Unique Identifiers vs. ID Bridging: The Key Differences Every Publisher Should Know

As privacy rules tighten and third-party cookies are fading away ever more, advertisers are shifting their budgets to reach valuable audiences—those who can be targeted effectively through first-party data which publishers collect and share with them in an encrypted way. This shift means that identity solutions have become essential for publishers because they enable them to uphold and even increase their programmatic revenue while respecting user privacy in a cookie-free world.

When publishers look for ways to make their audiences more valuable to advertisers, terms like ID Bridging and Unique Identifiers often come up. The two concepts are sometimes getting mixed up though, leading to confusion about their distinct roles, effectiveness and especially their significance for the publisher-side of the programmatic advertising industry. However, understanding the differences between these concepts is crucial—especially for publishers aiming to maximize the value of their audience and still keep up with privacy standards.

In this article, we’ll break down the differences between these two concepts and explain why Unique Identifiers, especially those based on real data like hashed emails, are superior for both publishers and advertisers. 

What is ID Bridging? (Probabilistic)

ID Bridging is a process that attempts to “bridge” together different pieces of information about a user—such as device type, browser, or IP address—to identify them across different websites. The key thing to understand is that ID Bridging is probabilistic in about 99% of the cases, meaning it mostly relies on signals and assumptions rather than solid, direct data from the user. In other words, it’s making an educated guess about who the user might be.

For example, imagine a user in Oakland accesses Site 1 (e-commerce) using an iPhone 13 Pro on a Verizon network, from IP address X, at a specific time on a Monday morning and buys a product. Then, on Site 2 (content), a visit occurs with the same parameters—an iPhone 13 Pro, Verizon network, IP address X, and a similar time of day. Based on these similar data points, ID Bridging might assume this is the same user visiting both sites, even though it’s only relying on inferred signals.

In some cases, ID Bridging can incorporate deterministic data (we´ll explain what that is in the next paragraph)—such as a hashed email—as an anchor to improve accuracy. However, most of the time, ad tech vendors rely on probabilistic indicators alone, which can make ID Bridging less reliable. This is especially true as privacy regulations become stricter and limit the types of tracking data that can be used without the need to ask for additional consent from the site visitor. Without a dependable foundation of first-party data, ID Bridging becomes increasingly difficult to use effectively in today’s privacy-focused landscape.

What are Unique Identifiers? (Deterministic)

A Unique Identifier (UID) is a string of numbers or letters associated with a single user within a system. UIDs are part of identity solutions that help publishers and advertisers recognize users across different sites and devices with accuracy. Unlike the inferred matches used in ID Bridging, UIDs are based on deterministic data from direct user information, like hashed emails or phone numbers. This makes them way more reliable while still ensuring user privacy through different encryption algorithms.

For example, a user buys shoes from an online shop on their home computer. To complete the purchase, they enter their email address to receive an order confirmation. The email automatically gets added to the shops´ customer list. This customer list is then synced with platforms like Google Ads to run a display ad campaign aimed at previous buyers, allowing the shop to retarget them with ads for their latest best-selling shoes.

A week later, the same user visits a content site (here´s where the publisher comes into play), such as an online newspaper, on their phone. To view exclusive articles, they log in using the same email address. The site has ID solutions in place that encrypt the email and attach it to a UID, maintaining user privacy. This UID signals that the user is the same person who made a purchase on the e-commerce site.

When the online shop's retargeting display ad campaign is able to match this UID through its ad platform, it automatically places a higher bid to show the new shoe ad to this user, who is very likely to convert and make a purchase on the shop again. It is important to note that the ad platform isn´t able to see the user’s actual email or any personal details—only a signal that indicates this visitor matches the advertisers´ target audience. This fully automated, privacy-compliant process ensures that user data is protected while still allowing effective, targeted advertising in a cookie-less environment.

The value for publishers becomes clear: by implementing ID solutions like ezID that utilize UIDs based on hashed data, publishers can make their audience more valuable to advertisers without having to do any extra work. This results in higher bids and more revenue, as advertisers are willing to pay more to target verified, high-value users. The entire process is seamless and compliant with privacy standards, providing benefits to all parties involved.

It´s simply a win-win: users see relevant ads that enhance their experience, advertisers effectively reach their target audience, and publishers see increased revenue from premium bids.

Why Utilizing First-Party Data Through Unique Identifiers is Important for Publishers

Leveraging first-party data through Unique Identifiers (UIDs) gives publishers a clear advantage. Unlike probabilistic methods like ID Bridging, UIDs are based on verified, deterministic data, such as hashed emails. This allows publishers to offer advertisers more reliable and precise audience targeting, resulting in higher ad bids and increased programmatic revenue. UIDs also ensure compliance with privacy regulations, providing a seamless, secure way to maximize audience value without sacrificing user trust.

The best possible setup combines both UIDs and ID Bridging. While ID Bridging can help bring revenue back up to the levels seen with third-party cookies in some environments, UIDs take it further by enabling publishers to increase their CPMs well beyond that point. When a UID based on hashed data is present, advertisers place higher bids due to the certainty of targeting. When UIDs aren’t available because the user doesn´t log in or come from a newsletter, ID Bridging still enables advertisers to bid more effectively than when they have no data at all, allowing them to target users more generously in a cookie-less environment. End-to-end Identity solutions like ezID enable publishers to benefit from these innovations, making them competitive and well-positioned in an evolving digital landscape.

Conclusion: Clear Up the Confusion and Future-Proof Your Identity Strategy

While some still confuse the two concepts of ID Bridging and Unique Identifiers (UIDs), it’s essential to understand that ID Bridging relies on probabilistic data, making educated guesses based on user signals such as device type and IP address. In contrast, UIDs are built on deterministic data from verified sources like hashed emails, providing a precise, reliable way to recognize users across platforms.

UIDs offer significant advantages because they give advertisers greater certainty about the audience they’re targeting. This leads to higher ad bids, as advertisers value the assurance that their ads are reaching specific, high-value users who are more likely to engage or convert. The result is increased revenue for publishers who implement identity solutions that support UIDs based on hashed user data.

While ID Bridging can still play a role by supporting data needs when UIDs aren’t available, it’s clear that UIDs provide a more effective and valuable approach. Comprehensive identity solutions like ezID allow publishers to maximize their programmatic revenue while maintaining privacy compliance, positioning them for long-term success in a world without third-party cookies.