The Privacy Pass Architecture
draft-ietf-privacypass-architecture-09
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| Last updated | 2022-12-08 | ||
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draft-ietf-privacypass-architecture-09
Network Working Group A. Davidson
Internet-Draft LIP
Intended status: Informational J. Iyengar
Expires: 11 June 2023 Fastly
C. A. Wood
Cloudflare
8 December 2022
The Privacy Pass Architecture
draft-ietf-privacypass-architecture-09
Abstract
This document specifies the Privacy Pass architecture and
requirements for its constituent protocols used for constructing
anonymous-credential authentication mechanisms. It provides
recommendations on how the architecture should be deployed to ensure
the privacy of clients and the security of all participating
entities.
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
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on 11 June 2023.
Copyright Notice
Copyright (c) 2022 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
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extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Redemption Protocol . . . . . . . . . . . . . . . . . . . 4
3.2. Issuance Protocol . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Attester Role . . . . . . . . . . . . . . . . . . . . 8
3.2.2. Issuer Role . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. Metadata . . . . . . . . . . . . . . . . . . . . . . 10
3.2.4. Issuance Protocol Extensibility . . . . . . . . . . . 10
4. Deployment Considerations . . . . . . . . . . . . . . . . . . 11
4.1. Shared Origin, Attester, Issuer . . . . . . . . . . . . . 11
4.2. Joint Attester and Issuer . . . . . . . . . . . . . . . . 12
4.3. Joint Origin and Issuer . . . . . . . . . . . . . . . . . 13
4.4. Split Origin, Attester, Issuer . . . . . . . . . . . . . 14
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
5.1. Metadata Privacy Implications . . . . . . . . . . . . . . 15
5.2. Issuer Key Rotation . . . . . . . . . . . . . . . . . . . 16
5.3. Issuer Selection . . . . . . . . . . . . . . . . . . . . 16
5.4. Side-Channel Attacks . . . . . . . . . . . . . . . . . . 18
6. Centralization . . . . . . . . . . . . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Privacy Pass is an architecture for authorization based on anonymous-
credential authentication mechanisms. Typical approaches for
authorizing clients, such as through the use of long-term cookies,
are not privacy-friendly since they allow servers to track clients
across sessions and interactions. Privacy Pass takes a different
approach: instead of presenting linkable state carrying information
to servers, e.g., a cookie indicating whether or not the client is an
authorized user or has completed some prior challenge, clients
present unlinkable proofs that attest to this information. These
proofs, or tokens, are anonymous in the sense that a given token
cannot be linked to the protocol instance in which that token was
initially issued.
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At a high level, the Privacy Pass architecture consists of two
protocols: issuance and redemption. The issuance protocol runs
between an endpoint referred to as a Client and two functions in the
Privacy Pass architecture: Attestation and Issuance. These two
network functions can be implemented by the same protocol
participant, but can also be implemented separately. The entity that
implements Issuance, referred to as the Issuer, is responsible for
issuing tokens in response to requests from Clients. The entity that
implements Attestation, referred to as the Attester, is responsible
for attesting to properties about the Client for which tokens are
issued. The Issuer needs to be trusted by the server that later
redeems the token. Attestation can be performed by the Issuer or by
an Attester that is trusted by the Issuer. Clients might prefer to
select different Attesters, separate from the Issuer, to be able to
use preferred authentication methods or to improve privacy by not
directly communicating with an Issuer. Depending on the attestation,
Attesters can store state about a Client, such as the number of
overall tokens issued thus far. As an example of an issuance
protocol, in the original Privacy Pass protocol [PPSRV][PPEXT],
tokens were only issued to Clients that solved CAPTCHAs. In this
context, the Attester attested that some client solved a CAPTCHA and
the resulting token produced by the Issuer was proof of this fact.
The redemption protocol runs between Client and Origin (server). It
allows Origins to challenge Clients to present one or more tokens for
authorization. Depending on the type of token, e.g., whether or not
it can be cached, the Client either presents a previously obtained
token or invokes the issuance protocol to acquire one for
authorization.
The issuance and redemption protocols operate in concert as shown in
the figure below.
