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title: "Privacy Pass Issuance Protocol" abbrev: PP Issuance docname: draft-ietf-privacypass-protocol-latest date: category: info

ipr: trust200902 keyword: Internet-Draft

stand_alone: yes pi: [toc, sortrefs, symrefs]

author:

ins: S. Celi
name: Sofía Celi
org: Cloudflare
city: Lisbon
country: Portugal
email: sceli@cloudflare.com
  • ins: A. Davidson name: Alex Davidson org: Brave Software city: Lisbon country: Portugal email: alex.davidson92@gmail.com
  • ins: A. Faz-Hernandez name: Armando Faz-Hernandez org: Cloudflare street: 101 Townsend St city: San Francisco country: United States of America email: armfazh@cloudflare.com
  • ins: S. Valdez name: Steven Valdez org: Google LLC email: svaldez@chromium.org
  • ins: C. A. Wood name: Christopher A. Wood org: Cloudflare street: 101 Townsend St city: San Francisco country: United States of America email: caw@heapingbits.net

normative: RFC2119: HTTP-Authentication: title: The Privacy Pass HTTP Authentication Scheme target: https://datatracker.ietf.org/doc/html/draft-pauly-privacypass-auth-scheme-00 I-D.ietf-privacypass-architecture:

--- abstract

This document specifies two variants of the the two-message issuance protocol for Privacy Pass tokens: one that produces tokens that are privately verifiable, and another that produces tokens that are publicly verifiable. The privately verifiable issuance protocol optionally supports public metadata during the issuance flow.

--- middle

Introduction

The Privacy Pass protocol provides a privacy-preserving authorization mechanism. In essence, the protocol allows clients to provide cryptographic tokens that prove nothing other than that they have been created by a given server in the past {{I-D.ietf-privacypass-architecture}}.

This document describes the issuance protocol for Privacy Pass. It specifies two variants: one that is privately verifiable based on the oblivious pseudorandom function from {{!OPRF=I-D.irtf-cfrg-voprf}}, and one that is publicly verifiable based on the blind RSA signature scheme {{!BLINDRSA=I-D.irtf-cfrg-rsa-blind-signatures}}.

This document DOES NOT cover the architectural framework required for running and maintaining the Privacy Pass protocol in the Internet setting. In addition, it DOES NOT cover the choices that are necessary for ensuring that client privacy leaks do not occur. Both of these considerations are covered in {{I-D.ietf-privacypass-architecture}}.

Terminology

{::boilerplate bcp14}

The following terms are used throughout this document.

  • Client: An entity that provides authorization tokens to services across the Internet, in return for authorization.
  • Issuer: A service produces Privacy Pass tokens to clients.
  • Private Key: The secret key used by the Issuer for issuing tokens.
  • Public Key: The public key used by the Issuer for issuing and verifying tokens.

We assume that all protocol messages are encoded into raw byte format before being sent across the wire.

Issuance Protocol for Privately Verifiable Tokens with Public Metadata {#private-flow}

The Privacy Pass issuance protocol is a two message protocol that takes as input a challenge from the redemption protocol and produces a token, as shown in the figure below.

   Origin          Client                   Issuer
                (pkI, info)            (skI, pkI, info)
                  +------------------------------------\
  Challenge   ----> TokenRequest ------------->        |
                  |                       (evaluate)   |
    Token    <----+     <--------------- TokenResponse |
                  \------------------------------------/

Issuers provide a Private and Public Key, denoted skI and pkI, respectively, used to produce tokens as input to the protocol. See {{issuer-configuration}} for how this key pair is generated.

Clients provide the following as input to the issuance protocol:

  • Issuer name, identifying the Issuer. This is typically a host name that can be used to construct HTTP requests to the Issuer.
  • Issuer Public Key pkI, with a key identifier key_id computed as described in {{issuer-configuration}}.
  • Challenge value challenge, an opaque byte string. For example, this might be provided by the redemption protocol in {{HTTP-Authentication}}.

Both Client and Issuer also share a common public string called info.

