On Tue 2019-04-16 16:45:45 -0400, Daniel Kahn Gillmor wrote:
I think the next draft will use "certificate discovery" to refer only
to this third case, and will rename lookup-by-user-id to "certificate
lookup". Does that make sense?
I've just released version 03, which makes this change, and also adds
more nuance related to the ecosystem around keystores and how that might
have to change in the face of abuse-resistant keystores. Identifying
and naming this distinct interface helped to surface another attack,
fingerprint flooding, which is also now documented.
The new version is here:
https://tools.ietf.org/html/draft-dkg-openpgp-abuse-resistant-keystore-03
From the document's changelog:
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 (sub)key
* new attack: fingerprint flooding
* new retrieval-time mitigations -- tighter filters on discovery and update
* 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
I really appreciate the feedback i've gotten in the course of this
writeup, and welcome more.
In particular, if someone has any pointers to how to think about e-mail
address canonicalization within an OpenPGP User ID (which is a UTF-8
string), especially in the context of IDN and non-ASCII local-parts
(does this mean RFC2047-decoding or encoding?) i'd love to see some
pointers (or even better, proposed text).
The full text of the markdown source for -03 is attached below.
--dkg
---
title: Abuse-Resistant OpenPGP Keystores
docname: draft-dkg-openpgp-abuse-resistant-keystore-03
date: 2019-04-19
category: info
ipr: trust200902
area: int
workgroup: openpgp
keyword: Internet-Draft
stand_alone: yes
pi: [toc, sortrefs, symrefs]
author:
-
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: dkg(_at_)fifthhorseman(_dot_)net
informative:
RFC4366:
RFC5322:
RFC6960:
RFC6962:
RFC7929:
I-D.shaw-openpgp-hkp:
I-D.koch-openpgp-webkey-service:
I-D.mccain-keylist:
SKS:
target: https://bitbucket.org/skskeyserver/sks-keyserver/wiki/Home
title: SKS Keyserver Documentation
author:
-
name: Yaron Minsky
ins: Y. Minsky
org: SKS development team
-
name: Kristian Fiskerstrand
ins: K. Fiskerstrand
org: sks-keyservers.net pool operator
-
name: Phil Pennock
ins: P. Pennock
date: 2018-03-25
GnuPG:
target: https://www.gnupg.org/documentation/manuals/gnupg.pdf
title: Using the GNU Privacy Guard
author:
name: Werner Koch
ins: W. Koch
org: GnuPG development team
date: 2019-04-04
MAILVELOPE-KEYSERVER:
target: https://github.com/mailvelope/keyserver/
title: Mailvelope Keyserver
author:
name: Thomas Oberndörfer
ins: T. Oberndörfer
AUTOCRYPT:
target: https://autocrypt.org/
title: Autocrypt - Convenient End-to-End Encryption for E-Mail
author:
-
name: Vincent Breitmoser
ins: V. Breitmoser
-
name: Holger Krekel
ins: H. Krekel
-
name: Daniel Kahn Gillmor
ins: D. K. Gillmor
DEBIAN-KEYRING:
target: https://keyring.debian.org/
title: Debian Keyring
author:
name: Jonathan McDowell
ins: J. McDowell
org: Debian
TOR:
target: https://www.torproject.org/
title: The Tor Project
PARCIMONIE:
target: https://gaffer.ptitcanardnoir.org/intrigeri/code/parcimonie/
title: Parcimonie
author:
name: Intrigeri
PGP-GLOBAL-DIRECTORY:
target: https://keyserver.pgp.com/vkd/VKDVerificationPGPCom.html
title: PGP Global Directory Key Verification Policy
date: 2011
author:
org: Symantec Corporation
MONKEYSPHERE:
target: https://web.monkeysphere.info/
title: Monkeysphere
author:
-
name: Daniel Kahn Gillmor
ins: D. K. Gillmor
-
name: Jameson Rollins
ins: J. Rollins
KEY-TRANSPARENCY:
target: https://keytransparency.org/
title: Key Transparency, a transparent and secure way to look up public keys
author:
-
name: Gary Belvin
ins: G. Belvin
org: Google
-
name: Ryan Hurst
ins: R. Hurst
org: Google
CONIKS:
target: https://coniks.cs.princeton.edu/
title: CONIKS Key Management System
author:
-
name: Edward Felten
ins: E. Felten
org: Princeton University
-
name: Michael Freedman
ins: M. Freedman
org: Princeton University
-
name: Marcela Melara
ins: M. Melara
org: Princeton University
-
name: Aaron Blankstein
ins: A. Blankstein
org: Princeton University
-
name: Joseph Bonneau
ins: J. Bonneau
org: Stanford University/Electronic Frontier Foundation
BITCOIN:
target: https://bitcoin.org/
title: Bitcoin
UNICODE-NORMALIZATION:
target: https://unicode.org/reports/tr15/
title: Unicode Normalization Forms
date: 2019-02-04
author:
name: Ken Whistler
ins: K. Whistler
org: Unicode Consortium
normative:
RFC2047:
RFC2119:
RFC4880:
RFC8174:
I-D.ietf-openpgp-rfc4880bis:
--- abstract
OpenPGP transferable public keys are composite certificates, made up
of primary keys, direct key signatures, user IDs, identity
certifications ("signature packets"), subkeys, and so on. They are