Origin Client Attester Issuer
/---------------------------------------------------------------------
| /-----------------------------------------\
| TokenChallenge --> Attest ---> |
| | TokenRequest ---------------> |
| Redemption | (validate) | Issuance
| Flow | (evaluate) | Flow
| | <------------------- TokenResponse |
| <---- Token | |
| \-----------------------------------------/
\---------------------------------------------------------------------
Figure 1: Privacy Pass Architectural Components
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This document describes requirements for both issuance and redemption
protocols. It also provides recommendations on how the architecture
should be deployed to ensure the privacy of clients and the security
of all participating entities.
The privacypass working group is working on [HTTP-Authentication] as
an instantiation of a redemption protocol and [ISSUANCE] as an
instantiation of the issuance protocol.
2. Terminology
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.
The following terms are used throughout this document.
* Client: An entity that seeks authorization to an Origin.
* Origin: An entity that redeems tokens presented by Clients.
* Issuer: An entity that issues tokens to Clients for properties
attested to by the Attester.
* Attester: An entity that attests to properties of Client for the
purposes of token issuance.
3. Architecture
The Privacy Pass architecture consists of four logical entities --
Client, Origin, Issuer, and Attester -- that work in concert as shown
in Section 1 for token issuance and redemption. This section
describes the purpose of token issuance and redemption and the
requirements therein on the relevant participants.
3.1. Redemption Protocol
The redemption protocol is an authorization protocol wherein Clients
present tokens to Origins for authorization. Normally, redemption
follows a challenge-response flow, wherein the Origin challenges
Clients for a token and, if possible, Clients present a valid token
for the challenge in response. Alternatively, when configured to do
so, Clients may opportunistically present tokens to Origins without a
corresponding challenge.
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The context in which an Origin challenges a Client for a token is
referred to as the redemption context. This context includes all
information associated with the redemption event, such as the
timestamp of the event, Client visible information (including the IP
address), and the Origin name.
The challenge controls the type of token that the Origin will accept
for the given resource. As described in [HTTP-Authentication], there
are a number of ways in which the token may vary, including:
* Issuance protocol. The token identifies the type of issuance
protocol required for producing the token. Different issuance
protocols have different security properties, e.g., some issuance
protocols may produce tokens that are publicly verifiable, whereas
others may not have this property.
* Issuer identity. Tokens identify which Issuers are trusted for a
given issuance protocol. Each Issuer, in turn, determines which
Attesters it is willing to accept in the issuance protocol. This
means that if an Origin origin.example accepts tokens issued by
Issuer issuer.example, and that Issuer in turn accepts different
types of attestation from more than one trusted Attester, then a
Client may use either of these trusted Attesters to issue and
redeem tokens for origin.example. However, origin.example neither
explicitly specifies nor learns the Attesters or their attestation
formats used for token issuance.
* Redemption context. Tokens can be bound to a given redemption
context, which influences a client's ability to pre-fetch and
cache tokens. For example, an empty redemption context always
allows tokens to be issued and redeemed non-interactively, whereas
a fresh and random redemption context means that the redeemed
token must be issued only after the client receives the challenge.
See Section 2.1.1 of [HTTP-Authentication] for more details.
* Per-Origin or cross-Origin. Tokens can be constrained to the
Origin for which the challenge originated (referred to as per-
Origin tokens), or can be used across multiple Origins (referred
to as cross-Origin tokens). The set of Origins for which a cross-
Origin token is applicable is referred to as the cross-Origin set.
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Origins that admit cross-Origin tokens bear some risk of allowing
tokens issued for one Origin to be spent in an interaction with
another Origin. In particular, depending on the use case, Origins
may need to maintain state to track redeemed tokens. For example,
Origins that accept cross-Origin tokens across shared redemption
contexts SHOULD track which tokens have been redeemed already in
those redemption contexts, since these tokens can be issued and then
spent multiple times in response to any such challenge. See
Section 2.1.1 of [HTTP-Authentication] for discussion.
3.2. Issuance Protocol
The issuance protocol embodies the core of Privacy Pass. It takes as
input a challenge from the redemption protocol and produces a token,
as shown in the figure below.
