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[Sks-devel] Modelling an abuse-resistant OpenPGP keyserver

From: Daniel Kahn Gillmor
Subject: [Sks-devel] Modelling an abuse-resistant OpenPGP keyserver
Date: Thu, 04 Apr 2019 18:41:14 -0400

[ mail sent to both OpenPGP and SKS mailing lists; please respect
  Mail-Followup-To: address@hidden, since it is more than just SKS ] 

Hi OpenPGP and SKS folks--

As you may or may not have heard, the venerable OpenPGP keyserver
network is dying.  This has implications for key discovery, revocation,
subkey rollover, expiration update, etc. across the ecosystem of tools
that use OpenPGP.

The keyserver network dying because of several reasons, some of which
are discussed in a thread over at [0] -- but one main
issue is that the SKS keyserver network allows anyone to attach
arbitrary data to any OpenPGP certificate, bloating that certificate to
the point of being impossible to effectively retrieve.  SKS isn't the
only keyserver that is vulnerable to this kind of attack either [1].

I wanted to put forward a "simple proposal" (ha ha) about how to think
about a keyserver (or other public keystore) that would be more
resistant to this kind of abuse.

Such a keystore is unlikely to be able to synchronize with the existing
keyserver network, and need not be a synchronizing keyserver at all --
these rules could just as well apply to a centralized keyserver that
valdiates e-mail addresses, or any other authority.

I've documented some thoughts on how to resist this abuse in a new
Internet Draft:

That's being developed in git at:

I welcome feedback and edits.

The markdown source of the current draft is attached below.




title: Abuse-Resistant OpenPGP Keystores
docname: draft-dkg-openpgp-abuse-resistant-keystore-00
date: 2019-04-04
category: info

ipr: trust200902
area: int
workgroup: openpgp
keyword: Internet-Draft

stand_alone: yes
pi: [toc, sortrefs, symrefs]

    ins: D. K. Gillmor
    name: Daniel Kahn Gillmor
    org: American Civil Liberties Union
    street: 125 Broad St.
    city: New York, NY
    code: 10004
    country: USA
    abbrev: ACLU
    email: address@hidden
    title: SKS Keyserver Documentation
      name: Phil Pennock
      ins: P. Pennock
      org: SKS development team
    date: 2018-03-25
    title: Using the GNU Privacy Guard
      name: Werner Koch
      ins: W. Koch
      org: GnuPG development team
      date: 2019-04-04
    title: Mailvelope Keyserver
      name: Thomas Oberndörfer
      ins: T. Oberndörfer
--- abstract

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

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

This draft documents techniques that an archive of OpenPGP
certificates can use to mitigate the impact of these third-party
certificate flooding attacks.

--- middle


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.


 * "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".
 * 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

 * 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 discovery" is the process whereby a user retrieves an
   OpenPGP certificate based on user ID.  A user attempting to
   discover a certificate from a keystore will search for a substring
   of the known user IDs, most typically an e-mail address if the user
   ID is an {{RFC5322}} name-addr or addr-spec.  Some certificate
   discovery mechanisms look for an exact match on the known user IDs.
   {{I-D.koch-openpgp-webkey-service}} and {{}}
   are both certificate discovery mechanisms.

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

 * "Certificate update" is the process whereby a user fetches new
   information about a certificate, potentially merging those OpnePGP
   packets to change the status of the certificate.  Updates might
   include adding or revoking user IDs or subkeys, updating expiration
   dates, or even revoking the entire certificate by revoking the
   primary key directly.  A user attempting to update a certificate
   typically queries a keystore based on the certificate's

 * A "keyserver" is a particular kind of keystore, typically means of
   publicly distributing OpenPGP certificates or updates to them.
   Examples of keyserver software include {{SKS}} and
   {{MAILVELOPE-KEYSERVER}}.  One common HTTP interface for keyservers
   is {{}}.
 * A "synchronizing keyserver" is a keyserver which gossips with other
   peers, and typically acts as an append-only log.  Such a keyserver
   is typically useful for certificate discovery, certificate updates,
   and revocation information.  They are typically *not* useful for
   certificate validation, since they make no assertions about whether
   the identities in the certificates they server are accurate. As of
   the writing of this document, {{SKS}} is the canonical
   synchronizing keyserver implementation, though other
   implementations exist.
 * An "e-mail-validating keyserver" is a keyserver which attempts to
   verify the identity in an OpenPGP certificate's user ID by
   confirming access to the e-mail account, and possibly by confirming
   access to the secret key.  Some implementations permit removal of a
   certificate by anyone who can prove access to the e-mail address in
   question.  They are useful for certificate discovery based on
   e-mail address and certificate validation (by users who trust the
   operator), but some may not be useful for certificate update or
   revocation, since a certificate could be simply replaced by an
   adversary who also has access to the e-mail address in question.
   {{MAILVELOPE-KEYSERVER}} is an example of such a keyserver.

