Internet DRAFT - draft-pauly-privacypass-auth-scheme
draft-pauly-privacypass-auth-scheme
Network Working Group T. Pauly
Internet-Draft Apple Inc.
Intended status: Standards Track S. Valdez
Expires: 4 August 2022 Google LLC
C.A. Wood
Cloudflare
31 January 2022
The Privacy Pass HTTP Authentication Scheme
draft-pauly-privacypass-auth-scheme-00
Abstract
This document defines an HTTP authentication scheme that can be used
by clients to redeem Privacy Pass tokens with an origin. It can also
be used by origins to challenge clients to present an acceptable
Privacy Pass token.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/tfpauly/privacy-proxy.
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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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 4 August 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. HTTP Authentication Scheme . . . . . . . . . . . . . . . . . 4
2.1. Token Challenge . . . . . . . . . . . . . . . . . . . . . 4
2.2. Token Redemption . . . . . . . . . . . . . . . . . . . . 6
3. Issuance Protocol Requirements . . . . . . . . . . . . . . . 7
4. User Interaction . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Authentication Scheme . . . . . . . . . . . . . . . . . . 9
6.2. Token Type Registry . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Privacy Pass tokens are unlinkable authenticators that can be used to
anonymously authorize a client (see
[I-D.ietf-privacypass-architecture]). A client possessing such a
token is able to prove that it was able to get a token issued by a
token issuer -- based on some check from a token issuer, such as
authentication or solving a CAPTCHA -- without allowing the relying
party redeeming the client's token (the origin) to link it with
issuance flow.
Different types of authenticators, using different token issuance
protocols, can be used as Privacy Pass tokens.
This document defines a common HTTP authentication scheme
([RFC7235]), PrivateToken, that allows clients to redeem various
kinds of Privacy Pass tokens.
Clients and relying parties interact using this scheme to perform the
token challenge and token redemption flow. Clients use a token
issuance protocol to actually fetch tokens to redeem.
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Client Relying Party (Origin)
<------------------------------ Challenge \
|
+----------------------------------\ |
| | |
| Issuance Protocol | |
| | |
+----------------------------------/ |
|
Redemption -------------------------- > /
Figure 1: Token Architectural Components
In addition to working with different token issuance protocols, this
scheme supports both interactive (online challenges) and non-
interactive (pre-fetched) token redemption, as well as the ability to
scope a token to a specific resource or origin. Relying parties that
request and redeem tokens can choose a specific kind of token, as
appropriate for its use case. For example, non-interactive token
redemption that is not scoped to a specific origin can be used as a
replacement for CAPTCHAs, as exemplified by the original Privacy Pass
work [DGSTV18].
1.1. 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.
This document uses the following terms to refer to various roles and
functions in the Privacy Pass architecture:
* Client: A client is a application or device, generally operated by
a user, that can be issued tokens via an issuance protocol, and
can redeem these tokens with an origin.
* Origin: An HTTP server that acts as the relying party and can
redeem tokens presented by a client. When used in a web context,
this represents the origin the client is accessing.
* Token: A signed message that can be issued to a client and
redeemed without allowing token redemption to be linked to
issuance.
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* Interactive / non-interactive token: An interactive token signs a
nonce generated by an origin as part of a challenge for tokens.
This means that the client needs to fetch a new token for this
challenge in order to redeem it and cannot use a pre-fetched
token. A non-interactive token is one that can be pre-fetched.
* Issuance protocol: A protocol by which the client fetches tokens.
Every issuance protocol includes two functions: validating or
authenticating the client, and issuing a token to the client.
* Issuer: An entity that generates tokens for clients using one or
more issuance protocols. An Issuer is identified by an Issuer
name.
* Issuer key: Keying material that can be used with an issuance
protocol to create a signed token.
* Token challenge: A requirement for tokens sent from an origin to a
client, using the "WWW-Authenticate" HTTP header. This may be a
challenge for an interactive token or a non-interactive token. A
challenge defines the issuance protocol and issuer name to use for
a token.
* Token redemption: An action by which a client presents a token to
an origin, using the "Authorization" HTTP header.
2. HTTP Authentication Scheme
Token redemption is performed using HTTP Authentication ([RFC7235]),
with the scheme "PrivateToken". Origins challenge clients to present
a token from a specific issuer (Section 2.1). Once a client has
received a token from that issuer, or already has a valid token
available, it presents the token to the origin (Section 2.2).
2.1. Token Challenge
Origins send a token challenge to Clients in an "WWW-Authenticate"
header with the "PrivateToken" scheme. This challenge includes a
TokenChallenge message, along with information about what keys to use
when requesting a token from the issuer.