Origin Client Attester Issuer
+--------------------------------------\
Challenge ----> Attest -------> |
| TokenRequest ------------------> |
| (validate)|
| (evaluate)|
Token <----+ <------------------- TokenResponse |
|--------------------------------------/
Figure 2: Issuance Overview
Clients interact with the Attester and Issuer to produce a token in
response to a challenge. The context in which an Attester vouches
for a Client during issuance is referred to as the attestation
context. This context includes all information associated with the
issuance event, such as the timestamp of the event and Client visible
information, including the IP address or other information specific
to the type of attestation done.
Each issuance protocol may be different, e.g., in the number and
types of participants, underlying cryptographic constructions used
when issuing tokens, and even privacy properties.
Clients initiate the issuance protocol using the challenge, a
randomly generated nonce, and public key for the Issuer, all of which
are the Client's private input to the protocol and ultimately bound
to an output Token; see Section 2.2. of [HTTP-Authentication] for
details. Future specifications may change or extend the Client's
input to the issuance protocol to produce Tokens with a different
structure.
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The issuance protocol itself can be any interactive protocol between
Client, Issuer, or other parties that produces a valid authenticator
over the Client's private input, subject to the following security
requirements.
1. Unconditional input secrecy. The issuance protocol MUST NOT
reveal anything about the Client's private input, including the
challenge and nonce, to the Attester or Issuer, regardless of the
hardness assumptions of the underlying cryptographic protocol(s).
The issuance protocol can reveal the Issuer public key for the
purposes of determining which private key to use in producing the
token.
2. One-more forgery security. The issuance protocol MUST NOT allow
malicious Clients or Attesters (acting as Clients) to forge
tokens offline or otherwise without interacting with the Issuer
directly.
3. Concurrent security. The issuance protocol MUST be safe to run
concurrently with arbitrarily many Clients, Attesters and
Issuers.
See Section 3.2.4 for requirements on new issuance protocol variants
and related extensions.
Clients obtain the Issuer public key directly from the Origin using
the process described in [HTTP-Authentication]. Clients MAY apply
some form of key consistency check to determine if this public key is
consistent and correct for the specified Issuer. See [CONSISTENCY]
for example mechanisms. Depending on the deployment, the Attester
might assist the Client in applying these consistency checks across
clients.
Depending on the use case, issuance may require some form of Client
anonymization service, similar to an IP-hiding proxy, so that Issuers
cannot learn information about Clients. This can be provided by an
explicit participant in the issuance protocol, or it can be provided
via external means, such as through the use of an IP-hiding proxy
service like Tor. In general, Clients SHOULD minimize or remove
identifying information where possible when invoking the issuance
protocol.
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Issuers MUST NOT issue tokens for Clients through untrusted
Attesters. This is important because the Attester's role is to vouch
for trust in privacy-sensitive Client information, such as account
identifiers or IP address information, to the Issuer. Tokens
produced by an Issuer that admits issuance for any type of
attestation cannot be relied on for any specific property. See
Section 3.2.1 for more details.
3.2.1. Attester Role
Attestation is an important part of the issuance protocol.
Attestation is the process by which an Attester bears witness to,
confirms, or authenticates a Client so as to verify a property about
the Client that is required for Issuance. Examples of attestation
properties include, though are not limited to:
* Capable of solving a CAPTCHA. Clients that solve CAPTCHA
challenges can be attested to have this capability for the purpose
of being ruled out as a bot or otherwise automated Client.
* Client state. Clients can be associated with state and the
attester can attest to this state. Examples of state include the
number of issuance protocol invocations, the Client's geographic
region, and whether the client has a valid application-layer
account.
* Trusted device. Some Clients run on trusted hardware that are
capable of producing device-level attestation statements.
Each of these attestation types has different security properties.
For example, attesting to having a valid account is different from
attesting to running on trusted hardware. In general, minimizing the
set of attestation formats helps minimize the amount of information
leaked through a token.
Each attestation format also has an impact on the overall system
privacy. Requiring a conjunction of attestation types could decrease
the overall anonymity set size. For example, the number of Clients
that have solved a CAPTCHA in the past day, that have a valid
account, and that are running on a trusted device is less than the
number of Clients that have solved a CAPTCHA in the past day.
Attesters SHOULD not admit attestation types that result in small
anonymity sets.
