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2.5.5.3 Quality of Random Bits

Each use of a version 4 GUID has a measure of quality:

  • Uniqueness: The probability that some other system will happen to generate the same GUID (without any conscious attempt by an attacker to create that collision).

  • Nonce: The probability that some conscious attacker will be able to guess the generated GUID.

An ideal GUID that uses N bits of randomness collides with some chosen value by accident (violating uniqueness) with a probability proportional to 2-N and is guessable by an attacker with work proportional to 2N. This quality of randomness is measured by entropy. The ideal case that is described earlier is said to represent N bits of entropy.

If N allegedly random bits actually contain M<N bits of entropy, the probability of accidental collision is proportional to 2-M and the work for an attacker is proportional to 2M. This leads, in the case of GUIDs for uniqueness, to a higher-than-ideal probability of accidental collision. If such a collision occurs, the two different identified objects will have the same ID, possibly leading to confusion. In the case of GUIDs for use as nonces, the lower work by the attacker might result in a successful replay attack.

Neither of these flaws, if one occurs, changes any protocol that uses such a GUID. It might change the security claims that such a protocol might make, but not the state machine, packet sequence, or packet contents of the protocol. The same applies to GUIDs that are used for uniqueness. If two GUIDs that were supposed to be different are accidentally the same, then the protocol implementation is not changed, only the probability that a mistaken identity might occur.

Therefore, the implementer of a protocol is not required by the protocol specification to guarantee any quality of random bits. Nothing in the specification of any Microsoft Communications Protocol Program (MCPP) protocol directs a conformant implementation to look for, much less detect, any use of low-quality random bits in GUID generation. This means that even a very low-entropy random bit stream can be used to generate GUIDs that will allow a protocol to interoperate and be indistinguishable from any other protocol implementation, no matter what the quality of random bits in that other implementation.

Of course, implementers who prefer to minimize the confusion that would result from nonunique GUIDs or the replay attacks that would result from guessable nonces are well advised to use the best-quality random bit sources that they can find.

 
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