Given this configuration and these inputs, the two messages exchanged in this protocol are described below. This section uses notation described in {{OPRF, Section 4}}, including SerializeElement and DeserializeElement, SerializeScalar and DeserializeScalar, and DeriveKeyPair.

Client-to-Issuer Request {#client-to-issuer}

The Client first creates a context as follows:

client_context = SetupPOPRFClient(0x0004, pkI)

Here, 0x0004 is the two-octet identifier corresponding to the OPRF(P-384, SHA-384) ciphersuite in {{OPRF}}. SetupPOPRFClient is defined in {{OPRF, Section 3.2}}.

The Client then creates an issuance request message for a random value nonce using the input challenge and Issuer key identifier as follows:

nonce = random(32)
context = SHA256(challenge)
token_input = concat(0x0001, nonce, context, key_id)
blind, blinded_msg, tweaked_key = client_context.Blind(nonce, info)

The Blind function is defined in {{OPRF, Section 3.3.3}}. If the Blind function fails, the Client aborts the protocol. Otherwise, the Client then creates a TokenRequest structured as follows:

struct {
   uint16_t token_type = 0x0001;
   uint8_t token_key_id;
   uint8_t blinded_request[Ne];
} TokenRequest;

The structure fields are defined as follows:

  • "token_type" is a 2-octet integer, which matches the type in the challenge.

  • "token_key_id" is the least significant byte of the key_id.

  • "blinded_request" is the Ne-octet blinded message defined above, computed as SerializeElement(blinded_msg). Ne is as defined in {{OPRF, Section 4}}.

The value tweaked_key is stored locally and used later as described in {{finalization}}. The Client then generates an HTTP POST request to send to the Issuer, with the TokenRequest as the body. The media type for this request is "message/token-request". An example request is shown below.

:method = POST
:scheme = https
:authority = issuer.net
:path = /token-request
accept = message/token-response
cache-control = no-cache, no-store
content-type = message/token-request
content-length = <Length of TokenRequest>

<Bytes containing the TokenRequest>

Upon receipt of the request, the Issuer validates the following conditions:

  • The TokenRequest contains a supported token_type.
  • The TokenRequest.token_key_id corresponds to a key ID of a Public Key owned by the issuer.
  • The TokenRequest.blinded_msg is of the correct size.

If any of these conditions is not met, the Issuer MUST return an HTTP 400 error to the Client, which will forward the error to the client.

Issuer-to-Client Response {#issuer-to-client}

If the Issuer is willing to produce a token token to the Client, the Issuer completes the issuance flow by computing a blinded response as follows:

server_context = SetupPOPRFServer(0x0004, skI, pkI)
evaluate_msg, proof = server_context.Evaluate(skI,
    TokenRequest.blinded_message, info)

SetupPOPRFServer is in {{OPRF, Section 3.2}} and Evaluate is defined in {{OPRF, Section 3.3.3}}. The Issuer then creates a TokenResponse structured as follows:

struct {
   uint8_t evaluate_response[Nk];
   uint8_t evaluate_proof[Ns+Ns];
} TokenResponse;

The structure fields are defined as follows:

  • "evaluate_response" is the Ne-octet evaluated messaged, computed as SerializeElement(evaluated_msg).

  • "evaluate_proof" is the (Ns+Ns)-octet serialized proof, which is a pair of Scalar values, computed as concat(SerializeScalar(proof[0]), SerializeScalar(proof[1])), where Ns is as defined in {{OPRF, Section 4}}.

The Issuer generates an HTTP response with status code 200 whose body consists of TokenResponse, with the content type set as "message/token-response".