The TokenChallenge message has the following structure:
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struct {
uint16_t token_type;
opaque issuer_name<1..2^16-1>;
opaque redemption_nonce<0..32>;
opaque origin_name<0..2^16-1>;
} TokenChallenge;
The structure fields are defined as follows:
* "token_type" is a 2-octet integer, in network byte order. This
type indicates the issuance protocol used to generate the token.
Values are registered in an IANA registry, Section 6.2.
Challenges with unsupported token_type values MUST be ignored.
* "issuer_name" is a string containing the name of the issuer. This
is a hostname that is used to identify the issuer that is allowed
to issue tokens that can be redeemed by this origin.
* "redemption_nonce" is an optional field. If present, it indicates
that a client needs to present an interactive token, generated
specifically in response to this challenge. If empty, the client
can use a non-interactive token. When present, this valid is a
fresh 32-byte nonce generated by the origin for each challenge.
Valid lengths for this field are either 0 or 32 bytes. Challenges
with redemption_nonce values of invalid lengths MUST be ignored.
* "origin_name" is an optional string containing the name of the
origin. This allows a token to be scoped to a specific origin.
If empty, any non-origin specific token can be redeemed.
When used in an authentication challenge, the "PrivateToken" scheme
uses the following attributes:
* "challenge", which contains a base64url-encoded [RFC4648]
TokenChallenge value. This MUST be unique for every 401 HTTP
response to prevent replay attacks. This attribute is required
for all challenges.
* "token-key", which contains a base64url encoding of the public key
for use with the issuance protocol indicated by the challenge.
This attribute MAY be omitted in deployments where clients are
able to retrieve the issuer key using an out-of-band mechanism.
* "max-age", an optional attribute that consists of the number of
seconds for which the challenge will be accepted by the Origin.
Clients can ignore the challenge if the token-key is invalid or
otherwise untrusted.
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Origins MAY also include the standard "realm" attribute, if desired.
Issuance protocols MAY require other attributes.
As an example, the WWW-Authenticate header could look like this:
WWW-Authenticate: PrivateToken challenge=abc..., token-key=123...
Upon receipt of this challenge, a client uses the message and keys in
the issuance protocol indicated by the token_type. If the
TokenChallenge has a token_type the client does not recognize or
support, it MUST NOT parse or respond to the challenge.
Note that it is possible for the WWW-Authenticate header to include
multiple challenges, in order to allow the Client to fetch a batch of
multiple tokens for future use.
For example, the WWW-Authenticate header could look like this:
WWW-Authenticate: PrivateToken challenge=abc..., token-key=123...,
PrivateToken challenge=def..., token-key=234...
2.2. Token Redemption
The output of the issuance protocol is a token that corresponds to
the origin's challenge (see Section 2.1). A token is a structure
that begins with a two-octet field that indicates a token type, which
MUST match the token_type in the TokenChallenge structure.
struct {
uint16_t token_type;
uint8_t nonce[32];
uint8_t context[32];
uint8_t token_key_id[Nid];
uint8_t authenticator[Nk];
} Token;
The structure fields are defined as follows:
* "token_type" is a 2-octet integer, in network byte order. This
value must match the value in the challenge (Section 2.1).
* "nonce" is a 32-octet message containing a client-generated random
nonce.
* "context" is a 32-octet message containing the hash of the
original TokenChallenge, SHA256(TokenChallenge).
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* "token_key_id" is an Nid-octet identifier for the the token
authentication key. The value of this field is defined by the
token_type and corresponding issuance protocol.
* "authenticator" is a Nk-octet authenticator that covers the
preceding fields in the token. The value of this field is defined
by the token_type and corresponding issuance protocol.
The authenticator value in the Token structure is computed over the
token_type, nonce, context, and token_key_id fields.
When used for client authorization, the "PrivateToken" authentication
scheme defines one parameter, "token", which contains the base64url-
encoded Token struct. All unknown or unsupported parameters to
"PrivateToken" authentication credentials MUST be ignored.
Clients present this Token structure to Origins in a new HTTP request
using the Authorization header as follows:
Authorization: PrivateToken token=abc...
For token types that support public verifiability, origins verify the
token authenticator using the public key of the issuer, and validate
that the signed message matches the concatenation of the client nonce
and the hash of a valid TokenChallenge. For interactive tokens,
origins store the nonces of previous TokenChallenge structures in
order to validate uniqueness. A TokenChallenge MAY be bound to a
specific HTTP session with client, but origins can also accept tokens
for valid challenges in new sessions. For non-interactive tokens,
origins SHOULD implement some form of double spend prevention that
prevents a token with the same nonce from being redeemed twice. This
prevents clients from "replaying" tokens for previous challenges.
If a client is unable to fetch a token, it MUST react to the
challenge as if it could not produce a valid Authorization response.