often assembled by merging multiple certificates that all share the
same primary key, and are distributed in public keystores.
Unfortunately, since 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.
--- middle
Introduction
============
Requirements Language
---------------------
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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.
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".
* 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
<alice(_at_)example(_dot_)org>` and the user wants to send an e-mail to
`alice(_at_)example(_dot_)org`, 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 {{certificate-validation}} 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 {{certificate-lookup}} for more details.
* "Certificate update" refers to retrieval of a certificate from a
keystore based on the fingerprint of the primary key. See
{{certificate-update}} 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 {{certificate-discovery}} 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 {{I-D.shaw-openpgp-hkp}}.
* 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 update (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 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 lookup based on e-mail
address and certificate validation (by users who trust the
operator), but some may not be useful for certificate update 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.
* "Cryptographic validity" refers to mathematical evidence that a
signature came from the secret key associated with the public key
it claims to come from. Note that a certification may be
cryptographically valid without the signed data being true (for
example, a given certificate with the user ID `Alice
<alice(_at_)example(_dot_)org>` might not belong to the person who controls
the e-mail address `alice(_at_)example(_dot_)org` even though the self-sig is
cryptographically valid). In particular, cryptographic validity
for user ID in a certificate is typically insufficient evidence for
certificate validation. Also note that knowledge of the public key
of the issuer is necessary to determine whether any given signature
is cryptographically valid. Some keyservers perform cryptographic
validation in some contexts. Other keyservers (like {{SKS}})
perform no cryptographic validation whatsoever.
* 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 {{revocations}} for more
detail.
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.
The rest of the document describes some mitigations that can be used
by keystores that are concerned about these problems but want to
continue to offer some level of service for certificate lookup,
certificate update, certificate discovery, or certificate validation.
Certificate Flooding {#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 update. 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 information.
Additionally, even without malice, OpenPGP certificates can
potentially grow without bound.
User ID Flooding {#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
`alice(_at_)example(_dot_)org`, the attacker may upload hundreds or thousands of
certificates containing user IDs that match that address. Even if
those certificates would not be accepted by a client (e.g., because
they were not certified by a known-good authority), the client
typically still has to wade through all of them in order to find the
non-malicious certificates.
If the keystore does not offer a lookup interface at all (that is,
if clients cannot search it by user ID), then user ID flooding is of
less consequence.
Fingerprint Flooding {#fingerprint-flooding}
--------------------
A malicious actor who wants to render a certificate unavailable for
update 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 update, then the
certificate update 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
interest.
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.
Keystore Flooding {#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.
Toxic Data {#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.
Keystore Interfaces {#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.
Certificate Update {#certificate-update}
------------------
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_update(primary_fpr) -> certificate?
A client uses certificate update to retrieve the full details of a
certificate that it already knows about. For example, it might be
interested in updates to the certificate known to the keystore,
including revocations, expiration updates, new third-party
certifications, etc.
