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moderation endorsement unlinkability privacy Moderation of unLinkable Endorsements (MoLE) is an architecture that lets a party performing access control (a Moderator) bootstrap trust in a client from a third party (an Anchor) that already has a trust relationship with that client, and then adjust that trust over time in response to the client's behaviour, for example by dynamically rate-limiting access. MoLE targets open deployments, in which independent parties may be responsible for access control and for vouching for clients, whilst maintaining strong privacy protections for clients. These protections are designed to hold even if participants in the ecosystem collude or otherwise misbehave. This document specifies the roles, the information flows between them, the privacy and security requirements, and deployment considerations. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction Traditional approaches to access control rely on long-term identifiers such as user IDs or cookies, which allow clients to be tracked as they interact with the system. Newer approaches like Privacy Pass enable access control without identification, but they provide no privacy-preserving way to bootstrap trust in a client and cannot adjust that trust dynamically over time. MoLE aims to address these gaps and enable privacy-preserving access control in an open ecosystem. This means supporting a deployment where clients are trying to access resources from multiple independent services, each performing independent access control and potentially relying on different third parties to vouch for the trustworthiness of clients. Building a system which can support both openness and privacy is a key requirement and differentiator of MoLE. MoLE has three distinct roles. Clients seek to access resources protected by Moderators, who set and enforce access control policy. A Moderator may have no direct trust relationship with a Client, but instead trusts Anchors, who do have a relationship with some Clients, to vouch for them. A given Moderator may trust multiple Anchors in order to cover more of its user base. An Anchor may vouch for Clients that a Moderator deems untrustworthy; the Moderator can mitigate this either by withdrawing trust in those specific Clients or by withdrawing trust in the Anchor. These roles interact through three distinct flows: Endorsement, Redeem & Issue, and Presentation. In Endorsement, an Anchor grants a Client an Endorsement, which conveys the Anchor's trust in the Client to other parties. In Redeem & Issue, a Client redeems an Endorsement at a Moderator and receives a Credential, allowing the Moderator to bootstrap its trust in the Client. In Presentation, the Client presents a Credential to a Moderator, allowing the Moderator to make an access control decision and to adjust its level of trust in the Client over time, including by dynamically rate-limiting access. These flows are designed to protect the Client's privacy. Redeeming an Endorsement does not reveal the granting Anchor, only that it is a member of the Moderator's Anchor Set, so it does not leak information about the Client such as which Anchor's policy the Client satisfies. In addition, a Client's interactions across Endorsement, Redeem & Issue and Presentation are unlinkable, preventing Anchors and Moderators from tracking Clients through their participation in the system. These privacy properties are designed to hold even when Anchors and Moderators collude. This document describes the requirements for these flows and how they interact, as well as the anticipated deployment model.

Use Cases MoLE is applicable to any system where Moderators wish to rate-limit access to resources without having a direct relationship with those clients, instead trusting third parties to vouch for trustworthy users. MoLE tolerates third parties making incorrect decisions, e.g. trusting a malicious client, and enables Moderators to respond dynamically to limit abuse. MoLE targets an open system like the web where a population of Moderators and Anchors make independent decisions about their access control policy, which Anchors they trust, and which users are Endorsed. This contrasts with a closed system where the parties participating in the system are known a priori and can be expected to coordinate their deployment configuration.

Reduced-Friction Challenges A user visits a website for the first time, with no cookies and possibly through a VPN, shared NAT, or privacy-preserving proxy that obscures network-layer identifiers. The site has no per-user history and limited reputational signal from the network path. The user then bears the friction if the site responds with measures intended to mitigate volumetric abuse --- repeated CAPTCHAs, silent rejection, or a degraded experience --- which also harm users trying to access the site legitimately. However, other sites may have access to relatively rich context about a user, e.g. because the user maintains an account, made a payment, or provided some other scarce signal to the site. MoLE enables such sites to act as Anchors which grant Endorsements to users, suitable for bootstrapping trust when users visit new sites for the first time. In this use case, the site acts as or works with a Moderator. The Client presents a Credential whose underlying Endorsement attests to a scarce property accepted under the Moderator's policy. The site combines this signal with its existing inputs to decide whether to admit, challenge, or reject the request. A Credential may be one input among several; it does not entitle the Client to admission. The Moderator can then go on to adjust the Client's Credential in response to the Client's behaviour, allowing access to be upgraded or removed over time.

