openpgp                                                    D. K. Gillmor
Internet-Draft                                                      ACLU
Intended status: Informational                             28 April 2022
Expires: 30 October 2022

                   Abuse-Resistant OpenPGP Keystores


   OpenPGP transferable public keys are composite certificates, made up
   of primary keys, revocation signatures, 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

   Unfortunately, since many keystores permit any third-party to add a
   certification with any content to any OpenPGP certificate, the
   assembled/merged form of a certificate can become unwieldy or
   undistributable.  Furthermore, keystores that are searched by user ID
   or fingerprint can be made unusable for specific searches by public
   submission of bogus certificates.  And finally, keystores open to
   public submission can also face simple resource exhaustion from
   flooding with bogus submissions, or legal or other risks from uploads
   of toxic data.

   This draft documents techniques that an archive of OpenPGP
   certificates can use to mitigate the impact of these various attacks,
   and the implications of these concerns and mitigations for the rest
   of the OpenPGP ecosystem.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   Status information for this document may be found at

   Discussion of this document takes place on the OpenPGP Working Group
   mailing list (, which is archived at

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   Source for this draft and an issue tracker can be found at

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   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 30 October 2022.

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   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   8
     2.1.  Certificate Flooding  . . . . . . . . . . . . . . . . . .   8
     2.2.  User ID Flooding  . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Fingerprint Flooding  . . . . . . . . . . . . . . . . . .   9
     2.4.  Keystore Flooding . . . . . . . . . . . . . . . . . . . .   9
     2.5.  Toxic Data  . . . . . . . . . . . . . . . . . . . . . . .  10
   3.  Keystore Interfaces . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Certificate Refresh . . . . . . . . . . . . . . . . . . .  10
     3.2.  Certificate Discovery . . . . . . . . . . . . . . . . . .  11

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     3.3.  Certificate Lookup  . . . . . . . . . . . . . . . . . . .  12
       3.3.1.  Full User ID Lookup . . . . . . . . . . . . . . . . .  12
       3.3.2.  E-mail Address Lookup . . . . . . . . . . . . . . . .  12
       3.3.3.  Other Lookup Mechanisms . . . . . . . . . . . . . . .  13
     3.4.  Certificate Validation  . . . . . . . . . . . . . . . . .  13
     3.5.  Certificate Submission  . . . . . . . . . . . . . . . . .  14
   4.  Simple Mitigations  . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Decline Large Packets . . . . . . . . . . . . . . . . . .  15
     4.2.  Enforce Strict User IDs . . . . . . . . . . . . . . . . .  15
     4.3.  Scoped User IDs . . . . . . . . . . . . . . . . . . . . .  16
     4.4.  Strip or Standardize Unhashed Subpackets  . . . . . . . .  16
       4.4.1.  Issuer Fingerprint  . . . . . . . . . . . . . . . . .  16
       4.4.2.  Cross-sigs  . . . . . . . . . . . . . . . . . . . . .  16
     4.5.  Decline User Attributes . . . . . . . . . . . . . . . . .  16
     4.6.  Decline Non-exportable Certifications . . . . . . . . . .  17
     4.7.  Decline Data From the Future  . . . . . . . . . . . . . .  17
     4.8.  Accept Only Profiled Certifications . . . . . . . . . . .  17
     4.9.  Accept Only Certificates Issued by Designated
            Authorities  . . . . . . . . . . . . . . . . . . . . . .  17
     4.10. Decline Packets by Blocklist  . . . . . . . . . . . . . .  18
   5.  Retrieval-time Mitigations  . . . . . . . . . . . . . . . . .  19
     5.1.  Redacting User IDs  . . . . . . . . . . . . . . . . . . .  19
       5.1.1.  Certificate Refresh with Redacted User IDs  . . . . .  19
       5.1.2.  Certificate Discovery with Redacted User IDs  . . . .  20
       5.1.3.  Certificate Lookup with Redacted User IDs . . . . . .  20
       5.1.4.  Hinting Redacted User IDs . . . . . . . . . . . . . .  21
       5.1.5.  User ID Recovery by Client Brute Force  . . . . . . .  21
     5.2.  Primary-key Only Certificate Refresh  . . . . . . . . . .  21
     5.3.  Require Valid Cross-Sigs for Certificate Discovery  . . .  22
   6.  Contextual Mitigations  . . . . . . . . . . . . . . . . . . .  23
     6.1.  Accept Only Cryptographically-verifiable
           Certifications  . . . . . . . . . . . . . . . . . . . . .  23
     6.2.  Accept Only Certificates Issued by Known Certificates . .  23
     6.3.  Rate-limit Submissions by IP Address  . . . . . . . . . .  24
     6.4.  Accept Certificates Based on Exterior Process . . . . . .  24
     6.5.  Accept Certificates by E-mail Validation  . . . . . . . .  24
   7.  Non-append-only mitigations . . . . . . . . . . . . . . . . .  25
     7.1.  Drop Superseded Signatures  . . . . . . . . . . . . . . .  25
     7.2.  Drop Expired Signatures . . . . . . . . . . . . . . . . .  26
     7.3.  Drop Dangling User IDs, User Attributes, and Subkeys  . .  26
     7.4.  Drop All Other Elements of a Directly-Revoked
           Certificate . . . . . . . . . . . . . . . . . . . . . . .  26
     7.5.  Implicit Expiration Date  . . . . . . . . . . . . . . . .  27
   8.  Primary Key Sovereignty . . . . . . . . . . . . . . . . . . .  27
     8.1.  Refresh-only Keystores  . . . . . . . . . . . . . . . . .  28
     8.2.  First-party-only Keystores  . . . . . . . . . . . . . . .  29
       8.2.1.  First-party-only Without User IDs . . . . . . . . . .  29
     8.3.  First-party-attested Third-party Certifications . . . . .  29

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       8.3.1.  Client Interactions . . . . . . . . . . . . . . . . .  30
       8.3.2.  Revoking Third-party Certifications . . . . . . . . .  30
   9.  Keystore Client Best Practices  . . . . . . . . . . . . . . .  32
     9.1.  Use Constrained Keystores for Lookup  . . . . . . . . . .  32
     9.2.  Normalize Addresses and User IDs for Lookup . . . . . . .  32
     9.3.  Avoid Fuzzy Lookups . . . . . . . . . . . . . . . . . . .  32
     9.4.  Prefer Full Fingerprint for Discovery and Refresh . . . .  33
     9.5.  Use Caution with Keystore-provided Validation . . . . . .  33
   10. Certificate Generation and Management Best Practices  . . . .  33
     10.1.  Canonicalized E-Mail Addresses . . . . . . . . . . . . .  34
     10.2.  Normalized User IDs  . . . . . . . . . . . . . . . . . .  34
     10.3.  Avoid Large User Attributes  . . . . . . . . . . . . . .  34
     10.4.  Provide Cross-Sigs . . . . . . . . . . . . . . . . . . .  34
     10.5.  Provide Issuer Fingerprint Subpackets  . . . . . . . . .  35
     10.6.  Put Cross-Sigs and Issuer Fingerprint in Hashed
            Subpackets . . . . . . . . . . . . . . . . . . . . . . .  35
     10.7.  Submit Certificates to Restricted, Lookup-Capable
            Keystores  . . . . . . . . . . . . . . . . . . . . . . .  35
   11. Side Effects and Ecosystem Impacts  . . . . . . . . . . . . .  35
     11.1.  Designated Revoker . . . . . . . . . . . . . . . . . . .  35
     11.2.  Key IDs vs. Fingerprints in Certificate Discovery  . . .  36
     11.3.  In-band Certificates . . . . . . . . . . . . . . . . . .  36
       11.3.1.  In-band Certificate Minimization and Validity  . . .  37
     11.4.  Certification-capable Subkeys  . . . . . . . . . . . . .  38
     11.5.  Assessing Certificates in the Past . . . . . . . . . . .  38
       11.5.1.  Point-in-time Certificate Evaluation . . . . . . . .  39
       11.5.2.  Signature Verification and Non-append-only
               Keystores . . . . . . . . . . . . . . . . . . . . . .  39
     11.6.  Global Append-only Ledgers ("Blockchain")  . . . . . . .  39
     11.7.  Certificate Lookup for Identity Monitoring . . . . . . .  41
   12. OpenPGP details . . . . . . . . . . . . . . . . . . . . . . .  41
     12.1.  Revocations  . . . . . . . . . . . . . . . . . . . . . .  41
     12.2.  User ID Conventions  . . . . . . . . . . . . . . . . . .  42
     12.3.  E-mail Address Canonicalization  . . . . . . . . . . . .  43
       12.3.1.  Disallowing Non-UTF-8 Local Parts  . . . . . . . . .  43
       12.3.2.  Domain Canonicalization  . . . . . . . . . . . . . .  43
       12.3.3.  Local Part Canonicalization  . . . . . . . . . . . .  43
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  43
     13.1.  Tension Between Unrestricted Uploads and Certificate
            Lookup . . . . . . . . . . . . . . . . . . . . . . . . .  44
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  44
     14.1.  Publishing Identity Information  . . . . . . . . . . . .  44
     14.2.  Social Graph . . . . . . . . . . . . . . . . . . . . . .  45
     14.3.  Tracking Clients by Queries  . . . . . . . . . . . . . .  45
     14.4.  "Live" Certificate Validation Leaks Client Activity  . .  46
     14.5.  Certificate Discovery Leaks Client Activity  . . . . . .  46
     14.6.  Certificate Refresh Leaks Client Activity  . . . . . . .  47

