Probabilistic Reveal Tokens

Introduction

IP Protection aims to provide better protection against cross-site tracking in Incognito mode, however, it could also impact sites' ability to measure and prevent fraud if done without appropriate mitigations for this use case. Many websites rely on IP addresses to defend against fraudulent and spammy behavior, so they can focus on providing a safe experience to their users. To ensure that all businesses can continue to estimate the amount of fraud on their systems, train models to defend against fraud, and analyze emerging fraudulent behavior while still mitigating the ability to track users at scale using IP addresses, we propose to introduce a delayed IP sampling mechanism called Probabilistic Reveal Tokens (PRTs) alongside IP Protection. PRTs attempt to drastically reduce the availability of IP addresses to websites that present a high risk of tracking, while preserving the ability to estimate fraud and gain visibility into new attack patterns.

PRTs will be included on proxied requests in a new HTTP header added by the browser for domains that indicate they want to receive them via a signup process. Each PRT will contain a ciphertext, generated by an Issuer and re-randomized for unlinkability by the browser prior to the request, that the recipient can decrypt after a delay. Google will be the issuer for Chrome's implementation. A minority of the decrypted PRTs contain the client's pre-proxy IP address (i.e. non-masked, and as observed by the token issuer), while the remaining PRTs provide no information about the client's original IP address. This results in only a small percent of PRTs containing and revealing the user's IP.

Clients will be able to verify the reveal rate by decrypting the PRTs they receive using the private keys published by the key coordinator at the end of each epoch's delay period.

Goals

Enable proxied services to measure invalid traffic (IVT) in aggregate
Enable actionable insights, like understanding if a publisher or content provider has a particularly high rate of invalid traffic
Enable services to update publisher reputation indicators for their traffic, that may be applied on un-proxied traffic
Enable fraud and spam detection while minimizing IP reveal rate
Ensure that cross-site tracking is not re-enabled by this mechanism

Non-Goals

Provide a new mechanism for cross-site tracking
Provide insights for ad measurement beyond IVT detection
Provide IVT annotations on all masked traffic
Provide real time IVT detection
Provide a mechanism to reveal the IP on chosen requests

Use Cases

Many organizations use IP addresses as critical signals for identifying and mitigating fraud. This signal is used widely across industries and verticals, and is useful in preventing a variety of attacks like denial of service, account takeovers, spamming, payment fraud, etc.

For general fraud detection use cases, organizations can supplement their fraud detection pipelines by updating logs with the revealed IP addresses, and rerunning models or heuristics to generate fraud insights and reports. Although only a small percentage of requests will be annotated with an IP address, this sampled data can be used for manual analysis and help identify new threats.

The initial application of PRTs focuses on the use cases most likely to be impacted by IP Protection: domains embedded in a third-party context and on the Masked Domain List (MDL). Many companies on the MDL are in the business of serving ads, ads targeting, measuring ad effectiveness, or commerce related activities. These use cases may rely on detecting fraudulent ad activity through identification of invalid traffic, bots, or blocklisted IPs. PRTs offer the ability to detect ad fraud at scale while ensuring that IP Protection works effectively for users. Embedded third parties on the MDL, like advertisers, remain unable to observe the IP during a browsing session, and can detect and measure fraud after the fact using sampled data, all while continuing to help publishers monetize their sites and continuing to allow users to browse privately.

Proposed Solution

Probabilistic Reveal Tokens help organizations measure fraud and investigate new types of attacks while respecting users' privacy during IP-protected browsing. PRTs utilize re-randomizable ElGamal encryption to make tokens, and repeated uses of the same issued token, unlinkable.

Participants

Issuer - The issuer is an internet-facing service from which the browser fetches PRTs. Following the end of the epoch and some delay period, the issuer collects the secret key from the key coordinator and publishes it along with its HMAC key for token validation.
- At launch, Google will be the token issuer
Key Coordinator - The key coordinator generates the cryptographic keypair necessary for token encryption and decryption, and is responsible for keeping the secret key material secret while tokens are eligible to be spent. This may be implemented as part of the issuer.
Public Key Repository - Location where secret keys are published to by the issuer. Should keep a history of all previously published keys.
- Google's issuer will publish secret keys to GitHub
Browser - The browser fetches tokens from the issuer, re-randomizes them to prevent linkability, and sends the tokens to websites. After the key material is published, the browser provides tools to allow discerning users to validate the privacy properties of the tokens.
Websites - Receive tokens from the browser. After the key material is published, websites validate the legitimacy of the tokens and leverage sampled signals.

Probabilistic Reveal Token Generation

Each epoch will last for one day. Prior to the start of each epoch, the key coordinator will generate a new ElGamal keypair for encryption, and the issuer will generate a secret HMAC key necessary for token validation. The public key will be available at the start of the epoch, and the secret cryptographic key and HMAC key will be published some time after participants have rotated to a different set of keys. This delayed release of secrets controls when the PRT can be decrypted and analyzed. To avoid a thundering herd problem at epoch change, there will be a small overlap in the active epochs, avoiding all clients having to fetch new tokens at the same time.