The trustworthiness of attesters depends on their ability to
correctly and reliably perform attestation during the issuance
protocol. However, certain types of attestation can vary in value
over time, e.g., if the attestation process is compromised or
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maliciously automated. These are considered exceptional events and
require configuration changes to address the underlying cause. For
example, if attestation is compromised because of a zero-day exploit
on compliant devices, then the corresponding attestation format
should be untrusted until the exploit is patched. Addressing changes
in attestation quality is therefore a deployment-specific task. In
Split Attester and Issuer deployments (see Section 4.4), Issuers can
choose to remove compromised Attesters from their trusted set until
the compromise is patched.
3.2.2. Issuer Role
Issuers MUST be uniquely identifiable by all Clients with a
consistent identifier. In a web context, this identifier might be
the Issuer host name. As discussed in Section 5, ecosystems that
admit a large number of Issuers can lead to privacy concerns for the
Clients in the ecosystem. Therefore, in practice, the number of
Issuers should be bounded. The actual Issuers can be replaced with
different Issuers as long as the total never exceeds these bounds.
Moreover, Issuer replacements also have an effect on client anonymity
that is similar to when a key rotation occurs. See Section 5 for
more details about maintaining privacy with multiple Issuers.
3.2.2.1. Key Management
Issuers maintain an issuance key pair for the issuance protocol. The
Issuer public key is made available to all Clients in such a way that
key rotations and other updates are publicly visible. See
Section 5.2 for more considerations around Issuer key rotation. The
key material and protocol configuration that an Issuer uses to
produce tokens corresponds to two different pieces of information.
* The issuance protocol in use; and
* The public keys that are active for the Issuer.
The way that the Issuer publishes and maintains this information
impacts the effective privacy of the clients; see Section 5 for more
details. The fundamental requirement for key management and
discovery is that Issuers cannot target specific clients with unique
keys without detection. There are a number of ways in which this
might be implemented:
* Servers use a verifiable, tamper-free registry from which clients
discover keys. Similar to related mechanisms and protocols such
as Certificate Transparency [RFC6962], this may require external
auditors or additional client behavior to ensure the registry
state is consistent for all clients.
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* Clients use an anonymity-preserving tool such as Tor to discover
keys from multiple network vantage points. This is done to ensure
consistent keys to seemingly different clients.
* Clients embed Issuer keys into software.
As above, specific mechanisms for key management and discovery are
out of scope for this document.
3.2.3. Metadata
Certain instantiations of the issuance protocol may permit public or
private metadata to be cryptographically bound to a token. As an
example, one trivial way to include public metadata is to assign a
unique issuer public key for each value of metadata, such that N keys
yields log2(N) bits of metadata. The total amount of metadata bits
included in a token is the sum of public and private metadata bits.
Public metadata is that which clients can observe as part of the
token issuance flow. Public metadata can either be transparent or
opaque. For example, transparent public metadata is a value that the
client either generates itself, or the Issuer provides during the
issuance flow and the client can check for correctness. Opaque
public metadata is metadata the client can see but cannot check for
correctness. As an example, the opaque public metadata might be a
"fraud detection signal", computed on behalf of the Issuer, during
token issuance. In normal circumstances, Clients cannot determine if
this value is correct or otherwise a tracking vector.
Private metadata is that which Clients cannot observe as part of the
token issuance flow. Such instantiations may be built on the Private
Metadata Bit construction from Kreuter et al. [KLOR20] or the
attribute-based VOPRF from Huang et al. [HIJK21].
Metadata may also be arbitrarily long or bounded in length. The
amount of permitted metadata may be determined by application or by
the underlying cryptographic protocol.
3.2.4. Issuance Protocol Extensibility
The Privacy Pass architecture and ecosystem are both intended to be
receptive to extensions that expand the current set of
functionalities through new issuance protocols. Each issuance
protocol MUST include a detailed analysis of the privacy impacts of
the extension, why these impacts are justified, and guidelines on how
to deploy the protocol to minimize any privacy impacts. Any
extension to the Privacy Pass protocol MUST adhere to the guidelines
specified in Section 3.2.2 for managing Issuer public key data.
