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openpgp D. Gillmor
Internet-Draft ACLU
Intended status: Informational April 19, 2019
Expires: October 21, 2019
Abuse-Resistant OpenPGP Keystores
draft-dkg-openpgp-abuse-resistant-keystore-03
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 21, 2019.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Certificate Flooding . . . . . . . . . . . . . . . . . . 7
2.2. User ID Flooding . . . . . . . . . . . . . . . . . . . . 8
2.3. Fingerprint Flooding . . . . . . . . . . . . . . . . . . 8
2.4. Keystore Flooding . . . . . . . . . . . . . . . . . . . . 9
2.5. Toxic Data . . . . . . . . . . . . . . . . . . . . . . . 9
3. Keystore Interfaces . . . . . . . . . . . . . . . . . . . . . 9
3.1. Certificate Update . . . . . . . . . . . . . . . . . . . 10
3.2. Certificate Discovery . . . . . . . . . . . . . . . . . . 10
3.3. Certificate Lookup . . . . . . . . . . . . . . . . . . . 11
3.3.1. Full User ID Lookup . . . . . . . . . . . . . . . . . 11
3.3.2. E-mail Address Lookup . . . . . . . . . . . . . . . . 12
3.3.3. Other Lookup Mechanisms . . . . . . . . . . . . . . . 12
3.4. Certificate Validation . . . . . . . . . . . . . . . . . 12
3.5. Certificate Submission . . . . . . . . . . . . . . . . . 14
4. Simple Mitigations . . . . . . . . . . . . . . . . . . . . . 14
4.1. Decline Large Packets . . . . . . . . . . . . . . . . . . 14
4.2. Enforce Strict User IDs . . . . . . . . . . . . . . . . . 15
4.3. Scoped User IDs . . . . . . . . . . . . . . . . . . . . . 15
4.4. Strip or Standardize Unhashed Subpackets . . . . . . . . 15
4.4.1. Issuer Fingerprint . . . . . . . . . . . . . . . . . 15
4.4.2. Cross-sigs . . . . . . . . . . . . . . . . . . . . . 15
4.4.3. First-party Attestations . . . . . . . . . . . . . . 16
4.5. Decline User Attributes . . . . . . . . . . . . . . . . . 16
4.6. Decline Non-exportable Certifications . . . . . . . . . . 16
4.7. Decline Data From the Future . . . . . . . . . . . . . . 16
4.8. Accept Only Profiled Certifications . . . . . . . . . . . 16
4.9. Accept Only Certificates Issued by Designated Authorities 17
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4.10. Decline Packets by Blocklist . . . . . . . . . . . . . . 17
5. Retrieval-time Mitigations . . . . . . . . . . . . . . . . . 18
5.1. Redacting User IDs . . . . . . . . . . . . . . . . . . . 19
5.1.1. Certificate Update with Redacted User IDs . . . . . . 19
5.1.2. Certificate Discovery with Redacted User IDs . . . . 19
5.1.3. Certificate Lookup with Redacted User IDs . . . . . . 20
5.1.4. Hinting Redacted User IDs . . . . . . . . . . . . . . 20
5.1.5. User ID Recovery by Client Brute Force . . . . . . . 21
5.2. Primary-key Only Certificate Update . . . . . . . . . . . 21
5.3. Require Valid Cross-Sigs for Certificate Discovery . . . 21
6. Contextual Mitigations . . . . . . . . . . . . . . . . . . . 22
6.1. Accept Only Cryptographically-verifiable Certifications . 22
6.2. Accept Only Certificates Issued by Known Certificates . . 23
6.3. Rate-limit Submissions by IP Address . . . . . . . . . . 23
6.4. Accept Certificates Based on Exterior Process . . . . . . 23
6.5. Accept Certificates by E-mail Validation . . . . . . . . 24
7. Non-append-only mitigations . . . . . . . . . . . . . . . . . 24
7.1. Drop Superseded Signatures . . . . . . . . . . . . . . . 25
7.2. Drop Expired Signatures . . . . . . . . . . . . . . . . . 25
7.3. Drop Dangling User IDs, User Attributes, and Subkeys . . 25
7.4. Drop All Other Elements of a Directly-Revoked Certificate 26
7.5. Implicit Expiration Date . . . . . . . . . . . . . . . . 26
8. Updates-only Keystores . . . . . . . . . . . . . . . . . . . 27
9. First-party-only Keystores . . . . . . . . . . . . . . . . . 27
9.1. First-party-only Without User IDs . . . . . . . . . . . . 28
10. First-party-attested Third-party Certifications . . . . . . . 28
10.1. Key Server Preferences "No-modify" . . . . . . . . . . . 29
10.2. Client Interactions . . . . . . . . . . . . . . . . . . 30
11. Keystore Client Best Practices . . . . . . . . . . . . . . . 30
11.1. Use Constrained Keystores for Lookup . . . . . . . . . . 30
11.2. Normalize Addresses and User IDs for Lookup . . . . . . 30
11.3. Avoid Fuzzy Lookups . . . . . . . . . . . . . . . . . . 31
11.4. Prefer Full Fingerprint for Discovery and Update . . . . 31
11.5. Use Caution with Keystore-provided Validation . . . . . 31
12. Certificate Generation and Management Best Practices . . . . 32
12.1. Canonicalized E-Mail Addresses . . . . . . . . . . . . . 32
12.2. Normalized User IDs . . . . . . . . . . . . . . . . . . 32
12.3. Avoid Large User Attributes . . . . . . . . . . . . . . 32
12.4. Provide Cross-Sigs . . . . . . . . . . . . . . . . . . . 33
12.5. Provide Issuer Fingerprint Subpackets . . . . . . . . . 33
12.6. Put Cross-Sigs and Issuer Fingerprint in Hashed
Subpackets . . . . . . . . . . . . . . . . . . . . . . . 33
12.7. Submit Certificates to Restricted, Lookup-Capable
Keystores . . . . . . . . . . . . . . . . . . . . . . . 33
13. Side Effects and Ecosystem Impacts . . . . . . . . . . . . . 33
13.1. Designated Revoker . . . . . . . . . . . . . . . . . . . 33
13.2. Key IDs vs. Fingerprints in Certificate Discovery . . . 34
13.3. In-band Certificates . . . . . . . . . . . . . . . . . . 34
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13.3.1. In-band Certificate Minimization and Validity . . . 35
13.4. Certification-capable Subkeys . . . . . . . . . . . . . 36
13.5. Assessing Certificates in the Past . . . . . . . . . . . 36
13.5.1. Point-in-time Certificate Evaluation . . . . . . . . 37
13.5.2. Signature Verification and Non-append-only Keystores 37
13.6. Global Append-only Ledgers ("Blockchain") . . . . . . . 37
13.7. Certificate Lookup for Identity Monitoring . . . . . . . 39
14. OpenPGP details . . . . . . . . . . . . . . . . . . . . . . . 39
14.1. Revocations . . . . . . . . . . . . . . . . . . . . . . 39
14.2. User ID Conventions . . . . . . . . . . . . . . . . . . 40
14.3. E-mail Address Canonicalization . . . . . . . . . . . . 41
14.3.1. Disallowing Non-UTF-8 Local Parts . . . . . . . . . 41
14.3.2. Domain Canonicalization . . . . . . . . . . . . . . 41
14.3.3. Local Part Canonicalization . . . . . . . . . . . . 41
15. Security Considerations . . . . . . . . . . . . . . . . . . . 41
15.1. Tension Between Unrestricted Uploads and Certificate
Lookup . . . . . . . . . . . . . . . . . . . . . . . . . 42
16. Privacy Considerations . . . . . . . . . . . . . . . . . . . 42
16.1. Publishing Identity Information . . . . . . . . . . . . 42
16.2. Social Graph . . . . . . . . . . . . . . . . . . . . . . 43
16.3. Tracking Clients by Queries . . . . . . . . . . . . . . 43
16.4. "Live" Certificate Validation Leaks Client Activity . . 44
16.5. Certificate Discovery Leaks Client Activity . . . . . . 44
16.6. Certificate Update Leaks Client Activity . . . . . . . . 45
16.7. Distinct Keystore Interfaces Leak Client Context and
Intent . . . . . . . . . . . . . . . . . . . . . . . . . 45
16.8. Cleartext Queries . . . . . . . . . . . . . . . . . . . 46
16.9. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 46
17. User Considerations . . . . . . . . . . . . . . . . . . . . . 46
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
19. Document Considerations . . . . . . . . . . . . . . . . . . . 47
19.1. Document History . . . . . . . . . . . . . . . . . . . . 47
20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 49
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
21.1. Normative References . . . . . . . . . . . . . . . . . . 50
21.2. Informative References . . . . . . . . . . . . . . . . . 50
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction
1.1. 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.
