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openpgp                                                       D. Gillmor
Internet-Draft                                                      ACLU
Intended status: Informational                            April 06, 2019
Expires: October 8, 2019

                   Abuse-Resistant OpenPGP Keystores


   OpenPGP transferable public keys are composite certificates, made up
   of primary keys, direct key signatures, user IDs, identity
   certifications ("signature packets"), subkeys, and so on.  They are
   often assembled by merging multiple certificates that all share the
   same primary key, and are distributed in public keystores.

   Unfortunately, since any third-party can add certifications with any
   content to any OpenPGP certificate, the assembled/merged form of a
   certificate can become unwieldy or undistributable.

   This draft documents techniques that an archive of OpenPGP
   certificates can use to mitigate the impact of these various forms of
   flooding attacks.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 8, 2019.

Copyright Notice

   Copyright (c) 2019 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 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Certificate Flooding  . . . . . . . . . . . . . . . . . .   6
     2.2.  User ID Flooding  . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Keystore Flooding . . . . . . . . . . . . . . . . . . . .   7
   3.  Simple Mitigations  . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Decline Large Packets . . . . . . . . . . . . . . . . . .   7
     3.2.  Enforce Strict User IDs . . . . . . . . . . . . . . . . .   8
     3.3.  Scoped User IDs . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Strip or Standardize Unhashed Subpackets  . . . . . . . .   8
     3.5.  Decline User Attributes . . . . . . . . . . . . . . . . .   9
     3.6.  Decline Non-exportable Certifications . . . . . . . . . .   9
     3.7.  Decline Data From the Future  . . . . . . . . . . . . . .   9
     3.8.  Accept Only Profiled Certifications . . . . . . . . . . .   9
     3.9.  Accept Only Certificates Issued by Designated Authorities  10
     3.10. Decline Packets by Blocklist  . . . . . . . . . . . . . .  10
   4.  Contextual Mitigations  . . . . . . . . . . . . . . . . . . .  11
     4.1.  Accept Only Cryptographically-verifiable Certifications .  11
     4.2.  Accept Only Certificates Issued by Known Certificates . .  11
     4.3.  Rate-limit Submissions by IP Address  . . . . . . . . . .  12
     4.4.  Accept Certiifcates Based on Exterior Process . . . . . .  12
     4.5.  Accept Certificates by E-mail Validation  . . . . . . . .  12
   5.  Non-append-only mitigations . . . . . . . . . . . . . . . . .  13
     5.1.  Drop Superseded Signatures  . . . . . . . . . . . . . . .  13
     5.2.  Drop Expired Signatures . . . . . . . . . . . . . . . . .  14
     5.3.  Drop Dangling User IDs, User Attributes, and Subkeys  . .  14
     5.4.  Drop All Other Elements of a Directly-Revoked Certificate  14
     5.5.  Implicit Expiration Date  . . . . . . . . . . . . . . . .  15
   6.  Updates-only Keystores  . . . . . . . . . . . . . . . . . . .  15
   7.  First-party-only Keystores  . . . . . . . . . . . . . . . . .  16
   8.  First-party-attested Third-party Certifications . . . . . . .  16
     8.1.  Key Server Preferences "No-modify"  . . . . . . . . . . .  17
     8.2.  Client Interactions . . . . . . . . . . . . . . . . . . .  18
   9.  Side Effects and Ecosystem Impacts  . . . . . . . . . . . . .  18

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     9.1.  Designated Revoker  . . . . . . . . . . . . . . . . . . .  18
     9.2.  Certification-capable Subkeys . . . . . . . . . . . . . .  18
     9.3.  Assessing Certificates in the Past  . . . . . . . . . . .  19
   10. OpenPGP details . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Revocations  . . . . . . . . . . . . . . . . . . . . . .  19
     10.2.  User ID Conventions  . . . . . . . . . . . . . . . . . .  20
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   12. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  21
     12.1.  Publishing Identity Information  . . . . . . . . . . . .  22
     12.2.  Social Graph . . . . . . . . . . . . . . . . . . . . . .  22
     12.3.  Tracking Clients by Queries  . . . . . . . . . . . . . .  22
     12.4.  Cleartext Queries  . . . . . . . . . . . . . . . . . . .  23
     12.5.  Traffic Analysis . . . . . . . . . . . . . . . . . . . .  23
   13. User Considerations . . . . . . . . . . . . . . . . . . . . .  24
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   15. Document Considerations . . . . . . . . . . . . . . . . . . .  24
     15.1.  Document History . . . . . . . . . . . . . . . . . . . .  24
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     17.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   o  "OpenPGP certificate" (or just "certificate") is used
      interchangeably with [RFC4880]'s "Transferable Public Key".  The
      term "certificate" refers unambiguously to the entire composite
      object, unlike "key", which might also be used to refer to a
      primary key or subkey.

   o  An "identity certification" (or just "certification") is an
      [RFC4880] signature packet that covers OpenPGP identity
      information - that is, any signature packet of type 0x10, 0x11,
      0x12, or 0x13.  Certifications are said to (try to) "bind" a
      primary key to a User ID.

