[openssl-dev] ecp_nistz256 correctness/constant-timedness

[Emilia Käsper]

On Fri, Apr 24, 2015 at 3:00 PM, Emilia Käsper <emilia at openssl.org> wrote:

> Thanks for the report! Let me clarify a few points, and then I'll fix it.
>
> On Thu, Apr 23, 2015 at 11:07 PM, Brian Smith <brian at briansmith.org>
> wrote:
>
>> Hi,
>>
>> I have some questions regarding this implementation.
>>
>>
>> https://github.com/openssl/openssl/blob/e0e920b1a063f14f36418f8795c96f2c649400e1/crypto/ec/ecp_nistz256.c
>>
>> 1. In ecp_nistz256_points_mul, we have this code:
>>
>>     if ((BN_num_bits(scalar) > 256) {
>>             ...
>>             if (!BN_nnmod(tmp_scalar, scalar, &group->order, ctx)) {...}
>>             scalar = tmp_scalar;
>>     }
>>
>> I think it would be useful to add a comment about why this is OK in terms
>> of the constant-time-correctness of the code, because it isn't obvious.
>>
>>
> Yes, this needs a comment. Basically we treat overlong scalars as
> malformed, and don't guarantee constant-timeness for them. (It'd be too
> hard.) In the ecp_nistp***.c modules, this is commented.
>
>
>
>> 2. Later in the same function, we have this code:
>>
>>     bn_correct_top(r->X);
>>     bn_correct_top(r->Y);
>>     bn_correct_top(r->Z);
>>
>> Again, it isn't clear why it is OK to call bn_correct_top given that
>> bn_correct_top isn't a constant-time function. I think either this code
>> should be changed so that it is constant time, or a comment should be added
>> explaining why it doesn't need to be.
>>
>
> I think that's fine and just needs a comment. It happens in the end and
> basically strips leading zero-words from the (presumably public) output.
> The ecp_nistp***.c code just calls
> EC_POINT_set_Jprojective_coordinates_GFp() at this point.
>
>
>>
>> 3. When doing the initial adaptation of the code to get it working inside
>> of BoringSSL, I had to make this change:
>>
>>     bn_correct_top(r->X);
>>     bn_correct_top(r->Y);
>>     bn_correct_top(r->Z);
>> +  r->Z_is_one = is_one(p.p.Z);
>>
>> (Alternatively, maybe one can change BoringSSL's implementation
>> of EC_POINT_is_at_infinity to ignore r->Z_is_one like OpenSSL's
>> implementation does.)
>>
>
> Yep, that's a bug because r->Z_is_one may remain incorrectly set.
>
> I don't know why BoringSSL does that extra check in
> EC_POINT_is_at_infinity, though; it precedes their public git history. I'll
> check with the authors.
> I agree with you that this appears an optimization flag, so what should
> always be guaranteed is that point->Z_is_one => BN_is_one(point->Z). But
> !point->Z_is_one => inconclusive.
>

Oh, I think it's just an optimization on their end.


>
>
>
>>
>> Looking at the OpenSSL code, I can see that Z_is_one is only used for
>> optimization purposes in the "simple" ECC implementation. Even ignoring
>> BoringSSL being different, I found it confusing that sometimes Z_is_one
>> *must* be set correctly and other times it *must* be ignored. Perhaps it is
>> worth renaming this member to "simple_Z_is_one" and/or documenting more
>> clearly when it is maintained and when it can and cannot be used.
>>
>> Alternatively, I noticed that BN_is_one is not a very expensive function,
>> and probably can be made even less expensive, so the optimization of using
>> the Z_is_one flag instead of just calling BN_is_one may not be worthwhile.
>> Perhaps it would be better to remove it completely?
>>
>
> Quite likely! This'll go on a TODO list somewhere...
>
>
>>
>> 4. There seems to be quite a bit of missing error checking in this code.
>> For example, the return values of calls to bn_wexpand are not checked in
>> multiple functions. Further, a lot of the error checking in the
>> probably-never-used ecp_nistz256_mult_precompute function is missing, e.g.
>> calls to EC_POINT_new, EC_POINT_copy, and
>> ec_GFp_simple_{add,dbl,make_affine}. I think this whole file needs to be
>> combed for missing error checks.
>>
>> 5. In ecp_nistz256_mult_precompute, if the ctx parameter is null, a new
>> context will be created with BN_CTX_new but BN_CTX_free is not called
>> anywhere in the file.
>>
>
> Yep, these need fixing.
>
> Cheers,
> Emilia
>
>
>> All that aside, this code is very fast, and it is awesome that it got
>> adapted to non-X64 platforms already!
>>
>> Cheers,
>> Brian
>>
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>>
>
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