The NIST SP 800-90 series is easier to understand when you stop thinking of it as one random-number-generator specification.
It separates random-bit generation into related layers:
- SP 800-90A Rev. 1 specifies deterministic random bit generator mechanisms.[1]
- SP 800-90B addresses entropy sources.[2]
- SP 800-90C specifies random bit generator constructions that bring those components together.[3]
For teams evaluating hardware entropy or QRNG, this separation is useful because the physical entropy source is only one part of the complete architecture.
In short
A simplified mental model is:
SP 800-90B entropy source → SP 800-90A DRBG mechanism → SP 800-90C RBG construction
That diagram is conceptual rather than a literal implementation recipe, but it captures the division of responsibilities.
SP 800-90A Rev. 1: deterministic generation
SP 800-90A Rev. 1 specifies mechanisms for generating random bits using deterministic methods based on approved cryptographic primitives.[1]
A deterministic random bit generator, or DRBG, maintains internal state and uses cryptographic operations to generate output. Because the mechanism is deterministic, its security depends on factors that include the entropy used to initialize or refresh its state, correct implementation, and the security assumptions of the mechanism.
NIST lists SP 800-90A Rev. 1 as the current final publication, dated June 2015. NIST also began pre-draft work toward Revision 2 in 2025, but that work does not replace Rev. 1 as the final publication.[5]
For a QRNG discussion, the key point is simple: a DRBG can expand secure state efficiently, but it still needs appropriate entropy somewhere in the system.
SP 800-90B: entropy sources
SP 800-90B addresses the source of unpredictability.
It specifies design principles and requirements for entropy sources used by random bit generators and includes requirements related to source validation, entropy estimation, conditioning, and health testing.[2]
This is the part of the series most directly relevant when discussing a hardware entropy source.
A classical hardware RNG or QRNG may provide the physical mechanism from which entropy is derived. But the existence of a physical source alone does not establish how much entropy can safely be credited to the digitized output.
The implementation still needs a defensible source model, measurement process, entropy estimate, operational monitoring, and failure behavior.
NIST lists SP 800-90B as final from January 2018 and notes identified errata intended for a future update or revision.[2]
SP 800-90C: complete RBG constructions
SP 800-90C specifies constructions for implementing random bit generators using the components addressed by 90A and 90B.[3]
NIST finalized SP 800-90C in September 2025. The final publication defines four classes of RBG construction: RBG1, RBG2, RBG3, and RBGC.[3]
The publication completes the current three-part SP 800-90 framework by describing how entropy sources and deterministic mechanisms can be assembled into complete RBG constructions.
The exact class and architecture that apply to a real system depend on the implementation and use case. Engineers should use the actual publication rather than treating a high-level diagram as a compliance checklist.
Side-by-side comparison
| Document | Primary concern | Simplified question |
|---|---|---|
| SP 800-90A Rev. 1 | DRBG mechanisms | How are random bits generated deterministically from internal state? |
| SP 800-90B | Entropy sources | Where does unpredictability come from, and how is the source evaluated? |
| SP 800-90C | RBG constructions | How are entropy sources and DRBG components arranged into random-bit generators? |
The distinction matters because “RNG”, “entropy source”, “DRBG”, and “QRNG” are not interchangeable terms.
Where does a QRNG fit?
A QRNG is most naturally discussed on the entropy-source side of the architecture.
Its physical quantum process can provide unpredictable observations. Those observations still need to be measured, modeled, monitored, and processed appropriately before they can support cryptographic claims.
Depending on the system design, entropy from a QRNG could be used to instantiate or reseed a DRBG, participate in another RBG construction, or feed an architecture with additional controls.
That is why the question “QRNG or DRBG?” can be misleading. In many designs, the technologies serve different roles rather than being mutually exclusive alternatives.
Does SP 800-90B define whether a generator is quantum?
No.
SP 800-90B focuses on entropy sources and their validation requirements. It does not serve as a scientific definition of whether the physical mechanism is quantum.
Likewise, describing a product as a QRNG does not establish that its entropy source has passed an SP 800-90B validation process.
Those are distinct claims:
- physical-origin claim: why the entropy mechanism is quantum;
- entropy-validation claim: how the source is characterized and validated;
- certification/compliance claim: whether a recognized program has formally assessed a specific implementation.
QRNG.io should keep those claims separate.
What changed in 2025 and 2026?