3. Issuance Protocol Requirements
Clients initiate the issuance protocol using a challenge, a randomly
generated nonce, and a public key for the issuer. The 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. The issuance protocol can reveal the issuer
public key for the purposes of determining which private key to
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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 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.
4. User Interaction
When used in contexts like websites, origins that challenge clients
for tokens need to consider how to optimize their interaction model
to ensure a good user experience.
Tokens challenges can be performed without explicit user involvement,
depending on the issuance protocol. If tokens are scoped to a
specific origin, there is no need for per-challenge user interaction.
Note that the issuance protocol may separately involve user
interaction if the client needs to be newly validated.
The use of interactive tokens can add user-perceivable latency, since
such tokens cannot be pre-fetched. Origins need not block useful
work on token authentication. Instead, token authentication can be
used in similar ways to CAPTCHA validation today, but without the
need for user interaction. If issuance is taking a long time, a
website could show an indicator that it is waiting, or fall back to
another method of user validation.
An origin MUST NOT issue more than one interactive challenge for a
given token type and issuer per client request. If an origin issues
a large number of challenges, such as more than once for each
request, this can indicate that the origin is either not functioning
correctly or is trying to attack or overload the client or issuance
server. In such cases, a client MUST ignore redundant token
challenges for the same request and SHOULD alert the user if
possible.
Origins MAY include multiple challenges, where each challenge refers
to a different issuer or a different token type, to allow clients to
choose a preferred issuer or type.
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5. Security Considerations
The security properties of token challenges vary depending on whether
the challenge is interactive or not, as well as whether the challenge
is per-origin or not. For example, non-interactive, cross-origin
tokens can be replayed from one party by another, as shown below.
Client Attacker Origin
<----------- Challenge \
|
<--------- Challenge |
|
Redemption ----> |
|
Redemption ----------> /
Figure 2: Token Architectural Components
Interactive token challenges require clients to obtain matching
tokens when challenged, rather than presenting a token that was
obtained in the past. This means that issuance and redemption events
will occur at approximately the same time. For example, if a client
is challenged for an interactive token at time T1 and then
subsequently obtains a token at time T2, a colluding issuer and
origin can link this to the same client if T2 is unique to the
client. This linkability is less feasible as the number of issuance
events at time T2 increases. Depending on the "max-age" token
challenge attribute, clients MAY try to augment the time between
getting challenged then redeeming a token so as to make this sort of
linkability more difficult. For more discussion on correlation risks
between token issuance and redemption, see
[I-D.ietf-privacypass-architecture].
Applications SHOULD constrain tokens to a single origin unless the
use case can accommodate such replay attacks.
All random values in the challenge and token MUST be generated using
a cryptographically secure source of randomness.
6. IANA Considerations
6.1. Authentication Scheme
This document registers the "PrivateToken" authentication scheme in
the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" established by [RFC7235].
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Authentication Scheme Name: PrivateToken
Pointer to specification text: Section 2 of this document
6.2. Token Type Registry
The "Token Type" registry lists identifiers for issuance protocols
defined for use with the Privacy Pass token authentication scheme.
These identifiers are two-byte values, so the maximum possible value
is 0xFFFF = 65535.
Template:
* Value: The two-byte identifier for the algorithm
* Name: Name of the issuance protocol
* Publicly Verifiable: A Y/N value indicating if the output tokens
are publicly verifiable
* Public Metadata: A Y/N value indicating if the output tokens can
contain public metadata.
* Private Metadata: A Y/N value indicating if the output tokens can
contain private metadata.
* Nk: The length in bytes of an output authenticator
* Nid: The length of the token key identifier
* Reference: Where this algorithm is defined
The initial contents for this registry are defined in the table
below.
+======+============+============+========+========+==+===+=========+
|Value | Name | Publicly |Public |Private |Nk|Nid|Reference|
| | | Verifiable |Metadata|Metadata| | | |
+======+============+============+========+========+==+===+=========+
|0x0000| (reserved) | N/A |N/A |N/A |N/|N/A|N/A |
| | | | | |A | | |
+------+------------+------------+--------+--------+--+---+---------+
Table 1: Token Types
7. References
7.1. Normative References
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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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/rfc/rfc7235>.
[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>.
7.2. Informative References
[DGSTV18] "Privacy Pass, Bypassing Internet Challenges Anonymously",
n.d., <https://petsymposium.org/2018/files/papers/issue3/
popets-2018-0026.pdf>.
[I-D.ietf-privacypass-architecture]
Davidson, A. and C. A. Wood, "Privacy Pass Architectural
Framework", Work in Progress, Internet-Draft, draft-ietf-
privacypass-architecture-01, 22 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-architecture-01>.
Authors' Addresses
Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Email: tpauly@apple.com
Steven Valdez
Google LLC
Email: svaldez@chromium.org
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Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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