TRANS (Public Notary Transparency) R. Stradling
Internet-Draft Comodo CA, Ltd.
Intended status: Experimental E. Messeri
Expires: July 21, 2017 Google UK Ltd.
January 17, 2017
Certificate Transparency: Domain Label Redaction
draft-strad-trans-redaction-01
Abstract
This document defines mechanisms to allow DNS domain name labels that
are considered to be private to not appear in public Certificate
Transparency (CT) logs, while still retaining most of the security
benefits that accrue from using Certificate Transparency.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Redaction Mechanisms . . . . . . . . . . . . . . . . . . . . 3
3.1. Using Wildcard Certificates . . . . . . . . . . . . . . . 3
3.2. Using a Name-Constrained Intermediate CA . . . . . . . . 4
3.2.1. Presenting SCTs, Inclusion Proofs and STHs . . . . . 5
3.2.2. Matching an SCT to the Correct Certificate . . . . . 6
3.3. Redacting Labels in Precertificates . . . . . . . . . . . 6
3.3.1. redactedSubjectAltName Certificate Extension . . . . 7
3.3.2. Verifying the redactedSubjectAltName extension . . . 8
3.3.3. Reconstructing the TBSCertificate . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4.1. Avoiding Overly Redacted Domain Names . . . . . . . . . . 8
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
5.1. Ensuring Effective Redaction . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Some domain owners regard certain DNS domain name labels within their
registered domain space as private and security sensitive. Even
though these domains are often only accessible within the domain
owner's private network, it's common for them to be secured using
publicly trusted Transport Layer Security (TLS) server certificates.
Certificate Transparency v1 [RFC6962] and v2
[I-D.ietf-trans-rfc6962-bis] describe protocols for publicly logging
the existence of TLS server certificates as they are issued or
observed. Since each TLS server certificate lists the domain names
that it is intended to secure, private domain name labels within
registered domain space could end up appearing in CT logs, especially
as TLS clients develop policies that mandate CT compliance. This
seems like an unfortunate and potentially unnecessary privacy leak,
because it's the registered domain names in each certificate that are
of primary interest when using CT to look for suspect certificates.
TODO: Highlight better the differences between registered domains and
subdomains, referencing the relevant DNS RFCs.
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2. Requirements Language
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. Redaction Mechanisms
We propose three mechanisms, in increasing order of implementation
complexity, to allow certain DNS domain name labels to not appear in
public CT logs:
o Using wildcard certificates (Section 3.1) is the simplest option,
but it only covers certain use cases.
o Logging a name-constrained intermediate CA certificate in place of
the end-entity certificate (Section 3.2) covers more, but not all,
use cases.
o Therefore, we define a domain label redaction mechanism
(Section 3.3) that covers all use cases, at the cost of
considerably increased implementation complexity.
We anticipate that TLS clients may develop policies that impose
additional compliancy requirements on the use of the Section 3.2 and
Section 3.3 mechanisms.
To ensure effective redaction, CAs and domain owners should note the
privacy considerations (Section 5).
TODO(eranm): Do we need to further expand (either here or in the
following subsections) on when each of the mechanisms is/isn't
suitable?
TODO: Previously, these mechanisms were defined in earlier revisions
of CTv2 [I-D.ietf-trans-rfc6962-bis], and nothing was said about
compatibility with CTv1. But now, given that these mechanisms have
been decoupled from [I-D.ietf-trans-rfc6962-bis], and given that at
least one major TLS client has announced a policy of mandatory CT
compliance that will almost certainly take effect before CTv2 is
widely deployed, we should consider making some or all of these
mechnanisms compatible with both CTv1 and CTv2.
3.1. Using Wildcard Certificates
A certificate containing a DNS-ID [RFC6125] of "*.example.com" could
be used to secure the domain "topsecret.example.com", without
revealing the label "topsecret" publicly.
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Since TLS clients only match the wildcard character to the complete
leftmost label of the DNS domain name (see Section 6.4.3 of
[RFC6125]), a different mechanism is needed when any label other than
the leftmost label in a DNS-ID is considered private (e.g.,
"top.secret.example.com"). Also, wildcard certificates are
prohibited in some cases, such as Extended Validation Certificates
[EV.Certificate.Guidelines].
3.2. Using a Name-Constrained Intermediate CA
An intermediate CA certificate or intermediate CA precertificate that
contains the Name Constraints [RFC5280] extension MAY be logged in
place of end-entity certificates issued by that intermediate CA, as
long as all of the following conditions are met:
o there MUST be a non-critical extension (OID 1.3.101.76, whose
extnValue OCTET STRING contains ASN.1 NULL data (0x05 0x00)).
This extension is an explicit indication that it is acceptable to
not log certificates issued by this intermediate CA.
o there MUST be a Name Constraints extension, in which:
* permittedSubtrees MUST specify one or more dNSNames.
* excludedSubtrees MUST specify the entire IPv4 and IPv6 address
ranges.