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     14.7.  Distinct Keystore Interfaces Leak Client Context and
            Intent . . . . . . . . . . . . . . . . . . . . . . . . .  47
     14.8.  Cleartext Queries  . . . . . . . . . . . . . . . . . . .  48
     14.9.  Traffic Analysis . . . . . . . . . . . . . . . . . . . .  48
   15. User Considerations . . . . . . . . . . . . . . . . . . . . .  49
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
     16.1.  Document History . . . . . . . . . . . . . . . . . . . .  49
   17. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  51
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  52
     18.2.  Informative References . . . . . . . . . . . . . . . . .  53
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  55

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

   *  "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.

   *  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.

   *  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".

   *  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".

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   *  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.

   *  All subkeys are bound to the primary key with an [RFC4880] Subkey
      Binding Signature.  Some subkeys also reciprocate by binding
      themselves back to the primary key with an [RFC4880] Primary Key
      Binding Signature.  The Primary Key Binding Signature is also
      known as a "cross-signature" or "cross-sig".

   *  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.

   *  "Certificate validation" is the process whereby a user decides
      whether a given user ID in an OpenPGP certificate is acceptable
      for use.  For example, if the certificate has a user ID of Alice
      <> 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.  Some clients may rely on specific keystores for
      certificate validation, but some keystores (e.g., [SKS]) make no
      assertions whatsoever about certificate validity, and others offer
      only very subtle guarantees.  See Section 3.4 for more details.

   *  "Certificate lookup" refers to the retrieval of a set of
      certificates from a keystore based on the user ID or some
      substring match of the user ID.  See Section 3.3 for more details.

   *  "Certificate refresh" refers to retrieval of a certificate from a
      keystore based on the fingerprint of the primary key.  See
      Section 3.1 for more details.

   *  "Certificate discovery" refers to the retrieval of a set of
      certificates from a keystore based on the fingerprint or key ID of
      any key in the certificate.  See Section 3.2 for more details.

   *  A "keyserver" is a particular kind of keystore, typically a 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 [].

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   *  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 lookup, certificate
      discovery, and certificate refresh (including 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.

   *  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 optionally 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
      lookup based on e-mail address and certificate validation (by
      users who trust the operator), but some may not be useful for
      certificate refresh or certificate discovery, 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.

   *  A "sovereignty-respecting" keystore is one that only distributes
      data associated with a given certificate that has been explicitly
      approved by the primary key of that certificate.  See Section 8
      for more details and example strategies.

   *  "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
      <> 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

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   *  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 12.1 for more

2.  Problem Statement

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

   This section describes five distinct attacks that public keystores
   should consider.

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 lookup, discovery, or refresh.  In the case of
   a revoked certificate that has been flooded, this potentially leaves
   the client of the keystore with the compromised certificate in an
   unrevoked state locally because it was unable to fetch the revocation

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

2.2.  User ID Flooding

   Public keystores that are used for certificate lookup 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
   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, the attacker may upload thousands of certificates

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   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 still has
   to iterate through all of them in order to find the non-malicious

   User ID flooding is only effective if the keystore offers a lookup
   interface at all.

2.3.  Fingerprint Flooding

   A malicious actor who wants to render a certificate unavailable for
   refresh may generate an arbitrary number of OpenPGP certificates with
   the targeted primary key attached as a subkey.  If they can convince
   a keystore to accept all of those certificates, and the keystore
   returns them by subkey match during certificate refresh, then the
   certificate refresh client will need to spend an arbitrary amount of
   bandwidth and processing power filtering out the irrelevant data, and
   may potentially give up before discovering the certificate of

   A malicious actor may also want to confuse a certificate discovery
   request that was targeted at a particular subkey, by binding that
   subkey to multiple bogus certificates.  If these bogus certificates
   are ingested and redistributed by the keystore, then a certificate
   discovery client may receive a set of certificates that cannot be
   adequately distinguished.

2.4.  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.

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2.5.  Toxic Data

   Like any large public dataset, it's possible that a keystore ends up
   hosting some content that is legally actionable in some
   jurisdictions, including libel, child pornography, material under
   copyright or other "intellectual property" controls, blasphemy, hate
   speech, etc.

   A public keystore that accepts and redistributes arbitrary content
   may face risk due to uploads of toxic data.

3.  Keystore Interfaces

   Some keystores have simple interfaces, like files present in a local
   filesystem.  But many keystores offer an API for certificate
   retrieval of different types.  This section documents a set of useful
   interactions that a client may have with such a keystore.

   They are represented in abstract form, and are not intended to be the
   full set of interfaces offered by any keystore, but rather a
   convenient way to think about the operations that make the keystore
   useful for its clients.

   Not all keystores may offer all of these interfaces, or they may
   offer them in subtly different forms, but clients will nevertheless
   try to perform something like these operations with keystores that
   they interact with.

3.1.  Certificate Refresh

   This is the simplest keystore operation.  The client sends the
   keystore the full fingerprint of the certificate's primary key, and
   the keystore sends the client the corresponding certificate (or
   nothing, if the keystore does not contain a certificate with a
   matching primary key).

   keystore.cert_refresh(primary_fpr) -> certificate?

   A client uses certificate refresh to retrieve the full details of a
   certificate that it already knows about.  For example, it might be
   interested in refreshs to the certificate known to the keystore,
   including revocations, expiration refreshs, new third-party
   certifications, etc.

   Upon successful refresh, the client SHOULD merge the retrieved
   certificate with its local copy.

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   Not all keystores offer this operation.  For example, clients cannot
   use WKD ([I-D.koch-openpgp-webkey-service]) or OPENPGPKEY ([RFC7929])
   for certificate refresh.

3.2.  Certificate Discovery

   If a client is aware of an OpenPGP signature or certification that it
   cannot verify because it does not know the issuing certificate, it
   may consult a keystore to try to discover the certificate based on
   the Issuer or Issuer Fingerprint subpacket in the signature or
   certification it is trying to validate.

   keystore.cert_discovery(keyid|fpr) -> certificate_list

   This is subtly different from certificate refresh (Section 3.1) in
   three ways:

   *  it may return more than one certificate (e.g., when multiple
      certificates share a subkey, or when a primary key on one
      certificate is a subkey on another)

   *  it is willing to accept searches by short key ID, not just

   *  it is willing to match against a subkey, not just a primary key

   While a certificate discovery client does not initially know the
   certificate it is looking for, it's possible that the returned
   certificate is one that the client already knows about.  For example,
   a new subkey may have been added to a certificate.

   Upon successful discovery, the client SHOULD merge any retrieved
   certificates with discovered local copies (as determined by primary
   key), and then evaluate the original signature against any retrieved
   certificate that appears to be valid and reasonable for use in the
   signing context.

   It is unclear what a client should do if multiple certificates do
   appear to be valid for a given signature, because of ambiguity this
   represents about the identity of the signer.  However, this ambiguity
   is similar to the ambiguity of a certificate with multiple valid user
   IDs, which the client already needs to deal with.

   Not all keystores offer this operation.  For example, clients cannot
   use WKD ([I-D.koch-openpgp-webkey-service]) or OPENPGPKEY ([RFC7929]
   for certificate discovery.

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3.3.  Certificate Lookup

   If a client wants to encrypt a message to a particular e-mail
   address, or wants to encrypt a backup to some identity that it knows
   of but does not have a certificate for, it may consult a keystore to
   discover certificates that claim that identity in their user ID
   packets.  Both [I-D.koch-openpgp-webkey-service] and
   [] offer certificate lookup mechanisms.

   [RFC4880] User IDs are constrained only in that they are a UTF-8
   string, but some conventions govern their practical use.  See
   Section 12.2 for more discussion of some common conventions around
   user ID structure.

   Note that lookup does not necessarily imply user ID or certificate
   validation.  It is entirely possible for a keystore to return a
   certificate during lookup that the client cannot validate.

   Abuse-resistant keystores that offer a lookup interface SHOULD
   distinguish interfaces that perform full-string-match lookup from
   interfaces that perform e-mail address based lookup.

