How to generate Public Key from the X, Y component and Curve for ECDH?

[shahdhiren]

I am trying to generate Public Key from the X, Y Component and Curve that I received from Server. Below code I am using but not able to generate Public Key.

void* getPublicKeyFromX_Y(const void *xInput, int xLen, const void *yInput, int yLen, int outputLength)
{
    EC_GROUP *group = EC_GROUP_new_by_curve_name(NID_X9_62_prime256v1);
    BN_CTX *ctx = BN_CTX_new();
    EC_POINT *point = EC_POINT_new(group);
    void *pubKey = NULL;

    BIGNUM *x = BN_bin2bn(xInput, xLen, NULL);
    BIGNUM *y = BN_bin2bn(yInput, yLen, NULL);


    if(EC_POINT_set_affine_coordinates_GFp(group, point, x, y, ctx))
    {
        BIGNUM publicKeyBn;
        BN_init(&publicKeyBn);
        EC_POINT_point2bn(group, point, POINT_CONVERSION_UNCOMPRESSED, &publicKeyBn, NULL);
        int length = 65; //Uncompressed, For compressed, it should be 33

        unsigned int offset = length - BN_num_bytes(&publicKeyBn);
        pubKey = calloc(1, length * sizeof(char *));
        outputLength = length;
        BN_bn2bin(&publicKeyBn, pubKey+offset);
        BN_clear(&publicKeyBn);
        EC_GROUP_free(group);
//        EC_KEY *publicKey = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
//        if(EC_KEY_set_public_key(publicKey, point))
//        {
//            printf("here");
//        }
    }
    BN_free(x);
    BN_free(y);

    return pubKey;

}

+(NSData *)getPublicKeyFromX:(NSData *)xInput Y:(NSData *)yInput
{
    NSData *pubKey = nil;

    int outputLength = 65;
    void *pubkey = getPublicKeyFromX_Y([xInput bytes],(int)[xInput length],[yInput bytes],(int)[yInput length], outputLength);

    pubKey = [[NSData alloc] initWithBytes:pubkey length:outputLength];
    return pubKey;
}

Later I found this Library and was exploring my solution but didn't found any thing related.

Does CoreBitcoin contains my solution? How and from where should I attack my problem?

If this question is irrelevant for CoreBitcoin, please feel free to close. But I found it to be somewhat related and hence posting it to find my solution.

Help is much appreciated. Thanks.

[dtco-kimi]

Hi Shahdhiren

@shahdhiren
Did you find any solutions for your question? I also need to generate ECDH share secret. It seems like CoreBitcoin doesn't support this, but I have no idea how to use openssl to do this. Could you give me some suggestions? I also use Obj-C on ios.

Thanks.

[brett-doffing]

Not exactly sure how you are looking to use ECDH, but there is ECDH functionality for a BTCKey. For example I used...
BTCKey *DH_NotificationTXKey = [bobNotificationKey diffieHellmanWithPrivateKey:aliceUnassociatedKey];

[liunina]

Not exactly sure how you are looking to use ECDH, but there is ECDH functionality for a BTCKey. For example I used...
BTCKey *DH_NotificationTXKey = [bobNotificationKey diffieHellmanWithPrivateKey:aliceUnassociatedKey];

why the function "diffieHellmanWithPrivateKey" require bobNotificationKey and aliceUnassociatedKey is Privatekey? I was Confused.