 * "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
   <address@hidden>" might not belong to the person who controls
   the e-mail address "address@hidden" 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 whatsoever.

Problem Statement

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

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

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

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

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 itself.

Limited Packet Sizes

While {{RFC4880}} permits OpenPGP packet sizes of arbitrary length,
OpenPGP certificates rarely need to be so large.  An abuse-resistant
keystore SHOULD reject any OpenPGP packet larger than 8383
octets. (This cutoff is chosen because it guarantees that the packet
size can be represented as a one- or two-octet {{RFC4880}} "New Format
Packet Length", but it could be reduced further)

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

Strict User IDs

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

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

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

Drop or Standardize Unhashed Subpackets

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

An abuse-resistant keysetore SHOULD strip out all unhashed subpackets.

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

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

Drop User Attributes

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

Drop Non-exportable Certifications

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

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.

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.

Contextual Mitigations

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

Note also that many of these mitigations depend on cryptographic

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

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

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

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 2019.

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

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

Drop Expired Signatures

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

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

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.

Drop All Other Elements of a Directly-Revoked Certificate {#only-revocation}

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

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

In the event that an abuse-resistant keystore is flooded with direct
key revocation signatures, it should retain the strongest, earliest

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

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

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

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

Implicit Expiration Date

A particularly aggressive abuse-resistant keystore MAY choose an
implicit expiration date for all signature packets.  For example, a
signature packet that claims no expiration could be treated by the
keystore as expiring 3 years after issuance.

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

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

First-party-only Keystores

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

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

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

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

First-party-attested Third-party Certifications {#fpatpc}

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

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

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

 * The third-party certification MUST have an unhashed subpacket of
   type Embedded Signature, the contents of which we'll call the
   "attestation".  This attestation is from the certificate's primary
   key over the third-party certification itself, as detailed in the
   steps below:

 * The attestation MUST be an OpenPGP signature packet of type 0x50
   (Third-Party Confirmation signature)
 * The attestation MUST contain a notation subpacket
 * The attestation MUST contain a hashed "Issuer Fingerprint"
   subpacket with the fingerprint of the primary key of the
   certificate in question.
 * The attestation MUST NOT be marked as non-exportable.
 * The attestation MUST contain a hashed Notation subpacket with the
   name "ksok", and an empty (0-octet) value.

 * The attestation MUST contain a hashed "Signature Target" subpacket
   with "public-key algorithm" that matches the public-key algorithm
   of the third-party certification.
 * The attestation's hashed "Signature Target" subpacket MUST use a
   reasonably strong hash algorithm (as of this writing, any
   {{RFC4880}} hash algorithm except MD5, SHA1, or RIPEMD160), and
   MUST have a hash value equal to the hash over the third-party
   certification with all unhashed subpackets removed.
 * The attestation MUST be cryptographically valid, verifiable by the
   primary key of the certificate in question.
What this means is that a third-party certificate will only be
accepted/distributed by the keystore if:

 * the keystore knows about both the first- and third-parties.
 * the third-party has made the identity assertion
 * the first-party has confirmed that they're OK with the third-party
   certification being distributed by any keystore.
FIXME: it's not clear whether the "ksok" notification is necessary --
it's in place to avoid some accidental confusion with any other use of
the Third-Party Confirmation signature packet type, but the author
does not know of any such use that might collide.

Key Server Preferences "No-modify"

{{RFC4880}} section ("Key Server Preferences") defines a
"No-modify" bit.  That bit has never been respected by any keyserver
implementation that the author is aware of.  This section effectively
asks an abuse-resistant keystore to treat that bit as always set,
whether it is present in the certificate or not.

Client Interactions {#client-interactions}

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

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

Side Effects and Ecosystem Impacts

Designated Revoker {#designated-revoker}

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

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 are much fuzzier.

The author of this draft recommends *not* considering any subkeys to
be certification-capable to avoid this headache.

Security Considerations

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

{{designated-revoker}} describes a potentially scary security problem
for designated revokers.

TODO (more security considerations)

Privacy Considerations

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

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

TODO (more privacy considerations)

User Considerations

{{client-interactions}} 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

IANA Considerations

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

Document Considerations

\[ RFC Editor: please remove this section before publication ]

This document is currently edited as markdown.  Minor editorial
changes can be suggested via merge requests at or by
e-mail to the author.  Please direct all significant commentary to the
public IETF OpenPGP mailing list: address@hidden

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