Updates to the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
Small but important fixes to the X.509 certificate profile: text encodings, internationalized names and trust anchors.
Yee, P., "Updates to the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January 2013, <https://www.rfc-editor.org/info/rfc6818>.
@misc{rfc6818,
series = {Request for Comments},
number = 6818,
howpublished = {RFC 6818},
publisher = {RFC Editor},
doi = {10.17487/RFC6818},
url = {https://www.rfc-editor.org/info/rfc6818},
author = {P. Yee},
title = {{Updates to the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile}},
pagetotal = 8,
year = 2013,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) P. Yee
Request for Comments: 6818 AKAYLA
Updates: 5280 January 2013
Category: Standards Track
ISSN: 2070-1721
Updates to the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile
Abstract
This document updates RFC 5280, the "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List (CRL)
Profile". This document changes the set of acceptable encoding
methods for the explicitText field of the user notice policy
qualifier and clarifies the rules for converting internationalized
domain name labels to ASCII. This document also provides some
clarifications on the use of self-signed certificates, trust anchors,
and some updated security considerations.
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/rfc6818.
Copyright Notice
Copyright (c) 2013 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
Provisions Relating to IETF Documents
(http://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 extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Update to RFC 5280, Section 3.2: "Certification Paths and Trust" 3
3. Update to RFC 5280, Section 4.2.1.4: "Certificate Policies" .....3
4. Update to RFC 5280, Section 6.2: "Using the Path Validation
Algorithm" ......................................................4
5. Update to RFC 5280, Section 7.3: "Internationalized
Domain Names in Distinguished Names" ............................5
6. Security Considerations .........................................5
7. Update to RFC 5280, Section 11.1: "Normative References" ........7
8. Update to RFC 5280, Section 11.2: "Informative References" ......7
9. References ......................................................7
9.1. Normative References .......................................7
9.2. Informative References .....................................7
10. Acknowledgements ................................................8
1. Introduction
This document updates the "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile" [RFC5280].
This document makes a recommendation that self-signed certificates
used to convey trust anchor data be marked as certificate authority
(CA) certificates, which is not always current practice.
The use of self-signed certificates as trust anchors in Section 6.2
of [RFC5280] is clarified. While it is optional to use additional
information in these certificates in the path validation process,
[RFC5937] is noted as providing guidance in that regard.
The acceptable and unacceptable encodings for the explicitText field
of the user notice policy qualifier are updated to bring them in line
with existing practice.
The rules in Section 7.3 of [RFC5280] for ASCII encoding of
Internationalized Domain Names (IDNs) as Distinguished Names are
aligned with the rules in Section 7.2 of that document that govern
IDN encoding as GeneralNames.
In light of some observed attacks [Prins], the Security
Considerations section now gives added depth to the consequences of
CA key compromise. This section additionally notes that collision
resistance is not a required property of one-way hash functions when
used to generate key identifiers.
This document also adds normative and informative references for
Trust Anchor formats and how they may be used to initialize the path
validation inputs. These are needed as a result of the changes made
in Section 4 of this document.
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 [RFC2119].
2. Update to RFC 5280, Section 3.2: "Certification Paths and Trust"
Add the following paragraph to the end of RFC 5280, Section 3.2:
| Consistent with Section 3.4.61 of X.509 (11/2008) [X.509], we note
| that use of self-issued certificates and self-signed certificates
| issued by entities other than CAs are outside the scope of this
| specification. Thus, for example, a web server or client might
| generate a self-signed certificate to identify itself. These
| certificates and how a relying party uses them to authenticate
| asserted identities are both outside the scope of RFC 5280.
3. Update to RFC 5280, Section 4.2.1.4: "Certificate Policies"
RFC 5280, Section 4.2.1.4, the tenth paragraph says:
An explicitText field includes the textual statement directly in
the certificate. The explicitText field is a string with a
maximum size of 200 characters. Conforming CAs SHOULD use the
| UTF8String encoding for explicitText, but MAY use IA5String.
| Conforming CAs MUST NOT encode explicitText as VisibleString or
| BMPString. The explicitText string SHOULD NOT include any control
| characters (e.g., U+0000 to U+001F and U+007F to U+009F). When
| the UTF8String encoding is used, all character sequences SHOULD be
normalized according to Unicode normalization form C (NFC) [NFC].