User Agents Acting Under Delegation A user delegates some of their interaction with a site to an automated agent running in, or alongside, their browser. Sites that lack a richer signal commonly treat the appearance of automation as grounds for friction or denial of service, blocking delegated browsing as a side effect of resisting unwanted automation. Here the agent presents a Credential on the user's behalf, and the site admits the request based on the user's standing without the agent surfacing a stable identity or correlatable state. From the Credential alone, the Moderator and site cannot tell agent-driven presentations from the user's own; distinguishability via request content, timing, or rate is the user agent's responsibility, not the Credential's. When delegated behaviour violates policy, the Moderator may update state bound to the user's Credential, so the user bears the consequences for the agent's actions --- yet those actions remain unlinkable to other sessions and to the user's own, and the user's identity is not revealed to the site. Alternatively, delegated agents may rely on a distinct pool of Anchors and Credentials, letting sites and Moderators differentiate them from traditional user agents. The same architecture can serve automation not acting under delegation, such as autonomous crawlers. In all configurations, clients enjoy strong privacy protections from sites and Moderators, while sites and Moderators are protected from abusive clients mounting volumetric attacks.

Private Access Control A user visits a site repeatedly, without persistent cookies, expecting that successive visits are not linkable to one another. The site gates some functionality on a non-public criterion such as a paid subscription, group membership, or a per-period quota of allowed operations. In this use case, the Anchor's attestation conveys eligibility under such a criterion, and the Moderator translates that eligibility into a Credential under a policy that may include rate or quota state. The successive presentations are unlinkable to each other and to Redeem & Issue. This use case is a secondary goal. More elaborate authorization policies, including rich attribute-based access control and multi-factor eligibility combinations, are out of scope of this architecture document but may be included in companion documents.

Conventions and Definitions 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 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 access to resources controlled by a Moderator.

Anchor:

An entity that grants Endorsements to Clients based on a trust relationship.

Moderator:

An entity that consumes Endorsement redemptions and Credential presentations from Clients and issues Credentials according to an access policy.

Endorsement:

A cryptographic object granted by an Anchor to a Client.

Credential:

A cryptographic object issued by a Moderator to a Client. This Credential has an associated state that is updated during presentation.

Predicate:

A boolean test that a Moderator evaluates against a Credential's hidden state during a Presentation. The Moderator learns only whether the state satisfies the Predicate, not the state itself.

Endorsement Flow:

The flow in which an Anchor grants an Endorsement to a Client, based on the Anchor's trust relationship with that Client.

Redeem & Issue Flow:

The flow in which a Client redeems an Endorsement at a Moderator and, in return, obtains a Credential, without revealing which Anchor granted the Endorsement.

Presentation Flow:

The flow by which a Client proves possession of a Credential whose state satisfies a Predicate specified by the Moderator.

Policy:

Rules used by a Moderator to evaluate presentations.

Anchor Set:

The set of Anchors whose Endorsements a Moderator will accept during Redeem & Issue.

Anonymity Set:

The set of Clients among which a given Client is indistinguishable during an interaction. The strength of MoLE's unlinkability and Anchor-hiding properties depends on the size of this set.

Overview MoLE is composed of three distinct flows: Endorsement, in which a Client obtains an Endorsement signifying its trust relationship with an Anchor; Redeem & Issue, in which a Client redeems an Endorsement from an Anchor to obtain a Credential from a Moderator without revealing which Anchor granted it; and Presentation, in which a Client uses a Credential from a Moderator to prove that it is presently in good standing with the Moderator in order to access a resource.

