Why identifier types matter
When you query data, you need to know what kind of identifier you’re working with:- A cookie represents a browser session on a specific domain
- A mobile ad ID represents a mobile device
- A hashed email represents a person across devices
- Match data across the right dimensions
- Estimate reach and overlap correctly
- Apply appropriate privacy controls
Identifier categories
Mobile Ad IDs (MAIDs)
Mobile advertising identifiers are device-level identifiers provided by mobile operating systems. They’re accessible through platform APIs and persist until the user resets them.| Platform | ID Type | Format |
|---|---|---|
| Apple iOS | idfa | Uppercase UUID: 918F1D4F-D195-4A8B-AF47-44683FE11DB9 |
| Google Android | adid (also called GAID) | Lowercase UUID: 3f097372-f01e-4b64-984c-395ae5828ee6 |
Mobile ad ID availability has decreased significantly due to privacy changes. Apple’s App Tracking Transparency (ATT) requires user opt-in, and many users decline.
Cookies
When data is collected in a web browser context (desktop or mobile web), the identifier is typically a cookie. Cookies are storage mechanisms tied to a specific domain—a cookie set by one website cannot be read by another. Because cookies are domain-specific, Narrative doesn’t have a single “cookie” identifier type. Instead, each data provider has their own cookie namespace. Cookie identifiers are useful for:- Retargeting within a publisher’s properties
- Frequency capping across a provider’s inventory
- Cross-device matching when linked to other identifiers
Hashed Emails
Email addresses are personally identifiable information (PII) and cannot be shared in raw form. To enable matching while preserving privacy, emails are hashed using one-way cryptographic functions before being shared.| ID Type | Hash Algorithm | Output Length |
|---|---|---|
md5_email | MD5 | 32 hex characters |
sha1_email | SHA-1 | 40 hex characters |
sha256_email | SHA-256 | 64 hex characters |
- Convert to lowercase before hashing
- Remove leading/trailing whitespace
- Apply the hash algorithm
- They persist across devices (people use the same email everywhere)
- They’re deterministic (same email always produces the same hash)
- They can’t be reversed to reveal the original email
Hashed Phone Numbers
Similar to emails, phone numbers are hashed to enable matching while protecting PII.| ID Type | Hash Algorithm | Format |
|---|---|---|
md5_phone | MD5 | E.164 format, then hash |
sha1_phone | SHA-1 | E.164 format, then hash |
sha256_phone | SHA-256 | E.164 format, then hash |
- Include country code
- Remove spaces, dashes, parentheses
- Example:
+14155551234
Universal IDs
Emerging universal identifier systems aim to provide privacy-preserving, cross-platform identity without relying on cookies or device IDs.| ID Type | Description |
|---|---|
uid2 | Unified ID 2.0 - generated from hashed emails, supports opt-out |
rampid | LiveRamp’s identity resolution identifier |
id5 | ID5’s probabilistic and deterministic identity solution |
Choosing the right identifier type
| Use Case | Recommended Identifier Types |
|---|---|
| Cross-device targeting | Hashed email, UID2 |
| Mobile app targeting | IDFA, GAID |
| Web retargeting | First-party cookies, hashed email |
| Offline-to-online matching | Hashed email, hashed phone |
| Privacy-first campaigns | UID2, first-party data |
Related content
ID Mapping
Connect identifiers across devices and channels
Mobile Ad IDs
Deep dive into IDFA and GAID
Data Pseudonymization
How hashing protects identifiers
Hashing PII for Upload
Step-by-step guide to formatting and hashing