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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.

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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   This Internet-Draft will expire on July 21, 2017.

Copyright Notice

   Copyright (c) 2017 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
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   described in the Simplified BSD License.



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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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