Internet DRAFT - draft-hendrickson-privacypass-public-metadata
draft-hendrickson-privacypass-public-metadata
Privacy Pass S. Hendrickson
Internet-Draft Google
Intended status: Informational C. A. Wood
Expires: 26 May 2024 Cloudflare, Inc.
23 November 2023
Public Metadata Issuance
draft-hendrickson-privacypass-public-metadata-03
Abstract
This document specifies Privacy Pass issuance protocols that encode
public information visible to the Client, Attester, Issuer, and
Origin into each token.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://smhendrickson.github.io/draft-hendrickson-privacypass-public-
metadata-issuance/draft-hendrickson-privacypass-public-metadata.html.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-hendrickson-privacypass-
public-metadata/.
Discussion of this document takes place on the Privacy Pass Working
Group mailing list (mailto:privacy-pass@ietf.org), which is archived
at https://mailarchive.ietf.org/arch/browse/privacy-pass/. Subscribe
at https://www.ietf.org/mailman/listinfo/privacy-pass/.
Source for this draft and an issue tracker can be found at
https://github.com/smhendrickson/draft-hendrickson-privacypass-
public-metadata-issuance.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Issuance Protocol for Privately Verifiable Tokens . . . . . . 4
5.1. Client-to-Issuer Request . . . . . . . . . . . . . . . . 5
5.2. Issuer-to-Client Response . . . . . . . . . . . . . . . . 6
5.3. Finalization . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Token Verification . . . . . . . . . . . . . . . . . . . 8
5.5. Issuer Configuration . . . . . . . . . . . . . . . . . . 8
6. Issuance Protocol for Publicly Verifiable Tokens . . . . . . 9
6.1. Client-to-Issuer Request . . . . . . . . . . . . . . . . 10
6.2. Issuer-to-Client Response . . . . . . . . . . . . . . . . 11
6.3. Finalization . . . . . . . . . . . . . . . . . . . . . . 12
6.4. Token Verification . . . . . . . . . . . . . . . . . . . 12
6.5. Issuer Configuration . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8.1. Privately Verifiable Token Type . . . . . . . . . . . . . 14
8.2. Publicly Verifiable Token Type . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
The basic Privacy Pass issuance protocols as specified in
[BASIC-PROTOCOL] and resulting tokens convey only a single bit of
information: whether or not the token is valid. However, it is
possible for tokens to be issued with additional information aggreed
upon by Client, Attester, and Issuer during issuance. This
information, sometimes referred to as public metadata, allows Privacy
Pass applications to encode deployment-specific information that is
necessary for their use case.
This document specifies two Privacy Pass issuance protocols that
encode public information visible to the Client, Attester, Issuer,
and Origin. One is based on the partially-oblivious PRF construction
from [POPRF], and the other is based on the partially-blind RSA
signature scheme from [PBRSA].
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.
3. Notation
The following terms are used throughout this document to describe the
protocol operations in this document:
* len(s): the length of a byte string, in bytes.
* concat(x0, ..., xN): Concatenation of byte strings. For example,
concat(0x01, 0x0203, 0x040506) = 0x010203040506
* int_to_bytes: Convert a non-negative integer to a byte string.
int_to_bytes is implemented as I2OSP as described in Section 4.1
of [RFC8017]. Note that these functions operate on byte strings
in big-endian byte order.
4. Motivation
Public metadata enables Privacy Pass deployments that share
information between Clients, Attesters, Issuers and Origins. In the
basic Privacy Pass issuance protocols (types 0x0001 and 0x0002), the
only information available to all parties is the choice of Issuer,
expressed through the TokenChallenge. If one wants to differentiate
bits of information at the origin, many TokenChallenges must be sent,
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one for each Issuer that attests to the bit required.
For example, if a deployment was built that attested to an app’s
published state in an app store, it requires 1 bit {published,
not_published} and can be built with a single Issuer. An app version
attester would require one Issuer for each app version and one
TokenChallenge per Issuer.
Taken further, the limitation of one bit of information in each
Privacy Pass token means that a distinct Issuer and Issuer public key
is needed for each unique value one wants to express with a token.