Upon successful update, the client SHOULD merge the retrieved
certificate with its local copy.
Not all keystores offer this operation. For example, clients cannot
use WKD ({{I-D.koch-openpgp-webkey-service}}) or OPENPGPKEY
({{RFC7929}} for certificate update.
Certificate Discovery {#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 update
({{certificate-update}}) 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
fingerprint
* 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.
Certificate Lookup {#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 {{I-D.shaw-openpgp-hkp}} 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
{{user-id-conventions}} 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.
### 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.
### 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
{{e-mail-address-canonicalization}}), and compare it to the
canonicalized form of of the `addr` query parameter.
### 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., {{identity-monitoring}}), but they
may also represent privacy concerns (e.g.,
{{publishing-identity-information}}), and they may impose additional
computational or indexing burdens on the keystore.
Certificate Validation {#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
address).
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.
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 {{in-the-past}}.
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 judgement.
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
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 {{cert-best-practices}}.
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.
Decline Large Packets {#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.
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
UTF-8.
Some abuse-resistant keystores MAY only accept User IDs that meet even
stricter conventions, such as an {{RFC5322}} `name-addr` or
`addr-spec`, or a URL like `ssh://host.example.org` (see
{{user-id-conventions}}).
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.
Scoped User IDs {#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.
Strip or Standardize Unhashed Subpackets {#standardize-unhashed}
----------------------------------------
{{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:
### 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.
### 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.
### First-party Attestations
Some third-party certifications are attested to by the certificate
primary key itself in an unhashed subpacket, as described in
{{fpatpc}}. A cryptographically-validating abuse-resistant keystore
SHOULD be willing to redistribute a valid first-party attestation as
an unhashed subpacket.
The redistributed first-party attestation itself should be stripped of
all unhashed subpackets.
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.
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 certifications.
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.
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 (please see {{non-append-only}}).
Accept Only Certificates Issued by Designated Authorities {#authorities}
---------------------------------------------------------
An abuse-resistant keystore capable of cryptographic validation MAY
retain a list of designated authorities, typically in the form of a
set of known public keys. Upon receipt of a new OpenPGP certificate,
the keystore can decide whether to accept or decline each user ID of
the certificate based whether that user ID has a certification that
was issued by one or more of the designated authorities.
If no user IDs are certified by designated authority, such a keystore
SHOULD decline the certificate and its primary key entirely. Such a
keystore SHOULD decline to retain or propagate all certifications
associated with each accepted user ID except for first-party
certifications and certifications by the designated authorities.
The operator of such a keystore SHOULD have a clear policy about its
set of designated authorities.
Given the ambiguities about expiration and revocation, such a
keyserver SHOULD ignore expiration and revocation of authority
certifications, and simply accept and retain as long as the
cryptographic signature is valid.
Note that if any key is removed from the set of designated
authorities, and that change is applied to the existing keystore, such
a keystore may no longer be append-only (please see
{{non-append-only}}).
Decline Packets by Blocklist {#blocklist}
----------------------------
The maintainer of the keystore may keep a specific list of "known-bad"
material, and decline to accept or redistribute items matching that
blocklist. The material so identified could be anything, but most
usefully, specific public keys or User IDs could be blocked.
Note that if a blocklist grows to include an element already present
in the keystore, it will no longer be append-only (please see
{{non-append-only}}).
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 {{retrieval-time-mitigations}} 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
({{toxic-data}}) 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.
Retrieval-time Mitigations {#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, update, or discovery).
Applying an abuse mitigation at retrieval time may help a client
defend against a user ID flooding ({{user-id-flooding}}), certificate
flooding ({{certificate-flooding}}), or fingerprint flooding
({{fingerprint-flooding}}) attack. It may also help a keystore limit
its liability for redistributing toxic data ({{toxic-data}}).
However, only mitigations applied at ingestion time are able to
mitigate keystore flooding attacks ({{keystore-flooding}}).
Some mitigations (like the non-append-only mitigations described in
{{non-append-only}}) 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 update).
Redacting User IDs {#user-id-redacting}
------------------
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.
### Certificate Update with Redacted User IDs
A user ID redacting keystore is useful for certificate update 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.