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   o  The primary key that makes the certification is known as the
      "issuer".  The primary key over which the certification is made is
      known as the "subject".

   o  A "first-party certification" is issued by the primary key of a
      certificate, and binds itself to a user ID in the certificate.
      That is, the issuer is the same as the subject.  This is sometimes
      referred to as a "self-sig".

   o  A "third-party certification" is a made over a primary key and
      user ID by some other certification-capable primary key.  That is,
      the issuer is different than the subject.  (The elusive "second-
      party" is presumed to be the verifier who is trying to interpret
      the certificate)

   o  A "keystore" is any collection of OpenPGP certificates.  Keystores
      typically receive mergeable updates over the course of their
      lifetime which might add to the set of OpenPGP certificates they
      hold, or update the certificates.

   o  "Certificate discovery" is the process whereby a user retrieves an
      OpenPGP certificate based on user ID (see Section 10.2).  A user
      attempting to discover a certificate from a keystore will search
      for a substring of the known user IDs, most typically an e-mail
      address if the user ID is an [RFC5322] name-addr or addr-spec.
      Some certificate discovery mechanisms look for an exact match on
      the known user IDs.  [I-D.koch-openpgp-webkey-service] and
      [I-D.shaw-openpgp-hkp] both offer certificate discovery

   o  "Certificate validation" is the process whereby a user decides
      whether a given user ID in an OpenPGP certificate is acceptable.
      For example, if the certificate has a user ID of "Alice
      <alice@example.org>" and the user wants to send an e-mail to
      "alice@example.org", the mail user agent might want to ensure that
      the certificate is valid for this e-mail address before encrypting
      to it.  This process can take different forms, and can consider
      many different factors, some of which are not directly contained
      in the certificate itself.  For example, certificate validation
      might consider whether the certificate was fetched via DANE
      ([RFC7929]) or WKD ([I-D.koch-openpgp-webkey-service]); or whether
      it has seen e-mails from that address signed by the certificate in
      the past; or how long it has known about certificate.

   o  "Certificate update" is the process whereby a user fetches new
      information about a certificate, potentially merging those OpnePGP
      packets to change the status of the certificate.  Updates might
      include adding or revoking user IDs or subkeys, updating

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      expiration dates, or even revoking the entire certificate by
      revoking the primary key directly.  A user attempting to update a
      certificate typically queries a keystore based on the
      certificate's fingerprint.

   o  A "keyserver" is a particular kind of keystore, typically means of
      publicly distributing OpenPGP certificates or updates to them.
      Examples of keyserver software include [SKS] and
      [MAILVELOPE-KEYSERVER].  One common HTTP interface for keyservers
      is [I-D.shaw-openpgp-hkp].

   o  A "synchronizing keyserver" is a keyserver which gossips with
      other peers, and typically acts as an append-only log.  Such a
      keyserver is typically useful for certificate discovery,
      certificate updates, and revocation information.  They are
      typically _not_ useful for certificate validation, since they make
      no assertions about whether the identities in the certificates
      they server are accurate.  As of the writing of this document,
      [SKS] is the canonical synchronizing keyserver implementation,
      though other implementations exist.

   o  An "e-mail-validating keyserver" is a keyserver which attempts to
      verify the identity in an OpenPGP certificate's user ID by
      confirming access to the e-mail account, and possibly by
      confirming access to the secret key.  Some implementations permit
      removal of a certificate by anyone who can prove access to the
      e-mail address in question.  They are useful for certificate
      discovery based on e-mail address and certificate validation (by
      users who trust the operator), but some may not be useful for
      certificate update or revocation, since a certificate could be
      simply replaced by an adversary who also has access to the e-mail
      address in question.  [MAILVELOPE-KEYSERVER] is an example of such
      a keyserver.

   o  "Cryptographic validity" refers to mathematical evidence that a
      signature came from the secret key associated with the public key
      it claims to come from.  Note that a certification may be
      cryptographically valid without the signed data being true (for
      example, a given certificate with the user ID "Alice
      <alice@example.org>" might not belong to the person who controls
      the e-mail address "alice@example.org" even though the self-sig is
      cryptographically valid).  In particular, cryptographic validity
      for user ID in a certificate is typically insufficient evidence
      for certificate validation.  Also note that knowledge of the
      public key of the issuer is necessary to determine whether any
      given signature is cryptographically valid.  Some keyservers
      perform cryptographic validation in some contexts.  Other

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      keyservers (like [SKS]) perform no cryptographic validation

   o  OpenPGP revocations can have "Reason for Revocation" (see
      [RFC4880]), which can be either "soft" or "hard".  The set of
      "soft" reasons is: "Key is superseded" and "Key is retired and no
      longer used".  All other reasons (and revocations that do not
      state a reason) are "hard" revocations.  See Section 10.1 for more

2.  Problem Statement

   OpenPGP keystores that handle submissions from the public are subject
   to a range of flooding attacks by malicious submitters.

   This section describes three distinct flooding attacks that public
   keystores should consider.

   The rest of the document describes some mitigations that can be used
   by keystores that are concerned about these problems but want to
   continue to offer some level of service for certificate discovery,
   certificate update, or certificate validation.

2.1.  Certificate Flooding

   Many public keystores (including both the [SKS] keyserver network and
   [MAILVELOPE-KEYSERVER]) allow anyone to attach arbitrary data (in the
   form of third-party certifications) to any certificate, bloating that
   certificate to the point of being impossible to effectively retrieve.
   For example, some OpenPGP implementations simply refuse to process
   certificates larger than a certain size.

   This kind of Denial-of-Service attack makes it possible to make
   someone else's certificate unretrievable from the keystore,
   preventing certificate discovery.  It also makes it possible to swamp
   a certificate that has been revoked, preventing certificate update,
   potentially leaving the client of the keystore with the compromised
   certificate in an unrevoked state locally.

   Additionally, even without malice, OpenPGP certificates can
   potentially grow without bound.

2.2.  User ID Flooding

   Public keystores that are used for certificate discovery may also be
   vulnerable to attacks that flood the space of known user IDs.  In
   particular, if the keystore accepts arbitrary certificates from the
   public and does no verification of the user IDs, then any client

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   searching for a given user ID may need to review and process an
   effectively unbounded set of maliciously-submitted certificates to
   find the non-malicious certificates they are looking for.