This many-key metadata deployment should provide metadata visible to
all parties in the same way as the [PBRSA] proposal outlined in this
document. However, it comes with practical reliability and
scalability tradeoffs. In particular, many simultaneous deployed
keys could be difficult to scale. Some HSM implementations have
fixed per-key costs, slow key generation, and minimum key lifetimes.
Quick key rotation creates reliability risk to the system, as a pause
or slowdown in key rotation could cause the system to run out of
active signing or verification keys. Issuance protocols that support
public metadata mitigate these tradeoffs by allowing deployments to
change metadata values without publishing new keys.
5. Issuance Protocol for Privately Verifiable Tokens
This section describes a variant of the issuance protocol in
Section 5 of [BASIC-PROTOCOL] that supports public metadata based on
the partially oblivious PRF (POPRF) from [POPRF]. Issuers provide a
Private and Public Key, denoted skI and pkI respectively, used to
produce tokens as input to the protocol. See Section 5.5 for how
this key pair is generated.
Clients provide the following as input to the issuance protocol:
* Issuer Request URI: A URI to which token request messages are
sent. This can be a URL derived from the "issuer-request-uri"
value in the Issuer's directory resource, or it can be another
Client-configured URL. The value of this parameter depends on the
Client configuration and deployment model. For example, in the
'Split Origin, Attester, Issuer' deployment model, the Issuer
Request URI might correspond to the Client's configured Attester,
and the Attester is configured to relay requests to the Issuer.
* Issuer name: An identifier for the Issuer. This is typically a
host name that can be used to construct HTTP requests to the
Issuer.
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* Issuer Public Key: pkI, with a key identifier token_key_id
computed as described in Section 6.5.
* Challenge value: challenge, an opaque byte string. For example,
this might be provided by the redemption protocol in [AUTHSCHEME].
* Extensions: extensions, an Extensions structure as defined in
[TOKEN-EXTENSION].
Given this configuration and these inputs, the two messages exchanged
in this protocol are described below. This section uses notation
described in [POPRF], Section 4, including SerializeElement and
DeserializeElement, SerializeScalar and DeserializeScalar, and
DeriveKeyPair.
The constants Ne and Ns are as defined in [POPRF], Section 4 for
OPRF(P-384, SHA-384). The constant Nk, which is also equal to Nh as
defined in [POPRF], Section 4, is defined in Section 8.
5.1. Client-to-Issuer Request
The Client first creates a context as follows:
client_context = SetupPOPRFClient("P384-SHA384", pkI)
Here, "P384-SHA384" is the identifier corresponding to the
OPRF(P-384, SHA-384) ciphersuite in [POPRF]. SetupPOPRFClient is
defined in [POPRF], Section 3.2.
The Client then creates an issuance request message for a random
value nonce with the input challenge and Issuer key identifier as
described below:
nonce = random(32)
challenge_digest = SHA256(challenge)
token_input = concat(0xDA7B, // Token type field is 2 bytes long
nonce,
challenge_digest,
token_key_id)
blind, blinded_element, tweaked_key = client_context.Blind(token_input, extensions, pkI)
The Blind function is defined in [POPRF], Section 3.3.3. If the
Blind function fails, the Client aborts the protocol. The Client
stores the nonce, challenge_digest, and tweaked_key values locally
for use when finalizing the issuance protocol to produce a token (as
described in Section 5.3).
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The Client then creates an ExtendedTokenRequest structured as
follows:
struct {
TokenRequest request;
Extensions extensions;
} ExtendedTokenRequest;
The contents of ExtendedTokenRequest.request are as defined in
Section 5 of [BASIC-PROTOCOL]. The contents of
ExtendedTokenRequest.extensions match the Client's configured
extensions value.
The Client then generates an HTTP POST request to send to the Issuer
Request URI, with the ExtendedTokenRequest as the content. The media
type for this request is "application/private-token-request". An
example request is shown below:
:method = POST
:scheme = https
:authority = issuer.example.net
:path = /request
accept = application/private-token-response
cache-control = no-cache, no-store
content-type = application/private-token-request
content-length = <Length of ExtendedTokenRequest>
<Bytes containing the ExtendedTokenRequest>
5.2. Issuer-to-Client Response
* The ExtendedTokenRequest.request contains a supported token_type.