MoLE Architecture Overview | | | | | +==== Endorsement ===>| | | | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | | | | |<~~~~~ Interaction ~~~~~>| | | | | +---- (If needed) -----------------. | | | | | | | +------ Endorsement ----->| | | | | | | | | |<----- Credential -------+ | | | | | | | `---------------------------------' | | | | +====== Presentation ====>| | | | ]]>

During Endorsement, a Client interacts with an Anchor with which it has a trust relationship. The nature of the trust relationship is specific to the Anchor and may be based on some kind of strong authentication, e.g. a login, or may be relatively weak, e.g. based on solving a CAPTCHA. The Anchor grants the Client one or more Endorsements to signify this trust relationship. Later, when trying to access a resource, a Client may be prompted by a Moderator to present a Credential. If the Client doesn't have a Credential for this Moderator it will then engage in the Redeem & Issue flow. In this flow, the Client redeems an Endorsement from the Moderator's Anchor Set and receives a Credential in return. Redeeming an Endorsement does not reveal which Anchor was used, only that it came from the Anchor Set. Once a Moderator has accepted a specific Endorsement, the Moderator can prevent a second use of that Endorsement in their system. This property prevents abusive Clients from constantly resetting their state to receive a fresh Credential from the Moderator in order to bypass rate limits. Finally, the Client can present the Credential to the Moderator, along with a credential update request. This presentation enables the Moderator to query the state of the Credential and receive a boolean value indicating whether the presented Credential meets the Moderator's chosen predicate. The Moderator can also update the state of the Credential, e.g. to increase or decrease access, but this process does not reveal any further information about the state of the Credential. Depending on the result of the applied predicate, the Moderator can make a decision about access control to the protected resource.

Privacy Goals and Threat Model MoLE deployments aim to provide three privacy properties for Clients:

1. Endorsement Grants and Redemptions are Unlinkable - During an interaction in which a Moderator issues a new Credential on the basis of an Endorsement obtained from an Anchor, neither the Moderator nor the Anchor can link the session in which the Endorsement was granted to the session in which it was redeemed.
2. Endorsement Redemptions are Anchor-hiding - During an interaction in which a Moderator issues a new Credential on the basis of an Endorsement from an Anchor, the Client hides the Anchor it obtained the Endorsement from among the Moderator's Anchor Set. Such Anchor-hiding makes an Endorsement redemption indistinguishable regardless of which Anchor in the Anchor Set the Client used, to both the Anchor and the Moderator.
3. Credential Issuances and Presentations are Unlinkable - If a Client engages in multiple presentations, then those presentations are unlinkable to each other and to the Redeem & Issue flow that produced the Credential.

A successful presentation tells the Moderator that the Client holds a Credential satisfying the Moderator's policy. It does not reveal the Client's identity, the underlying Anchor, or the state of the Credential. Collectively, these properties minimize the information Clients reveal to Anchors and Moderators beyond what is necessary to participate in the system. The strength of these unlinkability properties depends on anonymity-set size. For Credential Issuance and Presentation unlinkability, the relevant set is the population of Clients holding Credentials under the same Moderator policy. For Anchor-hiding, the relevant set is the Moderator's Anchor Set. A Moderator with only one Anchor in its Anchor Set provides no Anchor-hiding. Further considerations for maximizing the size of these sets are set out in the Privacy Considerations section. MoLE's privacy properties are intended to hold in the face of strong attackers who:

• May act in the role of Anchor, Moderator, or Client.
• May collude with other Anchors and Moderators in order to try to identify Clients.
• May have access to a cryptographically relevant quantum computer (CRQC).
• May deliberately deviate from the protocol and attempt to alter or forge messages.

Some attackers may exploit information not directly revealed by the protocol, for example: - Timing information - Network metadata like IP addresses - Implementation fingerprinting These side channels, which depend on details specific to each deployment, may compromise the privacy properties of MoLE and are discussed further in the Privacy Considerations.

Security Goals and Threat Model MoLE's security properties are intended for Moderators. Moderators can be assured:

• When they accept an Endorsement, that it corresponds to exactly one Endorsement granted by a trusted Anchor.
• When they accept a Credential under a given predicate, that the Credential is one they previously issued with a matching predicate. Once a Credential has been presented, it will not be accepted again unless the Moderator chooses to re-issue it.

Collectively, these two properties allow Moderators to dynamically rate-limit access to the population of Anchor-endorsed Clients. If Endorsements could be redeemed, or Credentials presented, multiple times, it would allow malicious Clients to obtain additional Credentials which could be used to bypass rate limits. MoLE's security properties are intended to hold in the face of coordinated attackers who: - Can control a number of Clients and deviate from the protocol. - Cannot violate the endorsement criteria of a Moderator's trusted Anchors but may control other Anchors. - Do not have access to a CRQC. MoLE considers a CRQC for its privacy properties like Anchor-hiding and unlinkability, because a CRQC deployed in the future could be used to analyse protocol transcripts recorded today and so identify Clients. By contrast, MoLE's security properties hold until an attacker gains access to a CRQC; once they do, only future protocol sessions are affected.

