Creating a public key from a private key using secp256k1 library in C++

chizisch

newbie

Activity: 3

Merit: 7

Thank you so much for your help Grin

pooya87

legendary

Activity: 3472

Merit: 10611

Quote from: chizisch on February 13, 2020, 07:12:47 PM

how do i convert a hex string to a 32bytes char array?

the stackoverflow website is where you should go for these basic programming questions. here is two links that would answer your question:
https://stackoverflow.com/questions/17261798/converting-a-hex-string-to-a-byte-array
https://stackoverflow.com/questions/3221170/how-to-turn-a-hex-string-into-an-unsigned-char-array

Quote from: chizisch on February 13, 2020, 07:12:47 PM

Also, if you could guide me for some ressources i could use to well understand these conversions etc.. i will be more than grateful.

just use google to find out what each encoding is. for example for hexadecimal or base-16 encoding you can read the wikipedia article: https://en.wikipedia.org/wiki/Hexadecimal

chizisch

newbie

Activity: 3

Merit: 7

Hello,
Thank you so much for your answer.

Also, I have an additional newbie question as i am still not so good with data representations in C++ and how to deal with them.
how do i convert a hex string to a 32bytes char array?
I mean, if i convert that private key to ascii format it is gonna be 32-bytes, however it is just garbage non printable symbols most of time.

Also, if you could guide me for some ressources i could use to well understand these conversions etc.. i will be more than grateful.

Thank you again.

A-Bolt

legendary

Activity: 2317

Merit: 2318

Private key must be a 32-byte char array, not 64-byte hex string.

pkey.data is an internal representation of pubkey object. Don't use it.

To get serialized pubkey use secp256k1_ec_pubkey_serialize function.

chizisch

newbie

Activity: 3

Merit: 7

Hello,

I'm trying to generate a public key from a private one using the bitcoin library secp256k1 https://github.com/bitcoin-core/secp256k1 in C++:

Code:

// private key
std::string privateKey = "baca891f5f0285e043496843d82341d15533f016c223d114e1e4dfd39e60ecb0";
const char* cPrivateKey = privateKey.c_str();

// creating public key from it
secp256k1_context *signingContext = secp256k1_context_create(SECP256K1_CONTEXT_SIGN);
secp256k1_pubkey pkey;

unsigned char *seckey = (unsigned char*) malloc(privateKey.size() * sizeof(unsigned char));
std::copy(cPrivateKey, cPrivateKey + privateKey.size(), seckey);

if (secp256k1_ec_pubkey_create(signingContext, &pkey, seckey) == 0) throw "Creation error";

// print the result in hex
std::stringstream ss;
for (int i = 0; i < 64; ++i)
{
ss << std::setw(2) << std::hex << (0xff & (unsigned int)pkey.data[i]);
}

std::cout << ss.str() << std::endl;

// parsing the result
std::string pkey_data = ss.str();
secp256k1_context *noneContext = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
secp256k1_pubkey pubkey;

if (secp256k1_ec_pubkey_parse(noneContext, &pubkey, pkey.data, 64) == 0) std::cout << "Couldnt parse using pkey.data" << std::endl;
if (secp256k1_ec_pubkey_parse(noneContext, &pubkey, pkay_data, pkey_data.size()) == 0) std::cout << "Couldnt parse using hex public key " << std::endl;

The output:

Code:

1b9e55408c5141414e8337adef57ead18f62444fdf5f8c897d0dc812696a6141a919254d3e750075a2a9ba32dc4ed30c84e65f27e431b59b94a2aafe3e80a974
Couldnt parse using pkey.data
Couldnt parse using hex public key

The idea is to parse the public key from the generated one to see if it is really correct. Also, i tried using openssl with the same private key and i get a different public key from the private one using ecdsa secp256k1..

Any help or example on how to use the library in C++ is more than welcome.

Thank you Grin

Topic: Creating a public key from a private key using secp256k1 library in C++ (Read 327 times)