This paragraph is replaced with:
An explicitText field includes the textual statement directly in
the certificate. The explicitText field is a string with a
maximum size of 200 characters. Conforming CAs SHOULD use the
| UTF8String encoding for explicitText. VisibleString or BMPString
| are acceptable but less preferred alternatives. Conforming CAs
| MUST NOT encode explicitText as IA5String. The explicitText string
| SHOULD NOT include any control characters (e.g., U+0000 to U+001F
| and U+007F to U+009F). When the UTF8String or BMPString encoding
is used, all character sequences SHOULD be normalized according
to Unicode normalization form C (NFC) [NFC].
4. Update to RFC 5280, Section 6.2: "Using the Path Validation
Algorithm"
RFC 5280, Section 6.2, the third paragraph says:
Where a CA distributes self-signed certificates to specify trust
anchor information, certificate extensions can be used to specify
recommended inputs to path validation. For example, a policy
constraints extension could be included in the self-signed
certificate to indicate that paths beginning with this trust anchor
should be trusted only for the specified policies. Similarly, a name
constraints extension could be included to indicate that paths
beginning with this trust anchor should be trusted only for the
specified name spaces. The path validation algorithm presented in
Section 6.1 does not assume that trust anchor information is provided
in self-signed certificates and does not specify processing rules for
| additional information included in such certificates.
| Implementations that use self-signed certificates to specify trust
| anchor information are free to process or ignore such information.
This paragraph is replaced with:
Where a CA distributes self-signed certificates to specify trust
anchor information, certificate extensions can be used to specify
recommended inputs to path validation. For example, a policy
constraints extension could be included in the self-signed
certificate to indicate that paths beginning with this trust anchor
should be trusted only for the specified policies. Similarly, a name
constraints extension could be included to indicate that paths
beginning with this trust anchor should be trusted only for the
specified name spaces. The path validation algorithm presented in
Section 6.1 does not assume that trust anchor information is provided
| in self-signed certificates and does not specify processing rules for
| additional information included in such certificates.
| However, [RFC5914] defines several formats for representing trust
| anchor information, including self-signed certificates, and [RFC5937]
| provides an example of how such information may be used to initialize
| the path validation inputs. Implementations are free to make use of
| any additional information that is included in a trust anchor
| representation, or to ignore such information.
5. Update to RFC 5280, Section 7.3: "Internationalized Domain Names in
Distinguished Names"
RFC 5280, Section 7.3, the first paragraph says:
Domain Names may also be represented as distinguished names using
domain components in the subject field, the issuer field, the
subjectAltName extension, or the issuerAltName extension. As with
the dNSName in the GeneralName type, the value of this attribute is
defined as an IA5String. Each domainComponent attribute represents a
single label. To represent a label from an IDN in the distinguished
name, the implementation MUST perform the "ToASCII" label conversion
| specified in Section 4.1 of RFC 3490. The label SHALL be considered
| a "stored string". That is, the AllowUnassigned flag SHALL NOT be
| set.
This paragraph is replaced with:
Domain Names may also be represented as distinguished names using
domain components in the subject field, the issuer field, the
subjectAltName extension, or the issuerAltName extension. As with
the dNSName in the GeneralName type, the value of this attribute is
defined as an IA5String. Each domainComponent attribute represents a
single label. To represent a label from an IDN in the distinguished
name, the implementation MUST perform the "ToASCII" label conversion
| specified in Section 4.1 of RFC 3490 with the UseSTD3ASCIIRules flag
| set. The label SHALL be considered a "stored string". That is, the
| AllowUnassigned flag SHALL NOT be set. The conversion process is the
| same as is performed in step 4 in Section 7.2.
6. Security Considerations
This document modifies the Security Considerations section of RFC
5280 as follows. The fifth paragraph of the Security Considerations
section of RFC 5280 says:
The protection afforded private keys is a critical security factor.