### 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
string.
* The resulting string should be an {{RFC5322}} `name-addr` or
`addr-spec`.
* 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 continue.
* Canonicalize the `addr-spec` according to
{{e-mail-address-canonicalization}}, 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.
### 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.
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 {{uploads-vs-lookup}}).
### Hinting Redacted User IDs {#uidhash}
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 update 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.
### 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
Primary-key Only Certificate Update {#primary-key-only-update}
-----------------------------------
Abuse-resistant keystores can defend against a fingerprint flooding
{{fingerprint-flooding}} attack during certificate update by
implementing a narrowly-constrained certificate update interface.
Such a keystore MUST accept only a full fingerprint as the search
parameter from the certificate update 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 {{I-D.shaw-openpgp-hkp}} does not offer the primitive
described in {{certificate-update}} 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.
Require Valid Cross-Sigs for Certificate Discovery
{#require-cross-sig-discovery}
--------------------------------------------------
By definition, certificate discovery needs to be able to match
subkeys, not just primary keys. This means that the mitigation in
{{primary-key-only-update}} 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
({{fingerprint-flooding}}) 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
certificate
* 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.
This is an acceptable loss, since the key ID in a PKESK is typically
unverifiable anyway.
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.
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 Certificates Issued by Known Certificates
-----------------------------------------------------
This is an extension of {{authorities}}, 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 {{blocklist}}). In
particular, if a public key is added to a blocklist for a keystore
implementing this mitigation, and it is removed from the keystore,
then all certificates that were only "reachable" from the blocklisted
certificate should also be simultaneously removed.
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.
Accept Certificates Based on Exterior Process {#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.
Accept Certificates by E-mail Validation
----------------------------------------
Some keystores resist abuse by declining any certificate until the
user IDs have been verified by e-mail. When these "e-mail validating"
keystores review a new certificate that has a user ID with an e-mail
address in it, they send an e-mail to the associated address with a
confirmation mechanism (e.g., a high-entropy HTTPS URL link) in it.
In some cases, the e-mail itself is encrypted to an encryption-capable
key found in the proposed certificate. If the keyholder triggers the
confirmation mechanism, then the keystore accepts the certificate.
Some e-mail validating keystores MAY choose to distribute
certifications over all user IDs for any given certificate, but will
redact (see {{user-id-redacting}}) those user IDs that have not been
e-mail validated.
{{PGP-GLOBAL-DIRECTORY}} describes some concerns held by a keystore
operator using this approach. {{MAILVELOPE-KEYSERVER}} is another
example.
Non-append-only mitigations {#non-append-only}
===========================
The following mitigations may cause some previously-retained packets
to be dropped after the keystore receives new information, or as time
passes. This is entirely reasonable for some keystores, but it may be
surprising for any keystore that expects to be append-only (for
example, some keyserver synchronization techniques may expect this
property to hold).
Furthermore, keystores that drop old data (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 {{in-the-past}} 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 update,
discovery, or lookup), and offer a more verbose (non-mitigated)
interface for auditors, as described in
{{retrieval-time-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
keyring.
Drop Superseded Signatures {#drop-superseded}
--------------------------
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 {#drop-expired}
-----------------------
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!
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 certificate is revoked via a direct key
signature, an abuse-resistant keystore SHOULD drop all the rest of the
associated data (user IDs, user attributes, and subkeys, and all
attendant certifications and subkey signatures). This defends against
an adversary who compromises a primary key and tries to flood the
certificate to hide the revocation.
Note that the direct key revocation signature MUST NOT be dropped.
In the event that an abuse-resistant keystore is flooded with direct
key revocation signatures, it should retain the hardest, earliest
revocation (see also {{revocations}}).
In particular, if any of the direct key revocation signatures is a
"hard" revocation, the abuse-resistant keystore SHOULD retain the
earliest such revocation signature (by signature creation date).
Otherwise, the abuse-resistant keystore SHOULD retain the earliest
"soft" direct key revocation signature it has seen.
If either of the above date comparisons results in a tie between two
revocation signatures of the same "hardness", an abuse-resistant
keystore SHOULD retain the signature that sorts earliest based on a
binary string comparison of the direct key revocation signature packet
itself.
