The UID (Unique Identifier) is a built-in identifier used by many RFID cards to distinguish one card from another within a compatible system. However, the identifier format depends on the RFID technology and communication protocol.
This guide focuses on the 4-byte, 7-byte, and 10-byte UID formats commonly found in ISO/IEC 14443 Type A RFID cards, explaining how these UID types affect card compatibility, selection, and deployment considerations.
Common Types of UIDs
Here’s a breakdown of the common ISO/IEC 14443 Type A UID formats and what they mean for RFID card selection.
Single Size UID (4 byte)
The traditional 4-byte UID format provides a limited identifier space of approximately 3.7 billion possible values. Due to this limitation and the use of non-unique or reused identifiers in some implementations, a 4-byte UID should not always be treated as globally unique. Therefore the UID at this range is not unique in some cases, like NUID (FNUID and ONUID), RID. It is recommended to using Double Size UIDs (7-byte) to guarantee the uniqueness of RFID card.
Single Size NUID (4 byte)
A 4 byte NUID (Non-Unique Identifier) is an identifier which may be assigned to more than one RFID chip over the production time of a product so that more then one RFID card with the same identified may be deployed into one particular RFID system. The probability to have 2 PICCs on one PCD at the same time with the same NUID is still extremely low. The MIFARE Plus® card or MIFARE Classic® card with Single Size NUID can be activated like a usual Single Size UID card. There are two types NUID:
- Fixed but non-unique ID (FNUID): Some 4-byte UIDs with UID0 values in the 0xF0–0xFF range are fixed identifiers similar to unique UIDs. However, the same identifier may be used for multiple PICCs, so RFID card systems should not rely on this PICC identifier as globally unique.
- Re-used UID (ONUID): The very old Single Size UIDs will be re-used, which means the same UID might be used for several PICCs. Therefore that RFID card systems cannot rely on the uniqueness of such a PICC identifier.
The Random ID (RID, 4 byte)
A single size UID with UID0 = 0x08 indicates a Random Identifier. The Random ID (RID) is dynamically generated, when the PICC powers up. Deselecting a PICC does not reset the RID, but a field reset does.
Double Size UID (7 byte)
In ISO/IEC 14443 Type A cards, double size UIDs use a 7-byte identifier format and include manufacturer-related information in the UID structure. With the double size UIDs each manufacturer can theoretically use up to 2.8 * 1014 UIDs.
Triple size UID (10 byte)
Triple Size UIDs always contain a manufacturer code in the UID0. Triple size UID (10-byte UID) is defined in ISO/IEC 14443 Type A specifications, although it is less commonly encountered in commercial RFID cards. However it is mandatory that every PCD supports Triple size UIDs according to ISO/IEC 14443.
ISO/IEC 14443 Type A UID Format Comparison Table
| UID Type | Length | Identifier Characteristics | Compatibility Considerations | Common Applications |
|---|---|---|---|---|
| Single UID | 4-byte | Identifier behavior depends on the specific chip and product implementation. | Confirm whether the reader and application support this UID format. | Legacy systems, specific applications, and testing environments |
| Double UID | 7-byte | A common UID format used by many RFID products. | Verify chip specifications and compatibility with the deployed system. | General RFID identification applications |
| Triple UID | 10-byte | Available in certain RFID products with specific identifier formats. | Confirm product availability and end-to-end system support before selection. | Applications requiring this specific UID format |
UID Read/Write and Security Considerations
For many ISO/IEC 14443 Type A RFID cards, the UID is a chip-provided identifier that is typically read-only. Its format and behavior depend on the specific IC, including whether the card uses a fixed UID or another identifier mechanism such as a random identifier.
The UID helps a system identify or distinguish cards, but UID length or uniqueness alone does not provide security protection. Security depends on the chip’s authentication features, encryption capabilities, access control settings, and overall system design.
- Single Size UID (4-byte, including FNUID/ONUID/RID) may not be truly unique. Multiple cards can share the same UID, which can cause issues in large-scale or high-security deployments.
- Double Size UID (7-byte) embeds a manufacturer code and provides a vast number of unique IDs, ensuring reliable identification across enterprise systems.
Why it matters: Understanding the read-only nature and uniqueness of UIDs helps prevent misconfiguration. Enterprise systems should not attempt to modify UIDs, avoiding potential failures or security breaches. Choosing Double Size UID over Single Size UID is a practical decision for reliability and security.
UID vs Writable Data Blocks
While the UID itself cannot be modified, most RFID cards include writable data blocks where system information can be stored.
- Writable data blocks allow you to store access permissions, user data, or other application-specific information.
- Implication for decision-making: If your system requires dynamic updates—such as changing access rights or recording user activity—focus on the card’s data block capabilities, not the UID itself.
Key takeaway: UID provides identification, while data blocks handle application-specific information. Understanding this distinction is essential for system design and card selection.
Security and Selection Guidance
Choosing the right UID type is critical for reliability and security:
- Single Size UID: Read-only, may duplicate → suitable only for small-scale or test systems.
- Double Size UID: Provides a larger identifier space and is commonly used in modern RFID card deployments. Security-sensitive applications should also consider authentication features and system-level protection.
- Triple Size UID: Read-only, future-proof → useful for large-scale or advanced deployments, though rarely deployed today.
Practical advice:
- For secure, large-scale deployments, combine Double Size UID with writable data blocks for maximum security and flexibility.
- Single Size UID is acceptable only for small-scale systems, legacy setups, or testing environments.
How to Determine the Right UID
Choosing the right UID is the first step in selecting the appropriate RFID chip for your system. Here’s a clear, practical approach:
System Scale
- Small/testing systems → Single UID may suffice.
- Medium/large systems → Double UID recommended.
- Enterprise/future-proof systems → Triple UID for flexibility.
Security Requirements
- High-security (corporate access, hotels, sensitive asset tracking) → Double or Triple UID.
- Low-security/experimental setups → Single UID acceptable.
Compatibility Check
- Legacy readers → may only support Single UID.
- Modern HF systems → support Double and Triple UID.
- Confirm your chosen UID is compatible with both hardware and software.
Your UID decision directly influences which RFID chip you should choose. For guidance on matching the right UID type to your chip and system requirements, contact us today. Our experts can help ensure your RFID solution is secure, compatible, and future-proof.
FAQs
Q1: Can Single Size UID be used for hotel access control?
Technically yes, but it’s not recommended. Single Size UIDs may duplicate, compromising security. Double Size UID is preferred.
Q2: Are Triple Size UIDs supported by all readers?
Yes. ISO/IEC 14443-compliant readers must support Triple Size UID, ensuring future-proof compatibility.
Q3: Which UID format provides a larger identifier range?
7-byte UID formats provide a much larger identifier space than 4-byte formats in ISO/IEC 14443 Type A cards. However, UID length alone does not determine security or prevent unauthorized duplication.
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