ID5 is a London-based company that issues a shared user identifier, the ID5 ID, and operates an identity graph linking it to cookies, mobile advertising IDs and connected television IDs. It exists because the third-party cookie, the mechanism the open web used for two decades to recognise the same person across unrelated sites, stopped working reliably. Safari and Firefox blocked third-party cookies years before Chrome debated it, and mobile applications and connected television never had them at all. Without a replacement, a publisher selling an impression in Safari can tell a buyer almost nothing about who is on the other side of the screen, and the impression sells for less. The ID5 ID is what the industry calls an alternative or universal ID: a single pseudonymous string, issued by a neutral third party, that any participating publisher, exchange or demand-side platform (DSP) can read and transact against.

How the identifier is created and distributed

Most publishers pick up the ID5 ID through Prebid.js, the open-source header bidding wrapper. The submodule is id5IdSystem, added at build time alongside the generic userId module. The only required field is params.partner, a numeric partner number issued on registration; Prebid documentation uses 173 as its placeholder. Storage is constrained: type must be html5, meaning local storage rather than a cookie, and the key must be id5id.

Freshness matters more than that brevity suggests. According to Prebid documentation, the identifier delivered into the auction is encrypted by ID5 with a rotating key, both to block unauthorised use and to carry current privacy signals, and publishers are advised to set refreshInSeconds to 7200 seconds or lower. An identifier encrypted under a stale key may not be transactable downstream. Storage expiry is typically 90 days.

Once retrieved, the identifier enters the bid request as an extended identifier. The EID source string is id5-sync.com, with atype 1 denoting a device-level identifier and an ext object carrying linkType, an integer describing how the identifier was resolved, and abTestingControlGroup, a boolean. Where the user has not consented or has opted out, ID5 returns 0 rather than omitting the field. Two further sources may appear: gpid.id5-sync.com, carrying the Guarded Publisher ID, and uidapi.com, meaning ID5 is distributing The Trade Desk's European identifier for that publisher.

The A/B testing parameter is unusual enough to note. Publishers set abTesting.controlGroupPct between 0.0 and 1.0, and users in the control group receive no ID5 ID in the auction while still being counted. Assignment is user-based rather than request-based, so a browser stays in or out across page views. An analytics adapter compares bid CPM and response rates across those groups, letting a publisher price the identifier rather than take the vendor's word for its value.

Signals, matching and the graph

Deterministic signals arrive through the optional pd parameter, a URL-safe base64 string of hashed emails, hashed phone numbers, IP address and user agent, which ID5 documentation requires publishers to normalise before hashing so the same person hashed by two partners produces the same value. Probabilistic resolution draws on IP address, user agent and browsing patterns.

According to ID5, deterministic signals reconcile users across domains and devices and then act as training data for machine learning models that resolve identity probabilistically, with probabilistic linkages used in turn to validate deterministic matches. The company brands the current version Adaptive Identity, introduced in June 2025.

A newer client-side method, TrueLink, is supported in Prebid from version 9.2.0. According to ID5 documentation, it redirects a consented user through an ID5-operated domain, writes an identifier there as a first-party cookie, and returns the user to the page in milliseconds, producing a cross-domain signal that does not depend on third-party cookies at all.

Origin and evolution

ID5 Technology Limited was incorporated in April 2017. Its own account names Mathieu Roche, previously at the French data company Weborama, and Pierre-Antoine Durgeat as co-founders; Crunchbase lists a third, Scott Menzer, and sources disagree on Durgeat's title.

The company spent its early years arguing a contested position: that login-based identifiers alone would not scale. In November 2020, Roche told AdExchanger that authentication was excellent for accuracy but that the market expected identification to work for users who were not logged in.

Google took the opposite view publicly. On 3 March 2021, David Temkin, then director of product management for ads privacy and trust, wrote that once third-party cookies were phased out Google would not build alternate identifiers to track individuals across the web, nor use them in its products, naming email-based graphs as an approach the company did not expect to survive regulatory pressure. That removed the largest buyer in the market from direct support of the category.

Interoperability came anyway. On 19 June 2023, The Trade Desk added ID5's graph to Identity Alliance, its coalition of identity graph providers, and enabled ID5-integrated publishers to generate European Unified ID (EUID) values for authenticated traffic. In April 2024 ID5 raised $20 million in Series B funding led by TransUnion and Sir Martin Sorrell's S4S Ventures, taking total funding to roughly $27 million. In November 2025 it acquired the US identity graph company TrueData, its first acquisition, lifting headcount to around 80. Roche told AdExchanger he expected a 30 to 40 percent incremental revenue increase and that the combined graphs recognised roughly 1.5 billion users across 665 million households.

Why it matters to media buyers and publishers

For a buyer, the practical question is what share of inventory carries the identifier and what it does to outcomes. ID5 cites Sincera, an independent crawler, putting its deployment near 100,000 sites in January 2025, 1.6 times the next most adopted identifier. Those remain vendor-selected citations rather than audited figures.

Distribution runs through most of the supply chain. PubMatic included ID5 among the partner identifiers in its Identity Hub built on the Prebid User ID module. TripleLift connected its 1,300-segment audience product to ID5's graph in January 2024, and Adobe Advertising integrated the identifier that March. Expansion followed into audio through Triton Digital's marketplace, gaming through a partnership with Anzu, and connected television through an arrangement with Future Today covering 1.5 billion impressions in June 2025.

