Key Agreement

keyagreement.c

#include <openssl/evp.h>
#include <stdio.h>

typedef enum Algo {
    AlgoDH, AlgoEC
} Algo;

int generate_shared_secret(EVP_PKEY *private_key,
    			EVP_PKEY *peer_public_key,
     			unsigned char **secret) {
    EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new(private_key, NULL);
    if (ctx == NULL) {
        printf("EVP_KEY_CTX_new failed\n");
    }

    if (EVP_PKEY_derive_init(ctx) <= 0) {
        printf("EVP_PKEY_derive_init failed\n");
    }

    if (EVP_PKEY_derive_set_peer(ctx, peer_public_key) <= 0) {
        printf("EVP_PKEY_derive_set_peer failed\n");
    }

    size_t len = 0;
    
    if (EVP_PKEY_derive(ctx, NULL, &len) <= 0) {
        printf("EVP_PKEY_derive with NULL buffer failed\n");
    }

    *secret = OPENSSL_malloc(len);

    if (!*secret) {
        printf("Could not allocate memory\n");
    }
    if (EVP_PKEY_derive(ctx, *secret, &len) <= 0) {
        printf("EVP_PKEY_dervice failed\n");
    }
    printf("Returning secret = %x, len = %ld\n", secret, len);
    return len;
}

void compare(unsigned char *s1, size_t l1, unsigned char *s2, size_t l2) {
    if (l1 != l2) {
        printf("The secrets don't match\n");
        return;
    }

    for (size_t i = 0; i < l1; i++) {
       if (s1[i] != s2[i]) {
           printf("The secrets don't match\n");
       }
    }
    printf("The secrets match!\n");
}
    
EVP_PKEY* create_key(Algo algo, EVP_PKEY ** ppkey) {
    EVP_PKEY_CTX *pctx = NULL;
    EVP_PKEY *key = NULL;
    OSSL_PARAM params[2];
    if (algo == AlgoDH) {
        if(NULL == (pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_DH, NULL)))
            printf("Error in DH key generation\n");
        params[0] = OSSL_PARAM_construct_utf8_string("group", "ffdhe2048", 0);
        params[1] = OSSL_PARAM_construct_end();
    } else {
        if(NULL == (pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_EC, NULL)))
            printf("Error in DH key generation\n");
        params[0] = OSSL_PARAM_construct_utf8_string("group", "prime256v1", 0);
        params[1] = OSSL_PARAM_construct_end();
    }

    if(EVP_PKEY_keygen_init(pctx) <= 0)
        printf("EVP_PKEY_keygen_init failed\n");
    EVP_PKEY_CTX_set_params(pctx, params);

    if(EVP_PKEY_keygen(pctx, &key) <= 0)
        printf("EVP_PKEY_keygen failed\n");

    *ppkey = key;
}

void test(Algo type) {
    EVP_PKEY *key1 = NULL, *key2 = NULL;
    unsigned char *secret1 = NULL, *secret2 = NULL;

    create_key(type, &key1);
    create_key(type, &key2);

    int len1 = generate_shared_secret(key1, key2, &secret1);
    int len2 = generate_shared_secret(key2, key1, &secret2);

    compare(secret1, len1, secret2, len2);
}

int main(int argc, char *argv[]) {
    test(AlgoEC);
    test(AlgoDH);
}