Need guidance to replace HMAC-SHA1 implementation via engine

[mhnrj79]

I have developed an openssl engine which replaces the AES implementation.
Currently, I am trying to replace the HMAC-SHA1 implementation. I would like
some advice whether my implementation is correct. Below is my sample code:

///////////////////////////////////////////////////////////
static int engine_digest_nids [] = { NID_sha1 };
static int engine_digest_nids_num = 1;

#define MYENGINE_ID     "myengine"
#define MYENGINE_NAME   "my custom engine"

const EVP_MD myengine_sha1 =
{   
     NID_sha1,
NID_undef,
SHA_DIGEST_LENGTH,
0,
myengine_digest_init,
myengine_digest_update,
myengine_digest_final,
NULL,
myengine_digest_cleanup,
EVP_PKEY_NULL_method,
SHA_CBLOCK,
sizeof(EVP_MD *) + sizeof(SHA_CTX),
}

void ENGINE_load_myengine(void)
{
ENGINE *eng = ENGINE_myengine();
if(!eng) return;
ENGINE_add(eng);
ENGINE_free(eng);
ERR_clear_error();
}

static int myengine_finish(ENGINE *e)
{
return 1;
}

static int myengine_init(ENGINE *e)
{
return 1;
}

static int myengine_bind_helper(ENGINE *e)
{
if(!ENGINE_set_id(e, MYENGINE_ID) ||
    !ENGINE_set_name(e, MYENGINE_NAME) ||
    !ENGINE_set_init_function(e, myengine_init) ||
    !ENGINE_set_finish(e, myengine_finish) ||
    !ENGINE_set_ciphers(e, myengine_ciphers) ||
    !ENGINE_set_digests(e, myengine_engine_digests))
{
    return 0;
}

return 1;
}

static ENGINE *ENGINE_myengine(void)
{
ENGINE *eng = ENGINE_new();

if(!eng)
{
    return NULL;
}

if(!myengine_bind_helper(eng))
{
    ENGINE_free(eng);
    return NULL;
}

return eng;
}

static int myengine_digest_init(EVP_MD_CTX *ctx)
{
memset(ctx->md_data, 0, ctx->digest->md_size);
return 1;
}

static int myengine_digest_update(EVP_MD_CTX *ctx, const void *data, size_t
count)
{
ctx->md_data = OPENSSL_malloc(ctx->digest->md_size);
myengine_hmac_sha1(data, count, ctx->md_data);
return 1;
}

static int myengine_digest_final(EVP_MD_CTX *ctx, unsigned char *md)
{
memcpy(md, ctx->md_data, ctx->digest->md_size);
return 1;
}

static int myengine_digest_cleanup(EVP_MD_CTX *ctx)
{
OPENSSL_free(ctx->md_data);
ctx->md_data = NULL;
return 1;
}

static int myengine_engine_digests(ENGINE *e, const EVP_MD **digest, const
int **nids, int nid)
{
if(!digest)
{
    *nids = engine_digest_nids;
    return engine_digest_nids_num;
}

switch(nid)
{
    case NID_sha1:
        *digest = &myengine_sha1;
        break;

    default:
        *digest = NULL;
        break;
}
return (*digest != NULL);
}




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