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1.2. Terminology
o "OpenPGP certificate" (or just "certificate") is used
interchangeably with [RFC4880]'s "Transferable Public Key". The
term "certificate" refers unambiguously to the entire composite
object, unlike "key", which might also be used to refer to a
primary key or subkey.
o An "identity certification" (or just "certification") is an
[RFC4880] signature packet that covers OpenPGP identity
information - that is, any signature packet of type 0x10, 0x11,
0x12, or 0x13. Certifications are said to (try to) "bind" a
primary key to a User ID.
o The primary key that makes the certification is known as the
"issuer". The primary key over which the certification is made is
known as the "subject".
o A "first-party certification" is issued by the primary key of a
certificate, and binds itself to a user ID in the certificate.
That is, the issuer is the same as the subject. This is sometimes
referred to as a "self-sig".
o A "third-party certification" is a made over a primary key and
user ID by some other certification-capable primary key. That is,
the issuer is different than the subject. (The elusive "second-
party" is presumed to be the verifier who is trying to interpret
the certificate)
o 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".
o A "keystore" is any collection of OpenPGP certificates. Keystores
typically receive mergeable updates over the course of their
lifetime which might add to the set of OpenPGP certificates they
hold, or update the certificates.
o "Certificate 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@example.org>" and the user wants to send an e-mail to
"alice@example.org", the mail user agent might want to ensure that
the certificate is valid for this e-mail address before encrypting
to it. Some clients may rely on specific keystores for
certificate validation, but some keystores (e.g., [SKS]) make no
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assertions whatsoever about certificate validity, and others offer
only very subtle guarantees. See Section 3.4 for more details.
o "Certificate lookup" refers to the retrieval of a set of
certificates from a keystore based on the user ID or some
substring match of the user ID. See Section 3.3 for more details.
o "Certificate update" refers to retrieval of a certificate from a
keystore based on the fingerprint of the primary key. See
Section 3.1 for more details.
o "Certificate discovery" refers to the retrieval of a set of
certificates from a keystore based on the fingerprint or key ID of
any key in the certificate. See Section 3.2 for more details.
o 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].
o A "synchronizing keyserver" is a keyserver which gossips with
other peers, and typically acts as an append-only log. Such a
keyserver is typically useful for certificate 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.
o An "e-mail validating keyserver" is a keyserver which attempts to
verify the identity in an OpenPGP certificate's user ID by
confirming access to the e-mail account, and possibly by
confirming access to the secret key. Some implementations permit
removal of a certificate by anyone who can prove access to the
e-mail address in question. They are useful for certificate
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.
o "Cryptographic validity" refers to mathematical evidence that a
signature came from the secret key associated with the public key
it claims to come from. Note that a certification may be
cryptographically valid without the signed data being true (for
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example, a given certificate with the user ID "Alice
<alice@example.org>" might not belong to the person who controls
the e-mail address "alice@example.org" even though the self-sig is
cryptographically valid). In particular, cryptographic validity
for user ID in a certificate is typically insufficient evidence
for certificate validation. Also note that knowledge of the
public key of the issuer is necessary to determine whether any
given signature is cryptographically valid. Some keyservers
perform cryptographic validation in some contexts. Other
keyservers (like [SKS]) perform no cryptographic validation
whatsoever.
o OpenPGP revocations can have "Reason for Revocation" (see
[RFC4880]), which can be either "soft" or "hard". The set of
"soft" reasons is: "Key is superseded" and "Key is retired and no
longer used". All other reasons (and revocations that do not
state a reason) are "hard" revocations. See Section 14.1 for more
detail.
2. Problem Statement
OpenPGP keystores that handle submissions from the public are subject
to a range of attacks by malicious submitters.
This section describes five distinct attacks that public keystores
should consider.
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.
2.1. Certificate Flooding
Many public keystores (including both the [SKS] keyserver network and
[MAILVELOPE-KEYSERVER]) allow anyone to attach arbitrary data (in the
form of third-party certifications) to any certificate, bloating that
certificate to the point of being impossible to effectively retrieve.
For example, some OpenPGP implementations simply refuse to process
certificates larger than a certain size.
This kind of Denial-of-Service attack makes it possible to make
someone else's certificate unretrievable from the keystore,
preventing certificate lookup, discovery, or 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.
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Additionally, even without malice, OpenPGP certificates can
potentially grow without bound.
2.2. User ID Flooding
Public keystores that are used for certificate lookup may also be
vulnerable to attacks that flood the space of known user IDs. In
particular, if the keystore accepts arbitrary certificates from the
public and does no verification of the user IDs, then any client
searching for a given user ID may need to review and process an
effectively unbounded set of maliciously-submitted certificates to
find the non-malicious certificates they are looking for.
For example, if an attacker knows that a given system consults a
keystore looking for certificates which match the e-mail address
"alice@example.org", the attacker may upload hundreds or thousands of
certificates containing user IDs that match that address. Even if
those certificates would not be accepted by a client (e.g., because
they were not certified by a known-good authority), the client
typically still has to wade through all of them in order to find the
non-malicious certificates.
If the keystore does not offer a lookup interface at all (that is, if
clients cannot search it by user ID), then user ID flooding is of
less consequence.
2.3. 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.
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2.4. Keystore Flooding
A public keystore that accepts arbitrary OpenPGP material and is
append-only is at risk of being overwhelmed by sheer quantity of
malicious uploaded packets. This is a risk even if the user ID space
is not being deliberately flooded, and if individual certificates are
protected from flooding by any of the mechanisms described later in
this document.
The keystore itself can become difficult to operate if the total
quantity of data is too large, and if it is a synchronizing
keyserver, then the quantities of data may impose unsustainable
bandwidth costs on the operator as well.
Effectively mitigating against keystore flooding requires either
abandoning the append-only property that some keystores prefer, or
imposing very strict controls on initial ingestion.
2.5. Toxic Data
Like any large public dataset, it's possible that a keystore ends up
hosting some content that is legally actionable in some
jurisdictions, including libel, child pornography, material under
copyright or other "intellectual property" controls, blasphemy, hate
speech, etc.
A public keystore that accepts and redistributes arbitrary content
may face risk due to uploads of toxic data.
3. Keystore Interfaces
Some keystores have simple interfaces, like files present in a local
filesystem. But many keystores offer an API for certificate
retrieval of different types. This section documents a set of useful
interactions that a client may have with such a keystore.
They are represented in abstract form, and are not intended to be the
full set of interfaces offered by any keystore, but rather a
convenient way to think about the operations that make the keystore
useful for its clients.
Not all keystores may offer all of these interfaces, or they may
offer them in subtly different forms, but clients will nevertheless
try to perform something like these operations with keystores that
they interact with.
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3.1. 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.
3.2. Certificate Discovery
If a client is aware of an OpenPGP signature or certification that it
cannot verify because it does not know the issuing certificate, it
may consult a keystore to try to discover the certificate based on
the Issuer or Issuer Fingerprint subpacket in the signature or
certification it is trying to validate.
keystore.cert_discovery(keyid|fpr) -> certificate_list
This is subtly different from certificate update (Section 3.1) in
three ways:
o 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)
o it is willing to accept searches by short key ID, not just
fingerprint
o 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
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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.
3.3. Certificate Lookup
If a client wants to encrypt a message to a particular e-mail
address, or wants to encrypt a backup to some identity that it knows
of but does not have a certificate for, it may consult a keystore to
discover certificates that claim that identity in their user ID
packets. Both [I-D.koch-openpgp-webkey-service] and
[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
Section 14.2 for more discussion of some common conventions around
user ID structure.
