Re: [PATCH v2 3/4] crypto: kdf - SP800-108 Key Derivation Function

[Stephan Mueller <smueller@chronox.de>]

Am Donnerstag, 2. Juni 2016, 16:55:26 schrieb Herbert Xu:

Hi Herbert,

> 
> Why don't you put the result in a temporary buffer and then copy
> it? These things are tiny, right?

The KDFs are usually used for output sizes between one and 4 keys. So, 
commonly it is expected that not more than 200 or 300 bytes are generated by 
one call. But you cannot be sure how much data a user wants. The spec allows 
that the user generates up to 2^50 or so bytes. The implementation I offer is 
limited to unsigned int bytes.

Note, if one would implement a key ladder, it can be expected that many keys 
are generated from one KDF seed.

I tried to avoid memcpy for speed purposes. And all the user needs to do is to 
not invoke an in-place crypto operation.

Maybe I should copy the input data into a private memory location so that the 
KDF can be used like any other cipher: the caller uses a reference to the 
instance to generate data where the caller does not need to ensure that some 
initial data must be left at some specific place.

Ciao
Stephan