:status = 200
content-type = message/token-response
content-length = <Length of TokenResponse>

<Bytes containing the TokenResponse>

Finalization

Upon receipt, the Client handles the response and, if successful, deserializes the body values TokenResponse.evaluate_response and TokenResponse.evaluate_proof. If deserialization of any value fails, the Client aborts the protocol. Otherwise, when deserialization yields evaluated_msg and proof, the Client processes the response as follows:

authenticator = client_context.Finalize(context, blind, pkI,
  evaluated_msg, blinded_msg, info, tweaked_key)

The Finalize function is defined in {{OPRF, Section 3.3.3}}. If this succeeds, the Client then constructs a Token as follows:

struct {
    uint16_t token_type = 0x0001
    uint8_t nonce[32];
    uint8_t context[32];
    uint8_t key_id[32];
    uint8_t authenticator[Nk];
} Token;

Otherwise, the Client aborts the protocol.

Issuer Configuration

Issuers are configured with Private and Public Key pairs, each denoted skI and pkI, respectively, used to produce tokens. Each key pair MUST be generated as follows:

seed = random(Ns)
(skI, pkI) = DeriveKeyPair(seed, "PrivacyPass")

The key identifier for this specific key pair, denoted key_id, is computed as follows:

key_id = SHA256(0x0001 || SerializeElement(pkI))

Issuance Protocol for Publicly Verifiable Tokens {#public-flow}

This section describes a variant of the issuance protocol in {{private-flow}} for producing publicly verifiable tokens. It differs from the previous variant in two important ways:

  1. The output tokens are publicly verifiable by anyone with the Issuer public key; and
  2. The issuance protocol does not admit public or private metadata to bind additional context to tokens.

Otherwise, this variant is nearly identical. In particular, Issuers provide a Private and Public Key, denoted skI and pkI, respectively, used to produce tokens as input to the protocol. See {{public-issuer-configuration}} for how this key pair is generated.

Clients provide the following as input to the issuance protocol:

  • Issuer name, identifying the Issuer. This is typically a host name that can be used to construct HTTP requests to the Issuer.
  • Issuer Public Key pkI, with a key identifier key_id computed as described in {{public-issuer-configuration}}.
  • Challenge value challenge, an opaque byte string. For example, this might be provided by the redemption protocol in {{HTTP-Authentication}}.

Given this configuration and these inputs, the two messages exchanged in this protocol are described below.

Client-to-Issuer Request {#public-request}

The Client first creates an issuance request message for a random value nonce using the input challenge and Issuer key identifier as follows:

nonce = random(32)
context = SHA256(challenge)
token_input = concat(0x0002, nonce, context, key_id)
blinded_msg, blind_inv = rsabssa_blind(pkI, token_input)

The rsabssa_blind function is defined in {{BLINDRSA, Section 5.1.1.}}. The Client then creates a TokenRequest structured as follows:

struct {
   uint16_t token_type = 0x0002
   uint8_t token_key_id;
   uint8_t blinded_msg[Nk];
} TokenRequest;

The structure fields are defined as follows:

  • "token_type" is a 2-octet integer, which matches the type in the challenge.

  • "token_key_id" is the least significant byte of the key_id.

  • "blinded_msg" is the Nk-octet request defined above.

The Client then generates an HTTP POST request to send to the Issuer, with the TokenRequest as the body. The media type for this request is "message/token-request". An example request is shown below, where Nk = 512.

:method = POST
:scheme = https
:authority = issuer.net
:path = /token-request
accept = message/token-response
cache-control = no-cache, no-store
content-type = message/token-request
content-length = <Length of TokenRequest>

<Bytes containing the TokenRequest>

Upon receipt of the request, the Issuer validates the following conditions:

  • The TokenRequest contains a supported token_type.
  • The TokenRequest.token_key_id corresponds to a key ID of a Public Key owned by the issuer.
  • The TokenRequest.blinded_msg is of the correct size.

If any of these conditions is not met, the Issuer MUST return an HTTP 400 error to the Client, which will forward the error to the client.

Issuer-to-Client Response {#public-response}

If the Issuer is willing to produce a token token to the Client, the Issuer completes the issuance flow by computing a blinded response as follows:

blind_sig = rsabssa_blind_sign(skI, TokenRequest.blinded_rmsg)

The rsabssa_blind_sign function is defined in {{BLINDRSA, Section 5.1.2.}}. The Issuer generates an HTTP response with status code 200 whose body consists of blind_sig, with the content type set as "message/token-response".