When a client requests a batch of Probabilistic Reveal Tokens, the issuer will record the client's IP address from the inbound connection. Within a batch of tokens, some percentage (based on the reveal rate) will contain the client's observed pre-proxy IP address. The remaining tokens will contain a NULL value instead of the IP address. The issuer will then generate a message containing the chosen IP address or NULL, some low-entropy metadata including a token version, token ordinal, and an HMAC of the token contents. Finally, the issuer will encrypt each token using ElGamal encryption and return the batch of tokens to the client.

Sending Probabilistic Reveal Tokens

When the client creates a request that will be proxied, it checks whether the destination origin is in the registration list. If the destination is in the registration list, the client checks whether the top level site already has an associated PRT. If the client has already associated a PRT with the top level site and this PRT has not expired, the token's cipher text is re-randomized and attached to the request. If there is no PRT associated with the top level site or the PRT is expired, the client randomly picks a PRT from pre-fetched PRTs, re-randomizes it, and associates it with the top level origin.
The client includes re-randomized PRTs in the Sec-Probabilistic-Reveal-Token header. Values are Structured Header Byte Sequences and hold a TLS Presentation Language serialized PRTStruct defined below.

struct {
 uint8 version;
 /* size of u and e depends on version, size is 33 for version 1 */
 opaque u<0..2^16-1>;
 opaque e<0..2^16-1>;
 /* Used to identify the corresponding key/secrets to decrypt */
 opaque epoch_id[8];
} PRTStruct;

Sec-Probabilistic-Reveal-Token: :AQAhA0YcSOPXwN8JkGJz2Rxe349sEOzwLcXnrU0/e5P1QUEEACECjvPnzEReeDlIkrDocZA5ZtiIptiG02YOOaNMJKyKZTdIXbE63QJtYA==:

If there are no valid PRTs to attach when composing a proxied request, the client will make the proxied request without the header. A well-behaved client should only fail to attach a PRT in exceptional circumstances, e.g. Issuer unavailability. As the lack of an attached PRT will not prevent a request from being proxied, the volume of exceptional circumstances must be minimized to ensure that a website can treat any non-trivial amount of proxied traffic missing a PRT as suspicious.

Making use of Probabilistic Reveal Tokens

After each epoch's delay period, the issuer will publish the private key needed to decrypt the corresponding batch of PRTs and the HMAC key used to sign contents. Any interested party can decrypt the PRTs they received during the corresponding period and verify the reveal rate, ensuring that the issuer is behaving honestly.

For Google's implementation of the PRT issuer, we will publish keys to Github. We will update the explainer with the Github repository at launch.

When an origin receives a PRT, it will store the PRT until the corresponding private key and HMAC secret are published by the issuer. The origin can then decrypt the PRT to recover either the client's true IP address or a NULL IP address. Origins can verify the contents of the token via the HMAC, and ensure clients are, in aggregate, selecting them randomly from tokens that were issued to them. Significant deviations from a uniform distribution of ordinal values is one indicator of potential malicious client activity. The origin can use the revealed IP addresses as input to their fraud detection pipeline to estimate the amount of fraud associated with different publishers or other entities and create denylists to mitigate that fraud.

For more technical information on PRT tokens, please see here.

Requesting access

By default, PRTs are not included on proxied requests. Organizations on the MDL must sign up to receive the header as part of the proxied requests using a forthcoming form. Google will verify that the domain is on the MDL, but will otherwise not restrict any organization that signs up to receive the tokens.

We use registration to avoid sending PRTs automatically on every proxied request. The encryption scheme, although performant, uses client resources. As sites will need to make additional investments to log and analyse PRTs, sending them automatically would likely waste client resources.

Requiring sites to request tokens for each client, e.g. through an HTTP request header, also doesn't match the expected usage. A site that makes use of PRTs likely wants them on all proxied connections, and cannot receive them on un-proxied connections.

Tunable Parameters

Reveal rate

We are seeking feedback on what a reasonable starting reveal rate should be. The reveal rate can be modified over time, and a stated goal is to minimize the reveal rate without impacting sufficient IVT measurement. Given organizations likely will have a large portion of their traffic unmasked as non-IP-protected traffic, we may be able to start at a reasonably small reveal rate around 5-15%.The reveal rate must be high enough to enable reasonable detection of attacks occurring exclusively in masked traffic.

Epoch & Delay Period length

We expect the epoch and delay period lengths to begin at one day each, but are open to feedback on reasonable lengths of time. Please leave your feedback in this github issue to explain your rationale for the timing. The delay period should be long enough such that it is impractical to associate revealed tokens back to a current user session, but not so long in combination with the epoch length that it is difficult to reprocess events and compile fraud measurements.

Privacy & Security Considerations

Ciphertext Re-randomization

The ability to re-randomize a token's ciphertext, without changing the underlying contents (a property of the ElGamal encryption scheme used), underpins many of the security and privacy properties of PRTs. The issuer cannot link a re-randomized token's ciphertext back to any tokens it issued, and origins that receive re-randomized versions of the same token cannot link them together.