Flows

Endorsement

MoLE Endorsement | +--- EndorsementRequest -->| |<-- EndorsementResponse --+ EndorsementFinalization | | | ]]>

The Endorsement flow allows an Anchor to provide a Client with an Endorsement, signalling its trust in the client. Depending on the deployment, the Endorsement flow might be triggered by the Anchor pro-actively, or it may be requested by the Client according to need. An Anchor will endorse a client according to its own criteria for trust in the client. This may be because of some kind of strong authentication like a login, weak authentication like a CAPTCHA or any other mechanism that the Anchor deems suitable. Moderators will choose whether to trust a specific Anchor on the basis of its criteria for endorsing users. In order to be useful, Anchors will need to constrain how many times they will Endorse a given user. This is because Endorsements are valuable to Clients and Moderators insofar as they are scarce. If an Anchor gave out a very large number of Endorsements, this would reduce the effectiveness of rate limiting applied by Moderators. Anchors furnish each Endorsement with the necessary metadata to identify the Anchor that used it and for the Client to evaluate whether a given Moderator can consume that Endorsement. This metadata may enumerate specific Moderators (in a small deployment) or identify authentication material like a public key which can be used later to evaluate whether a given Moderator is authorized. The specific properties of the Endorsement vary based on the exact protocol used. Important properties are described below:

1. Public Verifiability: any party with knowledge of the Anchor can verify that the Endorsement is valid without needing access to secret key material held by the Anchor.
2. Blind Redemption: a Client redeeming the Endorsement does not reveal any information other than that the Endorsement is from one of a set of Anchors trusted by a Moderator.
3. Unlinkable Redemption: even if the Anchor colludes with another party at which the Endorsement is redeemed, they can't link the redemption to the session in which it was granted.
4. Expiry: A Client redeeming the Endorsement can prove that the Endorsement was granted within a particular time period.
5. One-Time-Use: A Moderator can prevent the redemption of the same Endorsement twice.
6. One-More Unforgability: A Client granted an Endorsement cannot forge an another valid Endorsement.

Redeem & Issue

MoLE Redeem & Issue | EndorsementRedemption | +--- EndorsementToken+CredentialRequest --->| |<---------- CredentialResponse ------------+ CredentialFinalization | | | ]]>

The Redeem & Issue flow allows an Anchor's trust in a Client to be communicated to a Moderator, enabling a new Credential to be issued with an initial amount of trust. Redeem & Issue is likely to happen on-demand in response to a Moderator's challenge for a Client to present a valid Credential. The challenge should contain enough information to identify the Moderator and the Anchors whose Endorsements it will trust for redemption. If the Client wishes to complete the presentation and has suitable Endorsements, it will begin the Redeem & Issue flow. In some circumstances, the Moderator may not require any Anchors for redemption, i.e. it has some local means of establishing trust in this case. In Redeem & Issue, the Client redeems an Endorsement, proving to the Moderator that it holds a suitable Endorsement from an Anchor trusted by the Moderator without revealing which specific Anchor was used. In order to translate this into a tangible privacy property for clients, the see the Privacy Considerations section for a discussion around anonymity sets and maximizing them. The redemption must be unlinkable to the grant of the Endorsement so that even if the Anchor and Moderator collude they can't link the two sessions. Further, the redemption of the Endorsement must ensure that even in the presence of a dishonest client, a Moderator will only issue a Credential once for a given Endorsement in order to mitigate attempts to bypass rate limits. The properties of the Credential issued by the Moderator will vary depending on the exact protocol used. However, they must support:

1. Unlinkable Presentation: A Client presenting a Credential reveals nothing beyond the result of the Moderator's predicate. Two successful presentations cannot be linked to one another, to the Redeem & Issue session that produced the Credential, or to any prior update, even if the Moderator records every transcript.
2. One-More Unforgeability: Even a dishonest Client cannot forge a Credential or derive additional valid Credentials from those it holds.
3. Rate Limiting: The Credential enables the Moderator to apply a rate limiting policy to the client which may be dynamically adjusted.

