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A GUID has substructure, which a GUID generator needs to be aware of. A protocol stack implementation that uses a GUID, unless the individual protocol specification explicitly states the contrary, SHOULD treat that GUID as an opaque single quantity to which only equality or inequality tests are applied. Such a protocol implementation SHOULD NOT make decisions based upon the substructure of the GUID.

[RFC4122] defines five versions of GUID and specifies how they are constructed. The known substructure is represented by the version and variant fields. The remaining fields (what that RFC calls time stamp and node) are what make the GUID unique.

In a version 1 GUID, these fields carry a value that is derived from the time and the computer’s network node address, respectively. A side effect of a version 1 GUID is that it identifies the machine on which it was generated (barring replacement or transfer of its network interface card).

In a version 4 GUID, these fields have been replaced by random bits. In generating a version 4 GUID, an implementation SHOULD use a Federal Information Processing Standard (FIPS)-approved pseudo-random number generator (PRNG) (as specified in [FIPS140-2] or later), but any superior source of random bits (such as a true hardware PRNG) MAY be used instead. For more information about entropy sources, see [RFC4086]. If a PRNG is used as the source of random bits, it takes a parameter called a seed. Although many experts in the field have studied, tested, and approved each FIPS-approved PRNG algorithm, no approved or nonapproved PRNG output is or can be more unique or less guessable than its seed.<9><10>

Any implementation of version 4 GUIDs MAY implement two types of GUID: one GUID for uniqueness only and another GUID for use as a nonce. These types are characterized as follows:

  • Uniqueness-only: This GUID is not kept secret and it is not required that it be unguessable. Therefore, the PRNG that is used to generate a version 4 GUID needs to be seeded only with values that have a high probability of being unique for that machine and that session. Typically, such a seed is formed by the cryptographic hash of uniqueness values, such as the CPU ID, MAC address, time of day, processor tick count, system process table, and system state.

  • Nonce or authenticator: This GUID must be kept secret to preserve its security value. The PRNG that is used to generate a version 4 GUID for this purpose needs to be seeded with values that have a high probability not only of being unique but also of being unguessable by an attacker. Typically, such a seed is formed by the cryptographic hash of unguessable values of high entropy, such as the output of a hardware True Random Bit Generator, the system state readable in kernel mode, the history of system state over a long runtime, the accumulated entropy from earlier operation of the system (retained in a place that the attacker cannot access), the time of arrival of hardware interrupts, or the history of mouse positions as it moves.

A secret GUID that is good enough for a nonce is also good for uniqueness. Any system that generates cryptographic keys needs a source of true random or pseudo-random bits that are unguessable enough for building those keys. Therefore, it is common practice for an implementer to provide only one type of version 4 GUID, which is a type that is good enough for a nonce; and to use cryptographic-quality random bits for generating that GUID—even when building a GUID that is used only for uniqueness.<11>

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