Privacy Pass Architectural Framework
draft-ietf-privacypass-architecture-03
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draft-ietf-privacypass-architecture-03
Network Working Group A. Davidson
Internet-Draft LIP
Intended status: Informational J. Iyengar
Expires: 8 September 2022 Fastly
C. A. Wood
Cloudflare
7 March 2022
Privacy Pass Architectural Framework
draft-ietf-privacypass-architecture-03
Abstract
This document specifies the architectural framework for constructing
secure and anonymity-preserving instantiations of the Privacy Pass
protocol. It provides recommendations on how the protocol ecosystem
should be constructed 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
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 8 September 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Redemption Protocol . . . . . . . . . . . . . . . . . . . 5
3.2. Issuance Protocol . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Attester Role . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Issuer Role . . . . . . . . . . . . . . . . . . . . . 8
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 . . . . . . . . . . . . . . . . . . . 15
5.3. Large Number of Issuers . . . . . . . . . . . . . . . . . 16
5.3.1. Allowing More Issuers . . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6.1. Double-spend Protection . . . . . . . . . . . . . . . . . 18
6.2. Token Exhaustion . . . . . . . . . . . . . . . . . . . . 19
7. Protocol Parameterization . . . . . . . . . . . . . . . . . . 19
7.1. Justification . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Example parameterization . . . . . . . . . . . . . . . . 21
7.3. Allowing more Issuers . . . . . . . . . . . . . . . . . . 21
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
Privacy Pass is a protocol 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., whether or not the client is an authorized user or
has completed some prior challenge, clients present unlinkable proofs
that attest to this information.
The most basic Privacy Pass protocol provides a set of cross-origin
authorization tokens that protect the client's anonymity during
interactions with a server. This allows clients to communicate an
attestation of a previously authenticated server action, without
having to reauthenticate manually. The tokens retain anonymity in
the sense that the act of revealing them cannot be linked back to the
session where they were initially issued.
At a high level, Privacy Pass is composed of two protocols: issuance
and redemption.
The issuance protocol runs between a Client and two network 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 Issuer is
responsible for issuing tokens in response to requests from Clients.
The Attester is responsible for attesting 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 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],
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 is cross-origin or per-origin, and 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.
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The issuance and redemption protocols operate in concert as shown in
the figure below.
Origin Client Attester Issuer
/--------------------------------------------------------------------
| /-----------------------------------------\
| Challenge ----> Attest ---> |
| | TokenRequest ---------------> |
| Redemption | (validate) | Issuance
| Flow | (evaluate) | Flow
| | <------------------- TokenResponse |
| <--- Response | |
| \-----------------------------------------/
\--------------------------------------------------------------------
Figure 1: Privacy Pass Architectural Components
This document describes requirements for both issuance and redemption
protocols. This document also describes ecosystem considerations
that impact the stated privacy and security guarantees of the
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 challenges Clients for tokens.
* 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.
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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 a simple challenge-response based
authorization protocol between Client and Origin. Origins prompt
Clients with a token challenge and, if possible, Clients present a
valid token for the challenge in response. 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.
* Interactive or non-interactive. Tokens can either be interactive
or not. An interactive token is one which requires a freshly
issued token based on the challenge, whereas a non-interactive
token can be issued proactively and cached for future use.
* Per-origin or cross-origin. Tokens can be constrained to the
Origin for which the challenge originated, or can be used across
Origins.
Depending on the use case, Origins may need to maintain state to
track redeemed tokens. For example, Origins that accept non-
interactive, cross-origin tokens SHOULD track which tokens have been
redeemed already, since these tokens can be issued and then spent
multiple times in response to any such challenge. See Section 6.1
for discussion.
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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. If tokens protected with resources are unique to a
single Origin, then said Origin MUST NOT admit cross-origin tokens
for authorization.
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 ----> TokenRequest ---> |
| (attest) |
| TokenRequest ---> |
| (evaluate)|
| <--- TokenResponse |
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 Token issuance protocol using the challenge, a
randomly generated nonce, and public key for the Issuer. The Token
issuance protocol itself can be any interactive protocol between
Client, Issuer, or other parties that produces a valid authenticator
over the Client's 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. The issuance
protocol can reveal the Issuer public key for the purposes of
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determining which private key to use in producing the issuance
protocol. A result of this property is that the redemption flow
is unlinkable from the issuance flow.