   For example, if an attacker knows that a given system consults a
   keystore looking for certificates which match the e-mail address
   "alice@example.org", the attacker may upload hundreds or thousands of
   certificates containing user IDs that match that address.  Even if
   those certificates would not be accepted by a client (e.g., because
   they were not certified by a known-good authority), the client
   typically still has to wade through all of them in order to find the
   non-malicious certificates.

   If the keystore does not offer a discovery interface at all (that is,
   if clients cannot search it by user ID), then user ID flooding is of
   less consequence.

2.3.  Keystore Flooding

   A public keystore that accepts arbitrary OpenPGP material and is
   append-only is at risk of being overwhelmed by sheer quantity of
   malicious uploaded packets.  This is a risk even if the user ID space
   is not being deliberately flooded, and if individual certificates are
   protected from flooding by any of the mechanisms described later in
   this document.

   The keystore itself can become difficult to operate if the total
   quantity of data is too large, and if it is a synchronizing
   keyserver, then the quantities of data may impose unsustainable
   bandwidth costs on the operator as well.

   Effectively mitigating against keystore flooding requires either
   abandoning the append-only property that some keystores prefer, or
   imposing very strict controls on initial ingestion.

3.  Simple Mitigations

   These steps can be taken by any keystore that wants to avoid
   obviously malicious abuse.  They can be implemented on receipt of any
   new packet, and are based strictly on the structure of the packet

3.1.  Decline Large Packets

   While [RFC4880] permits OpenPGP packet sizes of arbitrary length,
   OpenPGP certificates rarely need to be so large.  An abuse-resistant
   keystore SHOULD reject any OpenPGP packet larger than 8383 octets.
   (This cutoff is chosen because it guarantees that the packet size can

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   be represented as a one- or two-octet [RFC4880] "New Format Packet
   Length", but it could be reduced further)

   This may cause problems for user attribute packets that contain large
   images, but it's not clear that these images are concretely useful in
   any context.  Some keystores MAY extend this limit for user attribute
   packets specifically, but SHOULD NOT allow even user attributes
   packets larger than 65536 octets.

3.2.  Enforce Strict User IDs

   [RFC4880] indicates that User IDs are expected to be UTF-8 strings.
   An abuse-resistant keystore MUST reject any user ID that is not valid

   Some abuse-resistant keystores MAY only accept User IDs that meet
   even stricter conventions, such as an [RFC5322] name-addr or addr-
   spec, or a URL like "ssh://host.example.org".

   As simple text strings, User IDs don't need to be nearly as long as
   any other packets.  An abuse-resistant keystore SHOULD reject any
   user ID packet larger than 1024 octets.

3.3.  Scoped User IDs

   Some abuse-resistant keystores may restrict themselves to publishing
   only certificates with User IDs that match a specific pattern.  For
   example, [RFC7929] encourages publication in the DNS of only
   certificates whose user IDs refer to e-mail addresses within the DNS
   zone.  [I-D.koch-openpgp-webkey-service] similarly aims to restrict
   publication to certificates relevant to the specific e-mail domain.

3.4.  Strip or Standardize Unhashed Subpackets

   [RFC4880] signature packets contain an "unhashed" block of
   subpackets.  These subpackets are not covered by any cryptographic
   signature, so they are ripe for abuse.

   An abuse-resistant keysetore SHOULD strip out all unhashed

   Note that some certifications only identify the issuer of the
   certification by an unhashed Issuer ID subpacket.  If a
   certification's hashed subpacket section has no Issuer ID or Issuer
   Fingerprint (see [I-D.ietf-openpgp-rfc4880bis]) subpacket, then an
   abuse-resistant keystore that has cryptographically validated the
   certification SHOULD make the unhashed subpackets contain only a

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   single subpacket.  That subpacket should be of type Issuer
   Fingerprint, and should contain the fingerprint of the issuer.

   A special exception may be made for unhashed subpackets in a third-
   party certification that contain attestations from the certificate's
   primary key as described in Section 8.

3.5.  Decline User Attributes

   Due to size concerns, some abuse-resistant keystores MAY choose to
   ignore user attribute packets entirely, as well as any certifications
   that cover them.

3.6.  Decline Non-exportable Certifications

   An abuse-resistant keystore MUST NOT accept any certification that
   has the "Exportable Certification" subpacket present and set to 0.
   While most keystore clients will not upload these "local"
   certifications anyway, a reasonable public keystore that wants to
   minimize data has no business storing or distributing these

3.7.  Decline Data From the Future

   Many OpenPGP packets have time-of-creation timestamps in them.  An
   abuse-resistant keystore with a functional real-time clock MAY decide
   to only accept packets whose time-of-creation is in the future.

   Note that some OpenPGP implementations may pre-generate OpenPGP
   material intended for use only in some future window (e.g.  "Here is
   the certificate we plan to use to sign our software next year; do not
   accept signatures from it until then."), and may use modified time-
   of-creation timestamps to try to acheive that purpose.  This material
   would not be distributable ahead of time by an abuse-resistant
   keystore that adopts this mitigation.

3.8.  Accept Only Profiled Certifications

   An aggressively abuse-resistant keystore MAY decide to only accept
   certifications that meet a specific profile.  For example, it MAY
   reject certifications with unknown subpacket types, unknown
   notations, or certain combinations of subpackets.  This can help to
   minimize the amount of room for garbage data uploads.

   Any abuse-resistant keystore that adopts such a strict posture should
   clearly document what its expected certificate profile is, and should
   have a plan for how to extend the profile if new types of
   certification appear that it wants to be able to distribute.