3.3.1.  Full User ID Lookup

   The most straightforward form of certificate lookup asks for the set
   of all certificates that contain a user ID that exactly and
   completely matches the query parameter supplied by the client.

   keystore.cert_lookup(uid) -> certificate_list

   In its simplest form, this match is done by a simple bytestring
   comparison.  More sophisticated keystores MAY perform the comparison
   after applying [UNICODE-NORMALIZATION] form NFC to both the uid query
   and the user IDs from the stored certificates.

3.3.2.  E-mail Address Lookup

   However, some common use cases look for specific patterns in the user
   ID rather than the entire user ID.  Most useful to many existing
   OpenPGP clients is a lookup by e-mail address.

   keystore.cert_lookup(addr) -> certificate_list

   For certificates with a user ID that matches the structure of an
   [RFC5322] name-addr or addr-spec, a keystore SHOULD extract the addr-
   spec from the user ID, canonicalize it (see Section 12.3), and
   compare it to the canonicalized form of of the addr query parameter.

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3.3.3.  Other Lookup Mechanisms

   Some keystores offer other forms of substring or regular expression
   matching against the stored user IDs.  These other forms of lookup
   may be useful in some contexts (e.g., Section 11.7), but they may
   also represent privacy concerns (e.g., Section 14.1), and they may
   impose additional computational or indexing burdens on the keystore.

3.4.  Certificate Validation

   An OpenPGP client may assess certificate and user ID validity based
   on many factors, some of which are directly contained in the
   certificate itself (e.g., third-party certifications), and some of
   which are based on the context known to the client, including:

   *  Whether it has seen e-mails from that address signed by that
      certificate in the past,

   *  How long it has known about the certificate,

   *  Whether the certificate was fetched from a keystore that asserts
      validity of the user ID or some part of it (such as the e-mail

   A keystore MAY facilitate clients pursuing this last point of
   contextual corroboration via a direct interface:

   keystore.cert_validate(primary_fpr, uid) -> boolean

   In an e-mail-specific context, the client might only care about the
   keystore's opinion about the validity of the certificate for the
   e-mail address portion of the user ID only:

   keystore.cert_validate(primary_fpr, addr) -> boolean

   For some keystores, the presence of a certificate in the keystore
   alone implies that the keystore asserts the validity of all user IDs
   in the certificate retrieved.  For others, the presence in the
   keystore applies only to some part of the user ID.  For example,
   [PGP-GLOBAL-DIRECTORY] will only return user IDs that have completed
   an e-mail validation step, so presence in that keystore implies an
   assertion of validity of the e-mail address part of the user IDs
   returned, but makes no claim about the display-name portion of any
   returned user IDs.  Note that a client retrieving a certificate from
   such a keystore may merge the certificate with a local copy -- but
   the validity asserted by the keystore of course has no bearing on the
   packets that the keystore did not return.

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   In a more subtle example, the retrieval of a certificate looked up
   via WKD ([I-D.koch-openpgp-webkey-service]) or DANE ([RFC7929])
   should only be interpreted as a claim of validity about any user ID
   which matches the e-mail address by which the certificate was looked
   up, with no claims made about any display-name portions, or about any
   user ID that doesn't match the queried e-mail address at all.

   A keystore that offers some sort of validation interface may also
   change its opinion about the validity of a given certificate or user
   ID over time; the interface described above only allows the client to
   ask about the keystore's current opinion, but a more complex
   interface might be capable of describing the keystore's assertion
   over time.  See also Section 11.5.

   An abuse-resistant keystore that clients rely on for any part of
   their certificate validation process SHOULD offer a distinct
   interface for making assertions about certificate and user ID
   validity to help clients avoid some of the subtleties involved with
   inference based on presence described above.

   Note that the certificate validation operation as described above has
   a boolean response.  While a "true" response indicates that keystore
   believes the user ID or e-mail address is acceptable for use with the
   certificate referred to by the public key fingerprint, a "false"
   response doesn't necessarily mean that the keystore actively thinks
   that the certificate is actively bad, or must not be used for the
   referenced identity.  Rather, "false" is the default state: no
   opinion is expressed by the keystore, and the client is left to make
   their own inference about validity based on other factors.  A
   keystore MAY offer a more nuanced validity interface; if it does, it
   SHOULD explicitly document the semantics of the different response
   types so that clients can make appropriate judgment.

3.5.  Certificate Submission

   Different keystores have different ways to submit a certificate for
   consideration for ingestion, including:

   *  a simple upload of a certificate via HTTP

   *  round-trip e-mail verification

   *  proof of presence in some other service

   *  vouching, or other forms of multi-party attestation

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   Because these schemes vary so widely, this document does not attempt
   to describe the keystore certificate submission process in detail.
   However, guidance can be found for implementations that generate,
   manage, and submit certificates in Section 10.

4.  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

4.1.  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
   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.

4.2.  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:// (see Section 12.2).

   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.

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4.3.  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.

4.4.  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 keystore SHOULD strip out all unhashed subpackets
   but the following exceptions:

4.4.1.  Issuer Fingerprint

   Some certifications only identify the issuer of the certification by
   an unhashed Issuer or Issuer Fingerprint subpacket.  If a
   certification's hashed subpacket section has no Issuer Fingerprint
   (see [I-D.ietf-openpgp-rfc4880bis]) subpacket, then an abuse-
   resistant keystore that has cryptographically validated the
   certification SHOULD synthesize an appropriate Issuer Fingerprint
   subpacket and include it in the certification's unhashed subpackets.

4.4.2.  Cross-sigs

   Some Primary Key Binding Signatures ("cross-sigs") are distributed as
   unhashed subpackets in a Subkey Binding Signature.  A
   cryptographically-validating abuse-resistant keystore SHOULD be
   willing to redistribute a valid cross-sig as an unhashed subpacket.

   The redistributed unhashed cross-sig itself should be stripped of all
   unhashed subpackets.

4.5.  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.

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4.6.  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

4.7.  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 past.

   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 achieve that purpose.  This material
   would not be distributable ahead of time by an abuse-resistant
   keystore that adopts this mitigation.

4.8.  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.

   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 (see Section 7).

4.9.  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.

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   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 (see Section 7).

4.10.  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 (see Section 7).

   Some keystores may choose to apply a blocklist only at retrieval time
   and not apply it at ingestion 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 (see Section 5 for more discussion).

   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 as it may describe data that is toxic
   (Section 2.5) in one community or jurisdiction but not another.
   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.

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5.  Retrieval-time Mitigations

   Most of the abuse mitigations described in this document are
   described as being applied at certificate ingestion time.  It's also
   possible to apply the same mitigations when a certificate is
   retrieved from the keystore (that is, during certificate lookup,
   refresh, or discovery).  Applying an abuse mitigation at retrieval
   time may help a client defend against a user ID flooding
   (Section 2.2), certificate flooding (Section 2.1), or fingerprint
   flooding (Section 2.3) attack.  It may also help a keystore limit its
   liability for redistributing toxic data (Section 2.5).  However, only
   mitigations applied at ingestion time are able to mitigate keystore
   flooding attacks (Section 2.4).

   Some mitigations (like the non-append-only mitigations described in
   Section 7) may be applied as filters at retrieval time, while still
   allowing access to the (potentially much larger) unfiltered dataset
   associated given certificate or user ID via a distinct interface.

   The rest of this section documents specific mitigations that are only
   relevant at retrieval time (certificate discovery, lookup, or

5.1.  Redacting User IDs

   Some abuse-resistant keystores may accept and store user IDs but
   decline to redistribute some or all of them, while still distributing
   the certifications that cover those redacted user IDs.  This draft
   refers to such a keystore as a "user ID redacting" keystore.

   The certificates distributed by such a keystore are technically
   invalid [RFC4880] "transferable public keys", because they lack a
   user ID packet, and the distributed certifications cannot be
   cryptographically validated independently.  However, an OpenPGP
   implementation that already knows the user IDs associated with a
   given primary key will be capable of associating each certification
   with the correct user ID by trial signature verification.

5.1.1.  Certificate Refresh with Redacted User IDs

   A user ID redacting keystore is useful for certificate refresh by a
   client that already knows the user ID it expects to see associated
   with the certificate.  For example, a client that knows a given
   certificate currently has two specific user IDs could access the
   keystore to learn that one of the user IDs has been revoked, without
   any other client learning the user IDs directly from the keystore.

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5.1.2.  Certificate Discovery with Redacted User IDs

   A user ID redacting keystore is somewhat less useful for clients
   doing certificate discovery.  Consider the circumstance of receiving
   a signed e-mail without access to the signing certificate.  If the
   verifier retrieves the certificate from a user ID redacting keystore
   by via the Issuer Fingerprint from the signature, and the signature
   validates, the received certificate might not be a valid
   "transferable public key" unless the client can synthesize the proper
   user ID.