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.
Updates-only Keystores {#updates-only}
======================
In addition to the mitigations above, some keystores may resist abuse
by declining to accept any user IDs or certifications whatsoever.
Such a keystore MUST be capable of cryptographic validation. It
accepts primary key packets, cryptographically-valid direct-key
signatures from a primary key over itself, subkeys and their
cryptographically-validated binding signatures (and cross-sigs, where
necessary).
A client of an updates-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 update, as it's possible
to ship revocations (which are direct key signatures), new subkeys,
updates to subkey expiration, subkey revocation, and direct key
signature-based certificate expiration updates.
Note that many popular OpenPGP 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
updates-only keystore.
Certificates shipped by an updates-only keystore are technically
invalid {{RFC4880}} "transferable public keys," because they lack a
user ID packet. However many OpenPGP implementations will accept such
a certificate if they already know of a user ID for the certificate,
because the composite certificate resulting from a merge will be a
standards-compliant transferable public key.
First-party-only Keystores {#first-party-only}
==========================
Slightly more permissive than the updates-only keystore described in
{{updates-only}} is a 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. {{fpatpc}} attempts to address this lack.
First-party-only Without User IDs
---------------------------------
It is possible to operate an first-party-only keystore that
redistributes certifications while declining to redistribute user IDs
(see {{user-id-redacting}}). This defends against concerns about
publishing identifiable information, while enabling full certificate
update for those keystore clients that already know the associated
user IDs for a given certificate.
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 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.
This means 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.
The "ksok" notification is not strictly necessary for this mitigation,
but it is intended to avoid potential accidental confusion with any
other use of the Third-Party Confirmation signature packet type. The
author does not know of any current use that might collide.
Key Server Preferences "No-modify"
----------------------------------
{{RFC4880}} defines "Key Server Preferences" with a "No-modify" bit.
That bit has never been respected by any keyserver implementation that
the author is aware of. An abuse-resistant keystore following
{{fpatpc}} effectively treats that bit as always set, whether it is
present in the certificate or not.
Client Interactions {#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.
* Finally, the first party then transfers the compound 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.
No current OpenPGP client can easily create the attestations 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
attestation.
Keystore Client Best Practices {#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 {{problem-statement}}. This section describes steps that
an abuse-resistant client can take.
Use Constrained Keystores for Lookup
------------------------------------
When performing certificate lookup, an abuse-resistant client SHOULD
prefer to query constrained keystores to avoid the risks described in
{{uploads-vs-lookup}}.
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 {{e-mail-address-canonicalization}}).
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
lookup.
Avoid Fuzzy Lookups
-------------------
Certificate lookup by arbitrary substring matching, or regular
expression is prone to abuse. An abuse-resistant client SHOULD prefer
exact-userid or exact-email match lookups where possible.
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.
Prefer Full Fingerprint for Discovery and Update
------------------------------------------------
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 keystore.
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
claims.
For example, a certificate published by WKD
({{I-D.koch-openpgp-webkey-service}}) at
`https://openpgpkey.example.org/.well-known/openpgpkey/hu/iy9q119eutrkn8s1mk4r39qejnbu3n5q?l=joe.doe`
offers a validation claim only for the e-mail address
`joe(_dot_)doe(_at_)example(_dot_)org`. 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 {{validation-privacy}}.
Certificate Generation and Management Best Practices {#cert-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.
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
{{e-mail-address-canonicalization}} 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`: `Alice(_at_)EXAMPLE(_dot_)org`, the implementation
SHOULD warn the user and, if possible, propose replacing the
`addr-spec` part of the user ID with `alice(_at_)example(_dot_)org`.
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 self-sig.
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.
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
{{large-packets}})
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
capabilities.
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.
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 {{id-vs-fingerprint-discovery}}.
Abuse-sensitive implementations SHOULD providue Issuer Fingerprint
subpackets.
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
{{standardize-unhashed}}.
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
{{uploads-vs-lookup}} 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 lookup.