Note that lookup does not necessarily imply user ID or certificate
validation. It is entirely possible for a keystore to return a
certificate during lookup that the client cannot validate.
Abuse-resistant keystores that offer a lookup interface SHOULD
distinguish interfaces that perform full-string-match lookup from
interfaces that perform e-mail address based lookup.
3.3.1. Full User ID Lookup
The most straightforward form of certificate lookup asks for the set
of all certificates that contain a user ID that exactly and
completely matches the query parameter supplied by the client.
keystore.cert_lookup(uid) -> certificate_list
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In its simplest form, this match is done by a simple bytestring
comparison. More sophisticated keystores MAY perform the comparison
after applying [UNICODE-NORMALIZATION] form NFC to both the "uid"
query and the user IDs from the stored certificates.
3.3.2. E-mail Address Lookup
However, some common use cases look for specific patterns in the user
ID rather than the entire user ID. Most useful to many existing
OpenPGP clients is a lookup by e-mail address.
keystore.cert_lookup(addr) -> certificate_list
For certificates with a user ID that matches the structure of an
[RFC5322] "name-addr" or "addr-spec", a keystore SHOULD extract the
"addr-spec" from the user ID, canonicalize it (see Section 14.3), and
compare it to the canonicalized form of of the "addr" query
parameter.
3.3.3. Other Lookup Mechanisms
Some keystores offer other forms of substring or regular expression
matching against the stored user IDs. These other forms of lookup
may be useful in some contexts (e.g., Section 13.7), but they may
also represent privacy concerns (e.g., Section 16.1), and they may
impose additional computational or indexing burdens on the keystore.
3.4. Certificate Validation
An OpenPGP client may assess certificate and user ID validity based
on many factors, some of which are directly contained in the
certificate itself (e.g., third-party certifications), and some of
which are based on the context known to the client, including:
o Whether it has seen e-mails from that address signed by that
certificate in the past,
o How long it has known about the certificate,
o 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
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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 Section 13.5.
An abuse-resistant keystore that clients rely on for any part of
their certificate validation process SHOULD offer a distinct
interface for making assertions about certificate and user ID
validity to help clients avoid some of the subtleties involved with
inference based on presence described above.
Note that the certificate validation operation as described above has
a boolean response. While a "true" response indicates that keystore
believes the user ID or e-mail address is acceptable for use with the
certificate referred to by the public key fingerprint, a "false"
response doesn't necessarily mean that the keystore actively thinks
that the certificate is actively bad, or must not be used for the
referenced identity. Rather, "false" is the default state: no
opinion is expressed by the keystore, and the client is left to make
their own inference about validity based on other factors. A
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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.
3.5. Certificate Submission
Different keystores have different ways to submit a certificate for
consideration for ingestion, including:
o a simple upload of a certificate via http
o round-trip e-mail verification
o proof of presence in some other service
o 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 Section 12.
4. Simple Mitigations
These steps can be taken by any keystore that wants to avoid
obviously malicious abuse. They can be implemented on receipt of any
new packet, and are based strictly on the structure of the packet
itself.
4.1. Decline Large Packets
While [RFC4880] permits OpenPGP packet sizes of arbitrary length,
OpenPGP certificates rarely need to be so large. An abuse-resistant
keystore SHOULD reject any OpenPGP packet larger than 8383 octets.
(This cutoff is chosen because it guarantees that the packet size can
be represented as a one- or two-octet [RFC4880] "New Format Packet
Length", but it could be reduced further)
This may cause problems for user attribute packets that contain large
images, but it's not clear that these images are concretely useful in
any context. Some keystores MAY extend this limit for user attribute
packets specifically, but SHOULD NOT allow even user attributes
packets larger than 65536 octets.
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4.2. Enforce Strict User IDs
[RFC4880] indicates that User IDs are expected to be UTF-8 strings.
An abuse-resistant keystore MUST reject any user ID that is not valid
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 Section 14.2).
As simple text strings, User IDs don't need to be nearly as long as
any other packets. An abuse-resistant keystore SHOULD reject any
user ID packet larger than 1024 octets.
4.3. Scoped User IDs
Some abuse-resistant keystores may restrict themselves to publishing
only certificates with User IDs that match a specific pattern. For
example, [RFC7929] encourages publication in the DNS of only
certificates whose user IDs refer to e-mail addresses within the DNS
zone. [I-D.koch-openpgp-webkey-service] similarly aims to restrict
publication to certificates relevant to the specific e-mail domain.
4.4. Strip or Standardize Unhashed Subpackets
[RFC4880] signature packets contain an "unhashed" block of
subpackets. These subpackets are not covered by any cryptographic
signature, so they are ripe for abuse.
An abuse-resistant keystore SHOULD strip out all unhashed subpackets
but the following exceptions:
4.4.1. Issuer Fingerprint
Some certifications only identify the issuer of the certification by
an unhashed Issuer or Issuer Fingerprint subpacket. If a
certification's hashed subpacket section has no Issuer Fingerprint
(see [I-D.ietf-openpgp-rfc4880bis]) subpacket, then an abuse-
resistant keystore that has cryptographically validated the
certification SHOULD synthesize an appropriate Issuer Fingerprint
subpacket and include it in the certification's unhashed subpackets.
4.4.2. Cross-sigs
Some Primary Key Binding Signatures ("cross-sigs") are distributed as
unhashed subpackets in a Subkey Binding Signature. A
cryptographically-validating abuse-resistant keystore SHOULD be
willing to redistribute a valid cross-sig as an unhashed subpacket.
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The redistributed unhashed cross-sig itself should be stripped of all
unhashed subpackets.
4.4.3. First-party Attestations
Some third-party certifications are attested to by the certificate
primary key itself in an unhashed subpacket, as described in
Section 10. 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.
4.5. Decline User Attributes
Due to size concerns, some abuse-resistant keystores MAY choose to
ignore user attribute packets entirely, as well as any certifications
that cover them.
4.6. Decline Non-exportable Certifications
An abuse-resistant keystore MUST NOT accept any certification that
has the "Exportable Certification" subpacket present and set to 0.
While most keystore clients will not upload these "local"
certifications anyway, a reasonable public keystore that wants to
minimize data has no business storing or distributing these
certifications.
4.7. Decline Data From the Future
Many OpenPGP packets have time-of-creation timestamps in them. An
abuse-resistant keystore with a functional real-time clock MAY decide
to only accept packets whose time-of-creation is in the past.
Note that some OpenPGP implementations may pre-generate OpenPGP
material intended for use only in some future window (e.g. "Here is
the certificate we plan to use to sign our software next year; do not
accept signatures from it until then."), and may use modified time-
of-creation timestamps to try to achieve that purpose. This material
would not be distributable ahead of time by an abuse-resistant
keystore that adopts this mitigation.
4.8. Accept Only Profiled Certifications
An aggressively abuse-resistant keystore MAY decide to only accept
certifications that meet a specific profile. For example, it MAY
reject certifications with unknown subpacket types, unknown
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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 Section 7).
4.9. Accept Only Certificates Issued by Designated Authorities
An abuse-resistant keystore capable of cryptographic validation MAY
retain a list of designated authorities, typically in the form of a
set of known public keys. Upon receipt of a new OpenPGP certificate,
the keystore can decide whether to accept or decline each user ID of
the certificate based whether that user ID has a certification that
was issued by one or more of the designated authorities.
If no user IDs are certified by designated authority, such a keystore
SHOULD decline the certificate and its primary key entirely. Such a
keystore SHOULD decline to retain or propagate all certifications
associated with each accepted user ID except for first-party
certifications and certifications by the designated authorities.
The operator of such a keystore SHOULD have a clear policy about its
set of designated authorities.
Given the ambiguities about expiration and revocation, such a
keyserver SHOULD ignore expiration and revocation of authority
certifications, and simply accept and retain as long as the
cryptographic signature is valid.
Note that if any key is removed from the set of designated
authorities, and that change is applied to the existing keystore,
such a keystore may no longer be append-only (please see Section 7).
4.10. Decline Packets by Blocklist
The maintainer of the keystore may keep a specific list of "known-
bad" material, and decline to accept or redistribute items matching
that blocklist. The material so identified could be anything, but
most usefully, specific public keys or User IDs could be blocked.