   A reasonable client that wants to validate a certification in the
   user ID redacted certificate SHOULD try to synthesize possible user
   IDs based on the value of the [RFC5322] From: header in the message:

   *  Decode any [RFC2047] encodings present in the raw header value,
      converting into UTF-8 [UNICODE-NORMALIZATION] form C (NFC),
      trimming all whitespace from the beginning and the end of the

   *  The resulting string should be an [RFC5322] name-addr or addr-

   *  If it is a name-addr, convert the UTF-8 string into an OpenPGP
      user ID and check whether the certification validates, terminating
      on success.

      -  If the test fails, extract the addr-spec from the name-addr and

   *  Canonicalize the addr-spec according to Section 12.3, and check
      whether the certification validates, terminating on success.

   *  If it doesn't validate wrap the canonicalized addr-spec in angle-
      brackets ("<" and ">") and test the resulting minimalist name-addr
      against the certification, terminating on success.

   *  If all of the above fails, synthesis has failed.

5.1.3.  Certificate Lookup with Redacted User IDs

   It's possible (though non-intuitive) to use a user ID redacting
   keystore for certificate lookup.  Since the keystore retains (but
   does not distribute) the user IDs, they can be used to select
   certificates in response to a search.  The OpenPGP certificates sent
   back in response to the search will not contain the user IDs, but a
   client that knows the full user ID they are searching for will be
   able to verify the returned certifications.

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   Certificate lookup from a user ID redacting keystore works better for
   certificate lookup by exact user ID match than it does for substring
   match, because a client that retrieves a certificate via a substring
   match may not be able to reconstruct the redacted user ID.

   However, without some additional restrictions on which certifications
   are redistributed (whether the user ID is redacted or not),
   certificate lookup can be flooded (see Section 13.1).

5.1.4.  Hinting Redacted User IDs

   To ensure that the distributed certificate is at least structurally a
   valid [RFC4880] transferable public key, a user ID redacting keystore
   MAY distribute an empty user ID (an OpenPGP packet of tag 13 whose
   contents are a zero-octet string) in place of the omitted user ID.
   This two-octet replacement user ID packet ("\xb4\x00") is called the
   "unstated user ID".

   To facilitate clients that match certifications with specific user
   IDs, a user ID redacting keystore MAY insert a non-hashed notation
   subpacket into the certification.  The notation will have a name of
   "uidhash", with 0x80 ("human-readable") flag unset.  The value of
   such a notation MUST be 32 octets long, and contains the SHA-256
   cryptographic digest of the UTF-8 string of the redacted user ID.

   A certificate refresh client which receives such a certification
   after the "unstated user ID" SHOULD compute the SHA-256 digest of all
   user IDs it knows about on the certificate, and compare the result
   with the contents of the "uidhash" notation to decide which user ID
   to try to validate the certification against.

5.1.5.  User ID Recovery by Client Brute Force

   User ID redaction is at best an imperfect process.  Even if a
   keystore redacts a User ID, if it ships a certification over that
   user ID, an interested client can guess user IDs until it finds one
   that causes the signature to verify.  This is even easier when the
   space of legitimate user IDs is relatively small, such as the set of
   commonly-used hostnames.

5.2.  Primary-key Only Certificate Refresh

   Abuse-resistant keystores can defend against a fingerprint flooding
   Section 2.3 attack during certificate refresh by implementing a
   narrowly-constrained certificate refresh interface.

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   Such a keystore MUST accept only a full fingerprint as the search
   parameter from the certificate refresh client, and it MUST return at
   most a single certificate whose primary key matches the requested
   fingerprint.  It MUST NOT return more than one certificate, and it
   MUST NOT return any certificate whose primary key does not match the
   fingerprint.  In particular, it MUST NOT return certificates where
   only the subkey fingerprint matches.

   Note that [] does not offer the primitive
   described in Section 3.1 exactly.  In that specification, the set of
   keys returned by a "get" operation with a "search" parameter that
   appears to be a full fingerprint is ambiguous.  Some popular
   implementations (e.g., [SKS]) do not currently implement this
   mitigation, because they return certificates with subkeys that match
   the fingerprint.

5.3.  Require Valid Cross-Sigs for Certificate Discovery

   By definition, certificate discovery needs to be able to match
   subkeys, not just primary keys.  This means that the mitigation in
   Section 5.2 is ineffective for a keystore that offers a certificate
   discovery interface.

   An abuse-resistant keystore that aims to defend its certificate
   discovery interface from a fingerprint flooding (Section 2.3) attack
   can follow the following procedure.

   Such a keystore MUST accept only a full fingerprint or a 64-bit key
   ID as the search parameter from the certificate discovery client.  It
   MUST only match that fingerprint against the following:

   *  the fingerprint or key ID of a primary key associated with a valid

   *  the fingerprint or key ID of a cryptographically-valid subkey that
      also has a cross-sig.

   This defends against the fingerprint flooding attack because a
   certificate will only be returned by subkey if the subkey has agreed
   to be associated with the primary key (and vice versa).

   Note that this mitigation means that certificate discovery will fail
   if used for subkeys that lack cross-sigs.  In particular, this means
   that a client that tries to use the certificate discovery interface
   to retrieve a certificate based on its encryption-capable subkey
   (e.g., taking the key ID from a Public Key Encrypted Session Key
   (PKESK) packet) will have no success.

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   This is an acceptable loss, since the key ID in a PKESK is typically
   unverifiable anyway.

6.  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 accesses or finds 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.

6.1.  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.

   As long as the keystore implements the verification algorithm, Any
   self-signature should always be cryptographically-verifiable, since
   the public key of the issuer is already present in the certificate
   under consideration.

6.2.  Accept Only Certificates Issued by Known Certificates

   This is an extension of Section 4.9, 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
   Section 4.10).  In particular, if a public key is added to a
   blocklist for a keystore implementing this mitigation, and it is

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   removed from the keystore, then all certificates that were only
   "reachable" from the blocklisted certificate should also be
   simultaneously removed.

   FIXME: There are complexities associated with this strategy when
   certificates expire or are revoked.  If expiration or revocation
   cause some certificates to become "unreachable", what should such a
   keystore do?

6.3.  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 behind a NAT, using a VPN, or accessing the
   keystore via Tor.

6.4.  Accept Certificates 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.

6.5.  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.  The e-mail itself MAY be encrypted to an encryption-
   capable key found in the proposed certificate.  If the keyholder
   triggers the confirmation mechanism, then the keystore accepts the

   Some e-mail validating keystores MAY choose to distribute
   certifications over all user IDs for any given certificate, but will
   redact (see Section 5.1) those user IDs that have not been e-mail

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

7.  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 clients of a keystore that expect the keystore to
   be append-only (for example, some keyserver synchronization
   techniques may expect this property to hold).

   Furthermore, keystores that drop old data (e.g., superseded
   certifications) may make it difficult or impossible for their users
   to reason about the validity of signatures that were made in the
   past.  See Section 11.5 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.  Alternately, a
   keystore may omit these mitigations at certificate ingestion time,
   but apply these mitigations at retrieval time (during certificate
   refresh, discovery, or lookup), and offer a more verbose (non-
   mitigated) interface for auditors, as described in Section 5.

   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

7.1.  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

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   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.

7.2.  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 functional 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!

7.3.  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.

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

   If the primary key of a certificate is revoked via a key revocation
   signature (type 0x20), 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 key revocation signature MUST NOT be dropped.

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   In the event that an abuse-resistant keystore is flooded with key
   revocation signatures, it should retain the hardest, earliest
   revocation (see also Section 12.1).

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

   Otherwise, the abuse-resistant keystore SHOULD retain the earliest
   "soft" 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 key revocation signature packet

7.5.  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.

   An abuse-resistant keystore that adopts this mitigation needs a
   policy for handling signature packets marked with an explicit
   expiration that is longer than implicit maximum.  The two obvious
   strategies are:

   *  cap the packet's expiration to the system's implicit expiration
      date, or

   *  accept the explicit expiration date.

   Warning: Any implementation of 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.

8.  Primary Key Sovereignty

   A keystore can defend against malicious external flooding by treating
   the "first party" of each certificate as "sovereign" over that
   certificate.  This means in practice that no part of the certificate
   will redistributed without explicit permission from the primary key.
   We call a keystore that aims to respect primary key sovereignty a
   "sovereignty-respecting" keystore.

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   [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.  A sovereignty-respecting keystore
   effectively treats that bit as always set, whether it is present in
   any part of the certificate or not.

