Globally Unique Identifier

A globally unique identifier or GUID (pronounced /ˈɡuːɪd/ or /ˈɡwɪd/) is a special type of identifier used in software applications to provide a reference number which is unique in any context (hence, "globally"), for example, in defining the internal reference for a type of access point in a software application, or for creating unique keys in a database. While each generated GUID is not guaranteed to be unique, the total number of unique keys (2¹²⁸ or 3.4×10³⁸) is so large that the probability of the same number being generated twice is extremely small.

The term GUID usually refers to Microsoft's implementation of the Universally Unique Identifier (UUID) standard. However, the term is common in applications not written by Microsoft, or for their operating systems. The GUID is also the basis of the GUID Partition Table, Intel's replacement for Master Boot Records under EFI, and of Preboot Execution Environment, Intel's environment to boot computers using a network interface.

Algorithm

The OSF-specified algorithm for generating new GUIDs has been widely criticized.^{[citation needed]} In these (V1) GUIDs, the user's network card MAC address is used as a base for the last group of GUID digits, which means, for example, that a document can be tracked back to the computer that created it. This privacy hole was used when locating the creator of the Melissa worm^[1]. Most of the other digits are based on the time while generating the GUID.

V1 GUIDs which contain a MAC address and time can be identified by the digit "1" in the first position of the third group of digits, for example {2f1e4fc0-81fd-11da-9156-00036a0f876a}.

V4 GUIDs use the later algorithm, which is a pseudo-random number. These have a "4" in the same position, for example {38a52be4-9352-453e-af97-5c3b448652f0}. More specifically, the 'data3' bit pattern would be 0001xxxxxxxxxxxx in the first case, and 0100xxxxxxxxxxxx in the second. Cryptanalysis of the WinAPI GUID generator shows that, since the sequence of V4 GUIDs is pseudo-random, given the initial state one can predict up to the next 250 000 GUIDs returned by the function UuidCreate^[2]. This is why GUIDs should not be used in cryptography, e.g., as random keys.

References

External links

• CLSID Registry Key Information
• International Standard "Generation and registration of Universally Unique Identifiers (UUIDs) and their use as ASN.1 Object Identifier components" (ITU-T Rec. X.667 | ISO/IEC 9834-8, technically compatible with IETF RFC 4122)
• DmaId for InstanceId Values (DCE Universally Unique IDentifiers, UUIDs)
• Syntax and semantics of the DCE variant of Universal Unique Identifiers (UUIDs)
• Generate and parse UUIDs (or GUIDs) in Java
• UUID Generator on the ITU-T website
• Creating GodMode Folders In Windows 7 & Vista