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   Note that if the profile is ever restricted (rather than extended),
   and the restriction is applied to the material already present, such
   a keystore is no longer append-only (please see Section 5).

3.9.  Accept Only Certificates Issued by Designated Authorities

   An abuse-resistant keystore capable of cryptographic validation MAY
   retain a list of designated authorities, typically in the form of a
   set of known public keys.  Upon receipt of a new OpenPGP certificate,
   the keystore can decide whether to accept or decline each user ID of
   the certificate based whether that user ID has a certification that
   was issued by one or more of the designated authorities.

   If no user IDs are certified by designated authority, such a keystore
   SHOULD decline the certificate and its primary key entirely.  Such a
   keystore SHOULD decline to retain or propagate all certifications
   associated with each accepted user ID except for first-party
   certifications and certifications by the designated authorities.

   The operator of such a keystore SHOULD have a clear policy about its
   set of designated authorities.

   Given the ambiguities about expiration and revocation, such a
   keyserver SHOULD ignore expiration and revocation of authority
   certifications, and simply accept and retain as long as the
   cryptographic signature is valid.

   Note that if any key is removed from the set of designated
   authorities, and that change is applied to the existing keystore,
   such a keystore may no longer be append-only (please see Section 5).

3.10.  Decline Packets by Blocklist

   The maintainer of the keystore may keep a specific list of "known-
   bad" material, and decline to accept or redistribute items matching
   that blocklist.  The material so identified could be anything, but
   most usefully, specific public keys or User IDs could be blocked.

   Note that if a blocklist grows to include an element already present
   in the keystore, it will no longer be append-only (please see
   Section 5).

   Some keystores may choose to apply a blocklist only at distribution
   time and not apply it at input time.  This allows the keystore to be
   append-only, and permits synchronization between keystores that don't
   share a blocklist, and somewhat reduces the attacker's incentive for
   flooding the keystore.

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   Note that development and maintenance of a blocklist is not without
   its own potentials for abuse.  For one thing, the blocklist may
   itself grow without bound.  Additionally, a blocklist may be socially
   or politically contentious.  There needs to be a clear policy about
   how it is managed, whether by delegation to specific decision-makers,
   or explicit tests.  Furthermore, the existence of even a well-
   intentioned blocklist may be an "attractive nuisance," drawing the
   interest of would-be censors or other attacker interested in
   controlling the ecosystem reliant on the keystore in question.

4.  Contextual Mitigations

   Some mitigations make the acceptance or rejection of packets
   contingent on data that is already in the keystore or the keystore's
   developing knowledge about the world.  This means that, depending on
   the order that the keystore encounters the various material, or how
   it discovers the material, the final set of material retained and
   distributed by the keystore might be different.

   While this isn't necessarily bad, it may be a surprising property for
   some users of keystores.

4.1.  Accept Only Cryptographically-verifiable Certifications

   An abuse-resistant keystore that is capable of doing cryptographic
   validation MAY decide to reject certifications that it cannot
   cryptographically validate.

   This may mean that the keystore rejects some packets while it is
   unaware of the public key of the issuer of the packet.

4.2.  Accept Only Certificates Issued by Known Certificates

   This is an extension of Section 3.9, but where the set of authorities
   is just the set of certificates already known to the keystore.  An
   abuse-resistant keystore that adopts this strategy is effectively
   only crawling the reachable graph of OpenPGP certificates from some
   starting core.

   A keystore adopting the mitigation SHOULD have a clear documentation
   of the core of initial certificates it starts with, as this is
   effectively a policy decision.

   This mitigation measure may fail due to a compromise of any secret
   key that is associated with a primary key of a certificate already
   present in the keystore.  Such a compromise permits an attacker to
   flood the rest of the network.  In the event that such a compromised
   key is identified, it might be placed on a blocklist (see

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   Section 3.10).  In particular, if a public key is added to a
   blocklist for a keystore implementing this mitigation, and it is
   removed from the keystore, then all certificates that were only
   "reachable" from the blocklisted certificate should also be
   simultaneously removed.

4.3.  Rate-limit Submissions by IP Address

   Some OpenPGP keystores accept material from the general public over
   the Internet.  If an abuse-resistant keystore observes a flood of
   material submitted to the keystore from a given Internet address, it
   MAY choose to throttle submissions from that address.  When receiving
   submissions over IPv6, such a keystore MAY choose to throttle entire
   nearby subnets, as a malicious IPv6 host is more likely to have
   multiple addresses.

   This requires that the keystore maintain state about recent
   submissions over time and address.  It may also be problematic for
   users who appear to share an IP address from the vantage of the
   keystore, including those beind a NAT, using a VPN, or accessing the
   keystore via Tor.

4.4.  Accept Certiifcates Based on Exterior Process

   Some public keystores resist abuse by explicitly filtering OpenPGP
   material based on a set of external processes.  For example,
   [DEBIAN-KEYRING] adjudicates the contents of the "Debian keyring"
   keystore based on organizational procedure and manual inspection.

4.5.  Accept Certificates by E-mail Validation

   Some keystores resist abuse by declining any certificate until the
   user IDs have been verified by e-mail.  When these "e-mail-
   validating" keystores review a new certificate that has a user ID
   with an e-mail address in it, they send an e-mail to the associated
   address with a confirmation mechanism (e.g., a high-entropy HTTPS URL
   link) in it.  In some cases, the e-mail itself is encrypted to an
   encryption-capable key found in the proposed certificate.  If the
   keyholder triggers the confirmation mechanism, then the keystore
   accepts the certificate.