   A sovereignty-respecting abuse-resistant keystore can apply other
   constraints in addition to primary-key sovereignty, of course, for
   reasons as diverse as performance concerns, storage capacity, legal
   regulation, cryptographic algorithm support, or project policy.  It
   will not redistribute anything that has not been explicitly approved
   by the primary key, but that does not mean it has to redistribute
   everything that has been explicitly approved by the primary key.

   The remaining subsections of Section 8 describe some sensible
   strategies for a sovereignty-respecting keystore.

8.1.  Refresh-only Keystores

   Some soveriegnty-respecting 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 and
   revocation signatures from a primary key over itself, subkeys and
   their cryptographically-validated binding signatures (and cross-sigs,
   where necessary).

   A client of a refresh-only keystore cannot possibly use the keystore
   for certificate lookup, because there are no user IDs to match.  And
   it is not particularly useful for certificate discovery, because the
   returned certificate would have no identity information.  However,
   such a keystore can be used for certificate refresh, as it's possible
   to ship revocations, new subkeys, updates to subkey expiration,
   subkey revocations, and updates of direct key signature-based
   certificate expiration or other OpenPGP properties.

   Note that many popular OpenPGP implementations do not implement
   direct primary key expiration mechanisms, relying instead on user ID
   expirations.  These user ID expiration dates or other metadata
   associated with a self-certification will not be distributed by an
   refresh-only keystore.

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   Certificates shipped by an refresh-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.

8.2.  First-party-only Keystores

   Slightly more permissive than the refresh-only keystore described in
   Section 8.1 is a sovereignty-respecting keystore that also permits
   user IDs and their self-sigs.

   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 avoids certificate flooding attacks, because the
   only party who can make a certificate overly large is the holder of
   the secret corresponding to the primary key itself.

   Note that a first-party-only keystore is still problematic for those
   people who rely on the keystore for retrieval of third-party
   certifications.  Section 8.3 attempts to address this lack.

8.2.1.  First-party-only Without User IDs

   It is possible to operate an first-party-only keystore that
   redistributes certifications (in particular, self-sigs) while
   declining to redistribute user IDs themselves (see Section 5.1).
   This defends against concerns about publishing identifiable
   information, while enabling full certificate refresh for those
   keystore clients that already know the associated user IDs for a
   given certificate.

8.3.  First-party-attested Third-party Certifications

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

   This can be done by placing an Attested Certifications subpacket in
   the most recent self-sig of the certificate (see

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8.3.1.  Client Interactions

   Creating such an attestation requires multiple steps by different
   parties, each of which is blocked by all prior steps:

   *  The first-party creates the certificate, and transfers it to the
      third party.

   *  The third-party certifies it, and transfers their certification
      back to the first party.

   *  The first party attests to the third party's certification by
      issuing a new self-sig.

   *  Finally, the first party then transfers the updated certificate to
      the keystore.

   The complexity and length of such a sequence may represent a
   usability obstacle to a user who needs a third-party-certified
   OpenPGP certificate.

   Few OpenPGP clients can currently create the attestations described
   in [Attested-Certifications].  None that the author is aware of are
   user-friendly.  More implementation work needs to be done to make it
   easy (and understandable) for a user to perform this kind of

8.3.2.  Revoking Third-party Certifications

   A sovereignty-respecting keystore distributes a third-party
   certification based on the desires of the first party, but the third-
   party themselves may change their mind about the certification that
   they issued.  In particular, they may revoke a previously attested
   third-party certification.  This causes some additional complexity.  Third-party Certification Revocations Aren't Shipped with the

   Distributing the third-party's revocation of their certification
   without the approval of the first party would arguably disrespect the
   first-party's sovereignty over their own certificate.  For example,
   consider an abusively large revocation, or a revocation which
   contains toxic data.

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   At the same time, if the first party were to revoke their
   attestation, then the third-party certification itself _and_ its
   third-party's revocation might not be distributed.  So distributing
   third-party certification revocations directly on the certificate
   they refer to doesn't seem to solve the problem for an abuse-
   resistant, sovereignty-respecting keystore.  Third-party Certification Revocations Ship With the Issuing

   Instead, a sovereignty-respecting keystore MAY ship a third-party
   certification revocation attached to the end of the issuing
   certificate, as this respects the sovereignty of all parties

   This means that the certifier's own OpenPGP certificate MAY be
   distributed like so:

   - A. Primary key
   - B. User ID
   - C. Self-sig (from A, binding A to B)
   - D. Subkey
   - E. Subkey binding signature (from A, binds D to A)
   - F. Certification revocation signature
        (from A over some other key+userID, targets other

   Note that OpenPGP packet K is unusual here, and augments the
   traditional Transferable Public Key structure from [RFC4880].

   A client that cares about third-party certifications SHOULD maintain
   an index of certifications based on the SHA256 digest of the
   certifications themselves (the "certification index").  The
   certification revocation packet SHOULD contain a Signature Target
   subpacket using SHA256 to identify the revoked certification.  The
   client can use this Signature Target subpacket and the certification
   index to identify the targeted certification and to compute the data
   over which the revocation is made.  This use of SHA256 is not used
   for cryptographic strength, but for indexing efficiency.

   A client that cares about third-party certifications from key A
   SHOULD refresh the certificate containing A from the keystore, and
   verify any discovered certification revocations correctly to the
   appropriate certificates, searching for the targeted revocation in
   its certification index.

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   A legacy client that is unaware of this augmentation of the
   Transferable Public Key structure is likely to consider packet K as
   out-of-order or inapplicable (it would typically expect only a Subkey
   Revocation Signature packet in this position), and so will discard

   In the event that the certificate has no subkeys (packets I and J are
   absent), a legacy client might consider F to be an attempt to revoke
   Self-Sig C.  However, F's Signature Target subpacket does not point
   to C, and the certification is not cryptographically valid over A and
   B, so it should be discarded/ignored safely in that case as well.

9.  Keystore Client Best Practices

   An OpenPGP client that needs to interact with an abuse-resistant
   keystore can take steps to minimize the extent that its interactions
   with a keystore can be abused by other parties via the attacks
   described in Section 2.  This section describes steps that an abuse-
   resistant client can take.

9.1.  Use Constrained Keystores for Lookup

   When performing certificate lookup, an abuse-resistant client SHOULD
   prefer to query abuse-resistant keystores to avoid the risks
   described in Section 13.1.  In particular, keystores that defend
   against User ID Flooding are significantly more reliable for
   certificate lookup.

9.2.  Normalize Addresses and User IDs for Lookup

   When performing lookup by e-mail address, an abuse-resistant client
   SHOULD consider canonicalizing the e-mail address before searching
   (see Section 12.3).

   When searching by full User ID, unless there is a strong reason to
   believe that a specific non-normalized form is preferable, an abuse-
   resistant client SHOULD normalize the entire user ID into
   [UNICODE-NORMALIZATION] Form C (NFC) before performing certificate

9.3.  Avoid Fuzzy Lookups

   Certificate lookup by arbitrary substring matching, or regular
   expression is prone to abuse.  An abuse-resistant client SHOULD
   prefer exact-uid or exact-email match lookups where possible.

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   In particular, an abuse-resistant client should avoid trying to offer
   reliable functionality that performs these sort of fuzzy lookups, and
   SHOULD warn the user about risks of abuse if the user triggers a
   codepath that unavoidably performs such a fuzzy lookup.

9.4.  Prefer Full Fingerprint for Discovery and Refresh

   Key IDs are inherently weaker and easier to spoof or collide than
   full fingerprints.  Where possible, an abuse-resistant keystore
   client SHOULD use the full fingerprint when interacting with the

9.5.  Use Caution with Keystore-provided Validation

   When an abuse-resistant client relies on a keystore for certificate
   validation, many things can go subtly wrong if the client fails to
   closely track the specific semantics of the keystore's validation

   For example, a certificate published by WKD
   ([I-D.koch-openpgp-webkey-service]) at
   iy9q119eutrkn8s1mk4r39qejnbu3n5q?l=joe.doe offers a validation claim
   only for the e-mail address  If the certificate
   retrieved at that address contains other user IDs, or if the user ID
   containing that e-mail address contains an [RFC5322] display-name,
   none of that information should be considered "validated" by the fact
   that the certificate was retrieved via certificate lookup by WKD.

   When certificate validation is represented more generally by a
   keystore via certificate retrieval (e.g. from an e-mail validating
   keyserver that has no distinct certificate validation interface), the
   thing validated is the certificate received from the keystore, and
   not the result of the merge into any local copy of the certificate
   already possessed by the client.

   Consider also timing and duration of these assertions of validity,
   which represent a subtle tradeoff between privacy and risk as
   described in Section 14.4.

10.  Certificate Generation and Management Best Practices

   An OpenPGP implementation that generates or manages certificates and
   expects to distribute them via abuse-resistant keystores can take
   steps to ensure that the certificates generated are more likely to be
   accessible when needed.  This section describes steps such an abuse-
   sensitive implementation can take.