Presentation and Updates

MoLE Presentation | | | CredentialPresentation | +----- Request+CredentialToken ------------>| | | |<---- Response+CredentialResponse ---------+ CredentialFinalization | | | ]]>

Moderators trigger Presentations when they want to perform an access control check. The client will consult its stored Credentials and identify if it has a Credential for this Moderator. If it doesn't, it will consider triggering the Redeem & Issue flow described above. The Client must never present a Credential to a party other than the issuing Moderator to avoid spending the Credential unnecessarily. When a Moderator credential is presented, the Moderator learns whether or not it satisfies the Moderator's policy. The presentation must not be linkable to past updates, or to the Redeem & Issue flow that produced it. After the presentation, the Moderator may provide a new Credential to the Client, which might be based on fresh state or derived from the Credential that the Client presented. To satisfy privacy requirements, Clients are limited in the number of Presentations they can satisfy within a particular context. This is discussed further in the . Moderators may also Update Client's Credentials, typically as part of the Presentation flow. The Update may allow the Client to modify its credential, or provide an additional Credential. This process must satisfy similar requirements to Credential issuance for unlinkability. Provided that the Update process does not reveal information about the previous state of the Credential, then it may be performed multiple times within a context.

Anchor Feedback TODO: Discuss use of DAP / PPM / Prio in Endorsements which feeds into Credentials to measure per-anchor abuse rates.

Deployment Considerations

Anchor Selection and Policy Moderators seek a deployment where honest users are able to pass as many challenges as possible, whereas the number of presentations that malicious users can pass is minimized. The exact weighting between the risk of excluding honest users vs including malicious users is deployment specific. This guides how Moderator's should select suitable Anchors, seeking to cover their user base whilst minimizing the number of Endorsements that users hold for a given time period. Each Endorsment can be converted into an independent credential, so if a malicious user can perform X queries before their credential is revoked and they have E endorsements then their total access is X * E.

Discriminatory Treatment In general servers already have the ability to deploy access control mechanisms to protect resources under their control. MoLE does not change this but does provide a new privacy preserving access control mechanism. In , Privacy Pass Issuers were encouraged to support a diverse range of attesters in order to reduce the possibility of discriminatory treatment by token verifiers. However, arranging many attesters to agree to use a single issuer is challenging. It requires coordinating changes in the trusted attesters with each downstream verifier. If downstream verifiers have differing token value requirements, this creates friction which leads to partitioning by issuer which reduces client privacy. There is also little incentive for high value attesters to share fate with low value attesters. The use of Anchors in MoLE and support for multiple Anchors mitigates this issue. Rather than requiring Issuers to coordinate with their Verifiers as to Attestor policy, instead Anchors can individually issue according to their own policies and Moderators can choose which anchors to trust. Moving the decision on aggregation from Issuers to Verifiers resolves the tension inherent in Privacy Pass. Further, the use of Anchor-Blinding prevents this from being a privacy risk for Clients, since even if Anchors are very fine grained and specific to a particular attestation type, the Moderator does not learn which Anchor was used and so cannot discriminate on this basis. This relies on the Moderator's Anchor set being large and not being partitioned on something specific to the client, which is discussed further in the Privacy Considerations section.

 Centralization identifies Centralization as a major risk from Privacy Pass, in large part due to the motivation for sharing Issuers as described in the previous section. MoLE aims to avoid the same centralization risk through a number of mechanisms. The Anchor / Endorsement mechanism means that parties that have user relationships and parties that provide access control can be distinct parties without compromising on user privacy. Without this mechanism, only parties which access to a scarce resource could also be effective Moderators, which would encourage centralization. MoLE allows each Moderator to make an independent decision about which anchors to trust rather than requiring shared Issuers to be established and coordinated. The dynamic rate limiting supported by MoLE enables lower-accuracy Anchors to be used than could otherwise be supported. The Feedback Mechanism also encourages experimentation with new Anchors by providing Moderators with insights into Anchor quality. However, there are residual risks. Moderators inherently benefit from scale which provides more insight into Client behavior and means that decisions to promote or restrict access are consequently more impactful. Sharing a rate limit across more sites means the amount of volumetric abuse that a attacker can inflict becomes smaller.