:status = 200
content-type = message/token-response
content-length = <Length of TokenResponse>

<Bytes containing the TokenResponse>

Finalization

Upon receipt, the Client handles the response and, if successful, processes the body as follows:

authenticator = rsabssa_finalize(pkI, nonce, blind_sig, blind_inv)

The rsabssa_finalize function is defined in {{BLINDRSA, Section 5.1.3.}}. If this succeeds, the Client then constructs a Token as described in {{HTTP-Authentication}} as follows:

struct {
    uint16_t token_type = 0x0002
    uint8_t nonce[32];
    uint8_t context[32];
    uint8_t key_id[32];
    uint8_t authenticator[Nk];
} Token;

Otherwise, the Client aborts the protocol.

Issuer Configuration {#public-issuer-configuration}

Issuers are configured with Private and Public Key pairs, each denoted skI and pkI, respectively, used to produce tokens. Each key pair MUST be generated as as a valid 4096-bit RSA private key according to [TODO]. The key identifier for a keypair (skI, pkI), denoted key_id, is computed as SHA256(encoded_key), where encoded_key is a DER-encoded SubjectPublicKeyInfo object carrying pkI.

Security considerations

This document outlines how to instantiate the Issuance protocol based on the VOPRF defined in {{OPRF}} and blind RSA protocol defnied in {{BLINDRSA}}. All security considerations described in the VOPRF document also apply in the Privacy Pass use-case. Considerations related to broader privacy and security concerns in a multi-Client and multi-Issuer setting are deferred to the Architecture document {{I-D.ietf-privacypass-architecture}}.

IANA considerations

Token Type

This document updates the "Token Type" Registry with the following values.

Value Name Publicly Verifiable Public Metadata Private Metadata Nk Reference
0x0001 OPRF(P-384, SHA-384) N Y N 48 {{private-flow}}
0x0002 Blind RSA, 4096 Y N N 512 {{public-flow}}
{: #aeadid-values title="Token Types"}

Media Types

This specification defines the following protocol messages, along with their corresponding media types:

  • TokenRequest: "message/token-request"
  • TokenResponse: "message/token-response"

The definition for each media type is in the following subsections.

"message/token-request" media type

Type name:

: message

Subtype name:

: token-request

Required parameters:

: N/A

Optional parameters:

: None

Encoding considerations:

: only "8bit" or "binary" is permitted

Security considerations:

: see {{security-considerations}}

Interoperability considerations:

: N/A

Published specification:

: this specification

Applications that use this media type:

: N/A

Fragment identifier considerations:

: N/A

Additional information:

:

Magic number(s):
N/A
Deprecated alias names for this type:
N/A
File extension(s):
N/A
Macintosh file type code(s):
N/A

Person and email address to contact for further information:

: see Authors' Addresses section

Intended usage:

: COMMON

Restrictions on usage:

: N/A

Author:

: see Authors' Addresses section

Change controller:

: IESG

"message/token-response" media type

Type name:

: message

Subtype name:

: access-token-response

Required parameters:

: N/A

Optional parameters:

: None

Encoding considerations:

: only "8bit" or "binary" is permitted

Security considerations:

: see {{security-considerations}}

Interoperability considerations:

: N/A

Published specification:

: this specification

Applications that use this media type:

: N/A

Fragment identifier considerations:

: N/A

Additional information:

:

Magic number(s):
N/A
Deprecated alias names for this type:
N/A
File extension(s):
N/A
Macintosh file type code(s):
N/A

Person and email address to contact for further information:

: see Authors' Addresses section

Intended usage:

: COMMON

Restrictions on usage:

: N/A

Author:

: see Authors' Addresses section

Change controller:

: IESG

--- back

Acknowledgements

The authors of this document would like to acknowledge the helpful feedback and discussions from Benjamin Schwartz, Joseph Salowey, Sofía Celi, and Tara Whalen.