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4. Deployment Considerations
A Client uses Privacy Pass to separate attestation context and
redemption context. Linking or combining these contexts can reveal
sensitive information about the Client, including their identity or
browsing history. Depending on the deployment model, separating
these contexts can take different forms. The Origin, Attester, and
Issuer portrayed in Figure 1 can be instantiated and deployed in a
number of ways. This section covers some expected deployment models
and their corresponding security and privacy considerations.
The discussion below assumes non-collusion between entities that have
access to the attestation context and entities that have access to
the redemption context, as collusion between such entities would
enable linking of these contexts. Generally, this means that
entities operated by separate parties do not collude. Mechanisms for
enforcing non-collusion are out of scope for this architecture.
4.1. Shared Origin, Attester, Issuer
In this model, the Origin, Attester, and Issuer are all operated by
the same entity, as shown in the figure below.
+------------------------------------------+
Client | Attester Issuer Origin |
| | |
| | Challenge |
<----------------------------------------------+ |
| | Attest |
+-----------------> |
| | TokenRequest |
+--------------------------------> |
| | TokenResponse |
<--------------------------------+ |
| | Redeem |
+----------------------------------------------> |
+------------------------------------------+
Figure 3: Shared Deployment Model
This model represents the initial deployment of Privacy Pass, as
described in [PPSRV]. In this model, the Attester, Issuer, and
Origin share the attestation and redemption contexts. As a result,
attestation mechanisms that can uniquely identify a Client, e.g.,
requiring that Clients authenticate with some type of application-
layer account, are not appropriate, as they could be used to learn or
reconstruct a Client's browsing history.
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Attestation and redemption context unlinkability requires that these
events be separated over time, such as through the use of tokens with
an empty redemption context, or be separated over space, such as
through the use of an anonymizing proxy when connecting to the
Origin.
4.2. Joint Attester and Issuer
In this model, the Attester and Issuer are operated by the same
entity that is separate from the Origin, as shown in the figure
below.
+-----------+
Client | Origin |
| Challenge | |
<-----------------------------------------------+ |
| | |
| +---------------------------+ | |
| | Attester Issuer | | |
| | | | |
| | Attest | | |
+-----------------> | | |
| | TokenRequest | | |
+--------------------------------> | | |
| | TokenResponse | | |
<--------------------------------+ | | |
| +---------------------------+ | |
| | |
| Redeem | |
+-----------------------------------------------> |
| |
+-----------+
Figure 4: Joint Attester and Issuer Deployment Model
This model is useful if an Origin wants to offload attestation and
issuance to a trusted entity. In this model, the Attester and Issuer
share an attestation context for the Client, which can be separate
from the Origin's redemption context.
For certain types of issuance protocols, this model separates
attestation and redemption contexts. However, issuance protocols
that require the Issuer to learn information about the Origin, such
as that which is described in [RATE-LIMITED], are not appropriate
since they could link attestation and redemption contexts through the
Origin name.
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4.3. Joint Origin and Issuer
In this model, the Origin and Issuer are operated by the same entity,
separate from the Attester, as shown in the figure below. The Issuer
accepts token requests that come from trusted Attesters. Depending
on the issuance protocol, the Attester may relay these requests
between Client and Issuer over an authenticated channel, or trust may
be established in some other manner.
+--------------------------+
Client | Issuer Origin |
| Challenge | |
<-----------------------------------------------+ |
| | |
| +-----------+ | |
| | Attester | | |
| | | | |
| | Attest | | |
+-----------------> | | |
| | | | |
| | TokenRequest |
+--------------------------------> |
| | | | |
| | TokenResponse |
<--------------------------------+ |
| | | | |
| +-----------+ | |
| | |
| Redeem | |
+-----------------------------------------------> |
+--------------------------+
Figure 5: Joint Origin and Issuer Deployment Model
This model is useful for Origins that require Client-identifying
attestation, e.g., through the use of application-layer account
information, but do not otherwise want to learn information about
individual Clients beyond what is observed during the token
redemption, such as Client IP addresses.
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In this model, attestation and redemption contexts are separate. As
a result, any type of attestation is suitable in this model.
Moreover, any type of token challenge is suitable assuming there is
more than one Origin involved, since no single party will have access
to the identifying Client information and unique Origin information.
