secp256k1 library does not create correct public key

Question

I am trying to get public key from private key using secp256k1 library. https://github.com/bitcoin-core/secp256k1

I set same private key(sk) in here. https://github.com/ethereumbook/ethereumbook/blob/develop/04keys-addresses.asciidoc#ethereum-addresses

unsigned char *sk = "f8f8a2f43c8376ccb0871305060d7b27b0554d2cc72bccf41b2705608452f315";
secp256k1_ec_pubkey_create(secp256k1_ctx, &pk, sk);
secp256k1_ec_pubkey_serialize(secp256k1_ctx, output, &outputlen, &pk, SECP256K1_EC_UNCOMPRESSED);

But serialized public key (output) which I got is

04a82b7752a73ab1de51ae4f614e7e9908fb891c1cdeeab96c0e7c237d3e3a0369daf8eab4e3864668575c5d10fc000a6897b10102c14671140c1cdf9765fd46b0

Not same public key in ethereum book. Am I writing wrong code to get correct public key ?

Answer 1

I solved this problem. Each byte in secret key(sk) must be a number.

void skstr_to_sk(const unsigned char *sk_str, unsigned char *sk)
{
  const unsigned char *sk_pos = sk_str;
  int i;

  for (i = 0; i < PRIVATE_KEY_SIZE; i++) {
      sscanf(sk_pos, "%2hhx", &sk[i]);
      sk_pos += 2;
  }
}

int main(void)
{
  ...

  unsigned char *sk_str = "f8f8a2f43c8376ccb0871305060d7b27b0554d2cc72bccf41b2705608452f315";
  unsigned char sk[PRIVATE_KEY_SIZE];  // PRIVATE_KEY_SIZE == 32
  ...

  skstr_to_sk(sk_str, sk);

  /* get public key and serialize it */
  secp256k1_ctx = secp256k1_context_create(flags);

  secp256k1_ec_pubkey_create(secp256k1_ctx, &pk, sk);
  secp256k1_ec_pubkey_serialize(secp256k1_ctx, output, &outputlen, &pk, SECP256K1_EC_UNCOMPRESSED);
  ...
}

skstr_to_sk() function converts sk_str to a corresponding hexadecimal number. For example, sk_str[0] and sk_str[1] have the value 'f' and '8' respectively. We must make it hexadecimal number 0xf8. such that

sk_str[0] = f, sk_str[1] = 8 -> sk[0] = 0xf8
sk_str[2] = f, sk_str[3] = 8 -> sk[1] = 0xf8
sk_str[4] = a, sk_str[5] = 2 -> sk[2] = 0xa2
sk_str[6] = f, sk_str[7] = 4 -> sk[3] = 0xf4
...