Clarification of RFC7030 CSR Attributes definition
draft-richardson-lamps-rfc7030-csrattrs-02
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Michael Richardson , Owen Friel , David von Oheimb , Dan Harkins | ||
| Last updated | 2022-03-07 | ||
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draft-richardson-lamps-rfc7030-csrattrs-02
LAMPS Working Group M. Richardson, Ed.
Internet-Draft Sandelman Software Works
Intended status: Standards Track O. Friel
Expires: 8 September 2022 Cisco
D. von Oheimb
Siemens
D. Harkins
The Industrial Lounge
7 March 2022
Clarification of RFC7030 CSR Attributes definition
draft-richardson-lamps-rfc7030-csrattrs-02
Abstract
Enrollment over Secure Transport (EST) is ambiguous in specification
of the CSR Attributes Response. This has resulted in implementation
challenges and implementor confusion. This document updates EST and
clarifies how the CSR Attributes Response can be used by an EST
server to specify both CSR attribute OIDs and also CSR attribute
values that the server expects the client to include in its CSR
request.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. CSR Attributes Handling . . . . . . . . . . . . . . . . . . . 3
2.1. Current EST Specification . . . . . . . . . . . . . . . . 3
3. Updated CSR Attributes Handling . . . . . . . . . . . . . . . 4
3.1. Option two: Extend CSR structure to allow values: . . . . 4
3.2. Option three: explicit content for the key
specification . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Option four: explicit members for unique attributes . . . 7
3.4. Option five: more specific structure, simpler
extensions . . . . . . . . . . . . . . . . . . . . . . . 7
4. Co-existence with existing implementations . . . . . . . . . 7
4.1. Use a new MIME type . . . . . . . . . . . . . . . . . . . 7
4.2. Use a new end point of the new format . . . . . . . . . . 8
4.3. Insist new format is upwardly compatible with old
format . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Return new format to new clients only . . . . . . . . . . 8
5. Whether or not to Base64 encoding of results . . . . . . . . 8
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. RFC8994/ACP subjectAltName with specific otherName
included . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. EST server requires public keys of a specific size . . . 8
6.3. EST server requires a public key of a specific algorithm/
curve . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.4. EST server requires a specific extension to be present . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7.1. Identity and Privacy Considerations . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Enrollment over Secure Transport [RFC7030] (EST) has been used in a
wide variety of applications. In particular, [RFC8994] and [RFC8995]
describe a way to use it in order to build out an autonomic control
plane (ACP) [RFC8368].
The ACP requires that each node be given a very specific
SubjectAltName. In the ACP specification, the solution was for the
EST server to use section 2.6 of [RFC7030] to convey to the EST
client the actual SubjectAltName that will end up in its certificate.
As a result of some implementation challenges, it came to light that
this particular way of using the CSR attributes was not universally
agreed upon, and in fact runs contrary to section 2.6. Section 2.6
says that the CSR attributes "provide additional descriptive
information that the EST server cannot access itself". This extends
to specifying that a particular attribute should exist, but not to
the point of having the EST server actually specify the value.
The way in which the CSRattributes were understood by [RFC8994] turns
out to be invalid. This document, therefore, updates section 2.6 to
define this behavior.
This document also updates section 4.5 to include revised ASN.1 that
covers all uses and is backward compatible with the existing use.
Additional examples are provided in an appendix.
2. CSR Attributes Handling
2.1. Current EST Specification
The ASN.1 for CSR Attributes as defined in EST section 4.5.2 is:
CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID
AttrOrOID ::= CHOICE (oid OBJECT IDENTIFIER, attribute Attribute }
Attribute { ATTRIBUTE:IOSet } ::= SEQUENCE {
type ATTRIBUTE.&id({IOSet}),
values SET SIZE(1..MAX) OF ATTRIBUTE.&Type({IOSet}{@type}) }
That section also states the following:
the values indicating the particular
attributes desired to be included in the resulting certificate's
extensions
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This has been interpreted by some implementations as meaning that the
CSR Attributes response can only include values for the attribute
OIDs that the client should include in its CSR, and cannot include
the actual values of those attributes. This is further reinforced by
the example:
Attribute: type = extensionRequest (1.2.840.113549.1.9.14)
value = macAddress (1.3.6.1.1.1.1.22)
This example illustrates that the 'value' specified is an attribute
OID, for example the macAddress OID, and not the value (such as
"10-00-00-12-23-45") of the attribute itself.