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10.1.  Canonicalized E-Mail Addresses

   E-mail addresses can be written in many different ways.  An abuse-
   sensitive implementation considering attaching a user ID containing
   an e-mail address on a certificate SHOULD ensure that the e-mail
   address is structured as simply as possible.  See Section 12.3 for
   details about e-mail address canonicalization.

   For example, if the e-mail domain considers the local part to be
   case-insensitive (as most e-mail domains do today), if a proposed
   user ID contains the addr-spec:, the implementation
   SHOULD warn the user and, if possible, propose replacing the addr-
   spec part of the user ID with

10.2.  Normalized User IDs

   User IDs are arbitrary UTF-8 strings, but UTF-8 offers several ways
   to represent the same string.  An abuse-sensitive implementation
   considering attaching a user ID to a certificate SHOULD normalize the
   string using [UNICODE-NORMALIZATION] Form C (NFC) before creating the

   At the same time, the implementation MAY also warn the user if the
   "compatibility" normalized form (NFKC) differs from the candidate
   user ID and, if appropriate, offer to convert the user ID to
   compatibility normalized form at the user's discretion.

10.3.  Avoid Large User Attributes

   An abuse-sensitive implementation SHOULD warn the user when attaching
   a user attribute larger than 8383 octets, and SHOULD refuse to attach
   user attributes entirely larger than 65536 octets.  (See Section 4.1)

10.4.  Provide Cross-Sigs

   [RFC4880] requires cross-sigs for all signing-capable subkeys, but is
   agnostic about the use of cross-sigs for subkeys of other

   An abuse-sensitive implementation that wants a certificate to be
   discoverable by subkey SHOULD provide cross-sigs for any subkey
   capable of making a cross-sig.

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10.5.  Provide Issuer Fingerprint Subpackets

   Issuer subpackets contain only 64-bit key IDs.  Issuer Fingerprint
   subpackets contain an unambiguous designator of the issuing key,
   avoiding the ambiguities described in Section 11.2.  Abuse-sensitive
   implementations SHOULD provide Issuer Fingerprint subpackets.

10.6.  Put Cross-Sigs and Issuer Fingerprint in Hashed Subpackets

   Unhashed subpackets may be stripped or mangled.  Placing cross-sigs
   and issuer fingerprint subpackets in the hashed subpackets will
   ensure that they are propagated by any cryptographically-validating
   keystore, even if that keystore fails to observe the exceptions in
   Section 4.4.

10.7.  Submit Certificates to Restricted, Lookup-Capable Keystores

   If an abuse-sensitive implementation wants other peers to be able to
   to retrieve the managed certificate by certificate lookup (that is,
   by searching based on user ID or e-mail address), it needs to be
   aware that submission to an unrestricted keystore is not reliable
   (see Section 13.1 for more details).

   Consequently, such an implementation SHOULD submit the managed
   certificate to restricted, lookup-capable keystores where possible,
   as those keystores are more likely to be able to offer reliable

11.  Side Effects and Ecosystem Impacts

11.1.  Designated Revoker

   A first-party-only keystore as described in Section 8.2 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.

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11.2.  Key IDs vs. Fingerprints in Certificate Discovery

   During signature verification, a user performing certificate
   discovery against a keystore SHOULD prefer to use the full
   fingerprint as an index, rather than the 64-bit key ID.  Using a
   64-bit key ID is more likely to run into collision attacks; and if
   the retrieved certificate has a matching key ID but the signature
   cannot be validated with it, the client is in an ambiguous state --
   did it retrieve the wrong certificate, or is the signature incorrect?
   Using the fingerprint resolves the ambiguity: the signature is
   incorrect, because the a fingerprint match is overwhelmingly stronger
   than a key ID match.

   Unfortunately, many OpenPGP implementations distribute signatures
   with only an Issuer subpacket, so a client attempting to find such a
   certificate MAY perform certificate discovery based on only the key

   A keystore that offers certificate discovery MAY choose to require
   full fingerprint, but such a keystore will not be useful for a client
   attempting to verify a bare signature from an unknown party if that
   signature only has an Issuer subpacket (and no Issuer Fingerprint

11.3.  In-band Certificates

   There are contexts where it is expected and acceptable that the
   signature appears without its certificate: for example, if the set of
   valid signers is already known (as in some OpenPGP-signed operating
   system updates), shipping the certificate alongside the signature
   would be pointless bloat.

   However, OpenPGP signatures are often found in contexts where the
   certificate is not yet known by the verifier.  For example, many
   OpenPGP-signed e-mails are not accompanied by the signing

   In another example, the use of authentication-capable OpenPGP keys in
   standard SSH connections do not contain the full OpenPGP
   certificates, which means that the SSH clients and servers need to
   resort to out-of-band processes if evaluation of the OpenPGP
   certificates is necessary.

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   The certificate discovery interface offered by keystores is an
   attempt to accommodate these situations.  But in the event that a
   keystore is unavailable, does not know the certificate, or suffers
   from a flooding attack, signature validation may fail unnecessarily.
   In an encrypted e-mail context specifically, such a failure may also
   limit the client's ability to reply with an encrypted e-mail.

   Certificate discovery also presents a potential privacy concern for
   the signature verifier, as noted in Section 14.5.

   These problematic situations can be mitigated by shipping the
   certificate in-band, alongside the signature.  Signers SHOULD adopt
   this practice where possible to reduce the dependence of the verifier
   on the keystores for certificate discovery.  [AUTOCRYPT] is an
   example of e-mail recommendations that include in-band certificates.

11.3.1.  In-band Certificate Minimization and Validity

   OpenPGP certificates are potentially large.  When distributing an in-
   band certificate alongside a signature in a context where size is a
   concern (e.g. bandwidth, latency, or storage costs are a factor), the
   distributor SHOULD reduce the size of the in-band certificate by
   stripping unnecessary packets.  For example, the distributor may:

   *  Strip certification and signature packets that (due to creation
      and expiration time) are not relevant to the time of the signature
      itself.  This ensures that the reduced certificate is
      contemporaneously valid with the signature.

   *  Strip irrelevant subkeys (and associated Subkey Binding Signature
      packets and cross-sigs).  If the signature was issued by a
      signing-capable subkey, that subkey (and its binding signature and
      cross-sig) are clearly relevant.  Other signing-capable subkeys
      are likely to be irrelevant.  But determining which other subkeys
      are relevant may be context-specific.  For example, in the e-mail
      context, an encryption-capable subkey is likely to be contextually
      relevant, because it enables the recipient to reply encrypted, and
      therefore should not be stripped.

   *  Strip user IDs (and associated certifications) that are unlikely
      to be relevant to the signature in question.  For example, in the
      e-mail context, strip any user IDs that do not match the declared
      sender of the message.

   *  Strip third-party certifications that are unlikely to be relevant
      to the verifier.  Doing this successfully requires some knowledge
      about what the third-parties the recipient is likely to care
      about.  Stripping all third-party certifications is a simple means

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      of certificate reduction.  The verifier of such a certificate may
      need to do certificate refresh against their preferred keystore to
      learn about third-party certifications useful to them.

11.4.  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 7) 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.

11.5.  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.  A verifier that does not already know the certificate
   that made the signature will need to perform certificate discovery
   against some set of keystores to find a certificate with which to
   check the signature.

   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.

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11.5.1.  Point-in-time Certificate Evaluation

   When evaluating a certificate at a time T in the past (for example,
   when trying to validate a data signature by that certificate that was
   created at time T), one approach is to discard all packets from the
   certificate if the packet has a creation time later than T.  Then
   evaluate the resulting certificate from the remaining packets in the
   context of time T.

   However, any such evaluation MUST NOT ignore "hard" OpenPGP key
   revocations, regardless of their creation date. (see Section 12.1).

11.5.2.  Signature Verification and Non-append-only Keystores

   If a non-append-only keystore (Section 7) has dropped superseded
   (Section 7.1) or expired (Section 7.2) 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.  If a user performs certificate discovery against such a
   keystore, the certificate it retrieves would be invalid according to
   [RFC4880], and consequently verification of any signature by that
   certificate would fail.

   One simple mitigation to this problem is to ship a contemporaneously-
   valid certificate in-band alongside the signature (see Section 11.3).

   If the distributor does this, then the verifier need only learn about
   revocations.  If knowledge about revocation is needed, the verifier
   might perform a certificate refresh (not "certificate discovery")
   against any preferred keystore, including non-append-only keystores,
   merging what it learns into the in-band contemporary certificate.

   Then the signature verifier can follow the certificate evaluation
   process outlined in Section 11.5.1, using the merged certificate.

