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NIST SP 800-90a defines the CTR_DRBG in section 10.2.

There is a comment within the document that I don't understand.

Let's assume we want to achieve a security strength of 256 bits, using as underlying algorithm AES-256 and we do use a derivation function.

Therefore in Table 3 the entropy string is supposed to be within $[256,2^{35}]$ bits and the personalization string is up to $2^{35}$ bits. The seedlen is equal to $384$ bits.

But in section 10.2.1.3.2 "Instantiation When a Derivation Function is Used" step 1 seed_material is defined as

seed_material = entropy_input || nonce || personalization_string

with the comment "Ensure that the length of the seed_material is exactly seedlen bits".

What is the sense of this comment ?

If personalization_string and entropy_input are allowed to grow up to $2^{35}$ bits, how can their concatenation (excluding the nonce) be forced to be exactly $384$ bits ?

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I believe the comment is explaining the next line:

seed_material = df(seed_material, seedlen)

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  • $\begingroup$ Ouch. Makes sense. $\endgroup$ – Ruggero Sep 6 '17 at 14:51

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