Side Effects and Ecosystem Impacts
==================================
Designated Revoker {#designated-revoker}
------------------
A first-party-only keystore as described in {{first-party-only}} 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 keystore.
FIXME: adjust this document to point out where revocations from a
designated revoker SHOULD be propagated, maybe even in
first-party-only keystores.
Key IDs vs. Fingerprints in Certificate Discovery {#id-vs-fingerprint-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
ID.
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
subpacket).
In-band Certificates {#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 certificate.
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.
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 {{discovery-privacy}}.
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.
### In-band Certificate Minimization and Validity
OpenPGP certificates are potenitally 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 of
certificate reduction. The verifier of such a certificate may need
to do certificate update against their preferred keystore to learn
about third-party certifications useful to them.
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
{{non-append-only}}) 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.
Assessing Certificates in the Past {#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.
### Point-in-time Certificate Evaluation {#point-in-time}
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 {{revocations}}).
### Signature Verification and Non-append-only Keystores
{#verification-and-non-append-only}
If a non-append-only keystore ({{non-append-only}}) has dropped
superseded ({{drop-superseded}}) or expired ({{drop-expired}})
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 {{in-band-certificates}}).
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 update (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 {{point-in-time}}, using the merged certificate.
Global Append-only Ledgers ("Blockchain") {#gaol}
-----------------------------------------
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 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://hkps.pool.sks-keyservers.net`, 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 {{authorities}}).
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.
Certificate Lookup for Identity Monitoring {#identity-monitoring}
------------------------------------------
A typical use case for certificate lookup is a user looking for a
certificate in order to be able to encrypt an outbound message
intended for a given e-mail address, but this is not the only use
case.
Another use caes is when the party in control of a particular identity
wants 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 {{gaol}}, but may also be useful for keystores
in general.
However, identity monitoring against a keystore that does not defend
against user ID flooding ({{user-id-flooding}}) 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.
OpenPGP details
===============
This section collects details about common OpenPGP implementation
behavior that are useful in evaluating and reasoning about OpenPGP
certificates.
Revocations {#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"
revocation.
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
date.
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.
User ID Conventions {#user-id-conventions}
-------------------
{{RFC4880}} requires a user ID to be a UTF-8 string, but does not
constrain it beyond that. In practice, a handful of conventions
predominate in how User IDs are formed.
The most widespread convention is a `name-addr` as defined in
{{RFC5322}}. For example:
Alice Jones <alice(_at_)example(_dot_)org>
But a growing number of OpenPGP certificates contain user IDs that are
instead a raw {{RFC5322}} `addr-spec`, omitting the `display-name` and
the angle brackets entirely, like so:
alice(_at_)example(_dot_)org
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:
ssh://foo.example.net
E-mail Address Canonicalization {#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.
### 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?
### 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?
### Local Part Canonicalization
FIXME: {{I-D.koch-openpgp-webkey-service}} recommends downcasing all
ASCII characters in the left-hand side, but leaves all
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 {{certificate-flooding}}).
But only some of these mitigations defend against flooding attacks
against the keystore itself (see {{keystore-flooding}}), or against
flooding attacks on the space of possible user IDs (see
{{user-id-flooding}}). Thoughtful threat modeling and monitoring of
the keystore and its defenses are probably necessary to maintain the
long-term health of the keystore.
{{designated-revoker}} describes a potentially scary security problem
for designated revokers.
TODO (more security considerations)
Tension Between Unrestricted Uploads and Certificate Lookup {#uploads-vs-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
({{user-id-flooding}}), 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 update.
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 update.
Publishing Identity Information {#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 {{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.
Some jurisdictions may present additional legal risk for keystore
operators that distribute names or e-mail addresses of non-consenting
parties.
Updates-only keystores ({{updates-only}}) and user ID redacting
keystores ({{user-id-redacting}}) may reduce this particular privacy
concern because they distribute no user IDs at all.
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 ({{first-party-only}}) avoids this
privacy concern because it distributes no third-party privacy concern.
First-party attested third-party certifications described in
{{fpatpc}} 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.