The standards landscape moved in ways that make this topic timely.
First, NIST finalized SP 800-90C in September 2025, completing the current A/B/C structure of the SP 800-90 series.[3]
Second, NIST and Germany's BSI published final NIST IR 8446 in January 2026. That report compares the NIST SP 800-90 series with BSI's AIS 20/31 framework, focusing on similarities and differences in terminology, assumptions, and requirements.[4]
Third, ETSI published TR 104 171 V1.1.1 in March 2026, a Technical Report titled Implementation Guidelines for Quantum Random Number Generators. Its scope covers implementation guidance and characteristics of QRNGs, with discussion of topics including entropy sources, testing, performance, and design trade-offs.[6]
ETSI's work-program record identifies TR 104 171 as a Technical Report and indicates that it is not a harmonised standard. It should therefore be described as implementation guidance rather than as a certification scheme.
Where NIST IR 8446 fits
NIST IR 8446 is not another member of the SP 800-90 A/B/C series.
It is a comparison document developed by authors from NIST and BSI. Its purpose is to bridge terminology and conceptual differences between the NIST SP 800-90 series and BSI AIS 20/31.[4]
For international engineering teams, this is useful because RNG terminology can differ across assurance frameworks. A future QRNG.io article can use IR 8446 as the primary source for a careful NIST-versus-AIS mapping.
Where ETSI TR 104 171 fits
ETSI TR 104 171 is particularly relevant to QRNG because it focuses specifically on implementation guidance for quantum random number generators.[6]
Its existence does not mean every QRNG conforms to an ETSI certification program. Rather, it gives implementers and users a current technical reference focused on QRNG characteristics and implementation concerns.
This makes it a strong future content hub for QRNG.io, especially when paired with the more general entropy-source and RBG framework from NIST.
Common misunderstandings
“SP 800-90A is the entropy standard.”
Not quite. 90A covers deterministic generator mechanisms. 90B is the entropy-source publication.
“If I have a hardware RNG, I do not need a DRBG.”
That depends on the complete architecture. Hardware entropy and deterministic generation can be complementary layers.
“If something is a QRNG, it is automatically SP 800-90B validated.”
No. QRNG describes the claimed physical origin of entropy. Validation status is a separate claim.
“Passing statistical tests means the system satisfies SP 800-90B.”
No. Statistical output tests are not a substitute for entropy-source modeling, estimation, health testing, and the other requirements of an entropy-source validation framework.
“SP 800-90C was only a draft.”
That is outdated. NIST finalized SP 800-90C in September 2025.[3]
Practical reading order for engineers
If you are new to the series, a useful order is:
- Read a plain-language overview of entropy sources.
- Understand min-entropy and conservative entropy accounting.
- Read the scope and terminology of SP 800-90B.
- Review the relevant DRBG concepts in SP 800-90A Rev. 1.
- Use SP 800-90C to understand complete RBG constructions.
- If your assurance context includes BSI, use IR 8446 to compare terminology and requirements.
- For QRNG-specific implementation guidance, review ETSI TR 104 171.
Summary
SP 800-90A, SP 800-90B, and SP 800-90C solve different parts of the random-bit-generation problem.
90A specifies deterministic generation mechanisms.
90B addresses entropy sources.
90C specifies random-bit-generator constructions that combine the relevant components.
For QRNG evaluation, the distinction matters because a quantum entropy source is one layer of the system — not the whole security architecture.
Evaluating how a hardware entropy source fits your RNG architecture? Explore the iQrypto Evaluation Kit.
Sources and further reading
- NIST, SP 800-90A Rev. 1 — https://csrc.nist.gov/pubs/sp/800/90/a/r1/final
- NIST, SP 800-90B — https://csrc.nist.gov/pubs/sp/800/90/b/final
- NIST, SP 800-90C — https://csrc.nist.gov/pubs/sp/800/90/c/final
- NIST, IR 8446: Bridging the Gap Between Standards on Random Number Generation: Comparison of SP 800-90 Series and AIS 20/31 — https://csrc.nist.gov/pubs/ir/8446/final
- NIST, Random Bit Generation publications — https://csrc.nist.gov/Projects/random-bit-generation/publications
- ETSI, TR 104 171 V1.1.1: Implementation Guidelines for Quantum Random Number Generators — https://www.etsi.org/deliver/etsi_tr/104100_104199/104171/01.01.01_60/tr_104171v010101p.pdf