There is no clearly documented mechanism with supporting examples
that specifies how a CSR Attributes response can include a value for
a given attribute such as SubjectAltName.
EST section 4.5.2 also states the following:
The structure of the CSR Attributes Response SHOULD, to the
greatest extent possible, reflect the structure of the CSR
it is requesting.
This statement aligns closely with the goal of this document.
Additionally, EST Extensions [RFC8295] Appendix A has an informative
appendix that outlines how a full CSR can be included in the CSR
Attributes response.
3. Updated CSR Attributes Handling
The WG will pick one option as part of the adoption call.
3.1. Option two: Extend CSR structure to allow values:
This ASN.1 needs fixing.
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CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID
AttrOrOID ::= CHOICE (oid OBJECT IDENTIFIER,
attribute Attribute,
value Value }
Attribute { ATTRIBUTE:IOSet } ::= SEQUENCE {
extType ATTRIBUTE.&id({IOSet}),
extAttr SET SIZE(1..MAX) OF ATTRIBUTE.&Type({IOSet}{@type})
}
Value { ATTRIBUTE:IOSet } ::= SEQUENCE {
extType ATTRIBUTE.&id({IOSet}),
type ATTRIBUTE.&Type({IOSet}{@type}),
value OCTET STRING
}
This would just add a value to the SEQUENCE:
OBJECT challengePassword
SEQUENCE
OBJECT subjectAltName
SET
OBJECT someACPgoo
SEQUENCE
OBJECT id-ecPublicKey
SET
OBJECT secp384r1
OBJECT ecdsa-with-SHA384
For example:
0 30: SEQUENCE {
2 28: SEQUENCE {
4 3: OBJECT IDENTIFIER subjectAltName (2 5 29 17)
9 21: SET {
11 19: [1] {
13 17: UTF8String 'hello@example.com'
: }
: }
: }
: }
3.2. Option three: explicit content for the key specification
The following options support complete and unambiguous specification
of
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* CSR ingredients optionally including values to use,
* the type of the public key, which is given in the form of a
public-key algorithm,
* and the hash algorithm to use for the self-signature.
CSR ingredients may be the subject DN, any X.509 extensions, and
special attributes like a challenge password.
For specifying the type of keys allowed in CSRs, they use a to-the-
point KeySpec type. It can be defined for instance as
KeySpec ::= CHOICE {
keyAlg AlgorithmIdentifier,
rsaKeyLen INTEGER
}
The keyAlg type use used to specify public-key alorithms and can
include parameters, such as the name of an elliptic curve. The
rsaKeyLen choice allows specifying the size of RSA keys, which it is
not possible using values of type AlgorithmIdentifier.
The keySpec could also be sequence of such specs, such that the
server can give several key types from which the client can choose,
e.g., EC keys on certain curves and/or RSA keys of certain sizes.
Stick for syntactic backward compatibility with
CsrAttrs ::= SEQUENCE SIZE (0..MAX) OF AttrOrOID
Each OID given in AttrOrOID must occur only once.
Plain OIDs are used mostly for challengePassword.
Attributes are used mostly for any X.509 extensions, subject DN, key
spec, and hash alg, while defining new generally usable OIDs for
* a subject DN of type Name
* a key spec of type KeySpec
* a hash alg spec of type AlgorithmIdentifier
to be given on demand as attribute IDs of type
ATTRIBUTE.&id({IOSet}).
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3.3. Option four: explicit members for unique attributes
Define a new and more to-the-point type, which does not require new
OIDs:
CsrAttrs ::= SEQUENCE {
oids SEQUENCE OF OBJECT IDENTIFIER,
attrs SEQUENCE OF Attribute,
subject [0] Name OPTIONAL,
keySpec [1] KeySpec OPTIONAL,
hashAlg [2] AlgorithmIdentifier OPTIONAL
}
Each OID given in oids or attrs must occur only once.
The oids are used mostly for requiring a challenge password.
The atttrs are used mostly for requiring certain X.509 extensions.
This is, typically just challengePassword and extensionRequest are
used.