Deployment in a Web Context In a Web setting, the MoLE architecture may be deployed in several different configurations. For example, the Moderator could be deployed in front of a site, mediating access to protected resources directly, or it could be deployed as an independent service which the site interacts with on the back-end, or the Moderator could be a third party service which communicates with the site through an information flow mediated by the user-agent. Many browsers limit the flow of information between distinct top-level origins, for example by partitioning cookies and other state. In a web context, MoLE endorsements and credentials may be used without partitioning to enable a limited form of cross-site information transfer. However, user-agents must employ suitable safeguards to ensure that the information flow is limited in line with the user-agent's privacy posture, specific recommendations are given in the Privacy Considerations.

Security Considerations

Key Compromise Anchor key compromise will enable an attacker to produce as many endorsement as they wish. For Moderators which trust this Anchor, attackers will be able to run the Redeem & Issue flow as many times as they wish. If any Moderator allocates some rate limited access to each initial Credential, this will enable the Attacker to bypass the rate limit. Moderator key compromise will enable an attack to produce as many credentials as they like with arbitrary state. However, this will impact only the Moderator and not any other parties.

 State Management MoLE requires all parties to maintain state in order to uphold its security and privacy properties. Anchors which grant Endorsements on the basis of some long term account will need to maintain state in order to record when a user has been granted an Endorsement (see ). Losing this state means over-granting, but not a privacy loss. Moderators require state in order to prevent the double-spend of Endorsements and Credentials. State should be bounded to uphold . Clients store their tokens and MUST burn them once redeemed or presented (see ). Ensuring that they do no replay state is their responsibility, as it puts them in control of the . If a Client is unsure that its state is valid SHOULD discard the affected tokens. This might affect their availability, but still uphold privacy and security.

DDOS Mitigations The server-side cost of Presentation flows needs to be as cheap as possible as malicious users who receive a challenge for a presentation may submit responses which will certainly fail but cost server-side resources to verify. The Redeem & Issue flow is particularly susceptible to DDOS as it is the only flow which is triggered by the client. Moderators should give consideration to suitable protection mechanisms for this endpoint, for example, using a client-side puzzle to ensure that clients spend at least as much CPU as it costs servers to reject a faulty Endorsement redemption. As the Endorsement flow is triggered by the Anchor in a context where it already trusts the user, it is less vulnerable to DDOS.

Privacy Considerations

Anonymity Sets and Minimum Thresholds Client anonymity sets depend on the total number of clients that share the same configuration metadata. During a Redeem & Issue flow, this corresponds to the number of other Clients that possess valid Endorsements for the specified anchor pool. During a Presentation flow, this corresponds to the number of other clients that possess valid Credentials for the specified Anchor pool. Client Vendors may act to limit the use of Anchors or Moderators where the total anonymity size is too low. For example, the Client Vendors may allow Endorsement flows to succeed but program Clients to flag the resulting Endorsement as unusable until a threshold is met across the fleet of clients. The same process may be applied to a Moderator's credential during Redeem & Issue, ensuring that a minimum size set is reached before the Client will present a Credential from a given Moderator.

 Configuration Consistency and Partitioning Attacks During Redeem & Issue, if Moderators can rotate their configuration material freely, they can enable the tracking of users, e.g. by restricting a user to a unique configuration which it can later detect. For this reason, Client Vendors should ensure that their clients receive consistent information from Anchors and Moderators. Deployments should make configuration material consistent across Clients and resistant to split views. A deployment can use a , , , or another mechanism which provides similar global consistency properties. It maybe that some deployments are comfortable with a weaker form of consistency where Clients manage configurations locally and limit how often they tolerate updates to it. This cannot entirely eliminate partitioning attacks but can make them less effective in practice.