   [PGP-GLOBAL-DIRECTORY] describes some concerns held by a keystore
   operator using this approach.  [MAILVELOPE-KEYSERVER] is another

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5.  Non-append-only mitigations

   The following mitigations may cause some previously-retained packets
   to be dropped after the keystore receives new information, or as time
   passes.  This is entirely reasonable for some keystores, but it may
   be surprising for any keystore that expects to be append-only (for
   example, some keyserver synchronization techniques may expect this
   property to hold).

   Furthermore, keystores that drop old data, or certifications that are
   superseded may make it difficult or impossible for their users to
   reason about the validity of signatures that were made in the past.
   See Section 9.3 for more considerations.

   Note also that many of these mitigations depend on cryptographic
   validation, so they're typically contextual as well.

   A keystore that needs to be append-only, or which cannot perform
   cryptographic validation MAY omit these mitigations.

   Note that [GnuPG] anticipates some of these suggestions with its
   "clean" subcommand, which is documented as:

   Compact  (by  removing all signatures except the selfsig)
   any user ID that is no longer usable  (e.g.  revoked,  or
   expired). Then, remove any signatures that are not usable
   by the trust calculations.   Specifically,  this  removes
   any  signature that does not validate, any signature that
   is superseded by a later signature,  revoked  signatures,
   and signatures issued by keys that are not present on the

5.1.  Drop Superseded Signatures

   An abuse-resistant keystore SHOULD drop all signature packets that
   are explicitly superseded.  For example, there's no reason to retain
   or distribute a self-sig by key K over User ID U from 2017 if the
   keystore have a cryptographically-valid self-sig over <K,U> from

   Note that this covers both certifications and signatures over
   subkeys, as both of these kinds of signature packets may be

   Getting this right requires a nuanced understanding of subtleties in
   [RFC4880] related to timing and revocation.

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5.2.  Drop Expired Signatures

   If a signature packet is known to only be valid in the past, there is
   no reason to distribute it further.  An abuse-resistant keystore with
   access to a functionally real-time clock SHOULD drop all
   certifications and subkey signature packets with an expiration date
   in the past.

   Note that this assumes that the keystore and its clients all have
   roughly-synchronized clocks.  If that is not the case, then there
   will be many other problems!

5.3.  Drop Dangling User IDs, User Attributes, and Subkeys

   If enough signature packets are dropped, it's possible that some of
   the things that those signature packets cover are no longer valid.

   An abuse-resistant keystore which has dropped all certifications that
   cover a User ID SHOULD also drop the User ID packet.

   Note that a User ID that becomes invalid due to revocation MUST NOT
   be dropped, because the User ID's revocation signature itself remains
   valid, and needs to be distributed.

   A primary key with no User IDs and no subkeys and no revocations MAY
   itself also be removed from distribution, though note that the
   removal of a primary key may make it impossible to cryptographically
   validate other certifications held by the keystore.

5.4.  Drop All Other Elements of a Directly-Revoked Certificate

   If the primary key of a certiifcate is revoked via a direct key
   signature, an abuse-resistant keystore SHOULD drop all the rest of
   the associated data (user IDs, user attributes, and subkeys, and all
   attendant certifications and subkey signatures).  This defends
   against an adversary who compromises a primary key and tries to flood
   the certificate to hide the revocation.

   Note that the direct key revocation signature MUST NOT be dropped.

   In the event that an abuse-resistant keystore is flooded with direct
   key revocation signatures, it should retain the hardest, earliest
   revocation (see also Section 10.1).

   In particular, if any of the direct key revocation signatures is a
   "hard" revocation, the abuse-resistant keystore SHOULD retain the
   earliest such revocation signature (by signature creation date).

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   Otherwise, the abuse-resistant keystore SHOULD retain the earliest
   "soft" direct key revocation signature it has seen.

   If either of the above date comparisons results in a tie between two
   revocation signatures of the same "hardness", an abuse-resistant
   keystore SHOULD retain the signature that sorts earliest based on a
   binary string comparison of the direct key revocation signature
   packet itself.

5.5.  Implicit Expiration Date

   In combination with some of the dropping mitigations above, a
   particularly aggressive abuse-resistant keystore MAY choose an
   implicit expiration date for all signature packets.  For example, a
   signature packet that claims no expiration could be treated by the
   keystore as expiring 3 years after issuance.  This would permit the
   keystore to eject old packets on a rolling basis.

   FIXME: it's not clear what should happen with signature packets
   marked with an explicit expiration that is longer than implicit
   maximum.  Should it be capped to the implicit date, or accepted?

   Warning: This idea is pretty radical, and it's not clear what it
   would do to an ecosystem that depends on such a keystore.  It
   probably needs more thinking.

6.  Updates-only Keystores

   In addition to the mitigations above, some keystores may resist abuse
   by declining to accept any user IDs or certifications whatsoever.

   Such a keystore MUST be capable of cryptographic validation.  It
   accepts primary key packets, cryptographically-valid direct-key
   signatures from a primary key over itself, subkeys and their
   cryptographically-validated binding signatures (and cross signatures,
   where necessary).

   Clients of an updates-only keystore cannot possibly use the keystore
   for certificate discovery, because there are no user IDs to match.
   However, they can use it for certificate update, as it's possible to
   ship revocations (which are direct key signatures), new subkeys,
   updates to subkey expiration, subkey revocation, and direct key
   signature-based certificate expiration updates.

   Note that many popular OpenPGP implemenations do not implement direct
   primary key expiration mechanisms, relying instead on user ID
   expirations.  These user ID expiration dates or other metadata

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   associated with a self-certification will not be distributed by an
   updates-only keystore.

   Certificates shipped by an updates-only keystore are technically
   invalid [RFC4880] "transferable public keys," because they lack a
   user ID packet.  However many OpenPGP implementations will accept
   such a certificate if they already know of a user ID for the
   certificate, because the composite certificate resulting from a merge
   will be a standards-compliant transferable public key.

