[openssl-users] How to provide KDF to ECDH key computation when using EVP API?

[Jakob Bohm]

On 28/06/2015 04:55, Reinier Torenbeek wrote:
> Hi again,
>
> After digging into the ECDH code a bit more, I (sort of) found an answer
> to my question.
>
> My reason to look at using the KDF is to apply a hash to the shared
> secret to compute a useable key within the derive function. There is a
> control value called EVP_PKEY_CTRL_MD which seems like it could be used
> for this purpose. However, for EC keys it looks like this control value
> only has a meaning for the signing functionality, not for the key
> derivation functionality. This looks like an omission to me. A small
> test showed that it would not be too hard to have the hash applied when
> doing key derivation as well.
>
> If the approach sketched above is not right or possible, then exposing
> the KDF function to the user of the EVP API seems a logical alternative.
> However, the KDF function prototype is rather limited, with only an in
> and out and no context at all. The latter would be required to make it
> useful.
>
> Since this functionality looks like it is a kind of half-finished to me,
> can anybody give some insight in its status or confirm/correct my
> conclusions?
>
> Thanks,
> Reinier
>
> On 6/19/15 4:23 PM, Reinier Torenbeek wrote:
>> Hi,
>>
>> My goal is to implement ECDH in my own engine. The snippet below shows
>> the struct that needs to be filled and set as the engine's ECDH method:
>>
>> struct ecdh_method {
>>      const char *name;
>>      int (*compute_key) (void *key, size_t outlen, const EC_POINT *pub_key,
>>                          EC_KEY *ecdh, void *(*KDF) (const void *in,
>>                                                      size_t inlen, void *out,
>>                                                      size_t *outlen));
>> # if 0
>>      int (*init) (EC_KEY *eckey);
>>      int (*finish) (EC_KEY *eckey);
>> # endif
>>      int flags;
>>      char *app_data;
>> };
>>
>> I intend to leverage the KDF mechanism, but it does not seem to be
>> exposed in the EVP API. Is that possible at all? If yes, how do I do
>> that? If no, what is the purpose of the KDF() parameter in compute_key?
How does this all compare to the EVP API for traditional
DH?,  I think this is a closer equivalent for API design
than ECDSA.
>>
>> (By the way, struct ecdh_method is in crypto/ecdh/ech_locl.h, which
>> seems to be a private header file. Am I supposed/allowed to include it
>> anyway?)
Unfortunately, someone thinks that OpenSSL should be made
as useless as possible, strangely, this all began shortly
after the heartbleed backdoor was closed.

Enjoy

Jakob
-- 
Jakob Bohm, CIO, Partner, WiseMo A/S.  http://www.wisemo.com
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