If there is only a single Origin, then per-Origin tokens are not
appropriate in this model, since the Attester can learn the
redemption context. However, the Attester does not learn whether a
token is per-Origin or cross-Origin.
4.4. Split Origin, Attester, Issuer
In this model, the Origin, Attester, and Issuer are all operated by
different entities, as shown in the figure below. As with the joint
Origin and Issuer model, the Issuer accepts token requests that come
from trusted Attesters, and the details of that trust establishment
depend on the issuance protocol.
+-----------+
Client | Origin |
| Challenge | |
<-----------------------------------------------+ |
| | |
| +-----------+ | |
| | Attester | | |
| | | | |
| | Attest | +----------+ | |
+-----------------> | | Issuer | | |
| | | | | | |
| | TokenRequest | | |
+--------------------------------> | | |
| | | | | | |
| | TokenResponse | | |
<--------------------------------+ | | |
| | | | | | |
| +-----------+ +----------+ | |
| | |
| Redeem | |
+-----------------------------------------------> |
| |
+-----------+
Figure 6: Split Deployment Model
This is the most general deployment model, and is necessary for some
types of issuance protocols where the Attester plays a role in token
issuance; see [RATE-LIMITED] for one such type of issuance protocol.
In this model, the Attester, Issuer, and Origin have a separate view
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of the Client: the Attester sees potentially sensitive Client
identifying information, such as account identifiers or IP addresses,
the Issuer sees only the information necessary for issuance, and the
Origin sees token challenges, corresponding tokens, and Client source
information, such as their IP address. As a result, attestation and
redemption contexts are separate, and therefore any type of token
challenge is suitable in this model as long as there is more than a
single Origin. As in the Joint Origin and Issuer model in
Section 4.3, if there is only a single Origin, then per-Origin tokens
are not appropriate.
5. Privacy Considerations
A Client uses Privacy Pass to separate attestation context and
redemption context. Depending on the deployment model, this can take
different forms. For example, any Client can only remain private
relative to the entire space of other Clients using the protocol.
Moreover, by owning tokens for a given set of keys, the Client's
anonymity set shrinks to the total number of Clients controlling
tokens for the same keys.
In the following, we consider the possible ways that Issuers can
leverage their position to try and reduce the size of the anonymity
sets to which Clients belong, often by segregating Clients. For each
case, we provide mitigations that the Privacy Pass ecosystem must
implement to prevent these actions.
5.1. Metadata Privacy Implications
Any metadata bits of information can be used to further segment the
size of the Client's anonymity set. Any Issuer that wanted to track
a single Client could add a single metadata bit to Client tokens.
For the tracked Client it would set the bit to 1, and 0 otherwise.
Adding additional bits provides an exponential increase in tracking
granularity similarly to introducing more Issuers (though with more
potential targeting).
For this reason, the amount of metadata used by an Issuer in creating
redemption tokens must be taken into account -- together with the
bits of information that Issuers may learn about Clients otherwise.
Since this metadata may be useful for practical deployments of
Privacy Pass, Issuers must balance this against the reduction in
Client privacy. In general, bounding the metadata permitted ensures
that it cannot uniquely identify individual Clients.
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5.2. Issuer Key Rotation
Issuer key rotation is important to hedge against long-term private
key compromise. If an Issuer realizes that a key compromise has
occurred then the Issuer should generate a new key and make it
available to Clients. If possible, it should invoke any revocation
procedures that may apply for the old key.
Key rotation can also be used to segment Client anonymity sets. In
particular, when an Issuer rotates their key, any Client that invokes
the issuance protocol in this key cycle will be part of a group of
possible Clients owning valid tokens for this key. To mechanize this
attack strategy, an Issuer could introduce a key rotation policy that
forces Clients into small key cycles, reducing the size of the
anonymity set for these Clients.
In general, key rotations represent a trade-off between Client
privacy and Issuer security. Therefore, it is still important that
key rotations occur on a regular cycle to reduce the harmfulness of
an Issuer key compromise. If there are multiple Issuer keys in
rotation, Clients can apply some form of consistency mechanism
[CONSISTENCY] to ensure that they receive the same key as other
Clients. Likewise, Origins can use one or more public keys for
redemption to support Issuer key rotation.