3.4. Option five: more specific structure, simpler extensions
Define a new fully to-the-point type, which does not require any
(direct) OIDs:
CsrAttrs ::= SEQUENCE {
subject Name OPTIONAL,
extensions SEQUENCE OF Extension,
challengePassword BOOLEAN,
keySpec [0] KeySpec OPTIONAL,
hashAlg [1] AlgorithmIdentifier OPTIONAL
}
4. Co-existence with existing implementations
There are some ways in which the new CSRattributes could co-exist
with RFC7030.
4.1. Use a new MIME type
The client can signal that it supports the new attribute format by
using an Accept: header in the transaction. This acts as a signal to
a server that it can/should return the attributes in the new format.
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4.2. Use a new end point of the new format
Clients that want to use the new format would use a new end point,
such as "csrvalues" which would only support the new format. A
client which supported both would have to try both "csrvalues" and
then fall back "csrattrs" if the EST server did not support the new
format. Some uses (such as [RFC8994]) require the new format, so if
it was not suppored, that would be a protocol error.
4.3. Insist new format is upwardly compatible with old format
ASN.1 encoding is self-describing, and some formats proposed above
could possibly be parsed by legacy clients without a problem.
4.4. Return new format to new clients only
The Registrar may know which clients are which by the kind of
authentication that they do. An [RFC8994] client which has just
performed a [RFC8995] enrollment would be assumed to require the new
format only. A client which authenticates with an LDevID for a
renewal would be strongly identified, and the Registrar could be
programmed whether to return new format, or legacy CSR attributes.
5. Whether or not to Base64 encoding of results
[RFC8951] clarified that the csrattrs end point was to be Base64
encoded even though the HTTP transport was 8-bit clean.
If this document establishes a new end point, then the new end point
will not be base64 encoded according to current HTTP usage.
6. Examples
6.1. RFC8994/ACP subjectAltName with specific otherName included
TBD
6.2. EST server requires public keys of a specific size
TBD
6.3. EST server requires a public key of a specific algorithm/curve
TBD
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6.4. EST server requires a specific extension to be present
TBD
7. Security Considerations
All security considertions from EST [RFC7030] section 6 are
applicable.
7.1. Identity and Privacy Considerations
An EST server may use this mechanism to instruct the EST client about
the identities it should include in the CSR it sends as part of
enrollment. The client may only be aware of its IDevID Subject,
which includes a manufacturer serial number. The EST server can use
this mechanism to tell the client to include a specific fully
qualified domain name in the CSR in order to complete domain
ownership proofs required by the CA. Additionally, the EST server
may deem the manufacturer serial number in an IDevID as personally
identifiable information, and may want to specify a new random opaque
identifier that the pledge should use in its CSR. This may be
desirable if the CA and EST server have different operators.
8. IANA Considerations
None.
9. Acknowledgements
TODO
10. Changelog
11. References
11.1. Normative References
[BCP14] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/info/rfc7030>.
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[RFC8994] Eckert, T., Ed., Behringer, M., Ed., and S. Bjarnason, "An
Autonomic Control Plane (ACP)", RFC 8994,
DOI 10.17487/RFC8994, May 2021,
<https://www.rfc-editor.org/info/rfc8994>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
11.2. Informative References
[RFC8295] Turner, S., "EST (Enrollment over Secure Transport)
Extensions", RFC 8295, DOI 10.17487/RFC8295, January 2018,
<https://www.rfc-editor.org/info/rfc8295>.
[RFC8368] Eckert, T., Ed. and M. Behringer, "Using an Autonomic
Control Plane for Stable Connectivity of Network
Operations, Administration, and Maintenance (OAM)",
RFC 8368, DOI 10.17487/RFC8368, May 2018,
<https://www.rfc-editor.org/info/rfc8368>.
[RFC8951] Richardson, M., Werner, T., and W. Pan, "Clarification of
Enrollment over Secure Transport (EST): Transfer Encodings
and ASN.1", RFC 8951, DOI 10.17487/RFC8951, November 2020,
<https://www.rfc-editor.org/info/rfc8951>.
Authors' Addresses
Michael Richardson (editor)
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
Owen Friel
Cisco
Email: ofriel@cisco.com
Dr. David von Oheimb
Siemens
Email: dev@ddvo.net
Dan Harkins
The Industrial Lounge
Email: dharkins@lounge.org
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