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Globally Unique Identifiers (GUID) are a grouped string with a specific format which I assume has a security reason.

A GUID is most commonly written in text as a sequence of hexadecimal digits separated into five groups, such as:


Why aren't GUID/UUID strings just random bytes encoded using hexadecimal of X length?

This text notation contains the following fields, separated by hyphens:

| Hex digits | Description
| 8            | Data1
| 4            | Data2
| 4            | Data3
| 4            | Initial two bytes from Data4
| 12           | Remaining six bytes from Data4

There are also several versions of the UUID standards.

Version 4 UUIDs are generally internally stored as a raw array of 128 bits, and typically displayed in a format something like:


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migrated from Oct 14 '12 at 9:18

This question came from our site for software developers, mathematicians and others interested in cryptography.

No, it probably isn't for security reasons, the bitstring has the same entropy with or without the dashes. I would think it is so that GUID's can be recognized at a glance instead of going "here's a bunch of hex characters, is that md5.. or perhaps sha1.. no, wait, it could be..." and so on. Also, GUID's are usually not just random bytes. – Thomas Oct 14 '12 at 2:35
Similar Question from SO UUID format: 8-4-4-4-12 - Why? – Praveen Sep 12 '14 at 10:55

From RfC4122 – A Universally Unique IDentifier (UUID) URN Namespace

The formal definition of the UUID string representation is provided by the following ABNF:

UUID                   = time-low "-" time-mid "-"
                         time-high-and-version "-"
                         clock-seq-low "-" node

So, those are just the different fields from the original time and MAC-based UUID. The RFC says it originates from the Apollo Network Computing System.

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The text representation with the dashes is separating the four fields of the Guid/UUID into five groups (with the last field being separated itself after the first two bytes): Guid Text Encoding

The representation doesn't have anything to do with security, as there are different methods of computing it and is intended to be a unique identifier not necessarily a secure one.

The most likely reason the fields are split (even though the standard doesn't mention it) is for readability/separation of the component parts.

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That tells us what the format is, information that was already in the question. It doesn't explain why, which is what the OP was asking. – Keith Thompson Oct 14 '12 at 19:49
It is just separating them into the fields, likely for better readability and identification. Maybe the last one was split further because of its length. – Turnkey Oct 14 '12 at 20:06
logical. Same reason phone numbers, credit card numbers, and many other long numbers are frequently split up in groups when printed or written down. – jwenting Oct 15 '12 at 5:39

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