* The ExtendedTokenRequest.request.truncated_token_key_id
corresponds to the truncated key ID of an Issuer Public Key.
* The ExtendedTokenRequest.request.blinded_msg is of the correct
size.
* The ExtendedTokenRequest.extensions value is permitted by the
Issuer's policy.
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.
Otherwise, if the Issuer is willing to produce a token token to the
Client for the provided extensions, the Issuer then tries to
deseralize ExtendedTokenRequest.request.blinded_msg using
DeserializeElement from Section 2.1 of [POPRF], yielding
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blinded_element. If this fails, the Issuer MUST return an HTTP 400
error to the client. Otherwise, the Issuer completes the issuance
flow by computing a blinded response as follows:
server_context = SetupPOPRFServer("P384-SHA384", skI, pkI)
evaluate_element, proof =
server_context.BlindEvaluate(skI, blinded_element, ExtendedTokenRequest.extensions)
SetupPOPRFServer is defined in [POPRF], Section 3.2 and BlindEvaluate
is defined in [POPRF], Section 3.3.3. The Issuer then creates a
TokenResponse structured as follows:
struct {
uint8_t evaluate_msg[Ne];
uint8_t evaluate_proof[Ns+Ns];
} TokenResponse;
The structure fields are defined as follows:
* "evaluate_msg" is the Ne-octet evaluated message, computed as
SerializeElement(evaluate_element).
* "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])).
The Issuer generates an HTTP response with status code 200 whose
content consists of TokenResponse, with the content type set as
"application/private-token-response".
:status = 200
content-type = application/private-token-response
content-length = <Length of TokenResponse>
<Bytes containing the TokenResponse>
5.3. Finalization
Upon receipt, the Client handles the response and, if successful,
deserializes the content values TokenResponse.evaluate_msg and
TokenResponse.evaluate_proof, yielding evaluated_element and proof.
If deserialization of either value fails, the Client aborts the
protocol. Otherwise, the Client processes the response as follows:
authenticator = client_context.Finalize(token_input, blind,
evaluated_element,
blinded_element,
proof, extensions, tweaked_key)
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The Finalize function is defined in [POPRF], Section 3.3.3. If this
succeeds, the Client then constructs a Token as follows:
struct {
uint16_t token_type = 0xDA7B; /* Type POPRF(P-384, SHA-384) */
uint8_t nonce[32];
uint8_t challenge_digest[32];
uint8_t token_key_id[32];
uint8_t authenticator[Nk];
} Token;
The Token.nonce value is that which was sampled in Section 5.1. If
the Finalize function fails, the Client aborts the protocol.
The Client will send this Token to Origins for redemption in the
"token" HTTP authentication parameter as specified in Section 2.2 of
[AUTHSCHEME]. The Client also supplies its extensions value as an
additional authentication parameter as specified in
[TOKEN-EXTENSION].
5.4. Token Verification
Verifying a Token requires creating a POPRF context using the Issuer
Private Key and Public Key, evaluating the token contents with the
corresponding extensions, and comparing the result against the token
authenticator value:
server_context = SetupPOPRFServer("P384-SHA384", skI)
token_authenticator_input =
concat(Token.token_type,
Token.nonce,
Token.challenge_digest,
Token.token_key_id)
token_authenticator =
server_context.Evaluate(skI, token_authenticator_input, extensions)
valid = (token_authenticator == Token.authenticator)
5.5. Issuer Configuration
Issuers are configured with Private and Public Key pairs, each
denoted skI and pkI, respectively, used to produce tokens. These
keys MUST NOT be reused in other protocols. A RECOMMENDED method for
generating key pairs is as follows:
seed = random(Ns)
(skI, pkI) = DeriveKeyPair(seed, "PrivacyPass-TypeDA7B")
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The DeriveKeyPair function is defined in [POPRF], Section 3.3.1. The
key identifier for a public key pkI, denoted token_key_id, is
computed as follows:
token_key_id = SHA256(SerializeElement(pkI))
Since Clients truncate token_key_id in each TokenRequest, Issuers
should ensure that the truncated form of new key IDs do not collide
with other truncated key IDs in rotation.