11.6.  Global Append-only Ledgers ("Blockchain")

   The append-only aspect of some OpenPGP keystores encourages a user of
   the keystore to rely on that keystore as a faithful reporter of
   history, and one that will not misrepresent or hide the history that
   they know about.  An unfaithful "append-only" keystore could abuse
   the trust in a number of ways, including withholding revocation
   certificates, offering different sets of certificates to different
   clients doing certificate lookup, and so on.

   However, the most widely used append-only OpenPGP keystore, the [SKS]
   keyserver pool, offers no cryptographically verifiable guarantees
   that it will actually remain append-only.  Users of the pool have

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   traditionally relied on its distributed nature, and the presumption
   that coordination across a wide range of administrators would make it
   difficult for the pool to reliably lie or omit data.  However, the
   endpoint most commonly used by clients to access the network is
   hkps://, the default for [GnuPG].  That
   endpoint is increasingly consolidated, and currently consists of
   hosts operated by only two distinct administrators, increasing the
   risk of potential misuse.

   Offering cryptographic assurances that a keystore could remain
   append-only is an appealing prospect to defend against these kinds of
   attack.  Many popular schemes for providing such assurances are known
   as "blockchain" technologies, or global append-only ledgers.

   With X.509 certificates, we have a semi-functional Certificate
   Transparency ([RFC6962], or "CT") ecosystem that is intended to
   document and preserve evidence of (mis)issuance by well-known
   certificate authorities (CAs), which implements a type of global
   append-only ledger.  While the CT infrastructure remains vulnerable
   to certain combinations of colluding actors, it has helped to
   identify and sanction some failing CAs.

   Like other global append-only ledgers, CT itself is primarily a
   detection mechanism, and has no enforcement regime.  If a widely-used
   CA were identified by certificate transparency to be untrustworthy,
   the rest of the ecosystem still needs to figure out how to impose
   sanctions or apply a remedy, which may or may not be possible.

   CT also has privacy implications -- the certificates published in the
   CT logs are visible to everyone, for the lifetime of the log.

   For spam abatement, CT logs decline all X.509 certificates except
   those issued by certain CAs (those in popular browser "root stores").
   This is an example of the strategy outlined in Section 4.9).

   Additional projects that provide some aspects of global append-only
   ledgers that try to address some of the concerns described here
   include [KEY-TRANSPARENCY] and [CONIKS], though they are not specific
   to OpenPGP.  Both of these systems are dependent on servers operated
   by identity providers, however.  And both offer the ability to detect
   a misbehaving identity provider, but no specific enforcement or
   recovery strategies against such an actor.

   It's conceivable that a keystore could piggyback on the CT logs or
   other blockchain/ledger mechanisms like [BITCOIN] to store
   irrevocable pieces of data (such as revocation certificates).
   Further work is needed to describe how to do this in an effective and
   performant way.

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11.7.  Certificate Lookup for Identity Monitoring

   While certificate lookup is classically used to find a key to encrypt
   an outbound message to, another use case for certificate lookup is
   for the party in control of a particular identity to determine
   whether anyone else is claiming that identity.

   That is, a client in control of the secret key material associated
   with a particular certificate with user ID X might search a keystore
   for all certificates matching X in order to find out whether any
   other certificates claim it.

   This is an important safeguard as part of the ledger-based detection
   mechanisms described in Section 11.6, but may also be useful for
   keystores in general.

   However, identity monitoring against a keystore that does not defend
   against user ID flooding (Section 2.2) is expensive and potentially
   of limited value.  In particular, a malicious actor with a
   certificate which duplicates a given User ID could flood the keystore
   with similar certificates, hiding whichever one is in malicious use.

   Since such a keystore is not considered authoritative by any
   reasonable client for the user ID in question, this attack forces the
   identity-monitoring defender to spend arbitrary resources fetching
   and evaluating each certificate in the flood, without knowing which
   certificate other clients might be evaluating.

12.  OpenPGP details

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

12.1.  Revocations

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

   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

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   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.

12.2.  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 <>

   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:

   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:


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12.3.  E-mail Address Canonicalization

   When an OpenPGP user IDs includes an addr-spec, there still may be
   multiple ways of representing the addr-spec that refer to the same
   underlying mailbox.  Having a truly canonical form of an addr-spec is
   probably impossible because of legacy deployments of mailservers that
   do odd things with the local part, but e-mail addresses used in an
   abuse-resistant ecosystem SHOULD be constrained enough to admit to
   some sensible form of canonicalization.

12.3.1.  Disallowing Non-UTF-8 Local Parts

   In [RFC5322], the local-part can be any dot-atom.  But if this is
   [RFC2047] decoded, it could be any arbitrary charset, not necessarily
   UTF-8.  FIXME: Do we convert from the arbitrary charset to UTF-8?

12.3.2.  Domain Canonicalization

   FIXME: should domain name be canonicalized into punycode form?  User
   IDs are typically user-facing, so i think the canonicalized form
   should be the [UNICODE-NORMALIZATION] Form C (NFC) of the domain
   name.  Can we punt to some other draft here?

12.3.3.  Local Part Canonicalization

   FIXME: [I-D.koch-openpgp-webkey-service] recommends downcasing all
   ASCII characters in the left-hand side, but leaves all

13.  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 2.4), 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 11.1 describes a potentially scary security problem for
   designated revokers.

   TODO (more security considerations)

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13.1.  Tension Between Unrestricted Uploads and Certificate Lookup

   Note that there is an inherent tension between accepting arbitrary
   certificate uploads and permitting effective certificate lookup.  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 lookup.

   In the broader ecosystem, it may be necessary to use gated/controlled
   certificate lookup mechanisms.  For example, both
   [I-D.koch-openpgp-webkey-service] and [RFC7929] enable the
   administrator of a DNS domain to distribute certificates associated
   with e-mail addresses within that domain, while excluding other
   parties.  As a rather different example, [I-D.mccain-keylist] offers
   certificate lookup on the basis of interest -- a client interested in
   an organization can use that mechanism to learn what certificates
   that organization thinks are worth knowing about, associated with a
   range of identities regardless of the particular DNS domain.  Note
   that [I-D.mccain-keylist] does not provide the certificates directly,
   but instead expects the client to be able to retrieve them by primary
   key fingerprint through some other keystore capable of (and
   responsible for) certificate refresh.

14.  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 lookup, certificate discovery,
   or certificate refresh.

14.1.  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 7.4 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.

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   Some jurisdictions may present additional legal risk for keystore
   operators that distribute names or e-mail addresses of non-consenting

   Refresh-only keystores (Section 8.1) and user ID redacting keystores
   (Section 5.1) may reduce this particular privacy concern because they
   distribute no user IDs at all.

14.2.  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 8.2) avoids this privacy
   concern because it distributes no third-party privacy concern.

   First-party attested third-party certifications described in
   Section 8.3 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.

14.3.  Tracking Clients by Queries

   Even without third-party certifications, the acts of certificate
   lookup, certificate discovery, and certificate refresh represent a
   potential privacy risk, because the keystore queried gets to learn
   which user IDs (in the case of lookup) or which certificates (in the
   case of refresh or discovery) the client is interested in.  In the
   case of certificate refresh, if a client attempts to refresh 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.

   A privacy-aware client which wants to to avoid such a tracking attack
   MAY try to perform certificate refresh 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
   refresh.  Such a client SHOULD also decline to use protocol features
   that permit linkability across interactions with the same keystore,
   such as TLS session resumption, HTTP cookies, and so on.

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   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.

14.4.  "Live" Certificate Validation Leaks Client Activity

   If a client relies on a keystore's certificate validation interface,
   or on the presence of a certificate in a keystore as a part of its
   validation calculations, it's unclear how long the assertion from the
   keystore is or should be considered to hold.  One seemingly simple
   approach is to simply query the keystore's validation interface each
   time that the client needs to validate the certificate.

   This "live" validation approach poses a quandary to the client in the
   event that the keystore is unavailable.  How should in interpret the
   "unknown" result?  In addition, live validation reveals the client's
   activity to the keystore with fine precision.

   A privacy-aware client that depends on keystores for certificate
   validation SHOULD NOT perform "live" certificate validation on each
   use it makes of the certificate.  Rather, it SHOULD cache the
   validation information for some period of time and make use of the
   cached copy where possible.  If such a client does a regular
   certificate refresh from the same keystore, it SHOULD also pre-
   emptively query the keystore for certificate validation at the same

   Choosing the appropriate time intervals for this kind of caching has
   implications for the windows of risk for the client that might use a
   revoked certificate.  Defining an appropriate schedule to make this
   tradeoff is beyond the scope of this document.

14.5.  Certificate Discovery Leaks Client Activity

   The act of doing certificate discovery on unknown signatures offers a
   colluding keystore and remote peer a chance to track a client's
   consumption of a given signature.

