cuid generator

CUID, short for Collision-Resistant Unique Identifier, is a unique ID format originally created by Eric Elliott in 2012 to solve a persistent problem in distributed web applications: generating identifiers that are guaranteed not to collide, even when multiple clients or servers produce them simultaneously without any coordination. The original CUID library was designed primarily for the JavaScript ecosystem, targeting both browser and Node.js environments where developers needed a simple, drop-in replacement for auto-incrementing database keys or overly complex UUID implementations. CUIDs gained significant traction in the web development community thanks to their short, URL-friendly output and their emphasis on practical collision avoidance rather than strict cryptographic guarantees.

It is important to note that CUID is now considered a legacy format. The original library has been deprecated by its creator in favor of CUID2, which addresses several security and design shortcomings discovered over the years. While existing systems that rely on CUID continue to function correctly, new projects are strongly encouraged to adopt CUID2 or another modern alternative such as Nano ID. Understanding how the original CUID works remains valuable, however, both for maintaining legacy systems and for appreciating the design evolution that led to its successor.

How CUID Works

The original CUID generation algorithm combines multiple sources of entropy into a single string to minimize the probability of collisions across distributed systems. A typical CUID looks like cjld2cyuq0000t3rmniod1foy and is composed of several concatenated segments, each contributing a different dimension of uniqueness.

The first character is always the letter c, serving as a fixed prefix that identifies the string as a CUID and ensures the identifier always starts with a letter, which is useful for compatibility with HTML element IDs and CSS selectors.

Following the prefix is a timestamp segment derived from the current time in milliseconds. This timestamp is converted to a base-36 string, giving CUIDs a rough chronological ordering. Because the timestamp occupies the leading portion of the identifier (after the prefix), CUIDs generated at different times will naturally sort in approximate creation order, though this ordering is not as precise as formats specifically engineered for lexicographic sorting.

Next comes a sequential counter that increments with each ID generated within the same process. This counter is critical for preventing collisions when multiple CUIDs are generated in rapid succession within the same millisecond. The counter resets when the process restarts, but because it is combined with other entropy sources, this reset does not compromise uniqueness.

The algorithm then appends a client fingerprint, a value derived from characteristics of the host machine or browser environment. In Node.js, this fingerprint is typically computed from the process ID and the hostname. In browser environments, it is derived from properties of the navigator object and other accessible client attributes. This fingerprint ensures that two different machines generating CUIDs at the exact same millisecond with the same counter value will still produce different identifiers.

Finally, two blocks of random characters are appended, each generated using Math.random(). These random segments provide an additional layer of collision resistance, acting as a safety net against the unlikely scenario where timestamp, counter, and fingerprint all align across two separate generators.

The resulting CUID is a lowercase alphanumeric string of approximately 25 characters. While this composite approach was effective for most practical scenarios, the reliance on Math.random() for the random segments became a point of criticism. Math.random() is not cryptographically secure, meaning that in adversarial contexts, an attacker could potentially predict or influence generated CUIDs. Additionally, the client fingerprint component raised privacy concerns because it could theoretically leak information about the host environment. These shortcomings were central motivations behind the development of CUID2, which replaced the concatenation approach with a hash-based design that eliminates fingerprinting entirely and uses cryptographically secure random number generation.

Use Cases

Legacy web application identifiers. Many web applications built between 2012 and 2022 adopted CUID as their primary identifier format for database records, user sessions, and API resources. These systems continue to operate with CUID-based identifiers, and developers maintaining such applications need to understand the format for debugging, data migration, and interoperability tasks. In these contexts, CUIDs remain perfectly functional and do not require immediate replacement.

Client-side ID generation in offline-first applications. One of CUID's original selling points was its ability to generate unique identifiers directly in the browser, without requiring a round-trip to a server. This made it popular for offline-first applications and progressive web apps where data records are created on the client and synchronized with a server later. The combination of timestamp, counter, fingerprint, and randomness provided sufficient collision resistance for most consumer-scale applications operating under this pattern.

Prototyping and development environments. For developers building prototypes, internal tools, or applications where adversarial collision attacks are not a concern, the original CUID offers a simple and lightweight solution. Its output is short, human-readable, and easy to work with in URLs, log files, and debugging sessions. The fixed c prefix also makes CUIDs easy to visually identify in mixed-format environments.

Distributed microservice architectures. Before the widespread adoption of newer time-sortable formats, CUID served as a practical identifier for distributed systems where multiple services needed to independently generate unique keys. The fingerprint component provided an extra layer of safety for multi-node deployments, ensuring that even services with synchronized clocks and similar counter states would produce distinct identifiers.

Comparison with Alternatives

Compared to CUID2, the original CUID has a fundamentally different internal architecture. CUID concatenates its entropy sources into a readable structure where individual components (timestamp, counter, fingerprint, random) can be visually identified and extracted. CUID2, by contrast, passes all entropy through a hash function, producing an opaque output that reveals nothing about the input data. This makes CUID2 more secure against information leakage and prediction attacks, which is why it officially replaced the original. For any new project, CUID2 is the recommended choice.

Compared to Nano ID, CUID offers rough time-based ordering due to its embedded timestamp, which Nano ID does not provide. However, Nano ID is significantly smaller in bundle size, uses the Web Crypto API for cryptographically secure randomness by default, and offers full customization of alphabet and length. For applications that need the shortest possible identifier with strong randomness and no sorting requirement, Nano ID is typically the better option.

Compared to UUID v4, CUID produces shorter, URL-safe strings without hyphens, making it more convenient for use in URLs and HTML attributes. Both formats rely on randomness for uniqueness, but UUID v4 uses 122 bits of random data from a cryptographically secure source (in most implementations), while the original CUID uses Math.random(), which provides weaker guarantees. UUID v4 benefits from universal tooling and standardization across every programming language and platform, whereas CUID was primarily a JavaScript-ecosystem solution.