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Note that if a blocklist grows to include an element already present
in the keystore, it will no longer be append-only (please see
Section 7).
Some keystores may choose to apply a blocklist only at retrieval time
and not apply it at ingestion time. This allows the keystore to be
append-only, and permits synchronization between keystores that don't
share a blocklist, and somewhat reduces the attacker's incentive for
flooding the keystore (see Section 5 for more discussion).
Note that development and maintenance of a blocklist is not without
its own potentials for abuse. For one thing, the blocklist may
itself grow without bound. Additionally, a blocklist may be socially
or politically contentious as it may describe data that is toxic
(Section 2.5) in one community or jurisdiction but not another.
There needs to be a clear policy about how it is managed, whether by
delegation to specific decision-makers, or explicit tests.
Furthermore, the existence of even a well-intentioned blocklist may
be an "attractive nuisance," drawing the interest of would-be censors
or other attacker interested in controlling the ecosystem reliant on
the keystore in question.
5. Retrieval-time Mitigations
Most of the abuse mitigations described in this document are
described as being applied at certificate ingestion time. It's also
possible to apply the same mitigations when a certificate is
retrieved from the keystore (that is, during certificate lookup,
update, or discovery). Applying an abuse mitigation at retrieval
time may help a client defend against a user ID flooding
(Section 2.2), certificate flooding (Section 2.1), or fingerprint
flooding (Section 2.3) attack. It may also help a keystore limit its
liability for redistributing toxic data (Section 2.5). However, only
mitigations applied at ingestion time are able to mitigate keystore
flooding attacks (Section 2.4).
Some mitigations (like the non-append-only mitigations described in
Section 7) may be applied as filters at retrieval time, while still
allowing access to the (potentially much larger) unfiltered dataset
associated given certificate or user ID via a distinct interface.
The rest of this section documents specific mitigations that are only
relevant at retrieval time (certificate discovery, lookup, or
update).
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5.1. Redacting User IDs
Some abuse-resistant keystores may accept and store user IDs but
decline to redistribute some or all of them, while still distributing
the certifications that cover those redacted user IDs. This draft
refers to such a keystore as a "user ID redacting" keystore.
The certificates distributed by such a keystore are technically
invalid [RFC4880] "transferable public keys", because they lack a
user ID packet, and the distributed certifications cannot be
cryptographically validated independently. However, an OpenPGP
implementation that already knows the user IDs associated with a
given primary key will be capable of associating each certification
with the correct user ID by trial signature verification.
5.1.1. Certificate 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.
5.1.2. Certificate Discovery with Redacted User IDs
A user ID redacting keystore is somewhat less useful for clients
doing certificate discovery. Consider the circumstance of receiving
a signed e-mail without access to the signing certificate. If the
verifier retrieves the certificate from a user ID redacting keystore
by via the Issuer Fingerprint from the signature, and the signature
validates, the received certificate might not be a valid
"transferable public key" unless the client can synthesize the proper
user ID.
A reasonable client that wants to validate a certification in the
user ID redacted certificate SHOULD try to synthesize possible user
IDs based on the value of the [RFC5322] From: header in the message:
o 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.
o The resulting string should be an [RFC5322] "name-addr" or "addr-
spec".
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o 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.
o Canonicalize the "addr-spec" according to Section 14.3, and check
whether the certification validates, terminating on success.
o 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.
o If all of the above fails, synthesis has failed.
5.1.3. Certificate Lookup with Redacted User IDs
It's possible (though non-intuitive) to use a user ID redacting
keystore for certificate lookup. Since the keystore retains (but
does not distribute) the user IDs, they can be used to select
certificates in response to a search. The OpenPGP certificates sent
back in response to the search will not contain the user IDs, but a
client that knows the full user ID they are searching for will be
able to verify the returned certifications.
Certificate lookup from a user ID redacting keystore works better for
certificate lookup by exact user ID match than it does for substring
match, because a client that retrieves a certificate via a substring
match may not be able to reconstruct the redacted user ID.
However, without some additional restrictions on which certifications
are redistributed (whether the user ID is redacted or not),
certificate lookup can be flooded (see Section 15.1).
5.1.4. Hinting Redacted User IDs
To ensure that the distributed certificate is at least structurally a
valid [RFC4880] transferable public key, a user ID redacting keystore
MAY distribute an empty user ID (an OpenPGP packet of tag 13 whose
contents are a zero-octet string) in place of the omitted user ID.
This two-octet replacement user ID packet ("\xb4\x00") is called the
"unstated user ID".
To facilitate clients that match certifications with specific user
IDs, a user ID redacting keystore MAY insert a non-hashed notation
subpacket into the certification. The notation will have a name of
"uidhash", with 0x80 ("human-readable") flag unset. The value of
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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.
5.1.5. User ID Recovery by Client Brute Force
User ID redaction is at best an imperfect process. Even if a
keystore redacts a User ID, if it ships a certification over that
user ID, an interested client can guess user IDs until it finds one
that causes the signature to verify. This is even easier when the
space of legitimate user IDs is relatively small, such as the set of
commonly-used hostnames
5.2. Primary-key Only Certificate Update
Abuse-resistant keystores can defend against a fingerprint flooding
Section 2.3 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 Section 3.1 exactly. In that specification, the set of
keys returned by a "get" operation with a "search" parameter that
appears to be a full fingerprint is ambiguous. Some popular
implementations (e.g., [SKS]) do not currently implement this
mitigation, because they return certificates with subkeys that match
the fingerprint.
5.3. Require Valid Cross-Sigs for Certificate Discovery
By definition, certificate discovery needs to be able to match
subkeys, not just primary keys. This means that the mitigation in
Section 5.2 is ineffective for a keystore that offers a certificate
discovery interface.
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An abuse-resistant keystore that aims to defend its certificate
discovery interface from a fingerprint flooding (Section 2.3) attack
can follow the following procedure.
Such a keystore MUST accept only a full fingerprint or a 64-bit key
ID as the search parameter from the certificate discovery client. It
MUST only match that fingerprint against the following:
o the fingerprint or key ID of a primary key associated with a valid
certificate
o 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.
6. Contextual Mitigations
Some mitigations make the acceptance or rejection of packets
contingent on data that is already in the keystore or the keystore's
developing knowledge about the world. This means that, depending on
the order that the keystore encounters the various material, or how
it accesses or finds the material, the final set of material retained
and distributed by the keystore might be different.
While this isn't necessarily bad, it may be a surprising property for
some users of keystores.
6.1. Accept Only Cryptographically-verifiable Certifications
An abuse-resistant keystore that is capable of doing cryptographic
validation MAY decide to reject certifications that it cannot
cryptographically validate.
This may mean that the keystore rejects some packets while it is
unaware of the public key of the issuer of the packet.
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6.2. Accept Only Certificates Issued by Known Certificates
This is an extension of Section 4.9, but where the set of authorities
is just the set of certificates already known to the keystore. An
abuse-resistant keystore that adopts this strategy is effectively
only crawling the reachable graph of OpenPGP certificates from some
starting core.
A keystore adopting the mitigation SHOULD have a clear documentation
of the core of initial certificates it starts with, as this is
effectively a policy decision.
This mitigation measure may fail due to a compromise of any secret
key that is associated with a primary key of a certificate already
present in the keystore. Such a compromise permits an attacker to
flood the rest of the network. In the event that such a compromised
key is identified, it might be placed on a blocklist (see
Section 4.10). In particular, if a public key is added to a
blocklist for a keystore implementing this mitigation, and it is
removed from the keystore, then all certificates that were only
"reachable" from the blocklisted certificate should also be
simultaneously removed.
6.3. Rate-limit Submissions by IP Address
Some OpenPGP keystores accept material from the general public over
the Internet. If an abuse-resistant keystore observes a flood of
material submitted to the keystore from a given Internet address, it
MAY choose to throttle submissions from that address. When receiving
submissions over IPv6, such a keystore MAY choose to throttle entire
nearby subnets, as a malicious IPv6 host is more likely to have
multiple addresses.