On a small scale, failure of users to protect their private keys will
permit an attacker to masquerade as them or decrypt their personal
information. On a larger scale, compromise of a CA's private signing
| key may have a catastrophic effect. If an attacker obtains the
| private key unnoticed, the attacker may issue bogus certificates and
| CRLs. Existence of bogus certificates and CRLs will undermine
| confidence in the system. If such a compromise is detected, all
| certificates issued to the compromised CA MUST be revoked, preventing
| services between its users and users of other CAs. Rebuilding after
| such a compromise will be problematic, so CAs are advised to
| implement a combination of strong technical measures (e.g., tamper-
| resistant cryptographic modules) and appropriate management
| procedures (e.g., separation of duties) to avoid such an incident.
This paragraph is replaced with:
The protection afforded private keys is a critical security factor.
On a small scale, failure of users to protect their private keys will
permit an attacker to masquerade as them or decrypt their personal
information. On a larger scale, compromise of a CA's private signing
key may have a catastrophic effect.
|
| If an attacker obtains the private key of a CA unnoticed, the
| attacker may issue bogus certificates and CRLs. Even if an attacker
| is unable to obtain a copy of a CA's private key, the attacker may be
| able to issue bogus certificates and CRLs by making unauthorized use
| of the CA's workstation or of an RA's workstation. Such an attack
| may be the result of an attacker obtaining unauthorized access to the
| workstation, either locally or remotely, or may be the result of
| inappropriate activity by an insider. Existence of bogus
| certificates and CRLs will undermine confidence in the system. Among
| many other possible attacks, the attacker may issue bogus
| certificates that have the same subject names as legitimate
| certificates in order impersonate legitimate certificate subjects.
| This could include bogus CA certificates in which the subject names
| in the bogus certificates match the names under which legitimate CAs
| issue certificates and CRLs. This would allow the attacker to issue
| bogus certificates and CRLs that have the same issuer names, and
| possibly the same serial numbers, as certificates and CRLs issued by
| legitimate CAs.
The following text is added to the end of the Security Considerations
section of 5280:
| One-way hash functions are commonly used to generate key identifier
| values (AKI and SKI), e.g., as described in Sections 4.1.1 and 4.1.2.
| However, none of the security properties of such functions are
| required for this context.
7. Update to RFC 5280, Section 11.1: "Normative References"
[RFC5914] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Format", RFC 5914, June 2010.
8. Update to RFC 5280, Section 11.2: "Informative References"
[RFC5937] Ashmore, S. and C. Wallace, "Using Trust Anchor
Constraints during Certification Path Processing",
RFC 5937, August 2010.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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, May 2008.
[RFC5914] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Format", RFC 5914, June 2010.
[X.509] ITU-T Recommendation X.509 (2008) | ISO/IEC 9594-8:2008,
Information Technology - Open Systems Interconnection -
The Directory: Public-key and attribute certificate
frameworks.
9.2. Informative References
[RFC5937] Ashmore, S. and C. Wallace, "Using Trust Anchor
Constraints during Certification Path Processing", RFC
5937, August 2010.
[Prins] Prins, J. R., "DigiNotar Certificate Authority breach
'Operation Black Tulip'", September 2011,
<http://www.rijksoverheid.nl/bestanden/
documenten-en-publicaties/rapporten/2011/
09/05/diginotar-public-report-version-1/
rapport-fox-it-operation-black-tulip-v1-0.pdf>.
[NFC] Davis, M. and M. Duerst, "Unicode Standard Annex #15:
Unicode Normalization Forms", October 2006,
<http://www.unicode.org/reports/tr15/>.
10. Acknowledgements
David Cooper is acknowledged for his fine work in editing previous
versions of this document.
Author's Address
Peter E. Yee
AKAYLA
7150 Moorland Drive
Clarksville, MD 21029
USA
EMail: peter@akayla.com
AI ELI5
On this page
§1What This Update Covers
RFC 6818 updates RFC 5280, the Internet X.509 Public Key Infrastructure Certificate and CRL Profile. It does not stand alone: each section gives text to add to, or replace in, a named place in RFC 5280.
The introduction lists the changes. It recommends that self-signed certificates used to convey trust anchor data be marked as CA certificates, which is not always current practice. It clarifies how such certificates are used in path validation. It changes the acceptable encodings for the explicitText field, aligns the IDN rules for distinguished names with those for GeneralNames, and expands the security considerations in light of observed attacks.