6. Issuance Protocol for Publicly Verifiable Tokens
This section describes a variant of the issuance protocol in
Section 6 of [BASIC-PROTOCOL] for producing publicly verifiable
tokens including public metadata using cryptography specified in
[PBRSA]. In particular, this variant of the issuance protocol works
for the RSAPBSSA-SHA384-PSSZERO-Deterministic or RSAPBSSA-SHA384-PSS-
Deterministic variant of the blind RSA protocol variants described in
Section 6 of [PBRSA].
The public metadata issuance protocol differs from the protocol in
Section 6 of [BASIC-PROTOCOL] in that the issuance and redemption
protocols carry metadata provided by the Client and visible to the
Attester, Issuer, and Origin. This means Clients can set arbitrary
metadata when requesting a token, but specific values of metadata may
be rejected by any of Attester, Issuer, or Origin. Similar to a
token nonce, metadata is cryptographically bound to a token and
cannot be altered.
Clients provide the following as input to the issuance protocol:
* Issuer Request URI: A URI to which token request messages are
sent. This can be a URL derived from the "issuer-request-uri"
value in the Issuer's directory resource, or it can be another
Client-configured URL. The value of this parameter depends on the
Client configuration and deployment model. For example, in the
'Split Origin, Attester, Issuer' deployment model, the Issuer
Request URI might be correspond to the Client's configured
Attester, and the Attester is configured to relay requests to the
Issuer.
* Issuer name: An identifier for 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 token_key_id
computed as described in Section 6.5.
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* Challenge value: challenge, an opaque byte string. For example,
this might be provided by the redemption protocol in [AUTHSCHEME].
* Extensions: extensions, an Extensions structure as defined in
[TOKEN-EXTENSION].
Given this configuration and these inputs, the two messages exchanged
in this protocol are described below. The constant Nk is defined as
256 for token type 0xDA7A.
6.1. Client-to-Issuer 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)
challenge_digest = SHA256(challenge)
token_input = concat(0xDA7A, // Token type field is 2 bytes long
nonce,
challenge_digest,
token_key_id)
blinded_msg, blind_inv = Blind(pkI, PrepareIdentity(token_input), extensions)
Where PrepareIdentity is defined in Section 6 of [PBRSA] and Blind is
defined in Section 4.2 of [PBRSA]
The Client stores the nonce, challenge_digest, and extensions values
locally for use when finalizing the issuance protocol to produce a
token (as described in Section 6.3).
The Client then creates an ExtendedTokenRequest structured as
follows:
struct {
TokenRequest request;
Extensions extensions;
} ExtendedTokenRequest;
The contents of ExtendedTokenRequest.request are as defined in
Section 6 of [BASIC-PROTOCOL]. The contents of
ExtendedTokenRequest.extensions match the Client's configured
extensions value.
The Client then generates an HTTP POST request to send to the Issuer
Request URI, with the ExtendedTokenRequest as the content. The media
type for this request is "application/private-token-request". An
example request is shown below:
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:method = POST
:scheme = https
:authority = issuer.example.net
:path = /request
accept = application/private-token-response
cache-control = no-cache, no-store
content-type = application/private-token-request
content-length = <Length of ExtendedTokenRequest>
<Bytes containing the ExtendedTokenRequest>
6.2. Issuer-to-Client Response
Upon receipt of the request, the Issuer validates the following
conditions:
* The ExtendedTokenRequest.request contains a supported token_type.
* The ExtendedTokenRequest.request.truncated_token_key_id
corresponds to the truncated key ID of an Issuer Public Key.
* The ExtendedTokenRequest.request.blinded_msg is of the correct
size.
* The ExtendedTokenRequest.extensions value is permitted by the
Issuer's policy.
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.