This requires that the keystore maintain state about recent
submissions over time and address. It may also be problematic for
users who appear to share an IP address from the vantage of the
keystore, including those behind a NAT, using a VPN, or accessing the
keystore via Tor.
6.4. Accept Certificates Based on Exterior Process
Some public keystores resist abuse by explicitly filtering OpenPGP
material based on a set of external processes. For example,
[DEBIAN-KEYRING] adjudicates the contents of the "Debian keyring"
keystore based on organizational procedure and manual inspection.
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6.5. Accept Certificates by E-mail Validation
Some keystores resist abuse by declining any certificate until the
user IDs have been verified by e-mail. When these "e-mail
validating" keystores review a new certificate that has a user ID
with an e-mail address in it, they send an e-mail to the associated
address with a confirmation mechanism (e.g., a high-entropy HTTPS URL
link) in it. 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 Section 5.1) 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.
7. Non-append-only mitigations
The following mitigations may cause some previously-retained packets
to be dropped after the keystore receives new information, or as time
passes. This is entirely reasonable for some keystores, but it may
be surprising for 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 Section 13.5 for more considerations.
Note also that many of these mitigations depend on cryptographic
validation, so they're typically contextual as well.
A keystore that needs to be append-only, or which cannot perform
cryptographic validation MAY omit these mitigations. Alternately, a
keystore may omit these mitigations at certificate ingestion time,
but apply these mitigations at retrieval time (during certificate
update, discovery, or lookup), and offer a more verbose (non-
mitigated) interface for auditors, as described in Section 5.
Note that [GnuPG] anticipates some of these suggestions with its
"clean" subcommand, which is documented as:
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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.
7.1. Drop Superseded Signatures
An abuse-resistant keystore SHOULD drop all signature packets that
are explicitly superseded. For example, there's no reason to retain
or distribute a self-sig by key K over User ID U from 2017 if the
keystore have a cryptographically-valid self-sig over <K,U> from
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.
7.2. Drop Expired Signatures
If a signature packet is known to only be valid in the past, there is
no reason to distribute it further. An abuse-resistant keystore with
access to a functional real-time clock SHOULD drop all certifications
and subkey signature packets with an expiration date in the past.
Note that this assumes that the keystore and its clients all have
roughly-synchronized clocks. If that is not the case, then there
will be many other problems!
7.3. Drop Dangling User IDs, User Attributes, and Subkeys
If enough signature packets are dropped, it's possible that some of
the things that those signature packets cover are no longer valid.
An abuse-resistant keystore which has dropped all certifications that
cover a User ID SHOULD also drop the User ID packet.
Note that a User ID that becomes invalid due to revocation MUST NOT
be dropped, because the User ID's revocation signature itself remains
valid, and needs to be distributed.
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A primary key with no User IDs and no subkeys and no revocations MAY
itself also be removed from distribution, though note that the
removal of a primary key may make it impossible to cryptographically
validate other certifications held by the keystore.
7.4. Drop All Other Elements of a Directly-Revoked Certificate
If the primary key of a certificate is revoked via a direct key
signature, an abuse-resistant keystore SHOULD drop all the rest of
the associated data (user IDs, user attributes, and subkeys, and all
attendant certifications and subkey signatures). This defends
against an adversary who compromises a primary key and tries to flood
the certificate to hide the revocation.
Note that the direct key revocation signature MUST NOT be dropped.
In the event that an abuse-resistant keystore is flooded with direct
key revocation signatures, it should retain the hardest, earliest
revocation (see also Section 14.1).
In particular, if any of the direct key revocation signatures is a
"hard" revocation, the abuse-resistant keystore SHOULD retain the
earliest such revocation signature (by signature creation date).
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.
7.5. Implicit Expiration Date
In combination with some of the dropping mitigations above, a
particularly aggressive abuse-resistant keystore MAY choose an
implicit expiration date for all signature packets. For example, a
signature packet that claims no expiration could be treated by the
keystore as expiring 3 years after issuance. This would permit the
keystore to eject old packets on a rolling basis.
An abuse-resistant keystore that adopts this mitigation needs a
policy for handling signature packets marked with an explicit
expiration that is longer than implicit maximum. The two obvious
strategies are:
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o cap the packet's expiration to the system's implicit expiration
date, or
o accept the explicit expiration date.
Warning: Any implementation of this idea is pretty radical, and it's
not clear what it would do to an ecosystem that depends on such a
keystore. It probably needs more thinking.
8. Updates-only Keystores
In addition to the mitigations above, some keystores may resist abuse
by declining to accept any user IDs or certifications whatsoever.
Such a keystore MUST be capable of cryptographic validation. It
accepts primary key packets, cryptographically-valid direct-key
signatures from a primary key over itself, subkeys and their
cryptographically-validated binding signatures (and cross-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.
9. First-party-only Keystores
Slightly more permissive than the updates-only keystore described in
Section 8 is a keystore that also permits user IDs and their self-
sigs.
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A first-party-only keystore only accepts and distributes
cryptographically-valid first-party certifications. Given a primary
key that the keystore understands, it will only attach user IDs that
have a valid self-sig, and will only accept and re-distribute subkeys
that are also cryptographically valid (including requiring cross-sigs
for signing-capable subkeys as recommended in [RFC4880]).
This effectively avoids certificate flooding attacks, because the
only party who can make a certificate overly large is the holder of
the secret corresponding to the primary key itself.
Note that a first-party-only keystore is still problematic for those
people who rely on the keystore for retrieval of third-party
certifications. Section 10 attempts to address this lack.
9.1. 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 Section 5.1). 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.
10. First-party-attested Third-party Certifications
We can augment a first-party-only keystore to allow it to distribute
third-party certifications as long as the first-party has signed off
on the specific third-party certification.
An abuse-resistant keystore SHOULD only accept a third-party
certification if it meets the following criteria:
o The third-party certification MUST be cryptographically valid.
Note that this means that the keystore needs to know the primary
key for the issuer of the third-party certification.
o The third-party certification MUST have an unhashed subpacket of
type Embedded Signature, the contents of which we'll call the
"attestation". This attestation is from the certificate's primary
key over the third-party certification itself, as detailed in the
steps below:
* The attestation MUST be an OpenPGP signature packet of type
0x50 (Third-Party Confirmation signature)
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* 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:
o the keystore knows about both the first- and third-parties.
o the third-party has made the identity assertion
o the first-party has confirmed that they're OK with the third-party
certification being distributed by any keystore.
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.
10.1. Key Server Preferences "No-modify"
[RFC4880] defines "Key Server Preferences" with a "No-modify" bit.
That bit has never been respected by any keyserver implementation
that the author is aware of. An abuse-resistant keystore following
Section 10 effectively treats that bit as always set, whether it is
present in the certificate or not.
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10.2. Client Interactions
Creating such an attestation requires multiple steps by different
parties, each of which is blocked by all prior steps:
o The first-party creates the certificate, and transfers it to the
third party.
o The third-party certifies it, and transfers their certification
back to the first party.
o The first party attests to the third party's certification.
o 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.
11. Keystore Client Best Practices
An OpenPGP client that needs to interact with an abuse-resistant
keystore can take steps to minimize the extent that its interactions
with a keystore can be abused by other parties via the attacks
described in Section 2. This section describes steps that an abuse-
resistant client can take.
11.1. 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
Section 15.1.
11.2. Normalize Addresses and User IDs for Lookup
When performing lookup by e-mail address, an abuse-resistant client
SHOULD consider canonicalizing the e-mail address before searching
(see Section 14.3).
When searching by full User ID, unless there is a strong reason to
believe that a specific non-normalized form is preferable, an abuse-
resistant client SHOULD normalize the entire user ID into
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[UNICODE-NORMALIZATION] Form C (NFC) before performing certificate
lookup.
11.3. 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.
11.4. 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.
11.5. Use Caution with Keystore-provided Validation
When an abuse-resistant client relies on a keystore for certificate
validation, many things can go subtly wrong if the client fails to
closely track the specific semantics of the keystore's validation
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.doe@example.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.
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Consider also timing and duration of these assertions of validity,
which represent a subtle tradeoff between privacy and risk as
described in Section 16.4.