Tracking Clients by Queries {#tracking-clients}
---------------------------
Even without third-party certifications, the acts of certificate
lookup, certificate discovery, and certificate update 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 update or discovery) the client is interested in. In the case
of certificate update, if a client attempts to update all of its known
certificates from the same keystore, that set is likely to be a unique
set, and therefore identifies the client. A keystore that monitors
the set of queries it receives might be able to profile or track those
clients who use it repeatedly.
A privacy-aware client which wants to to avoid such a tracking attack
MAY try to perform certificate update from multiple different
keystores. To hide network location, a client making a network query
to a keystore SHOULD use an anonymity network like {{TOR}}. Tools
like {{PARCIMONIE}} are designed to facilitate this type of
certificate update. 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.
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.
"Live" Certificate Validation Leaks Client Activity {#validation-privacy}
---------------------------------------------------
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 update from the same keystore, it SHOULD also
pre-emptively query the keystore for certificate validation at the
same time.
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.
Certificate Discovery Leaks Client Activity {#discovery-privacy}
-------------------------------------------
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.
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 {{in-band-certificates}}.
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 signature.
Certificate Update Leaks Client Activity {#update-privacy}
----------------------------------------
The act of doing certificate update 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 updated 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 update interfaces from
anonymity-preserving networks like {{TOR}} to obscure where they are
on the network, but if the certificate being updated is known to be
used only by a single client that may not help.
Privacy-aware clients MAY prefer to stage their certificate updates
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 {{in-band-certificates}}), and the OpenPGP client wants to
evaluate it for use in the immediate context.
Distinct Keystore Interfaces Leak Client Context and Intent
-----------------------------------------------------------
The distinct keystore interfaces documented in
{{keystore-interfaces}} offer subtly different semantics, and are
used by a reasonable keystore client at different times. A keystore
that offers distinct discovery and update interfaces may infer that a
client visiting the update interface already knows about the
certificate in question, or that a client visiting the discovery
interface is in the process of verifying a signature from a
certificate it has not seen before.
HKP itself ({{I-D.shaw-openpgp-hkp}}) conflates these two interfaces
-- the same HKP query is be used to perform both discovery and update
(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 update
and discovery, since the discovery interface is a proper superset of
the update interface.
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 {{tracking-clients}}.
Keystores which offer network access SHOULD provide encrypted
transport.
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 update, leaving open
the types of tracking attacks described in {{tracking-clients}}.
Clients of keystores SHOULD pad their queries to increase the size of
the anonymity set. And keystores SHOULD pad their responses.
The appropriate size of padding to effectively anonymize traffic to
and from keystores is likely to be mechanism- and cohort-specific.
For example, padding for keystores accessed via the DNS ({{RFC7929}}
may use different padding strategies that padding for keystores
accessed over WKD ({{I-D.koch-openpgp-webkey-service}}), which may in
turn be different from keystores accessed over HKPS
({{I-D.shaw-openpgp-hkp}}). A keystore which only accepts user IDs
within a specific domain (e.g., {{scoped-user-ids}}) or which uses
custom process ({{exterior-process}}) 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
document.
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
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.
IANA Considerations
===================
This document asks IANA to register two entries in the OpenPGP
Notation IETF namespace, both with a reference to this document:
* the "ksok" notation is defined in {{fpatpc}}.
* the "uidhash" notation is defined in {{uidhash}}.
Document Considerations
=======================
\[ RFC Editor: please remove this section before publication ]
This document is currently edited as markdown. Minor editorial
changes can be suggested via merge requests at
https://gitlab.com/dkg/draft-openpgp-abuse-resistant-keystore or by
e-mail to the author. Please direct all significant commentary to the
public IETF OpenPGP mailing list: openpgp(_at_)ietf(_dot_)org
Document History
----------------
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 (sub)key
* new attack: fingerprint flooding
* new retrieval-time mitigations -- tighter filters on discovery and update
* 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
* 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
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:
* Antoine Beaupré
* ilf
* Jamie McClelland
* Jonathan McDowell
* Justus Winter
* Marcus Brinkmann
* Micah Lee
* Neal Walfield
* Phil Pennock
* Philihp Busby
* vedaal
* Vincent Breitmoser
* Wiktor Kwapisiewicz
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