On Tue, Nov 12, 2013 at 4:56 PM, Niels Möller <[email protected]> wrote:
>> Attached is a patch that allows overwriting values (points and
>> scalars) in the ECC API.
> What's the use case? Just to clear sensitive memory, or do you do any
> operations on these values after zeroize?
The former.
> In the former case, maybe it's better to just let the ecc_*_clear
> functions do that? Cost should be negligible, compared to doing any real
> ecc operation.
That would be different to practices in other parts of nettle... but
would work as well.
> And struct ecc_scalar (and struct ecc_point) is different from both
> these examples, since the space is explicitly allocated in
> ecc_scalar_init.
Note that from the nettle user's perspective one cannot zeroize the
memory, as access to the structure that has the size requires
"ecc-internals.h" which isn't installed.
> Independently of what nettle does, I imagine you might want to use
> mp_set_memory_functions to make sure that all memory reallocated or
> freed by gmp is also overwritten.
That would work if I had full control on the application. If I use
mp_set_memory as a library, other parts of the application could crash
(we had such issue when gnutls was used together with another library
that was setting the gmp allocation functions). Reallocations are not
much of an issue to private keys, as they are typically added as a
singleton. You've got a point here though.
regards,
Nikos
> PS. I've started with integrating the poly1305 code. I think we need the
> internal representation of the key and state to be implementation
> dependent. The C code you sent me uses 5 uint32_t, with 26 bits used in
> each. x86_64 assembly most likely wants to use 2 fully used uint64_t and
> one (smaller) variable for the top two bits, and 32-bit assembly would
> want 4 full uint32_t + top bits. All variants could be accomodated with
> something like
[...]
> Everything depending on the internel representation would be collected
> into files poly1305-internal.{c,asm}, both the block processing, and
> code for installing a key and extracting a digest.
Sounds reasonable.
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