12. Certificate Generation and Management Best Practices
An OpenPGP implementation that generates or manages certificates and
expects to distribute them via abuse-resistant keystores can take
steps to ensure that the certificates generated are more likely to be
accessible when needed. This section describes steps such an abuse-
sensitive implementation can take.
12.1. Canonicalized E-Mail Addresses
E-mail addresses can be written in many different ways. An abuse-
sensitive implementation considering attaching a user ID containing
an e-mail address on a certificate SHOULD ensure that the e-mail
address is structured as simply as possible. See Section 14.3 for
details about e-mail address canonicalization.
For example, if the e-mail domain considers the local part to be
case-insensitive (as most e-mail domains do today), if a proposed
user ID contains the "addr-spec": "Alice@EXAMPLE.org", the
implementation SHOULD warn the user and, if possible, propose
replacing the "addr-spec" part of the user ID with
"alice@example.org".
12.2. Normalized User IDs
User IDs are arbitrary UTF-8 strings, but UTF-8 offers several ways
to represent the same string. An abuse-sensitive implementation
considering attaching a user ID to a certificate SHOULD normalize the
string using [UNICODE-NORMALIZATION] Form C (NFC) before creating the
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.
12.3. Avoid Large User Attributes
An abuse-sensitive implementation SHOULD warn the user when attaching
a user attribute larger than 8383 octets, and SHOULD refuse to attach
user attributes entirely larger than 65536 octets. (See Section 4.1)
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12.4. Provide Cross-Sigs
[RFC4880] requires cross-sigs for all signing-capable subkeys, but is
agnostic about the use of cross-sigs for subkeys of other
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.
12.5. Provide Issuer Fingerprint Subpackets
Issuer subpackets contain only 64-bit key IDs. Issuer Fingerprint
subpackets contain an unambiguous designator of the issuing key,
avoiding the ambiguities described in Section 13.2. Abuse-sensitive
implementations SHOULD providue Issuer Fingerprint subpackets.
12.6. Put Cross-Sigs and Issuer Fingerprint in Hashed Subpackets
Unhashed subpackets may be stripped or mangled. Placing cross-sigs
and issuer fingerprint subpackets in the hashed subpackets will
ensure that they are propagated by any cryptographically-validating
keystore, even if that keystore fails to observe the exceptions in
Section 4.4.
12.7. Submit Certificates to Restricted, Lookup-Capable Keystores
If an abuse-sensitive implementation wants other peers to be able to
to retrieve the managed certificate by certificate lookup (that is,
by searching based on user ID or e-mail address), it needs to be
aware that submission to an unrestricted keystore is not reliable
(see Section 15.1 for more details).
Consequently, such an implementation SHOULD submit the managed
certificate to restricted, lookup-capable keystores where possible,
as those keystores are more likely to be able to offer reliable
lookup.
13. Side Effects and Ecosystem Impacts
13.1. Designated Revoker
A first-party-only keystore as described in Section 9 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.
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FIXME: adjust this document to point out where revocations from a
designated revoker SHOULD be propagated, maybe even in first-party-
only keystores.
13.2. Key IDs vs. Fingerprints in Certificate Discovery
During signature verification, a user performing certificate
discovery against a keystore SHOULD prefer to use the full
fingerprint as an index, rather than the 64-bit key ID. Using a
64-bit key ID is more likely to run into collision attacks; and if
the retrieved certificate has a matching key ID but the signature
cannot be validated with it, the client is in an ambiguous state -
did it retrieve the wrong certificate, or is the signature incorrect?
Using the fingerprint resolves the ambiguity: the signature is
incorrect, because the a fingerprint match is overwhelmingly stronger
than a key ID match.
Unfortunately, many OpenPGP implementations distribute signatures
with only an Issuer subpacket, so a client attempting to find such a
certificate MAY perform certificate discovery based on only the key
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).
13.3. In-band Certificates
There are contexts where it is expected and acceptable that the
signature appears without its certificate: for example, if the set of
valid signers is already known (as in some OpenPGP-signed operating
system updates), shipping the certificate alongside the signature
would be pointless bloat.
However, OpenPGP signatures are often found in contexts where the
certificate is not yet known by the verifier. For example, many
OpenPGP-signed e-mails are not accompanied by the signing
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.
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The certificate discovery interface offered by keystores is an
attempt to accommodate these situations. But in the event that a
keystore is unavailable, does not know the certificate, or suffers
from a flooding attack, signature validation may fail unnecessarily.
In an encrypted e-mail context specifically, such a failure may also
limit the client's ability to reply with an encrypted e-mail.
Certificate discovery also presents a potential privacy concern for
the signature verifier, as noted in Section 16.5.
These problematic situations can be mitigated by shipping the
certificate in-band, alongside the signature. Signers SHOULD adopt
this practice where possible to reduce the dependence of the verifier
on the keystores for certificate discovery. [AUTOCRYPT] is an
example of e-mail recommendations that include in-band certificates.
13.3.1. 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:
o 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.
o 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.
o 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.
o Strip third-party certifications that are unlikely to be relevant
to the verifier. Doing this successfully requires some knowledge
about what the third-parties the recipient is likely to care
about. Stripping all third-party certifications is a simple means
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of certificate reduction. The verifier of such a certificate may
need to do certificate update against their preferred keystore to
learn about third-party certifications useful to them.
13.4. Certification-capable Subkeys
Much of this discussion assumes that primary keys are the only
certification-capable keys in the OpenPGP ecosystem. Some proposals
have been put forward that assume that subkeys can be marked as
certification-capable. If subkeys are certification-capable, then
much of the reasoning in this draft becomes much more complex, as
subkeys themselves can be revoked by their primary key without
invalidating the key material itself. That is, a subkey can be both
valid (in one context) and invalid (in another context) at the same
time. So questions about what data can be dropped (e.g. in
Section 7) are much fuzzier, and the underlying assumptions may need
to be reviewed.
If some OpenPGP implementations accept certification-capable subkeys,
but an abuse-resistant keystore does not accept certifications from
subkeys in general, then interactions between that keystore and those
implementations may be surprising.
13.5. Assessing Certificates in the Past
Online protocols like TLS perform signature and certificate
evaluation based entirely on the present time. If a certificate that
signs a TLS handshake message is invalid now, it doesn't matter
whether it was valid a week ago, because the present TLS session is
the context of the evaluation.
But OpenPGP signatures are often evaluated at some temporal remove
from when the signature was made. For example, software packages are
signed at release time, but those signatures are validated at
download time. A verifier that does not already know the certificate
that made the signature will need to perform certificate discovery
against some set of keystores to find a certificate with which to
check the signature.
Further complicating matters, the composable nature of an OpenPGP
certificate means that the certificate associated with any particular
signing key (primary key or subkey) can transform over time. So when
evaluating a signature that appears to have been made by a given
certificate, it may be better to try to evaluate the certificate at
the time the signature was made, rather than the present time.
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13.5.1. Point-in-time Certificate Evaluation
When evaluating a certificate at a time T in the past (for example,
when trying to validate a data signature by that certificate that was
created at time T), one approach is to discard all packets from the
certificate if the packet has a creation time later than T. Then
evaluate the resulting certificate from the remaining packets in the
context of time T.
However, any such evaluation MUST NOT ignore "hard" OpenPGP key
revocations, regardless of their creation date. (see Section 14.1).
13.5.2. Signature Verification and Non-append-only Keystores
If a non-append-only keystore (Section 7) has dropped superseded
(Section 7.1) or expired (Section 7.2) certifications, it's possible
for the certificate composed of the remaining packets to have no
valid first-party certification at the time that a given signature
was made. If a user performs certificate discovery against such a
keystore, the certificate it retrieves would be invalid according to
[RFC4880], and consequently verification of any signature by that
certificate would fail.
One simple mitigation to this problem is to ship a contemporaneously-
valid certificate in-band alongside the signature (see Section 13.3).
If the distributor does this, then the verifier need only learn about
revocations. If knowledge about revocation is needed, the verifier
might perform a certificate 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 Section 13.5.1, using the merged certificate.
