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Abuse-Resistant OpenPGP Keystores

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Author Daniel Kahn Gillmor
Last updated 2019-04-04
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openpgp                                                       D. Gillmor
Internet-Draft                                                      ACLU
Intended status: Informational                            April 04, 2019
Expires: October 6, 2019

                   Abuse-Resistant OpenPGP Keystores


   OpenPGP transferable public keys are composite certificates, made up
   of primary keys, 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
   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 third-party
   certificate 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

   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 6, 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
   ( 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 . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Simple Mitigations  . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Limited Packet Sizes  . . . . . . . . . . . . . . . . . .   6
     3.2.  Strict User IDs . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Drop or Standardize Unhashed Subpackets . . . . . . . . .   6
     3.4.  Drop User Attributes  . . . . . . . . . . . . . . . . . .   7
     3.5.  Drop Non-exportable Certifications  . . . . . . . . . . .   7
     3.6.  Accept Only Cryptographically-verifiable Certifications .   7
     3.7.  Accept Only Profiled Certifications . . . . . . . . . . .   7
   4.  Contextual Mitigations  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Drop Superseded Signatures  . . . . . . . . . . . . . . .   8
     4.2.  Drop Expired Signatures . . . . . . . . . . . . . . . . .   8
     4.3.  Drop Dangling User IDs, User Attributes, and Subkeys  . .   9
     4.4.  Drop All Other Elements of a Directly-Revoked Certificate   9
     4.5.  Implicit Expiration Date  . . . . . . . . . . . . . . . .  10
   5.  First-party-only Keystores  . . . . . . . . . . . . . . . . .  10
   6.  First-party-attested Third-party Certifications . . . . . . .  11
     6.1.  Key Server Preferences "No-modify"  . . . . . . . . . . .  12
     6.2.  Client Interactions . . . . . . . . . . . . . . . . . . .  12
   7.  Side Effects and Ecosystem Impacts  . . . . . . . . . . . . .  12
     7.1.  Designated Revoker  . . . . . . . . . . . . . . . . . . .  12
     7.2.  Certification-capable Subkeys . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13
   10. User Considerations . . . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   12. Document Considerations . . . . . . . . . . . . . . . . . . .  14
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     13.2.  Informative References . . . . . . . . . . . . . . . . .  15
     13.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

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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.

   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.  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

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      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 []
      are both certificate discovery mechanisms.

   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 [1]" and the user wants to send an e-mail to, 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
      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 [].

   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

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      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 [2]" might not belong to the person who controls
      the e-mail address "" 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
      keyservers (like [SKS]) perform no cryptographic validation

2.  Problem Statement

   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.

   The rest of this 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.

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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.  Limited Packet Sizes

   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
   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.  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://".

   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.  Drop 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

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   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
   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 6.

3.4.  Drop 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.

3.5.  Drop 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

3.6.  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.

3.7.  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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4.  Contextual Mitigations

   The following mitigations may cause some 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

   Note also that many of these mitigations depend on cryptographic

   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

4.1.  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.

4.2.  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

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   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!

4.3.  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.

4.4.  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 strongest, earliest

   In particular, if any of the revocation signatures has a "Reason for
   Revocation" of "Key material has been compromised", the keystore
   SHOULD retain the earliest such revocation signature (by signature
   creation date).

   If none have "Key material has been compromised", but some have "No
   reason specified", or lack a "Reason for Revocation" entirely, then
   the keystore SHOULD retain the earliest such revocation signature.

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

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

4.5.  Implicit Expiration Date

   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.

   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.

5.  First-party-only Keystores

   In addition to all of the mitigations above, some keystores may
   resist abuse by declining to carry third-party certifications

   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, first-party-only keystores also introduce new problems, for
   those people who rely on the keystore for discovery of third-party
   certifications.  Section 6 attempts to address this lack.

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6.  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
      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 notation subpacket

   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:

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   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.

6.1.  Key Server Preferences "No-modify"

   [RFC4880] section ("Key Server Preferences") defines 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-resistant keystore to treat that bit as always set,
   whether it is present in the certificate or not.

6.2.  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

7.  Side Effects and Ecosystem Impacts

7.1.  Designated Revoker

   A first-party-only keystore might decline to distribute revocations
   made by the designated revoker.  This is a risk to certificate-holder
   who depend on this mechanism.  Perhaps this document should be
   amended to include these

7.2.  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

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   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 are much fuzzier.

   The author of this draft recommends _not_ considering any subkeys to
   be certification-capable to avoid this headache.

8.  Security Considerations

   These mitigations defend individual OpenPGP certificates against
   bloating attacks.  They collectively reduce the amount of data that
   such a keystore needs to track over time, but given the near-infinite
   space of possible OpenPGP keys that can be generated, the keystore in
   aggregate can still be made to grow without bound.  This document
   proposes no clear measures to defend against such a denial of service
   attack against the keystore itself.

   Section 7.1 describes a potentially

   TODO (more security considerations)

9.  Privacy Considerations

   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 4.4 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.

   Third-party certifications effectively map out some sort of social
   graph.  While the certifications basically only assert a binding
   between user IDs, the parties those user IDs represent in the real
   world, and cryptographic key material, those connections may be
   potentially sensitive, and users may not want to see these maps

   TODO (more privacy considerations)

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10.  User Considerations

   Section 6.2 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.

11.  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 6.

12.  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 or by
   e-mail to the author.  Please direct all significant commentary to
   the public IETF OpenPGP mailing list:

13.  References

13.1.  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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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13.2.  Informative References

   [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.,

   [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,

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

13.3.  URIs



Author's Address

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


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