2. One-more forgery security. The issuance protocol MUST NOT allow
malicious Clients or Attesters (acting as Clients) to forge
tokens without interacting with the Issuer directly.
3. Concurrent security. The issuance protocol MUST be safe to run
concurrently with arbitrarily many Clients.
Each Issuance protocol MUST come with a detailed analysis of the
privacy impacts of the protocol, why these impacts are justified, and
guidelines on changes to the parametrization in Section 7.
The mechanism by which clients obtain the Issuer public key is not
specified. Clients may be configured with this key or they may
discover it via some other form. See [CONSISTENCY].
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, e.g., 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.
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 attest to this capability for the purposes of being
ruled out as a bot or otherwise automated Client.
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* 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 have different security properties.
For example, attesting to having a valid account is different from
attesting to be running on trusted hardware. In general, Attesters
should accept a limited form of attestation formats.
Each attestation format also has an impact on the overall system
privacy. For example, the number of users in possession of a single
class of trusted device might be lesser than the number of users that
can solve CAPTCHAs. Similarly, 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, have a valid account, and are running on a trusted device is
lesser 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.
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 later 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
To facilitate issuance, the Issuer MUST hold an Issuance key pair at
any given time. The Issuer public key MUST be made available to all
Clients in such a way that key rotations and other updates are
publicly visible. The key material and protocol configuration that
an Issuer uses to produce tokens corresponds to a number of different
pieces of information.
* The issuance protocol in use; and
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* 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 must be unable to 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.
* 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.
3.2.2.2. Key Rotation
Token issuance associates all issued tokens with a particular choice
of key. If an Issuer issues tokens with many keys, then this may
harm the anonymity of the Client. For example, they would be able to
map the Client's access patterns by inspecting which key each token
they possess has been issued under.
To prevent against this, Issuers MUST only use one private key for
issuing tokens at any given time. Servers MAY use one or more keys
for redemption to allow Issuers for seamless key rotation.
Servers may rotate keys as a means of revoking tokens issued under
old or otherwise expired keys. Alternatively, Issuers may include
expiration information as metadata alongside the token; See
Section 3.2.3 for more discussion about metadata constraints. Both
techniques are equivalent since they cryptographically bind
expiration to individual tokens.
Key rotations should be limited in frequency for similar reasons.
See Section 7 for guidelines on what frequency of key rotations are
permitted.
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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.
See Section 7 for discussion about metadata limits.
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 protocol and ecosystem are both intended to be
receptive to extensions that expand the current set of
functionalities through new issuance protocols. Each issuance
protocol SHOULD come with a detailed analysis of the privacy impacts
of the extension, why these impacts are justified, and guidelines on
changes to the parametrization in Section 7. 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
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 different ways. This section covers some expected
deployment models and their corresponding security and privacy
considerations. The discussion below assumes non-collusion between
entities when operated by separate parties. 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, e.g., through the use of non-
interactive tokens that can be issued without a fresh Origin
challenge, or over space, e.g., 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 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.
+--------------------------+
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.
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. (Note, however, that the Attester does not learn
whether a token is per-Origin or cross-Origin.)
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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.
+-----------+
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
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 assuming 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.
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5. Privacy Considerations
Client uses Private 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 anonymity sets that
Clients belong to (or, user segregation). 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 Issuer's 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, Issuers should permit no more than 32
bits of metadata, as this can uniquely identify each possible user.
We discuss this more in Section 7.
5.2. Issuer Key Rotation
Techniques to introduce Client "segregation" can be used to reduce
Client anonymity. Such techniques are closely linked to the type of
key schedule that is used by the Issuer. 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. Thus, reducing the size of the anonymity set for these
Clients.
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Issuers SHOULD invoke key rotation for a period of time between 1 and
12 weeks. Key rotations represent a trade-off between Client privacy
and continued Issuer security. Therefore, it is still important that
key rotations occur on a regular cycle to reduce the harmfulness of
an Issuer key compromise.
With a large number of Clients, a minimum of one week gives a large
enough window for Clients to participate in the issuance protocol and
thus enjoy the anonymity guarantees of being part of a larger group.