13.6. Global Append-only Ledgers ("Blockchain")
The append-only aspect of some OpenPGP keystores encourages a user of
the keystore to rely on that keystore as a faithful reporter of
history, and one that will not misrepresent or hide the history that
they know about. An unfaithful "append-only" keystore could abuse
the trust in a number of ways, including withholding revocation
certificates, offering different sets of certificates to different
clients doing certificate lookup, and so on.
However, the most widely used append-only OpenPGP keystore, the [SKS]
keyserver pool, offers no cryptographically verifiable guarantees
that it will actually remain append-only. Users of the pool have
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traditionally relied on its distributed nature, and the presumption
that coordination across a wide range of administrators would make it
difficult for the pool to reliably lie or omit data. However, the
endpoint most commonly used by clients to access the network is
"hkps://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 Section 4.9).
Additional projects that provide some aspects of global append-only
ledgers that try to address some of the concerns described here
include [KEY-TRANSPARENCY] and [CONIKS], though they are not specific
to OpenPGP. Both of these systems are dependent on servers operated
by identity providers, however. And both offer the ability to detect
a misbehaving identity provider, but no specific enforcement or
recovery strategies against such an actor.
It's conceivable that a keystore could piggyback on the CT logs or
other blockchain/ledger mechanisms like [BITCOIN] to store
irrevocable pieces of data (such as revocation certificates).
Further work is needed to describe how to do this in an effective and
performant way.
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13.7. Certificate Lookup for 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 Section 13.6, but may also be useful for
keystores in general.
However, identity monitoring against a keystore that does not defend
against user ID flooding (Section 2.2) is expensive and potentially
of limited value. In particular, a malicious actor with a
certificate which duplicates a given User ID could flood the keystore
with similar certificates, hiding whichever one is in malicious use.
Since such a keystore is not considered authoritative by any
reasonable client for the user ID in question, this attack forces the
identity-monitoring defender to spend arbitrary resources fetching
and evaluating each certificate in the flood, without knowing which
certificate other clients might be evaluating.
14. OpenPGP details
This section collects details about common OpenPGP implementation
behavior that are useful in evaluating and reasoning about OpenPGP
certificates.
14.1. Revocations
It's useful to classify OpenPGP revocations of key material into two
categories: "soft" and "hard".
If the "Reason for Revocation" of an OpenPGP key is either "Key is
superseded" or "Key is retired and no longer used", it is a "soft"
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"
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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.
14.2. User ID Conventions
[RFC4880] requires a user ID to be a UTF-8 string, but does not
constrain it beyond that. In practice, a handful of conventions
predominate in how User IDs are formed.
The most widespread convention is a "name-addr" as defined in
[RFC5322]. For example:
Alice Jones <alice@example.org>
But a growing number of OpenPGP certificates contain user IDs that
are instead a raw [RFC5322] "addr-spec", omitting the "display-name"
and the angle brackets entirely, like so:
alice@example.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
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participating in the [MONKEYSPHERE] might choose a user ID for the
OpenPGP representing the host like so:
ssh://foo.example.net
14.3. E-mail Address Canonicalization
When an OpenPGP user IDs includes an "addr-spec", there still may be
multiple ways of representing the addr-spec that refer to the same
underlying mailbox. Having a truly canonical form of an "addr-spec"
is probably impossible because of legacy deployments of mailservers
that do odd things with the local part, but e-mail addresses used in
an abuse-resistant ecosystem SHOULD be constrained enough to admit to
some sensible form of canonicalization.
14.3.1. Disallowing Non-UTF-8 Local Parts
In [RFC5322], the "local-part" can be any "dot-atom". But if this is
[RFC2047] decoded, it could be any arbitrary charset, not necessarily
UTF-8. FIXME: Do we convert from the arbitrary charset to UTF-8?
14.3.2. Domain Canonicalization
FIXME: should domain name be canonicalized into punycode form? User
IDs are typically user-facing, so i think the canonicalized form
should be the [UNICODE-NORMALIZATION] Form C (NFC) of the domain
name. Can we punt to some other draft here?
14.3.3. Local Part Canonicalization
FIXME: [I-D.koch-openpgp-webkey-service] recommends downcasing all
ASCII characters in the left-hand side, but leaves all
15. Security Considerations
This document offers guidance on mitigating a range of denial-of-
service attacks on public keystores, so the entire document is in
effect about security considerations.
Many of the mitigations described here defend individual OpenPGP
certificates against flooding attacks (see Section 2.1). But only
some of these mitigations defend against flooding attacks against the
keystore itself (see Section 2.4), or against flooding attacks on the
space of possible user IDs (see Section 2.2). Thoughtful threat
modeling and monitoring of the keystore and its defenses are probably
necessary to maintain the long-term health of the keystore.
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Section 13.1 describes a potentially scary security problem for
designated revokers.
TODO (more security considerations)
15.1. Tension Between Unrestricted Uploads and Certificate Lookup
Note that there is an inherent tension between accepting arbitrary
certificate uploads and permitting effective certificate lookup. If
a keystore accepts arbitrary certificate uploads for redistribution,
it appears to be vulnerable to user ID flooding (Section 2.2), which
makes it difficult or impossible to rely on for certificate lookup.
In the broader ecosystem, it may be necessary to use gated/controlled
certificate lookup mechanisms. For example, both
[I-D.koch-openpgp-webkey-service] and [RFC7929] enable the
administrator of a DNS domain to distribute certificates associated
with e-mail addresses within that domain, while excluding other
parties. As a rather different example, [I-D.mccain-keylist] offers
certificate lookup on the basis of interest - a client interested in
an organization can use that mechanism to learn what certificates
that organization thinks are worth knowing about, associated with a
range of identities regardless of the particular DNS domain. Note
that [I-D.mccain-keylist] does not provide the certificates directly,
but instead expects the client to be able to retrieve them by primary
key fingerprint through some other keystore capable of (and
responsible for) certificate update.
16. 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.
16.1. Publishing Identity Information
Public OpenPGP keystores often distribute names or e-mail addresses
of people. Some people do not want their names or e-mail addresses
distributed in a public keystore, or may change their minds about it
at some point. Append-only keystores are particularly problematic in
that regard. The mitigation in Section 7.4 can help such users strip
their details from keys that they control. However, if an OpenPGP
certificate with their details is uploaded to a keystore, but is not
under their control, it's unclear what mechanisms can be used to
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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 (Section 8) and user ID redacting keystores
(Section 5.1) may reduce this particular privacy concern because they
distribute no user IDs at all.
16.2. Social Graph
Third-party certifications effectively map out some sort of social
graph. A certification asserts a statement of belief by the issuer
that the real-world party identified by the user ID is in control of
the subject cryptographic key material. But those connections may be
potentially sensitive, and some people may not want these maps built.
A first-party-only keyserver (Section 9) avoids this privacy concern
because it distributes no third-party privacy concern.
First-party attested third-party certifications described in
Section 10 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.
16.3. Tracking Clients by Queries
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.
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Keystores which permit public access and want to protect the privacy
of their clients SHOULD NOT reject access from clients using [TOR] or
comparable anonymity networks. Additionally, they SHOULD minimize
access logs they retain.
Alternately, some keystores may distribute their entire contents to
any interested client, in what can be seen as the most trivial form
of private information retrieval. [DEBIAN-KEYRING] is one such
example; its contents are distributed as an operating system package.
Clients can interrogate their local copy of such a keystore without
exposing their queries to a third-party.
16.4. "Live" Certificate Validation Leaks Client Activity
If a client relies on a keystore's certificate validation interface,
or on the presence of a certificate in a keystore as a part of its
validation calculations, it's unclear how long the assertion from the
keystore is or should be considered to hold. One seemingly simple
approach is to simply query the keystore's validation interface each
time that the client needs to validate the certificate.
This "live" validation approach poses a quandary to the client in the
event that the keystore is unavailable. How should in interpret the
"unknown" result? In addition, live validation reveals the client's
activity to the keystore with fine precision.
A privacy-aware client that depends on keystores for certificate
validation SHOULD NOT perform "live" certificate validation on each
use it makes of the certificate. Rather, it SHOULD cache the
validation information for some period of time and make use of the
cached copy where possible. If such a client does a regular
certificate 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.