7.  First-party-only Keystores

   Slightly more permissive than the updates-only keystore described in
   Section 6 is a keystore that also permits user IDs and their self-

   A first-party-only keystore only accepts and distributes
   cryptographically-valid first-party certifications.  Given a primary
   key that the keystore understands, it will only attach user IDs that
   have a valid self-sig, and will only accept and re-distribute subkeys
   that are also cryptographically valid (including requiring cross-sigs
   for signing-capable subkeys as recommended in [RFC4880]).

   This effectively solves the problem of abusive bloating attacks on
   any certificate, because the only party who can make a certificate
   overly large is the holder of the secret corresponding to the primary
   key itself.

   However, a first-party-only keystore is still problematic for those
   people who rely on the keystore for discovery of third-party
   certifications.  Section 8 attempts to address this lack.

8.  First-party-attested Third-party Certifications

   We can augment a first-party-only keystore to allow it to distribute
   third-party certifications as long as the first-party has signed off
   on the specific third-party certification.

   An abuse-resistant keystore SHOULD only accept a third-party
   certification if it meets the following criteria:

   o  The third-party certification MUST be cryptographically valid.
      Note that this means that the keystore needs to know the primary
      key for the issuer of the third-party certification.

   o  The third-party certification MUST have an unhashed subpacket of
      type Embedded Signature, the contents of which we'll call the
      "attestation".  This attestation is from the certificate's primary

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      key over the third-party certification itself, as detailed in the
      steps below:

   o  The attestation MUST be an OpenPGP signature packet of type 0x50
      (Third-Party Confirmation signature)

   o  The attestation MUST contain a hashed "Issuer Fingerprint"
      subpacket with the fingerprint of the primary key of the
      certificate in question.

   o  The attestation MUST NOT be marked as non-exportable.

   o  The attestation MUST contain a hashed Notation subpacket with the
      name "ksok", and an empty (0-octet) value.

   o  The attestation MUST contain a hashed "Signature Target" subpacket
      with "public-key algorithm" that matches the public-key algorithm
      of the third-party certification.

   o  The attestation's hashed "Signature Target" subpacket MUST use a
      reasonably strong hash algorithm (as of this writing, any
      [RFC4880] hash algorithm except MD5, SHA1, or RIPEMD160), and MUST
      have a hash value equal to the hash over the third-party
      certification with all unhashed subpackets removed.

   o  The attestation MUST be cryptographically valid, verifiable by the
      primary key of the certificate in question.

   What this means is that a third-party certificate will only be
   accepted/distributed by the keystore if:

   o  the keystore knows about both the first- and third-parties.

   o  the third-party has made the identity assertion

   o  the first-party has confirmed that they're OK with the third-party
      certification being distributed by any keystore.

   FIXME: it's not clear whether the "ksok" notification is necessary -
   it's in place to avoid some accidental confusion with any other use
   of the Third-Party Confirmation signature packet type, but the author
   does not know of any such use that might collide.

8.1.  Key Server Preferences "No-modify"

   [RFC4880] defines "Key Server Preferences" with a "No-modify" bit.
   That bit has never been respected by any keyserver implementation
   that the author is aware of.  This section effectively asks an abuse-

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   resistant keystore to treat that bit as always set, whether it is
   present in the certificate or not.

8.2.  Client Interactions

   The multi-stage layer of creating such an attestation (certificate
   creation by the first-party, certification by the third-party,
   attestation by the first-party, then handoff to the keystore) may
   represent a usability obstacle to a user who needs a third-party-
   certified OpenPGP certificate.

   No current OpenPGP client can easily create the attestions described
   in this section.  More implementation work needs to be done to make
   it easy (and understandable) for a user to perform this kind of

9.  Side Effects and Ecosystem Impacts

9.1.  Designated Revoker

   A first-party-only keystore as described in Section 7 might decline
   to distribute revocations made by the designated revoker.  This is a
   risk to certificate-holder who depend on this mechanism, because an
   important revocation might be missed by clients depending on the

   FIXME: adjust this document to point out where revocations from a
   designated revoker SHOULD be propagated, maybe even in first-party-
   only keystores.

9.2.  Certification-capable Subkeys

   Much of this discussion assumes that primary keys are the only
   certification-capable keys in the OpenPGP ecosystem.  Some proposals
   have been put forward that assume that subkeys can be marked as
   certification-capable.  If subkeys are certification-capable, then
   much of the reasoning in this draft becomes much more complex, as
   subkeys themselves can be revoked by their primary key without
   invalidating the key material itself.  That is, a subkey can be both
   valid (in one context) and invalid (in another context) at the same
   time.  So questions about what data can be dropped (e.g. in
   Section 5) are much fuzzier, and the underlying assumptions may need
   to be reviewed.

   If some OpenPGP implementations accept certification-capable subkeys,
   but an abuse-resistant keystore does not accept certifications from
   subkeys in general, then interactions between that keystore and those
   implementations may be surprising.

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9.3.  Assessing Certificates in the Past

   Online protocols like TLS perform signature and certificate
   evaluation based entirely on the present time.  If a certificate that
   signs a TLS handshake message is invalid now, it doesn't matter
   whether it was valid a week ago, because the present TLS session is
   the context of the evaluation.

   But OpenPGP signatures are often evaluated at some temporal remove
   from when the signature was made.  For example, software packages are
   signed at release time, but those signatures are validated at
   download time.