Otherwise, if the Issuer is willing to produce a token token to the
Client for the provided extensions, the Issuer completes the issuance
flow by computing a blinded response as follows:
blind_sig = BlindSign(skI, ExtendedTokenRequest.request.blinded_msg, ExtendedTokenRequest.extensions)
Where BlindSign is defined in Section 4.3 of [PBRSA].
The result is encoded and transmitted to the client in a
TokenResponse structure as defined in Section 6 of [BASIC-PROTOCOL].
The Issuer generates an HTTP response with status code 200 whose
content consists of TokenResponse, with the content type set as
"application/private-token-response".
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:status = 200
content-type = application/private-token-response
content-length = <Length of TokenResponse>
<Bytes containing the TokenResponse>
6.3. Finalization
Upon receipt, the Client handles the response and, if successful,
processes the content as follows:
authenticator = Finalize(pkI, nonce, extensions, blind_sig, blind_inv)
Where Finalize function is defined in Section 4.4 of [PBRSA].
If this succeeds, the Client then constructs a Token as described in
[AUTHSCHEME] as follows:
struct {
uint16_t token_type = 0xDA7A; /* Type Partially Blind RSA (2048-bit) */
uint8_t nonce[32];
uint8_t challenge_digest[32];
uint8_t token_key_id[32];
uint8_t authenticator[Nk];
} Token;
The Token.nonce value is that which was sampled in Section 5.1 of
[BASIC-PROTOCOL]. If the Finalize function fails, the Client aborts
the protocol.
The Client will send this Token to Origins for redemption in the
"token" HTTP authentication parameter as specified in Section 2.2 of
[AUTHSCHEME]. The Client also supplies its extensions value as an
additional authentication parameter as specified in
[TOKEN-EXTENSION].
6.4. Token Verification
Verifying a Token requires checking that Token.authenticator is a
valid signature over the remainder of the token input with respect to
the corresponding Extensions value extensions using the Augmented
Issuer Public Key. This involves invoking the verification procedure
described in Section 4.5 of [PBRSA] using the following token_input
value as the input message, extensions as the input info (metadata),
the Issuer Public Key as the input public key, and the token
authenticator (Token.authenticator) as the signature.
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token_input = concat(0xDA7A, // Token type field is 2 bytes long
Token.nonce,
Token.challenge_digest,
Token.token_key_id)
6.5. Issuer Configuration
Issuers are configured with Private and Public Key pairs, each
denoted skI and pkI, respectively, used to produce tokens. Each key
pair SHALL be generated as as specified in FIPS 186-4 [DSS], where
the RSA modulus is 2048 bits in length. These key pairs MUST NOT be
reused in other protocols. Each key pair MUST comply with all
requirements as specified in Section 5.2 of [PBRSA].
The key identifier for a keypair (skI, pkI), denoted token_key_id, is
computed as SHA256(encoded_key), where encoded_key is a DER-encoded
SubjectPublicKeyInfo (SPKI) object carrying pkI. The SPKI object
MUST use the RSASSA-PSS OID [RFC5756], which specifies the hash
algorithm and salt size. The salt size MUST match the output size of
the hash function associated with the public key and token type.
Since Clients truncate token_key_id in each TokenRequest, Issuers
should ensure that the truncated form of new key IDs do not collide
with other truncated key IDs in rotation.
7. Security Considerations
By design, public metadata is known to both Client and Issuer. The
mechanism by which public metadata is made available to Client and
Issuer is out of scope for this document. The privacy considerations
in [ARCHITECTURE] offer a guide for determining what type of metadata
is appropriate to include, and in what circumstances.
Each metadata use case requires careful consideration to ensure it
does not regress the intended privacy properties of Privacy Pass. In
general, however, metadata is meant primarily for simplfiying Privacy
Pass deployments, and such simplifications require analysis so as to
not invalidate Client privacy. As an example of metadata that would
not regress privacy, consider the use case of metadata for
differentiating keys. It is currently possible for an Issuer to
assign a unique token key for each metadata value they support. This
design pattern yields an increase in keys and can therefore
complicate deployments. As an alternative, deployments can use one
of the issuance protocols in this document with a single issuance key
and different metadata values as the issuance public metadata.
