenge}
COUNTS MIN 1 MAX 1
IDENTIFIED BY id-aa-otpChallenge
}
ub-aa-revocationChallenge INTEGER ::= 255
id-aa-revocationChallenge OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 57
}
revocationChallenge ATTRIBUTE ::= {
TYPE DirectoryString {ub-aa-revocationChallenge}
COUNTS MIN 1 MAX 1
IDENTIFIED BY id-aa-revocationChallenge
}
ub-aa-est-identity-linking INTEGER ::= 255
id-aa-estIdentityLinking OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 58
}
estIdentityLinking ATTRIBUTE ::= {
TYPE DirectoryString {ub-aa-est-identity-linking}
COUNTS MIN 1 MAX 1
IDENTIFIED BY id-aa-estIdentityLinking
}
END
Acknowledgements
Thanks to Jim Schaad, Dan Harkins, Phil Scheffler, Geoff Beier, Mike
Jenkins, and Deb Cooley for their feedback.
Authors' Addresses
Max Pritikin
Cisco Systems, Inc.
510 McCarthy Drive
Milpitas, CA 95035
United States
Email: pritikin@cisco.com
Carl Wallace
Red Hound Software, Inc.
Email: carl@redhoundsoftware.com