   Further complicating matters, the composable nature of an OpenPGP
   certificate means that the certificate associated with any particular
   signing key (primary key or subkey) can transform over time.  So when
   evaluating a signature that appears to have been made by a given
   certificate, it may be better to try to evaluate the certificate at
   the time the signature was made, rather than the present time.

   When evaluating a certificate at a time T in the past, one approach
   is to discard all packets with a creation time later than T, and then
   evaluate the resulting certificate from the remaining packets in the
   context of time T.

   However, any such evaluator SHOULD NOT ignore "hard" OpenPGP key
   revocations, regardless of their creation date. (see Section 10.1).

   If a non-append-only keystore (Section 5) has dropped superseded
   (Section 5.1) or expired (Section 5.2) certifications, it's possible
   for the certificate composed of the remaining packets to have no
   valid first-party certification at the time that a given signature
   was made.  Such a certificate would be invalid according to

10.  OpenPGP details

   This section collects details about common OpenPGP implementation
   behavior that are useful in evaluating and reasoning about OpenPGP

10.1.  Revocations

   It's useful to classify OpenPGP revocations of key material into two
   categories: "soft" and "hard".

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   If the "Reason for Revocation" of an OpenPGP key is either "Key is
   superseded" or "Key is retired and no longer used", it is a "soft"

   An implementation that interprets a "soft" revocation will typically
   not invalidate signatures made by the associated key with a creation
   date that predates the date of the soft revocation.  A "soft"
   revocation in some ways behaves like a non-overridable expiration

   All other revocations of OpenPGP keys (with any other Reason for
   Revocation, or with no Reason for Revocation at all) should be
   considered "hard".

   The presence of a "hard" revocation of an OpenPGP key indicates that
   the user should reject all signatures and certifications made by that
   key, regardless of the creation date of the signature.

   Note that some OpenPGP implementations do not distinguish between
   these two categories.

   A defensive OpenPGP implementation that does not distinguish between
   these two categories SHOULD treat all revocations as "hard".

   An implementation aware of a "soft" revocation or of key or
   certificate expiry at time T SHOULD accept and process a "hard"
   revocation even if it appears to have been issued at a time later
   than T.

10.2.  User ID Conventions

   [RFC4880] requires a user ID to be a UTF-8 string, but does not
   constrain it beyond that.  In practice, a handful of conventions
   predominate in how User IDs are formed.

   The most widespread convention is a name-addr as defined in
   [RFC5322].  For example:

   Alice Jones <alice@example.org>

   But a growing number of OpenPGP certificates contain user IDs that
   are instead a raw [RFC5322] addr-spec, omitting the display-name and
   the angle brackets entirely, like so:


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   Some certificates have user IDs that are simply "normal" human names
   (perhaps display-name in [RFC5322] jargon, though not necessarily
   conforming to a specific ABNF).  For example:

   Alice Jones

   Still other certificates identify a particular network service by
   scheme and hostname.  For example, the administrator of an ssh host
   participating in the [MONKEYSPHERE] might choose a user ID for the
   OpenPGP representing the host like so:


11.  Security Considerations

   This document offers guidance on mitigating a range of denial-of-
   service attacks on public keystores, so the entire document is in
   effect about security considerations.

   Many of the mitigations described here defend individual OpenPGP
   certificates against flooding attacks (see Section 2.1).  But only
   some of these mitigations defend against flooding attacks against the
   keystore itself (see Section 2.3), or against flooding attacks on the
   space of possible user IDs (see Section 2.2).  Thoughtful threat
   modeling and monitoring of the keystore and its defenses are probably
   necessary to maintain the long-term health of the keystore.

   Section 9.1 describes a potentially scary security problem for
   designated revokers.

   Note that there is an inherent tension between accepting arbitrary
   certificate uploads and permitting effective certificate discovery.
   If a keystore accepts arbitrary certificate uploads for
   redistribution, it appears to be vulnerable to user ID flooding
   (Section 2.2), which makes it difficult or impossible to rely on for
   certificate discovery.

   TODO (more security considerations)

12.  Privacy Considerations

   Keystores themselves raise a host of potential privacy concerns.
   Additional privacy concerns are raised by traffic to and from the
   keystores.  This section tries to outline some of the risks to the
   privacy of people whose certificates are stored and redistributed in
   public keystores, as well as risks to the privacy of people who make
   use of the key stores for certificate discovery or certificate

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   TODO (more privacy considerations)

12.1.  Publishing Identity Information

   Public OpenPGP keystores often distribute names or e-mail addresses
   of people.  Some people do not want their names or e-mail addresses
   distributed in a public keystore, or may change their minds about it
   at some point.  Append-only keystores are particularly problematic in
   that regard.  The mitigation in Section 5.4 can help such users strip
   their details from keys that they control.  However, if an OpenPGP
   certificate with their details is uploaded to a keystore, but is not
   under their control, it's unclear what mechanisms can be used to
   remove the certificate that couldn't also be exploited to take down
   an otherwise valid certificate.

   An updates-only keyserver (Section 6) avoids this particular privacy
   concern because it distributes no user IDs at all.

12.2.  Social Graph

   Third-party certifications effectively map out some sort of social
   graph.  A certification asserts a statement of belief by the issuer
   that the real-world party identified by the user ID is in control of
   the subject cryptographic key material.  But those connections may be
   potentially sensitive, and some people may not want these maps built.

   A first-party-only keyserver (Section 7) avoids this privacy concern
   because it distribues no third-party privacy concern.

   First-party attested third-party certifications described in
   Section 8 are even more relevant edges in the social graph, because
   their bidirectional nature suggests that both parties are aware of
   each other, and see some value in mutual association.