A maximum of 12 weeks limits the damage caused by a 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.
5.3. Large Number of Issuers
Similarly to the Issuer rotation dynamic that is raised above, if
there are a large number of Issuers, and Origins accept all of them,
segregation can occur. For example, 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 learn. Therefore, there
is an exponential loss in anonymity relative to the number of Issuers
that there are.
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
one. If the distribution of Issuer trust is anything close to a
uniform distribution, then this is likely to uniquely identify any
Client amongst all other Internet users. Assuming a uniform
distribution is clearly the worst-case scenario, and unlikely to be
accurate, but it provides a stark warning against allowing too many
Issuers at any one time.
In cases where clients can hold tokens for all Issuers at any given
time, a strict bound SHOULD be applied to the active number of
Issuers in the ecosystem. We propose that allowing no more than 4
Issuers at any one time is highly preferable (leading to a maximum of
64 possible user segregations). However, as highlighted in
Section 7, having a very large user base (> 5 million users), 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.
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In addition, we RECOMMEND that trusted registries indicate at all
times which Issuers are deemed to be active. If a Client is asked to
invoke any Privacy Pass exchange for an Issuer that is not declared
active, then the client SHOULD refuse to retrieve the Issuer public
key during the protocol.
5.3.1. Allowing More Issuers
The bounds on the numbers of Issuers that this document proposes
above are very restrictive. This is because this document considers
a situation where a Client could be challenged (and asked to redeem)
tokens for any Issuer.
An alternative system is to ensure a robust strategy for ensuring
that Clients only possess redemption tokens for a similarly small
number of Issuers at any one time. This prevents a malicious
verifier from being able to invoke redemptions for many Issuers since
the Client would only be holding redemption tokens for a small set of
Issuers. When a Client is issued tokens from a new Issuer and
already has tokens from the maximum number of Issuers, it simply
deletes the oldest set of redemption tokens in storage and then
stores the newly acquired tokens.
For example, if Clients ensure that they only hold redemption tokens
for 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 they possess tokens for is still revealing. Understanding
this trade-off is important in deciding the effective anonymity of
each Client in the system.
5.3.1.1. Redemption Partitions
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 each other, the verifier will only be able to
learn a number of bits of information up to the limits within that
"redemption partitions".
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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:
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
6. Security Considerations
We present a number of security considerations that prevent malicious
Clients from abusing the protocol.
6.1. Double-spend Protection
When applicable for non-interactive tokens, all Origins SHOULD
implement a robust storage-query mechanism for checking that tokens
sent by clients have not been spent before. Such tokens only need to
be checked for each Origin individually. But all Origins must
perform global double-spend checks to avoid clients from exploiting
the possibility of spending tokens more than once against distributed
token checking systems. For the same reason, the global data storage
must have quick update times. While an update is occurring it may be
possible for a malicious client to spend a token more than once.
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6.2. Token Exhaustion
When a Client holds tokens for an Issuer, it is possible for any
verifier to invoke that client to redeem tokens for that Issuer.
This can lead to an attack where a malicious verifier can force a
Client to spend all of their tokens from a given Issuer. To prevent
this from happening, tokens can be scoped to single Origins such that
they can only be redeemed within for a single Origin.
If tokens are cross-Origin, Clients should 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.
7. Protocol Parameterization
This section provides a summary of the parameters used in the Privacy
Pass protocol ecosystem. These parameters are informed by both
privacy and security considerations that are highlighted in Section 5
and Section 6, respectively. These parameters are intended as a
single reference point for those implementing the protocol.
Firstly, let U be the total number of Clients (or users), I be the
total number of Issuers. We let M be the total number of metadata
bits that are allowed to be added by any given Issuer. Assuming that
each user accept tokens from a uniform sampling of all the possible
Issuers, as a worst-case analysis, this segregates Clients into a
total of 2^I buckets. As such, we see an exponential reduction in
the size of the anonymity set for any given user. This allows us to
specify the privacy constraints of the protocol below, relative to
the setting of A.
