I'm trying to understand if the XNU underpinnings of macOS (BSD) still use the Yarrow cryptographically secure pseudorandom number generator?

I have looked through the source and it seems as if they switched to a DRBG NIST generator?


There are very few references to Yarrow in the 10.14 XNU code compared with older versions of the Kernel, but 10.14 still contains this string in key.c :

/* Our PRNG is based on Yarrow and doesn't need to be seeded */

That said if you looked at older XNU Yarrow is littered everywhere.

Source code for XNU:

Lots of Yarrow references in XNU 2782 - macOS 10.11

Very few Yarrow references in XNU 4903 - macOS 10.14

So, are Apple still using Yarrow?

I ask because Yarrow is only capable of generating 160bit strong keys.


Yarrow is gone.

Ok! So I mailed Craig Federighi recently about this, concerned that macOS wasn’t capable of generating >128bit quality keys.

He responded!

‘The source you were referencing is out of date. You can find more current informations here: FIPS certification document, section 7: "The NDRNG feeds entropy from the pool into the DRBG on demand. The NDRNG provides 256-bits of entropy.”’

What a legend.

enter image description here

So yup, macOS and iOS are both capable of generating 256 bit entropy and creating quality CS keys for both ECC and Symmetric Key crypto.

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