View source for Creating an OpenSSL Engine to use indigenous ECDH ECDSA and HASH Algorithms

==Digests==
Now we want OpenSSL not only to use our own random function but also to use our sha2 family hash functions.  We first implement a digest selector function, which tells OpenSSL which digests are available in our engine. This kind of implementation is adapted from the OpenSSL`s build-in engine <tt>ccghost</tt>.  We will implement only one hash function namely <tt>SHA256</tt>. Following the the <tt>ossl_typ.h</tt> header  to the <tt>evp.h</tt> header one can see that the Message Digest Structure is defined as follows:
<pre>
struct env_md_st {
    int type;
    int pkey_type;
    int md_size;
    unsigned long flags;
    int (*init) (EVP_MD_CTX *ctx);
    int (*update) (EVP_MD_CTX *ctx, const void *data, size_t count);
    int (*final) (EVP_MD_CTX *ctx, unsigned char *md);
    int (*copy) (EVP_MD_CTX *to, const EVP_MD_CTX *from);
    int (*cleanup) (EVP_MD_CTX *ctx);
    /* FIXME: prototype these some day */
    int (*sign) (int type, const unsigned char *m, unsigned int m_length,
                 unsigned char *sigret, unsigned int *siglen, void *key);
    int (*verify) (int type, const unsigned char *m, unsigned int m_length,
                   const unsigned char *sigbuf, unsigned int siglen,
                   void *key);
    int required_pkey_type[5];  /* EVP_PKEY_xxx */
    int block_size;
    int ctx_size;               /* how big does the ctx->md_data need to be */
    /* control function */
    int (*md_ctrl) (EVP_MD_CTX *ctx, int cmd, int p1, void *p2);
} /* EVP_MD */ ;
</pre>

(Commentaries are original form OpenSSL code).
From this we use our own message digest hash SHA256 declaration:

<pre>
static EVP_MD oezgan_engine_sha256_method=  {
        NID_sha256,
        NID_undef,
        32,
        EVP_MD_FLAG_PKEY_METHOD_SIGNATURE,
        oezgan_engine_sha256_init,
        oezgan_engine_sha256_update,
        oezgan_engine_sha256_final,
        oezgan_engine_sha256_copy,
        oezgan_engine_sha256_cleanup,
        /* FIXME: prototype these some day */
        NULL,
        NULL,
        {NID_undef, NID_undef, 0, 0, 0},
        64, /*Block Size*/
        32, /* how big does the ctx->md_data need to be */
        /* control function */
        NULL,
} ;
</pre>
SHA256 uses a block size of 512 Bit = 64 byte and resulting digest is 256 Bit = 32 byte long. You can also use your own block size and outcome size but for a realistic approach we stick to the original sha256 standard.  Now we need to implement the init, update, final and copy functions for our own sha256 implementation. In our implementation the SHA256 Value is always 2222…

'''Note'''
<pre>
Since 1.1.0, EVP_MD cannot be directly accessed. Users have to call EVP_MD_meth_new to get EVP_MD instance, and use EVP_MD_meth_set_* to set functions and properties list above.
</pre>
'''OID and NID'''
<pre>
In above example, we reused sha256's nid. For those who want to implement new algorithms, use OBJ_create to create new nid and OBJ_*2nid to translate names to nids.
</pre>
<pre>
static int oezgan_engine_sha256_init(EVP_MD_CTX *ctx) {
    ctx->update = &oezgan_engine_sha256_update;
    printf("initialized! SHA256\n");
    return 1;
}

static int oezgan_engine_sha256_update(EVP_MD_CTX *ctx,const void *data,size_t count) 
{
    printf("SHA256 update \n");
    unsigned char * digest256 = (unsigned char*) malloc(sizeof(unsigned char)*32);
    memset(digest256,2,32);
    count = 32;
    ctx->md_data = digest256;
    return 1;
}