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+==========================================+==================+
| parameter | value |
+==========================================+==================+
| Minimum anonymity set size (A) | 5000 |
+------------------------------------------+------------------+
| Recommended key lifetime (L) | 2 - 24 weeks |
+------------------------------------------+------------------+
| Recommended key rotation frequency (F) | L/2 |
+------------------------------------------+------------------+
| Maximum additional metadata bits (M) | 1 |
+------------------------------------------+------------------+
| Maximum allowed Issuers (I) | (log_2(U/A)-1)/2 |
+------------------------------------------+------------------+
| Maximum active issuance keys | 1 |
+------------------------------------------+------------------+
| Maximum active redemption keys | 2 |
+------------------------------------------+------------------+
| Minimum cryptographic security parameter | 128 bits |
+------------------------------------------+------------------+
Table 1
7.1. Justification
We make the following assumptions in these parameter choices.
* Inferring the identity of a user in a 5000-strong anonymity set is
difficult.
* After 2 weeks, all Clients in a system will have rotated to the
new key.
In terms of additional metadata, the only concrete applications of
Privacy Pass that use additional metadata require just a single bit.
Therefore, we set the ceiling of permitted metadata to 1 bit for now,
this may be revisited in future revisions.
The maximum choice of I is based on the equation 1/2 * U/2^(2I) = A.
This is derived from the fact that permitting I Issuers lead to 2^I
segregations of the total user-base U. Moreover, if we permit M = 1,
then this effectively halves the anonymity set for each Issuer, and
thus we incur a factor of 2I in the exponent. By reducing I, we
limit the possibility of performing the attacks mentioned in
Section 5.
We must also account for each user holding issued data for more then
one possible active keys. While this may also be a vector for
monitoring the access patterns of Clients, it is likely to
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unavoidable that Clients hold valid issuance data for the previous
key epoch. This also means that the Issuer can continue to verify
redemption data for a previously used key. This makes the rotation
period much smoother for Clients.
For privacy reasons, it is recommended that key epochs are chosen
that limit Clients to holding issuance data for a maximum of two
keys. By choosing F = L/2 then the minimum value of F is a week,
since the minimum recommended value of L is 2 weeks. Therefore, by
the initial assumption, then all users should only have access to
only two keys at any given time. This reduces the anonymity set by
another half at most.
Finally, the minimum security parameter size is related to the
cryptographic security offered by the protocol that is run. This
parameter corresponds to the number of operations that any adversary
has in breaking one of the security guarantees in the Privacy Pass
protocol [I-D.ietf-privacypass-protocol].
7.2. Example parameterization
Using the specification above, we can give some example
parameterizations. For example, the current Privacy Pass browser
extension [PPEXT] has nearly 300000 active users (from Chrome and
Firefox). As a result, log_2(U/A) is approximately 6 and so the
maximum value of I should be 3.
If the value of U is much bigger (e.g. 5 million) then this would
permit I = (log_2(5000000/5000)-1)/2 ~= 4 Issuers.
7.3. Allowing more Issuers
Using the recommendations in Section 5.3.1, it is possible to
tolerate larger number of Issuers if Clients in the ecosystem ensure
that they only store tokens for a small number of them. In
particular, if Clients limit their storage of redemption tokens to
the bound implied by I, then prevents a malicious verifier from
triggering redemptions for all Issuers in the ecosystem.
8. References
8.1. Normative References
[HTTP-Authentication]
"The Privacy Pass HTTP Authentication Scheme", n.d.,
<https://datatracker.ietf.org/doc/html/draft-pauly-
privacypass-auth-scheme-00>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
8.2. Informative References
[CONSISTENCY]
Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
"Key Consistency and Discovery", Work in Progress,
Internet-Draft, draft-wood-key-consistency-02, 4 March
2022, <https://datatracker.ietf.org/doc/html/draft-wood-
key-consistency-02>.
[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/wp-content/uploads/2021/01/
PrivateStats-De-Identified-Authenticated-Logging-at-
Scale_final.pdf>.
[I-D.ietf-privacypass-protocol]
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-
02, 31 January 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-protocol-02>.
[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>.
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[PPSRV] Sullivan, N., "Cloudflare Supports Privacy Pass", n.d.,
<https://blog.cloudflare.com/cloudflare-supports-privacy-
pass/>.
[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 Scott Hendrickson, Tommy Pauly,
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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