Before sending a token on a connection to a particular 1P/3P pair, the client re-randomizes the ciphertext for that token to make it unlinkable to any other usage of the token for other 1P/3P pair requests. The client will cache the cyphertexts for each 1P/3P pairing and reuse them on subsequent requests while the underlying token remains valid. Received tokens may be stored by origins in partitioned storage, and so more granular re-randomization (e.g. per request) is unnecessary.

As re-randomization is a non-trivial computational operation the client may, without loss of privacy or security, choose to pre-randomize tokens, ready for attaching to connections.

PRT affinity to Top-Frame Site During a Delay Period

A single issued PRT is assigned to a top-frame site during a user session to a maximum of the delay period length, and all proxied requests to the embeds on the site will have the same PRT. As the delay period is much longer than typical user sessions, effectively all embedded origins will either see the revealed IP or receive NULL (at roughly the reveal rate) for that session. A user session is defined by the lifetime of site accessible storage, which in Chrome will be the length of the Incognito session.

If the client were to choose independent PRTs for each proxied request and embedded domain, it would be possible for websites to improve their odds of getting a PRT with the user's original IP address by increasing the number of origins they have on a page, i.e., multiple requests with PRTs attached, or by partnering with other embedded domains and sharing their data.

Although every embedded origin will receive the same underlying token, the re-randomization of the token's ciphertext ensures that these tokens remain unlinkable outside the context of the page. Within the context of a page, origins may choose to share via postMessage() - but because of the 1P affinity, they do not gain any additional information by doing so.

Token Ordinal Metadata

Token ordinals prevent a malicious client from choosing one token and re-randomizing it an arbitrary number of times. This would enable an attacker to generate an unbounded set of tokens that all either reveal the IP, or NULL. The attacker will not know which property the set of tokens has, but being able to know that they all share the same property, would make some attacks more feasible.

To mitigate these types of attack, for each requested batch of tokens, the issuer will record the order of the token as the token's "ordinal" (i.e. the first token has ordinal 1, the second token ordinal 2, etc.) and then shuffle the tokens. This field does not convey information about the client requesting tokens, since each batch of tokens has the full set of ordinal IDs. An origin can keep the complete set of unique token ciphertexts it receives during an epoch (noting that ciphertexts are reused on requests for the same 1P/3P pair), and look at the total, and per top frame, ordinal value distribution.

Websites will expect a uniformly random distribution of token ordinal values across their users and spikes of specific token ordinals may be indicative of an attacker re-randomizing the same token across different sessions they control. Websites can then apply additional scrutiny to activities associated with these spikes in the distribution. We expect token batch sizes (and so the maximum ordinal) to initially be 100, which will enable websites that receive thousands of tokens to build meaningful distributions.

IP Address Mismatch

The IP address a client fetches tokens from, and so is seen to the issuer and probabilistically included in tokens, may be different from the IP address used to later connect to un-proxied origins, or the IP proxy itself. There are many reasons this could occur, from a user changing WiFi networks, to an ISP dynamically assigning a new IP. This drift in IP address may result in a PRT revealing an IP address that is unexpected from the user's perspective (e.g. the user's work vs. personal network IP address).

Browsers can reduce the chance of IP address mismatch by fetching tokens close to when they are spent, and closely aligned with user expectations. In Chrome, tokens will be fetched at the start of each new Incognito session.

Timing Attacks

Web APIs that rely on allocating resources to a specific site must take care not to introduce any timing attacks. Specifically it is critical to ensure that there is no observable timing difference between retrieving an already allocated PRT and allocating a new PRT. Implementations should make sure that the pathways for retrieving already assigned tokens and allocating new random tokens are indistinguishable timing wise. Similarly, implementations should ensure that new PRT batches are automatically requested sufficiently in advance of a new epoch to ensure that allocating a PRT is always a constant-time operation.

Browser-Specific Considerations

User Control

PRTs are only sent when IP Protection is enabled, and only on requests that IP protection causes to be proxied. When a user disables IP Protection this also disables PRTs. The ability to detect and act on fraud using our additional privacy protections is a key component of responsibly launching those protections.

Developer Tools & Token Inspection

Developers and users can view PRTs passed in the header for proxied requests, if passed, using the Network tab of Developer Tools.

Some advanced users may wish to record their own PRT activity for better transparency. When the IPPrivacyStoreProbabilisticRevealToken feature flag is enabled, Chrome stores all PRTs received in the IPPrivacy/ProbabilisticRevealToken/ directory. Chrome will store this to the disk even in incognito sessions. As this data reveals information about Incognito usage, exported data will be limited to the fetched tokens themselves, and not any information about where they were sent. This feature is disabled by default. We will release tools that make this analysis easier.

Have Feedback?

We welcome your feedback on this proposal. Please use the following link to provide your input in our GitHub repository:

https://github.com/explainers-by-googlers/prtoken-reference

Provide feedback

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