Performance claims from those integrations are supplier-reported and uncontrolled. OpenX and ID5 reported a 58 percent reach increase on Safari desktop and 37 percent on mobile web against cookie-based solutions in December 2024. An Australian test with Audience360 and Adform reported 42 percent of budget delivered in cookieless browsers, 24 percent higher viewability and 40 percent higher click-through rate. Each figure comes from a party that benefits from it.

Limitations, criticisms and open disputes

The core criticism is accuracy, and probabilistic resolution was measured repeatedly through 2026 with poor results. A study commissioned by the Coalition for Innovative Media Measurement and Go Addressable, published on 5 November 2025, put IP-to-postal linkage accuracy at 13 percent and IP-to-email at 16 percent across nearly a billion records. Adstra and InterMedia Advertising research reported on 15 July 2026 found only 23 percent of residential IP addresses reached their intended geographic target. LiveRamp and the Mobile Marketing Association found identity imprecision cutting campaign return on investment by around 70 percent in simulation. None tested the ID5 ID specifically, and all came from parties selling identity or validation services, but they bear on any system resolving identity from IP address and device signals.

The second is regulatory. Probabilistic device recognition sits close to fingerprinting, which the UK Information Commissioner's Office described in December 2024 as a technique users cannot clear or control the way they can clear cookies. ID5 argues its signals are consented and its identifier encrypted so only permissioned vendors can decrypt it, but that rests on consent capture at the publisher, which the identity provider does not control.

Third, the economics are set by buyers. European publishers have historically resisted identifiers operated outside their control, ID5 included, and The Trade Desk restructured Identity Alliance payouts in early 2026 to compensate partners on incremental value rather than data volume, a change Roche called directionally sensible as DSPs tightened their grip on identity economics.

Disambiguation

ID5 ID and ID5 Graph are different products. The ID is a single string passed in the bid stream. The Graph is a mapping between that ID and platform cookies, MAIDs and CTV IDs, delivered offline or as real-time bid enrichment.

ID5 ID and UID2 or EUID. Unified ID 2.0 and its European variant are seeded from hashed, salted email addresses and require authentication. The ID5 ID needs no login and can be issued for unauthenticated traffic, which is why the two are complementary and why ID5 distributes EUID for its publishers.

ID5 ID and Guarded Publisher ID. The GPID derives from the ID5 ID but is scoped to a single media owner and carries no cross-publisher reconciliation. It can be passed to Google Ad Manager as a publisher provided identifier.

ID5 ID and RampID. LiveRamp's identifier is people-based and anchored in offline data. ID5's is device-level by default, with individual and household resolution through the graph.

Recent developments

The identity market consolidated sharply through 2026. Publicis acquired LiveRamp in May 2026, narrowing the field of independent providers and sharpening ID5's neutrality claim that it transacts in neither media nor data. The company has since pushed toward machine-readable environments, publishing on identity in agentic contexts as large language models intercept traffic that previously reached publisher pages. In June 2026 it added support for the IAB Tech Lab's ID provenance fields, exposing a matchmethod field stating how each link between an ID5 ID and a platform cookie was established. That answers the accuracy criticism directly: not a claim of better matching, but a disclosure of which matches are deterministic and which are not.

Timeline

  • April 2017: ID5 Technology Limited incorporated in London
  • March 2018: first funding round closes
  • 3 March 2021: Google states it will not build alternate identifiers to track individuals across the web
  • 19 June 2023: The Trade Desk adds ID5's graph to Identity Alliance and enables EUID distribution through ID5 publishers
  • 9 April 2024: ID5 announces $20 million Series B led by TransUnion and S4S Ventures
  • 11 December 2024: OpenX and ID5 report reach gains in Safari and mobile web
  • January 2025: Sincera data cited by ID5 puts deployment near 100,000 sites
  • 6 February 2025: Piemme becomes the first Italian media platform to run the combined ID5 and EUID integration
  • June 2025: ID5 rebrands its technology as Adaptive Identity
  • 29 September 2025: 2025 State of Digital Identity report published, based on 250 respondents
  • 5 November 2025: CIMM and Go Addressable publish accuracy research on IP-based linkage
  • 4 November 2025: ID5 announces the acquisition of TrueData
  • Early 2026: The Trade Desk revises Identity Alliance compensation toward incremental value
  • June 2026: ID5 documents support for IAB Tech Lab ID provenance fields

Summary

Who: ID5 Technology Limited, an independent identity provider founded in 2017 and led by co-founder and chief executive Mathieu Roche, serving publishers, supply-side platforms, demand-side platforms, data companies and measurement vendors.

What: The ID5 ID, an encrypted pseudonymous identifier passed in OpenRTB bid requests under the EID source id5-sync.com, and the ID5 Graph, which links that identifier to cookies, mobile advertising IDs and connected television IDs at device, individual and household level. Related products include the Guarded Publisher ID, TrueLink and real-time bid enrichment.

When: Incorporated April 2017. The Trade Desk integration June 2023, Series B April 2024, Adaptive Identity rebrand June 2025, TrueData acquisition November 2025, ID provenance support June 2026.

Where: Headquartered in London with operations in New York and Paris, and staff across North America, Europe and Asia following the TrueData deal. Deployment is concentrated on the open web through Prebid, with growing presence in audio, gaming and connected television. Adoption is strongest in Europe, where the company's GDPR posture is its principal differentiator, though the majority of clients and revenue come from the United States.

Why: Third-party cookies never worked in Safari, Firefox, mobile applications or connected television, and impressions without a recognisable user sell at a discount. A neutral identifier that any participant can read restores some of that addressability. The unresolved question is accuracy: independent 2026 research has found probabilistic identity linkages correct far less often than buyers assume, and the identity industry's response has shifted from claiming higher match rates to disclosing how each match was made.