Below is an example Name Constraints extension that meets these
conditions:
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SEQUENCE {
OBJECT IDENTIFIER '2 5 29 30'
BOOLEAN TRUE
OCTET STRING, encapsulates {
SEQUENCE {
[0] {
SEQUENCE {
[2] 'example.com'
}
}
[1] {
SEQUENCE {
[7] 00 00 00 00 00 00 00 00
}
SEQUENCE {
[7]
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
}
}
}
}
}
3.2.1. Presenting SCTs, Inclusion Proofs and STHs
Each SCT (and optional corresponding inclusion proof and STH)
presented by a TLS server MAY correspond to an intermediate CA
certificate or intermediate CA precertificate (to which the server
certificate chains) that meets the requirements in Section 3.2. This
extends section TBD of CT v2 [I-D.ietf-trans-rfc6962-bis], which
specifies that each SCT always corresponds to the server certificate
or to a precertificate that corresponds to that certificate.
Each SCT (and optional corresponding inclusion proof and STH)
included by a certification authority in a Transparency Information
X.509v3 extension in the "singleExtensions" of a "SingleResponse" in
an OCSP response MAY correspond to an intermediate CA certificate or
intermediate CA precertificate (to which the certificate identified
by the "certID" of that "SingleResponse" chains) that meets the
requirements in Section 3.2. This extends section TBD of CT v2
[I-D.ietf-trans-rfc6962-bis], which specifies that each SCT always
corresponds to the certificate identified by the "certID" of that
"SingleResponse" or to a precertificate that corresponds to that
certificate.
Each SCT (and optional corresponding inclusion proof and STH)
included by a certification authority in a Transparency Information
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X.509v3 extension in a certificate MAY correspond to an intermediate
CA certificate or intermediate CA precertificate (to which the
certificate chains) that meets the requirements in Section 3.2. This
extends section TBD of CT v2 [I-D.ietf-trans-rfc6962-bis], which
specifies that each SCT always corresponds to a precertificate that
corresponds to that certificate.
TODO: Refactor this section to avoid repetition.
3.2.2. Matching an SCT to the Correct Certificate
Before considering any SCT to be invalid, a TLS client MUST attempt
to validate it against the server certificate and against each of the
zero or more suitable name-constrained intermediates in the chain.
These certificates may be evaluated in the order they appear in the
chain, or indeed, in any order.
TODO: Shall we specify that there MUST be no more than ONE name-
constrained intermediate in the chain?
TODO: Shall we specify that all presented SCTs MUST correspond to the
same (end-entity or name-constrained intermediate) certificate?
3.3. Redacting Labels in Precertificates
When creating a precertificate, the CA MAY include a
redactedSubjectAltName (Section 3.3.1) extension that contains, in a
redacted form, the same entries that will be included in the
certificate's subjectAltName extension. When the
redactedSubjectAltName extension is present in a precertificate, the
subjectAltName extension MUST be omitted (even though it MUST be
present in the corresponding certificate).
Wildcard "*" labels MUST NOT be redacted, but one or more non-
wildcard labels in each DNS-ID [RFC6125] can each be replaced with a
redacted label as follows:
REDACT(label) = prefix || BASE32(index || _label_hash)
_label_hash = LABELHASH(keyid_len || keyid || label_len || label)
"label" is the case-sensitive label to be redacted.
"prefix" is the "?" character (ASCII value 63).
"index" is the 1 byte index of a hash function in the CT hash
algorithm registry (section TBD of [I-D.ietf-trans-rfc6962-bis]).
The value 255 is reserved.
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"keyid_len" is the 1 byte length of the "keyid".
"keyid" is the keyIdentifier from the Subject Key Identifier
extension (section 4.2.1.2 of [RFC5280]), excluding the ASN.1 OCTET
STRING tag and length bytes.
"label_len" is the 1 byte length of the "label".
"||" denotes concatenation.
"BASE32" is the Base 32 Encoding function (section 6 of [RFC4648]).
Pad characters MUST NOT be appended to the encoded data.
"LABELHASH" is the hash function identified by "index".
3.3.1. redactedSubjectAltName Certificate Extension
The redactedSubjectAltName extension is a non-critical extension (OID
1.3.101.77) that is identical in structure to the subjectAltName
extension, except that DNS-IDs MAY contain redacted labels
(Section 3.3).
When used, the redactedSubjectAltName extension MUST be present in
both the precertificate and the corresponding certificate.
This extension informs TLS clients of the DNS-ID labels that were
redacted and the degree of redaction, while minimizing the complexity
of TBSCertificate reconstruction (Section 3.3.3). Hashing the
redacted labels allows the legitimate domain owner to identify
whether or not each redacted label correlates to a label they know
of.
TODO: Consider the pros and cons of this 'un'redaction feature. If
the cons outweigh the pros, switch to using Andrew Ayer's alternative
proposal of hashing a random salt and including that salt in an
extension in the certificate (and not including the salt in the
precertificate).