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8. IANA Considerations
This document extends the token type registry defined in
Section 8.2.1 of [BASIC-PROTOCOL] with two new entries described in
the following sub-sections.
8.1. Privately Verifiable Token Type
The contents of this token type registry entry are as follows:
* Value: 0xDA7B
* Name: Partially Oblivious PRF, OPRF(P-384, SHA-384)
* Token Structure: As defined in Section 5.3
* TokenChallenge Structure: As defined in Section 2.1 of
[AUTHSCHEME]
* Publicly Verifiable: N
* Public Metadata: Y
* Private Metadata: N
* Nk: 48
* Nid: 32
* Notes: N/A
8.2. Publicly Verifiable Token Type
The contents of this token type registry entry are as follows:
* Value: 0xDA7A
* Name: Partially Blind RSA (2048-bit)
* Token Structure: As defined in Section 6.3
* TokenChallenge Structure: As defined in Section 2.1 of
[AUTHSCHEME]
* Publicly Verifiable: Y
* Public Metadata: Y
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* Private Metadata: N
* Nk: 256
* Nid: 32
* Notes: The RSAPBSSA-SHA384-PSS-Deterministic and RSAPBSSA-SHA384-
PSSZERO-Deterministic variants are supported; see Section 6 of
[PBRSA]
9. References
9.1. Normative References
[ARCHITECTURE]
Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
Pass Architecture", Work in Progress, Internet-Draft,
draft-ietf-privacypass-architecture-16, 25 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-architecture-16>.
[AUTHSCHEME]
Pauly, T., Valdez, S., and C. A. Wood, "The Privacy Pass
HTTP Authentication Scheme", Work in Progress, Internet-
Draft, draft-ietf-privacypass-auth-scheme-15, 23 October
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-auth-scheme-15>.
[BASIC-PROTOCOL]
Celi, S., Davidson, A., Valdez, S., and C. A. Wood,
"Privacy Pass Issuance Protocol", Work in Progress,
Internet-Draft, draft-ietf-privacypass-protocol-16, 3
October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-privacypass-protocol-16>.
[PBRSA] Amjad, G. A., Hendrickson, S., Wood, C. A., and K. W. L.
Yeo, "Partially Blind RSA Signatures", Work in Progress,
Internet-Draft, draft-amjad-cfrg-partially-blind-rsa-01, 6
July 2023, <https://datatracker.ietf.org/doc/html/draft-
amjad-cfrg-partially-blind-rsa-01>.
[POPRF] Davidson, A., Faz-Hernandez, A., Sullivan, N., and C. A.
Wood, "Oblivious Pseudorandom Functions (OPRFs) using
Prime-Order Groups", Work in Progress, Internet-Draft,
draft-irtf-cfrg-voprf-21, 21 February 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
voprf-21>.
Hendrickson & Wood Expires 26 May 2024 [Page 15]
Internet-Draft Public Metadata Issuance November 2023
[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>.
[RFC5756] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Updates for RSAES-OAEP and RSASSA-PSS Algorithm
Parameters", RFC 5756, DOI 10.17487/RFC5756, January 2010,
<https://www.rfc-editor.org/rfc/rfc5756>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/rfc/rfc8017>.
[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>.
[TOKEN-EXTENSION]
"The PrivateToken HTTP Authentication Scheme Extensions
Parameter", n.d., <https://chris-wood.github.io/draft-
wood-privacypass-extensible-token/draft-wood-privacypass-
extensible-token.html>.
9.2. Informative References
[DSS] "Digital Signature Standard (DSS)", National Institute of
Standards and Technology, DOI 10.6028/nist.fips.186-4,
July 2013, <https://doi.org/10.6028/nist.fips.186-4>.
Acknowledgments
This work benefited from input from Ghous Amjad and Kevin Yeo.
Authors' Addresses
Scott Hendrickson
Google
Email: scott@shendrickson.com
Christopher A. Wood
Cloudflare, Inc.
Email: caw@heapingbits.net
Hendrickson & Wood Expires 26 May 2024 [Page 16]