5.3. Issuer Selection
Similarly to the Issuer rotation dynamic discussed above, if there
are a large number of Issuers, and Origins accept all of them,
segregation can occur. If Clients obtain tokens from many Issuers,
and Origins later challenge a Client for a token from each Issuer,
Origins can learn information about the Client. Each per-Issuer
token that a Client holds essentially corresponds to a bit of
information about the Client that Origin learns. Therefore, there is
an exponential loss in anonymity relative to the number of Issuers.
For example, if there are 32 Issuers, then Origins learn 32 bits of
information about the Client if a valid token is presented for each
Issuer. As a contrasting example, if Clients ensure that they only
hold tokens issued from 4 Issuers, then this increases the potential
size of the anonymity sets that the Client belongs to. However, this
doesn't protect Clients completely as it would if only 4 Issuers were
permitted across the whole system. For example, these 4 Issuers
could be different for each Client. Therefore, the selection of
Issuers for which a Client possesses tokens is still revealing. This
trade-off is important in deciding the effective anonymity of each
Client in the system.
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Clients SHOULD bound the number of Issuers they are willing to
request tokens from at any given time. The exact bound depends on
the deployment model and number of Clients, i.e., having a very large
Client base could potentially allow for larger values. Issuer
replacements should only occur with the same frequency as config
rotations as they can lead to similar losses in anonymity if clients
still hold redemption tokens for previously active Issuers.
Alternatively, when applicable, trusted registries can indicate which
Issuers are deemed to be active. If a Client is asked to invoke the
issuance protocol for an Issuer that is not declared active, then the
client can refuse to run the protocol and obtain a token.
Another option to allow a large number of Issuers in the ecosystem,
while preventing the joining of a number of different tokens, is for
the Client to maintain sharded "redemption partitions". This would
allow the Client to redeem the tokens it wishes to use in a
particular context, while still allowing the Client to maintain a
large variety of tokens from many Issuers. Within a redemption
partition, the Client limits the number of different Issuers used to
a small number to maintain the privacy properties the Client
requires. As long as each redemption partition maintains a strong
privacy boundary with the others, the number of bits of information
the Origin can learn is bounded by the number of "redemption
partitions".
To support this strategy, the client keeps track of a partition which
contains the set of Issuers that redemptions have been attempted
against. An empty redemption is returned when the limit has been
hit:
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Client(partition, issuer) Issuer(skS, pkS)
------------------------------------------------------------
if issuer not in partition {
if partition.length > REDEEM_LIMIT {
Output {}
return
}
partition.push(issuer)
}
token = store[issuer.id].pop()
req = Redeem(token, info)
req
------------------>
if (dsIdx.includes(req.data)) {
raise ERR_DOUBLE_SPEND
}
resp = Verify(pkS, skS, req)
if resp.success {
dsIdx.push(req.data)
}
resp
<------------------
Output resp
5.4. Side-Channel Attacks
Side-channel attacks, such as those based on timing correlation,
could be used to link attestation and redemption contexts together.
In particular, for interactive tokens that are bound to a Client-
specific redemption context, the anonymity set of Clients during the
issuance protocol consists of those Clients that started issuance
between the time of the Origin's challenge and the corresponding
token redemption. Depending on the number of Clients using a
particular Issuer during that time window, the set can be small.
Appliations should take such side channels into consideration before
choosing a particular deployment model and type of token challenge
and redemption context.
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6. Centralization
A consequence of limiting the number of participants (Attesters or
Issuers) in Privacy Pass deployments for meaningful privacy is that
it forces concentrated centralization amongst those participants.
[CENTRALIZATION] discusses several ways in which this might be
mitigated. For example, a multi-stakeholder governance model could
be established to determine what candidate participants are fit to
operate as participants in a Privacy Pass deployment. This is
precisely the system used to control the Web's trust model.
Alternatively, Privacy Pass deployments might mitigate this problem
through implementation. For example, rather than centralize the role
of attestation in one or few entities, attestation could be a
distributed function performed by a quorum of many parties, provided
that neither Issuers nor Origins learn which attester implementations
were chosen. As a result, clients could have more opportunities to
switch between attestation participants.