This memo is a list of fix-it notes for an older rulebook. Each note says which page to change and what the new words are.
§2Self-Signed Certificates Outside the Scope
A paragraph is added to RFC 5280, Section 3.2. It notes that self-issued certificates, and self-signed certificates issued by entities other than CAs, are outside the specification, consistent with X.509. For example, a web server or client might generate a self-signed certificate to identify itself.
Both those certificates and how a relying party uses them to authenticate asserted identities are outside the scope of RFC 5280.
If a computer makes its own ID card and signs it itself, the rulebook does not say how to decide whether to trust it. That is left to someone else.
§3Encoding of explicitText
The explicitText field of a user notice policy qualifier holds a textual statement directly in a certificate, up to a maximum of 200 characters. The old rule said CAs SHOULD use UTF8String, MAY use IA5String, and MUST NOT use VisibleString or BMPString.
The replacement text says:
- Conforming CAs SHOULD use
UTF8String. VisibleStringorBMPStringare acceptable but less preferred.- Conforming CAs MUST NOT use
IA5String. - The string SHOULD NOT include control characters (U+0000 to U+001F and U+007F to U+009F).
- With UTF8String or BMPString, character sequences SHOULD be normalized to Unicode normalization form C (NFC).
The introduction says this brings the text in line with existing practice.
There is a note box on the ID card. The new rule says which kinds of handwriting are allowed in it, and bans one style that was allowed before.
| Encoding | RFC 5280 original | After RFC 6818 |
|---|---|---|
| UTF8String | SHOULD use | SHOULD use |
| IA5String THIS RFC | MAY use | MUST NOT use |
| VisibleString THIS RFC | MUST NOT use | Acceptable, less preferred |
| BMPString THIS RFC | MUST NOT use | Acceptable, less preferred |
§4Trust Anchors in Path Validation
RFC 5280, Section 6.2, explains that a CA may distribute self-signed certificates to carry trust anchor information. Extensions such as policy constraints or name constraints can then recommend inputs to path validation. The path validation algorithm in Section 6.1 does not assume that trust anchor information comes in self-signed certificates, and it does not specify processing for that extra information.
The update adds that RFC 5914 defines several formats for trust anchor information, including self-signed certificates, and that RFC 5937 shows how such information may be used to initialize path validation inputs. Implementations are free to use any additional information in a trust anchor representation, or to ignore it.
A trusted starting point can come with extra instructions, like only trusting it for certain topics. Programs may follow those instructions or skip them.
§5Internationalized Domain Names in Distinguished Names
Domain names can be written as distinguished names using domain components, and each domainComponent holds one label as an IA5String. To store a label from an internationalized domain name, an implementation MUST perform the ToASCII conversion from Section 4.1 of RFC 3490.
The update adds that the conversion MUST be done with the UseSTD3ASCIIRules flag set. The label is still a stored string, so the AllowUnassigned flag SHALL NOT be set. The process is the same as step 4 in Section 7.2 of RFC 5280, which aligns the two places where domain names are encoded.
Names with special letters are turned into plain letters and numbers before they go on the card. Now there is one rule for doing it in both places where names appear.
§6Security Considerations
The update rewrites a paragraph about protecting private keys. It adds that an attacker may issue bogus certificates and CRLs even without a copy of a CA's private key, by misusing the CA's workstation or an RA's workstation, either through unauthorized access or through inappropriate activity by an insider. Bogus certificates could copy the subject names of legitimate ones, including bogus CA certificates, and could even share issuer names and possibly serial numbers with real ones. The introduction says this was prompted by observed attacks.
A new paragraph notes that one-way hash functions are commonly used to generate key identifiers (AKI and SKI), but none of the security properties of such functions are required for that purpose.
A forger does not need to steal the stamp itself. Sneaking into the stamp office, or help from someone inside, also works. Also, the number on a key label does not need to be secret-proof.
§7Reference Changes
The update adds RFC 5914, Trust Anchor Format, to the normative references of RFC 5280, and RFC 5937, Using Trust Anchor Constraints during Certification Path Processing, to the informative references. Both are needed because of the trust anchor change described in Section 4.
Two new books are added to the reading list because the new notes refer to them.