Question about EVP API interaction

[Matt Caswell]

On 10/11/2023 16:05, Juan di Mauro wrote:
> Dear all,
> 
> I'm coding an external provider for OpenSSL to incorporate a KEM 
> algorithm (the provider will be compiled as a .so as usual). I have the 
> KEM part of the code
> and the Key management module (the OSSL_ALGORITHM dispatch tables and so 
> on, corresponding to the OSSL_OP_KEM and OSSL_OP_KEYMGMT query types).
> 
> However, I have problems making things interact ok. It's clear that
> I do not fully understand the way the API should work in this case so my 
> first question is: Is there a complete documented guide about that?
> 
> Secondly, to state things clearly: I want to make key encapsulation and 
> I'm following this sequence of API calls to interact with my provider:
> 
> 
>   EVP_PKEY_CTX *ctx;
>   ctx = EVP_PKEY_CTX_new_from_name(libctx,<Name of my Keymgmt 
> algorithm>,NULL);
>      if (!ctx){
>          printf("The context can't be created. Exiting\n");
>          return -1;
>      }
>      //generate the key
>      EVP_PKEY *key = NULL;
>      EVP_PKEY_keygen_init(ctx);
>      EVP_PKEY_keygen(ctx, &key);
>      // Here, since the pkey in context is NULL, fails
>      int res_encaps_init = EVP_PKEY_encapsulate_init(ctx,NULL);

Since you are starting a new operation here you should probably use a 
new pkey ctx created via EVP_PKEY_CTX_new_from_pkey() rather than trying 
to reuse the old one.

Matt

> 
> 
> So, maybe the sequence of steps is wrong or my code doesn't satisfy the 
> API requirements.
> 
> Thanks in advance,
> 
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