12.3.  Tracking Clients by Queries

   Even without third-party certifications, the acts of certificate
   discovery and certificate update represent a potential privacy risk,
   because the keystore queried gets to learn which user IDs (in the
   case of discovery) or which certificates (in the case of update) the
   client is interested in.  In the case of certificate update, if a
   client attempts to update all of its known certificates from the same
   keystore, that set is likely to be a unique set, and therefore
   identifies the client.  A keystore that monitors the set of queries
   it receives might be able to profile or track those clients who use
   it repeatedly.

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   Clients which want to to avoid such a tracking attack MAY try to
   perform certificate update from multiple different keystores.  To
   hide network location, a client making a network query to a keystore
   SHOULD use an anonymity network like [TOR].  Tools like [PARCIMONIE]
   are designed to facilitate this type of certificate update.

   Keystores which permit public access and want to protect the privacy
   of their clients SHOULD NOT reject access from clients using [TOR] or
   comparable anonymity networks.  Additionally, they SHOULD minimize
   access logs they retain.

   Alternately, some keystores may distribute their entire contents to
   any interested client, in what can be seen as the most trivial form
   of private information retrieval.  [DEBIAN-KEYRING] is one such
   example; its contents are distributed as an operating system package.
   Clients can interrogate their local copy of such a keystore without
   exposing their queries to a third-party.

12.4.  Cleartext Queries

   If access to the keystore happens over observable channels (e.g.,
   cleartext connections over the Internet), then a passive network
   monitor could perform the same type profiling or tracking attack
   against clients of the keystore described in Section 12.3.  Keystores
   which offer network access SHOULD provide encrypted transport.

12.5.  Traffic Analysis

   Even if a keystore offers encrypted transport, the size of queries
   and responses may provide effective identification of the specific
   certificates fetched during discovery or update, leaving open the
   types of tracking attacks described in Section 12.3.  Clients of
   keystores SHOULD pad their queries to increase the size of the
   anonymity set.  And keystores SHOULD pad their responses.

   The appropriate size of padding to effectively anonymize traffic to
   and from keystores is likely to be mechanism- and cohort-specific.
   For example, padding for keystores accessed via the DNS ([RFC7929]
   may use different padding strategies that padding for keystores
   accessed over WKD ([I-D.koch-openpgp-webkey-service]), which may in
   turn be different from keystores accessed over HKPS
   ([I-D.shaw-openpgp-hkp]).  A keystore which only accepts user IDs
   within a specific domain (e.g., Section 3.3) or which uses custom
   process (Section 4.4) for verification might have different padding
   criteria than a keystore that serves the general public.

   Specific padding policies or mechanisms are out of scope for this

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13.  User Considerations

   Section 8.2 describes some outstanding work that needs to be done to
   help users understand how to produce and distribute a third-party-
   certified OpenPGP certificate to an abuse-resistant keystore.

14.  IANA Considerations

   This document asks IANA to register the "ksok" notation name in the
   OpenPGP Notation IETF namespace, with a reference to this document,
   as defined in Section 8.

15.  Document Considerations

   [ RFC Editor: please remove this section before publication ]

   This document is currently edited as markdown.  Minor editorial
   changes can be suggested via merge requests at
   https://gitlab.com/dkg/draft-openpgp-abuse-resistant-keystore or by
   e-mail to the author.  Please direct all significant commentary to
   the public IETF OpenPGP mailing list: openpgp@ietf.org

15.1.  Document History

   substantive changes between -00 and -01:

   o  split out Contextual and Non-Append-Only mitigations

   o  documented several other mitigations, including:

      *  Decline Data From the Future

      *  Blocklist

      *  Exterior Process

      *  Designated Authorities

      *  Known Certificates

      *  Rate-Limiting

      *  Scoped User IDs

   o  documented Updates-Only Keystores

   o  consider three different kinds of flooding

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   o  deeper discussion of privacy considerations

   o  better documentation of Reason for Revocation

   o  document user ID conventions

16.  Acknowledgements

   This document is the result of years of operational experience and
   observation, as well as conversations with many different people -
   users, implementors, keystore operators, etc.  A non-exhaustive list
   of people who have contriubuted ideas or nuance to this document
   specifically includes:

   o  Antoine Beaupre

   o  Jamie McClelland

   o  Jonathan McDowell

   o  Justus Winter

   o  Neal Walfield

   o  vedaal

   o  Vincent Breitmoser

   o  Wiktor Kwapisiewicz

17.  References

17.1.  Normative References

              Koch, W., carlson, b., Tse, R., and D. Atkins, "OpenPGP
              Message Format", draft-ietf-openpgp-rfc4880bis-06 (work in
              progress), November 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,

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   [RFC8174]  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>.

17.2.  Informative References

              McDowell, J., "Debian Keyring", n.d.,

   [GnuPG]    Koch, W., "Using the GNU Privacy Guard", n.d.,

              Koch, W., "OpenPGP Web Key Directory", draft-koch-openpgp-
              webkey-service-07 (work in progress), November 2018.

              Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
              draft-shaw-openpgp-hkp-00 (work in progress), March 2003.

              Oberndoerfer, T., "Mailvelope Keyserver", n.d.,

              Gillmor, D. and J. Rollins, "Monkeysphere", n.d.,

              Intrigeri, ., "Parcimonie", n.d.,

              Symantec Corporation, "PGP Global Directory Key
              Verification Policy", 2011,

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,

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   [SKS]      Pennock, P., "SKS Keyserver Documentation", March 2018,

   [TOR]      "The Tor Project", n.d., <https://www.torproject.org/>.

Author's Address

   Daniel Kahn Gillmor
   American Civil Liberties Union
   125 Broad St.
   New York, NY  10004

   Email: dkg@fifthhorseman.net

Gillmor                  Expires October 8, 2019               [Page 27]

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