Only DNS-ID labels can be redacted using this mechanism. However,
CAs can use the Section 3.2 mechanism to allow DNS domain name labels
in other subjectAltName entries to not appear in logs.
TODO: Should we support redaction of SRV-IDs and URI-IDs using this
mechanism?
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3.3.2. Verifying the redactedSubjectAltName extension
If the redactedSubjectAltName extension is present, TLS clients MUST
check that the subjectAltName extension is present, that the
subjectAltName extension contains the same number of entries as the
redactedSubjectAltName extension, and that each entry in the
subjectAltName extension has a matching entry at the same position in
the redactedSubjectAltName extension. Two entries are matching if
either:
o The two entries are identical; or
o Both entries are DNS-IDs, have the same number of labels, and each
label in the subjectAltName entry has a matching label at the same
position in the redactedSubjectAltName entry. Two labels are
matching if either:
* The two labels are identical; or,
* Neither label is "*" and the label from the
redactedSubjectAltName entry is equal to REDACT(label from
subjectAltName entry) (Section 3.3).
If any of these checks fail, the certificate MUST NOT be considered
compliant.
3.3.3. Reconstructing the TBSCertificate
Section TBD of [I-D.ietf-trans-rfc6962-bis] describes how TLS clients
can reconstruct the TBSCertificate component of a precertificate from
a certificate, so that associated SCTs may be verified.
If the redactedSubjectAltName extension (Section 3.3.1) is present in
the certificate, TLS clients MUST also:
o Verify the redactedSubjectAltName extension against the
subjectAltName extension according to Section 3.3.2.
o Once verified, remove the subjectAltName extension from the
TBSCertificate.
4. Security Considerations
4.1. Avoiding Overly Redacted Domain Names
Redaction of domain name labels (Section 3.3) carries the same risks
as the use of wildcards (e.g., section 7.2 of [RFC6125]). If the
entirety of the domain space below the unredacted part of a domain
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name is not registered by a single domain owner (e.g.,
REDACT(label).com, REDACT(label).co.uk and other [Public.Suffix.List]
entries), then the domain name may be considered by clients to be
overly redacted.
CAs should take care to avoid overly redacting domain names in
precertificates. It is expected that monitors will treat
precertificates that contain overly redacted domain names as
potentially misissued. TLS clients MAY consider a certificate to be
non-compliant if the reconstructed TBSCertificate (Section 3.3.3)
contains any overly redacted domain names.
TODO(eranm): Describe how the CT ecosystem would be harmed if the use
of redaction becomes too widespread.
5. Privacy Considerations
5.1. Ensuring Effective Redaction
Although the mechanisms described in this document remove the need
for private labels to appear in CT logs, they do not guarantee that
this will never happen. For example, anyone who encounters a
certificate could choose to submit it to one or more logs, thereby
rendering the redaction futile.
Domain owners are advised to take the following steps to minimize the
likelihood that their private labels will become known outside their
closed communities:
o Avoid registering private labels in public DNS.
o Avoid using private labels that are predictable (e.g., "www",
labels consisting only of numerical digits, etc). If a label has
insufficient entropy then redaction will only provide a thin layer
of obfuscation, because it will be feasible to recover the label
via a brute-force attack.
o Avoid using publicly trusted certificates to secure private domain
space.
o Avoid enabling unrestricted access for DNS zone transfer (AXFR)
requests (see section 5 of [RFC5936]).
CAs are advised to carefully consider each request to redact a label
using the Section 3.3 mechanism. When a CA believes that redacting a
particular label would be futile, we advise rejecting the redaction
request. TLS clients may have policies that forbid redaction, so
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label redaction should only be used when it's absolutely necessary
and likely to be effective.
6. Acknowledgements
The authors would like to thank Andrew Ayer and TBD for their
valuable contributions.
A big thank you to Symantec for kindly donating the OIDs from the
1.3.101 arc that are used in this document.
7. References
7.1. Normative References
[I-D.ietf-trans-rfc6962-bis]
Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
Stradling, "Certificate Transparency Version 2.0", draft-
ietf-trans-rfc6962-bis-24 (work in progress), December
2016.
[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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[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>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<http://www.rfc-editor.org/info/rfc5936>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
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[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
7.2. Informative References
[EV.Certificate.Guidelines]
CA/Browser Forum, "Guidelines For The Issuance And
Management Of Extended Validation Certificates", 2007,
<https://cabforum.org/wp-content/uploads/
EV_Certificate_Guidelines.pdf>.
[Public.Suffix.List]
Mozilla Foundation, "Public Suffix List", 2016,
<https://publicsuffix.org>.
Authors' Addresses
Rob Stradling
Comodo CA, Ltd.
Email: rob.stradling@comodo.com
Eran Messeri
Google UK Ltd.
Email: eranm@google.com
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