Side Channels MoLE does not address all channels that can identify Clients. For example, side channels may reveal information about the user's Client, associated user-agent, hardware configuration, network address or similar information which can be used to cut the user's practical anonymity set. This type of side channel exists largely outside of the architecture's deployment and needs deployment-specific mitigations. Timing side channels may also reduce the practical anonymity set. For example, Redeem & Issue may be performed in response to a challenge for a Presentation by a Moderator. Based on the latency which the Client takes to answer the Presentation Challenge and can likely infer that the Client's presentation can be linked to just-issued Credential. Further, the Moderator can even control how long Redeem & Issue takes in order to actively cut the anonymity set. Clients can reduce their vulnerability to this type of attack by imposing a limit after which they will not attempt a presentation with a fresh credential after Redeem & Issue. Depending on the configuration of clients, other timing side channels may exist in the protocol. For example, if the Client is programmed to fetch Anchor endorsements on-demand (rather than pro-actively provided by an Anchor), a similar timing side channel exists between Endorsement and Redeem & Issue. This timing side channel is particularly severe as the Endorsement session may have context which identifies the user or their device. For this reason, deployments are discouraged from fetching Endorsements in direct response to a Redeem & Issue session. Particular consideration should be given to designing flows in which Moderators provide all the information that Clients need during Redeem & Issue and Presentation pro-actively, rather than requiring clients to interact with third parties, the Moderator or their Anchors in ways in which might lead to timing side channels.

Multiple Presentations in Concurrent Contexts Credentials can only be used a single time. Consider a malicious Moderator which begins a presentation with a Client in one context. From the moment the Client begins to present their credential, they must mark it as burned and cannot use it in another session. If the Moderator simultaneously issues a challenge for a presentation in a different session, the Moderator can conclude that the Client cannot also answer that presentation (assuming it has a single Credential). A similar pattern holds for Updates. A Credential being Updated cannot be Updated or Presented in any other session. In order to avoid this behavior being exploited for a timing side channel. E.g. an attacker holds an Update on one client and challenges for Updates in other sessions in order to link two sessions. The client must only never offer the same Credential in concurrent contexts. That is, once a Credential has been used in one context, it must remain locked to that context for a period of a time.

Multiple Presentations in the Same Context Every successful presentation of a Credential reveals at least one bit of information about a client. Answering Multiple presentations for the same or different moderators within one context can lead to greater inferences about the Client. For example, consider a Credential Instantiation where the Moderator assigns each user a unique integer in their credential, a Presentation operation which checks the integer is > 0 and an Update operation which reduces the integer by 1. If the Moderator can repeatedly issue Presentations and Updates, it can recover each user's unique integer in a given context, allowing their sessions to be linked. Consequently, the Clients should limit the number of presentations they'll make in any given context. A context is determined by the boundary of information flow. For example, in a web context, a context is governed not only by the top level origin but also carries on through navigations to different sites as bounce tracking or use of tracking URLs can enable information to cross to new pages.

IANA Considerations This document has no IANA actions.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document explains why the IAB believes that, when there is a conflict between the interests of end users of the Internet and other parties, IETF decisions should favor end users. It also explores how the IETF can more effectively achieve this. This document describes Oblivious HTTP, a protocol for forwarding encrypted HTTP messages. Oblivious HTTP allows a client to make multiple requests to an origin server without that server being able to link those requests to the client or to identify the requests as having come from the same client, while placing only limited trust in the nodes used to forward the messages. Pervasive monitoring is a technical attack that should be mitigated in the design of IETF protocols, where possible. Mozilla Apple Inc. Google LLC Cloudflare Apple Inc. This document describes the mirror protocol, an HTTP-based protocol for fetching mirrored HTTP resources. The primary use case for the mirror protocol is to support HTTP resource consistency checks in protocols that require clients have a consistent view of some protocol-specific resource (typically, a public key) for security or privacy reasons, including Privacy Pass and Oblivious HTTP. To that end, this document also describes how to use the mirror protocol to implement these consistency checks. This document defines the terminology and interaction patterns involved in the deployment of Key Transparency in a general secure group messaging infrastructure, and specifies the security properties that the protocol provides. It also gives more general, non- prescriptive guidance on how to securely apply Key Transparency to a number of common applications. Fraunhofer SIT Microsoft Research Microsoft Research ARM Traceability in supply chains is a growing security concern. While verifiable data structures have addressed specific issues, such as equivocation over digital certificates, they lack a universal architecture for all supply chains. This document defines such an architecture for single-issuer signed statement transparency. It ensures extensibility, interoperability between different transparency services, and compliance with various auditing procedures and regulatory requirements. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. This document specifies the Privacy Pass architecture and requirements for its constituent protocols used for authorization based on privacy-preserving authentication mechanisms. It describes the conceptual model of Privacy Pass and its protocols, its security and privacy goals, practical deployment models, and recommendations for each deployment model, to help ensure that the desired security and privacy goals are fulfilled.

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