static int oezgan_engine_sha256_final(EVP_MD_CTX *ctx,unsigned char *md) {
    printf("SHA256 final size of EVP_MD: %d\n", sizeof(EVP_MD));
    memcpy(md,(unsigned char*)ctx->md_data,32);
    return 1;
}

int oezgan_engine_sha256_copy(EVP_MD_CTX *to, const EVP_MD_CTX *from)
{
    printf("Copy SHA256\n");
    if (to->md_data && from->md_data) {
        memcpy(to->md_data, from->md_data,sizeof(from->md_data));
    }
    return 1;
}

static int oezgan_engine_sha256_cleanup(EVP_MD_CTX *ctx) {
    printf("SHA256 cleanup\n");
    if (ctx->md_data)
        memset(ctx->md_data, 0, 32);
    return 1;
}
</pre>

Now we have to tell OpenSSL that whenever a SHA256 digest is requested use the engine implementation of sha256 this will be the digest selector function.
<pre>
static int oezgan_digest_ids[] = {NID_sha256};

static int oezgan_engine_digest_selector(ENGINE *e, const EVP_MD **digest,
        const int **nids, int nid) {
    int ok = 1;
    if (!digest) {
        *nids = oezgan_digest_ids;
        printf("\n Digest is empty! Nid:%d\n", nid);
        return 2;
    }
    printf("Digest no %d requested\n",nid);
    if (nid == NID_sha256) {
        *digest = &oezgan_engine_sha256_method;
    }
    else {
        ok = 0;
        *digest = NULL;
    }
    return ok;
}
</pre>

We also need to modify the bind_helper function again: 
<pre>
int bind_helper(ENGINE * e, const char *id)
{
    if (!ENGINE_set_id(e, engine_oezgan_id) ||
        !ENGINE_set_name(e, engine_oezgan_name) ||
        !ENGINE_set_RAND(e, &oezgan_random_method) ||
        !ENGINE_set_init_function(e, oezgan_init) ||
        !ENGINE_set_digests(e, &oezgan_engine_digest_selector)
    )
        return 0;
    return 1;
}
</pre>

And we modify our tester to generate us a SHA256 Hash function.
<pre>
    char * str = "Fraunhofer FKIE Wachtberg!";
    int str_len =  26;
    int er = ENGINE_set_default_digests(oezgan_engine);
    printf("ENGINE SETTING DEFAULT DIGESTS %d\n",er);

    unsigned char * digest = new unsigned char[32];
    unsigned int digestSize = -1;

    EVP_MD_CTX *evp_ctx;
    evp_ctx = EVP_MD_CTX_create();
    er = EVP_DigestInit_ex(evp_ctx, EVP_sha256(),oezgan_engine);
    printf("Digest INIT %d\n",er);
    er = EVP_DigestUpdate(evp_ctx, (unsigned char*)str, str_len);
    printf("Digest Update %d\n",er);
    er = EVP_DigestFinal(evp_ctx, digest, &digestSize);
    printf("Digest Final %d Digest size:%d\n",er,digestSize);
    for(int i= 0; i< digestSize; i++) {
        printf("%x", digest[i]);
    }
    printf("\n");
    EVP_MD_CTX_destroy(evp_ctx);
</pre>

The result is:
<pre>
oezgan@kehf-dev1:~/workspace3/oezganEngineTester/Debug$ ./oezganEngineTester 
Oezgan Engine successfully loaded
Oezgan Engine Initializatzion!
Engine name: oezgan engine by Fraunhofer FKIE init result : 786 
oezgan engine random length 5
11111
ENGINE SETTING DEFAULT DIGESTS 1
initialized! SHA256
Digest INIT 1
SHA256 update 
Digest Update 1
SHA256 final size of EVP_MD: 76
SHA256 cleanup
Digest Final 1 Digest size:32
22222222222222222222222222222222
</pre>

Similarly one can also implement other digest functions. Now we move on to more complicated things like the ECDSA or the ECDH algorithms.