I haven't been able to find the beginning of this thread, so apologies
if I've misunderstood what this is for, but it _sounds_ like we're
re-inventing HKDF.

I'd recommend reading the paper about HKDF. It stands out among crypto
papers for having a nice clear justification for each of its design
decisions, so you can see why they did it (very slightly) differently
than the various constructions proposed up-thread.

https://eprint.iacr.org/2010/264

Also, of course, it is a great idea to re-use a standard
(https://tools.ietf.org/html/rfc5869) and widely-understood crypto
algorithm to reduce risk of both cryptographer errors and implementor
errors.

Of course, the cost of that is the you sometimes end up computing
something that is a tiny bit more complicated or inefficient than a
custom algorithm for our current use case. IMHO that's a cheap price
to pay.

Regards,

Zooko

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