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   An attacker with access to keystore logs could sign a message with a
   unique key, and then watch the keystore activity to determine when a
   client consumes the signature.  This is potentially a tracking or
   "phone-home" situation.

   A signer that has no interest in this particular form of tracking can
   mitigate this concern by shipping their certificate in-band,
   alongside the signature, as recommended in Section 11.3.

   A privacy-aware client MAY insist on in-band certificates by
   declining to use any certificate discovery interface at all, and
   treat a bare signature by an unknown certificate as an invalid

14.6.  Certificate Refresh Leaks Client Activity

   The act of doing certificate refresh itself reveals some information
   that the client is interested in a given certificate and how it may
   have changed since the last time the client refreshed it, or since it
   was first received by the client.

   This is essentially the same privacy problem presented by OCSP
   [RFC6960] in the X.509 world.  In the online world of TLS, this
   privacy leak is mitigated by the CertificateStatus TLS handshake
   extension ([RFC4366]), a.k.a.  "OCSP stapling".  There is no
   comparable solution for the store-and-forward or non-online scenarios
   where OpenPGP is often found.

   Privacy-aware clients MAY prefer to access refresh interfaces from
   anonymity-preserving networks like [TOR] to obscure where they are on
   the network, but if the certificate being refreshed is known to be
   used only by a single client that may not help.

   Privacy-aware clients MAY prefer to stage their certificate refreshes
   over time, but longer delays imply greater windows of vulnerability
   for use of an already-revoked certificate.  This strategy also does
   not help when a previously-unknown certificate is encountered in-band
   (see Section 11.3), and the OpenPGP client wants to evaluate it for
   use in the immediate context.

14.7.  Distinct Keystore Interfaces Leak Client Context and Intent

   The distinct keystore interfaces documented in Section 3 offer subtly
   different semantics, and are used by a reasonable keystore client at
   different times.  A keystore that offers distinct discovery and
   refresh interfaces may infer that a client visiting the refresh
   interface already knows about the certificate in question, or that a
   client visiting the discovery interface is in the process of

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   verifying a signature from a certificate it has not seen before.

   HKP itself ([]) conflates these two interfaces --
   the same HKP query is be used to perform both discovery and refresh
   (though implementations like [SKS] are not at all abuse-resistant for
   either use), which may obscure the context and intent of the client
   from the keystore somewhat.

   A privacy-aware client that can afford the additional bandwidth and
   complexity MAY use the keystore's discovery interface for both
   refresh and discovery, since the discovery interface is a proper
   superset of the refresh interface.

14.8.  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 14.3.  Keystores
   which offer network access SHOULD provide encrypted transport.

14.9.  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 lookup, discovery, or refresh, leaving
   open the types of tracking attacks described in Section 14.3.
   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
   ([]).  A keystore which only accepts user IDs
   within a specific domain (e.g., Section 4.3) or which uses custom
   process (Section 6.4) 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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15.  User Considerations

   Section 8.3.1 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.

   Additionally, some keystores present directly user-facing
   affordances.  For example, [SKS] keyservers typically offer forms and
   listings that can be viewed directly in a web browser.  Such a
   keystore SHOULD be as clear as possible about what abuse mitigations
   it takes (or does not take), to avoid user confusion.

   Keystores which do not expect to be used directly as part of a
   certificate validation calculation SHOULD advise clients as
   explicitly as possible that they offer no assertions of validity.

   Experience with the [SKS] keyserver network shows that many users
   treat the keyserver web interfaces as authoritative.  That is, users
   act as though the keyserver network offers some type of certificate
   validation.  Unfortunately, The developer and implementor communities
   explicitly disavow any authoritative role in the ecosystem, and the
   implementations attempt very few mitigations against abuse,
   permitting redistribution of even cryptographically invalid OpenPGP
   packets.  Clearer warnings to end users might reduce this kind of
   misperception.  Or the community could encourage the removal of
   frequently misinterpreted user interfaces entirely.

16.  IANA Considerations

   This document asks IANA to register two entries in the OpenPGP
   Notation IETF namespace, both with a reference to this document:

   *  the "uidhash" notation is defined in Section 5.1.4.

16.1.  Document History

   substantive changes bewteen -04 and -05:

   *  Clarify distinctions between different signature types

   *  Point to Attested Certifications proposal

   *  Formatting changes: use xml2rfc v3, publish editor's copy

   substantive changes bewteen -03 and -04:

   *  change "certificate update" to "certificate refresh" for clarity

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   *  relax first-party-attested third-party certification constraints
      at the suggestion of Valodim

   *  introduce "primary key sovereignty" concept explicitly

   *  describe how to distribute and consume attestation revocations

   *  introduce augmentation to TPK for third-party certification
      revocation distribution

   substantive changes between -02 and -03:

   *  new sections:

      -  Keystore Interfaces

      -  Keystore Client Best Practices

      -  Certificate Generation and Management Best Practices

   *  rename "certificate discovery" to "certificate lookup"

   *  redefine "certificate discovery" to refer to lookup by signing

   *  new attack: fingerprint flooding

   *  new retrieval-time mitigations -- tighter filters on discovery and

   *  recommend in-band certificates where possible to avoid discovery
      and lookup

   *  new privacy considerations:

      -  distinct keystore interfaces

      -  certificate update

      -  certificate discovery

      -  certificate validation

   *  more nuance about unhashed subpacket filtering

   substantive changes between -01 and -02:

   *  distinguish different forms of flooding attack

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   *  distinguish toxic data as distinct from flooding

   *  retrieval-time mitigations

   *  user ID redaction

   *  references to related work (CT, keylist, CONIKS, key transparency,
      ledgers/"blockchain", etc)

   *  more details about UI/UX

   substantive changes between -00 and -01:

   *  split out Contextual and Non-Append-Only mitigations

   *  documented several other mitigations, including:

      -  Decline Data From the Future

      -  Blocklist

      -  Exterior Process

      -  Designated Authorities

      -  Known Certificates

      -  Rate-Limiting

      -  Scoped User IDs

   *  documented Updates-Only Keystores

   *  consider three different kinds of flooding

   *  deeper discussion of privacy considerations

   *  better documentation of Reason for Revocation

   *  document user ID conventions

17.  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 contributed ideas or nuance to this document
   specifically includes:

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   *  Antoine Beaupre

   *  Heiko Stamer

   *  ilf

   *  Jamie McClelland

   *  Jon Callas

   *  Jonathan McDowell

   *  Justus Winter

   *  Marcus Brinkmann

   *  Micah Lee

   *  Neal Walfield

   *  Phil Pennock

   *  Philihp Busby

   *  vedaal

   *  Vincent Breitmoser

   *  Wiktor Kwapisiewicz

18.  References

18.1.  Normative References

              Koch, W., carlson, B. M., Tse, R. H., Atkins, D., and D.
              K. Gillmor, "OpenPGP Message Format", Work in Progress,
              Internet-Draft, draft-ietf-openpgp-rfc4880bis-10, 31
              August 2020, <

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,

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   [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, <>.

18.2.  Informative References

              "Documentation of the Attested Certifications subpacket",
              n.d., <

              Breitmoser, V., Krekel, H., and D. K. Gillmor, "Autocrypt
              - Convenient End-to-End Encryption for E-Mail", n.d.,

   [BITCOIN]  "Bitcoin", n.d., <>.

   [CONIKS]   Felten, E., Freedman, M., Melara, M., Blankstein, A., and
              J. Bonneau, "CONIKS Key Management System", n.d.,

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

   [GnuPG]    Koch, W., "Using the GNU Privacy Guard", 4 April 2019,

              Koch, W., "OpenPGP Web Key Directory", Work in Progress,
              Internet-Draft, draft-koch-openpgp-webkey-service-13, 14
              November 2021, <

              McCain, R. M., Lee, M., and N. Welch, "Distributing
              OpenPGP Key Fingerprints with Signed Keylist

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              Subscriptions", Work in Progress, Internet-Draft, draft-
              mccain-keylist-05, 2 September 2019,

              Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
              Work in Progress, Internet-Draft, draft-shaw-openpgp-hkp-
              00, 20 March 2003, <

              Belvin, G. and R. Hurst, "Key Transparency, a transparent
              and secure way to look up public keys", n.d.,

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

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

              Intrigeri, "Parcimonie", n.d.,

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

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,

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

Gillmor                  Expires 30 October 2022               [Page 54]

Internet-Draft      Abuse-Resistant OpenPGP Keystores         April 2022

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,

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

   [SKS]      Minsky, Y., Fiskerstrand, K., and P. Pennock, "SKS
              Keyserver Documentation", 25 March 2018,

   [TOR]      "The Tor Project", n.d., <>.

              Whistler, K., "Unicode Normalization Forms", 4 February
              2019, <>.

Author's Address

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

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