7. Security Considerations
Beyond the aforementioned security goals for the issuance protocol
(Section 3.2), it is important for Privacy Pass deployments to
mitigate the risk of abuse by malicious Origins.
For example, when a Client holds cross-Origin tokens for an Origin,
it is possible for any Origin in the cross-Origin set to deplete that
Client set of tokens. To prevent this from happening, tokens can be
scoped to single Origins such that they can only be redeemed for a
single Origin. Alternatively, if tokens are cross-Origin, Clients
can use alternate methods to prevent many tokens from being redeemed
at once. For example, if the Origin requests an excess of tokens,
the Client could choose to not present any tokens for verification if
a redemption had already occurred in a given time window.
8. References
8.1. Normative References
[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/rfc/rfc2119>.
[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/rfc/rfc8174>.
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8.2. Informative References
[CENTRALIZATION]
Nottingham, M., "Internet Consolidation: What can
Standards Efforts Do?", Work in Progress, Internet-Draft,
draft-nottingham-avoiding-internet-centralization-06, 3
December 2022, <https://datatracker.ietf.org/doc/html/
draft-nottingham-avoiding-internet-centralization-06>.
[CONSISTENCY]
Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
"Key Consistency and Discovery", Work in Progress,
Internet-Draft, draft-wood-key-consistency-03, 17 August
2022, <https://datatracker.ietf.org/doc/html/draft-wood-
key-consistency-03>.
[HIJK21] Huang, S., Iyengar, S., Jeyaraman, S., Kushwah, S., Lee,
C. K., Luo, Z., Mohassel, P., Raghunathan, A., Shaikh, S.,
Sung, Y. C., and A. Zhang, "PrivateStats: De-Identified
Authenticated Logging at Scale", January 2021,
<https://research.fb.com/privatestats>.
[HTTP-Authentication]
Pauly, T., Valdez, S., and C. A. Wood, "The Privacy Pass
HTTP Authentication Scheme", Work in Progress, Internet-
Draft, draft-ietf-privacypass-auth-scheme-06, 28 November
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-auth-scheme-06>.
[ISSUANCE] Celi, S., Davidson, A., Faz-Hernandez, A., Valdez, S., and
C. A. Wood, "Privacy Pass Issuance Protocol", Work in
Progress, Internet-Draft, draft-ietf-privacypass-protocol-
06, 6 July 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-privacypass-protocol-06>.
[KLOR20] Kreuter, B., Lepoint, T., Orrù, M., and M. Raykova,
"Anonymous Tokens with Private Metadata Bit", Advances in
Cryptology - CRYPTO 2020 pp. 308-336,
DOI 10.1007/978-3-030-56784-2_11, 2020,
<https://doi.org/10.1007/978-3-030-56784-2_11>.
[PPEXT] "Privacy Pass Browser Extension", n.d.,
<https://github.com/privacypass/challenge-bypass-
extension>.
[PPSRV] Sullivan, N., "Cloudflare Supports Privacy Pass", n.d.,
<https://blog.cloudflare.com/cloudflare-supports-privacy-
pass/>.
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[RATE-LIMITED]
Hendrickson, S., Iyengar, J., Pauly, T., Valdez, S., and
C. A. Wood, "Rate-Limited Token Issuance Protocol", Work
in Progress, Internet-Draft, draft-privacypass-rate-limit-
tokens-03, 6 July 2022,
<https://datatracker.ietf.org/doc/html/draft-privacypass-
rate-limit-tokens-03>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/rfc/rfc6962>.
Appendix A. Acknowledgements
The authors would like to thank Eric Kinnear, Scott Hendrickson,
Tommy Pauly, Christopher Patton, Benjamin Schwartz, Steven Valdez and
other members of the Privacy Pass Working Group for many helpful
contributions to this document.
Authors' Addresses
Alex Davidson
LIP
Lisbon
Portugal
Email: alex.davidson92@gmail.com
Jana Iyengar
Fastly
Email: jri@fastly.com
Christopher A. Wood
Cloudflare
101 Townsend St
San Francisco,
United States of America
Email: caw@heapingbits.net
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