EVP_MAC way of setting the digest algorithm

[Olivier Mascia]

Thanks Tomáš for this acknowledgement.
I will use that workaround.

> Le 27 sept. 2023 à 11:02, Tomas Mraz <tomas at openssl.org> a écrit :
> 
> Yes, unfortunately there is an inherent inefficiency in this design and
> we are working on ways how to fix it.
> 
> In the mean time there is one possibility to workaround this - you can
> duplicate the EVP_MAC_CTX after you call EVP_MAC_CTX_set_params() with
> the digest name being set. This way the internal EVP_MD will be already
> fetched when you later use the duplicate MAC context.
> 
> Tomas Mraz, OpenSSL

______________
Olivier Mascia

> On Wed, 2023-09-27 at 10:53 +0200, Olivier Mascia via openssl-users
> wrote:
>> Dear,
>> 
>> The documentation recommends not to use, for instance, EVP_sha256()
>> __each time__ it is needed when setting up a EVP_MD_CTX like in this
>> partial code (intentionally without error conditions tests):
>> 
>>         EVP_MD_CTX* ctx = EVP_MD_CTX_new();
>>         EVP_DigestInit_ex(ctx, EVP_sha256(), nullptr);
>> 
>> But that it would be wiser to use EVP_MD_fetch() and cache that while
>> it fits, like in some cache structure which would hold a hand of
>> algorithms useful to the repetitive task:
>> 
>>         EVP_MD* evp_sha256 = EVP_MD_fetch(nullptr, "SHA-256",
>> nullptr);
>> 
>> And so, reuse that pointer in EVP_DigestInit_ex() to spare a call to
>> EVP_sha256() or EVP_MD_fetch() each time.
>> 
>> When using the EVP_MAC_ interfaces, I can follow the same principle
>> for the EVP_MAC* itself which will be passed to EVP_MAC_CTX_new(),
>> after calling and caching EVP_MAC_fetch(nullptr, "HMAC", nullptr). 
>> Yet, I find no way of setting the digest algorithm to use for this
>> hmac, other than by text through OSSL_PARAM:
>> 
>>         const OSSL_PARAM params[] = {
>>                 {"digest", OSSL_PARAM_UTF8_STRING, (void*)"SHA-256",
>> 7, 0},
>>                 {nullptr, 0, nullptr, 0, 0}
>>         };
>> 
>> Is this really intentional? Is there an other way? If, for a hmac, it
>> is perfectly acceptable, having no other way, to select the hash
>> algorithm through text search each time it is needed, then why would
>> this be less acceptable for EVP_MD_ interfaces themselves?
>> 
>> I have no functional issue.  It works.  The API just leaves me
>> puzzled.
>> 
>> ______________
>> Olivier Mascia
>> 
> 
> -- 
> Tomáš Mráz, OpenSSL
>