16.5. Certificate Discovery Leaks Client Activity
The act of doing certificate discovery on unknown signatures offers a
colluding keystore and remote peer a chance to track a client's
consumption of a given signature.
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
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client consumes the signature. This is potentially a tracking or
"phone-home" situation.
A signer that has no interest in this particular form of tracking can
mitigate this concern by shipping their certificate in-band,
alongside the signature, as recommended in Section 13.3.
A privacy-aware client MAY insist on in-band certificates by
declining to use any certificate discovery interface at all, and
treat a bare signature by an unknown certificate as an invalid
signature.
16.6. Certificate Update Leaks Client Activity
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 Section 13.3), and the OpenPGP client wants to evaluate it for
use in the immediate context.
16.7. Distinct Keystore Interfaces Leak Client Context and Intent
The distinct keystore interfaces documented in Section 3 offer subtly
different semantics, and are used by a reasonable keystore client at
different times. A keystore that offers distinct discovery and
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.
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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.
16.8. Cleartext Queries
If access to the keystore happens over observable channels (e.g.,
cleartext connections over the Internet), then a passive network
monitor could perform the same type profiling or tracking attack
against clients of the keystore described in Section 16.3. Keystores
which offer network access SHOULD provide encrypted transport.
16.9. Traffic Analysis
Even if a keystore offers encrypted transport, the size of queries
and responses may provide effective identification of the specific
certificates fetched during lookup, discovery, or update, leaving
open the types of tracking attacks described in Section 16.3.
Clients of keystores SHOULD pad their queries to increase the size of
the anonymity set. And keystores SHOULD pad their responses.
The appropriate size of padding to effectively anonymize traffic to
and from keystores is likely to be mechanism- and cohort-specific.
For example, padding for keystores accessed via the DNS ([RFC7929]
may use different padding strategies that padding for keystores
accessed over WKD ([I-D.koch-openpgp-webkey-service]), which may in
turn be different from keystores accessed over HKPS
([I-D.shaw-openpgp-hkp]). A keystore which only accepts user IDs
within a specific domain (e.g., Section 4.3) or which uses custom
process (Section 6.4) for verification might have different padding
criteria than a keystore that serves the general public.
Specific padding policies or mechanisms are out of scope for this
document.
17. User Considerations
Section 10.2 describes some outstanding work that needs to be done to
help users understand how to produce and distribute a third-party-
certified OpenPGP certificate to an abuse-resistant keystore.
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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.
18. IANA Considerations
This document asks IANA to register two entries in the OpenPGP
Notation IETF namespace, both with a reference to this document:
o the "ksok" notation is defined in Section 10.
o the "uidhash" notation is defined in Section 5.1.4.
19. Document Considerations
[ RFC Editor: please remove this section before publication ]
This document is currently edited as markdown. Minor editorial
changes can be suggested via merge requests at
https://gitlab.com/dkg/draft-openpgp-abuse-resistant-keystore or by
e-mail to the author. Please direct all significant commentary to
the public IETF OpenPGP mailing list: openpgp@ietf.org
19.1. Document History
substantive changes between -02 and -03:
o new sections:
* Keystore Interfaces
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* Keystore Client Best Practices
* Certificate Generation and Management Best Practices
o rename "certificate discovery" to "certificate lookup"
o redefine "certificate discovery" to refer to lookup by signing
(sub)key
o new attack: fingerprint flooding
o new retrieval-time mitigations - tighter filters on discovery and
update
o recommend in-band certificates where possible to avoid discovery
and lookup
o new privacy considerations:
* distinct keystore interfaces
* certificate update
* certificate discovery
* certificate validation
o more nuance about unhashed subpacket filtering
substantive changes between -01 and -02:
o distinguish different forms of flooding attack
o distinguish toxic data as distinct from flooding
o retrieval-time mitigations
o user ID redaction
o references to related work (CT, keylist, CONIKS, key transparency,
ledgers/"blockchain", etc)
o more details about UI/UX
substantive changes between -00 and -01:
o split out Contextual and Non-Append-Only mitigations
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o documented several other mitigations, including:
* Decline Data From the Future
* Blocklist
* Exterior Process
* Designated Authorities
* Known Certificates
* Rate-Limiting
* Scoped User IDs
o documented Updates-Only Keystores
o consider three different kinds of flooding
o deeper discussion of privacy considerations
o better documentation of Reason for Revocation
o document user ID conventions
20. 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:
o Antoine Beaupre
o ilf
o Jamie McClelland
o Jonathan McDowell
o Justus Winter
o Marcus Brinkmann
o Micah Lee
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o Neal Walfield
o Phil Pennock
o Philihp Busby
o vedaal
o Vincent Breitmoser
o Wiktor Kwapisiewicz
21. References
21.1. Normative References
[I-D.ietf-openpgp-rfc4880bis]
Koch, W., carlson, b., Tse, R., and D. Atkins, "OpenPGP
Message Format", draft-ietf-openpgp-rfc4880bis-06 (work in
progress), November 2018.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, DOI 10.17487/RFC2047, November 1996,
<https://www.rfc-editor.org/info/rfc2047>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
21.2. Informative References
[AUTOCRYPT]
Breitmoser, V., Krekel, H., and D. Gillmor, "Autocrypt -
Convenient End-to-End Encryption for E-Mail", n.d.,
<https://autocrypt.org/>.
[BITCOIN] "Bitcoin", n.d., <https://bitcoin.org/>.
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[CONIKS] Felten, E., Freedman, M., Melara, M., Blankstein, A., and
J. Bonneau, "CONIKS Key Management System", n.d.,
<https://coniks.cs.princeton.edu/>.
[DEBIAN-KEYRING]
McDowell, J., "Debian Keyring", n.d.,
<https://keyring.debian.org/>.
[GnuPG] Koch, W., "Using the GNU Privacy Guard", n.d.,
<https://www.gnupg.org/documentation/manuals/gnupg.pdf>.
[I-D.koch-openpgp-webkey-service]
Koch, W., "OpenPGP Web Key Directory", draft-koch-openpgp-
webkey-service-07 (work in progress), November 2018.
[I-D.mccain-keylist]
McCain, R., Lee, M., and N. Welch, "Distributing OpenPGP
Key Fingerprints with Signed Keylist Subscriptions",
draft-mccain-keylist-04 (work in progress), March 2019.
[I-D.shaw-openpgp-hkp]
Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
draft-shaw-openpgp-hkp-00 (work in progress), March 2003.
[KEY-TRANSPARENCY]
Belvin, G. and R. Hurst, "Key Transparency, a transparent
and secure way to look up public keys", n.d.,
<https://keytransparency.org/>.
[MAILVELOPE-KEYSERVER]
Oberndoerfer, T., "Mailvelope Keyserver", n.d.,
<https://github.com/mailvelope/keyserver/>.
[MONKEYSPHERE]
Gillmor, D. and J. Rollins, "Monkeysphere", n.d.,
<https://web.monkeysphere.info/>.
[PARCIMONIE]
Intrigeri, ., "Parcimonie", n.d.,
<https://gaffer.ptitcanardnoir.org/intrigeri/code/
parcimonie/>.
[PGP-GLOBAL-DIRECTORY]
Symantec Corporation, "PGP Global Directory Key
Verification Policy", 2011,
<https://keyserver.pgp.com/vkd/
VKDVerificationPGPCom.html>.
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[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
<https://www.rfc-editor.org/info/rfc4366>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities
(DANE) Bindings for OpenPGP", RFC 7929,
DOI 10.17487/RFC7929, August 2016,
<https://www.rfc-editor.org/info/rfc7929>.
[SKS] Minsky, Y., Fiskerstrand, K., and P. Pennock, "SKS
Keyserver Documentation", March 2018,
<https://bitbucket.org/skskeyserver/sks-keyserver/wiki/
Home>.
[TOR] "The Tor Project", n.d., <https://www.torproject.org/>.
[UNICODE-NORMALIZATION]
Whistler, K., "Unicode Normalization Forms", February
2019, <https://unicode.org/reports/tr15/>.
Author's Address
Daniel Kahn Gillmor
American Civil Liberties Union
125 Broad St.
New York, NY 10004
USA
Email: dkg@fifthhorseman.net
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