public key in HEX format to EC_KEY using c++ - c++

Question

I have a public key in hex format in c++ code. I have to convert this public key to EC_KEY structure which is used by openssl to verify the signature. The problem is that I am not able to convert hex string to pem format from which openssl can generate the public key.
Tried:
1. I convert the hex string to bytes. (I guess this converts to ascii format)
2. this byte string to converted to base64 (ascii format to base64 encoding)
3. use PEM_read_bio_EC_PUBKEY to get the public key (read as pem)
4. EC_KEY_check_key fails.
static const char b64_table[65] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static std::string base64_encode(const ::std::string& bindata)
{
using ::std::string;
using ::std::numeric_limits;
if (bindata.size() > ((numeric_limits<string::size_type>::max)() / 4u) * 3u)
{
throw ::std::length_error("Converting too large a string to base64.");
}
const ::std::size_t binlen = bindata.size();
// Use = signs so the end is properly padded.
string retval((((binlen + 2) / 3) * 4), '=');
::std::size_t outpos = 0;
int bits_collected = 0;
unsigned int accumulator = 0;
const string::const_iterator binend = bindata.end();
for (string::const_iterator i = bindata.begin(); i != binend; ++i) {
accumulator = (accumulator << 8) | (*i & 0xffu);
bits_collected += 8;
while (bits_collected >= 6) {
bits_collected -= 6;
retval[outpos++] = b64_table[(accumulator >> bits_collected) & 0x3fu];
}
}
if (bits_collected > 0) { // Any trailing bits that are missing.
assert(bits_collected < 6);
accumulator <<= 6 - bits_collected;
retval[outpos++] = b64_table[accumulator & 0x3fu];
}
assert(outpos >= (retval.size() - 2));
assert(outpos <= retval.size());
return retval;
}
std::string HexToBytes(const std::string& hex)
{
std::string bytes;
for (unsigned int i = 0; i < hex.length(); i += 2) {
std::string byteString = hex.substr(i, 2);
char byte = (char)strtol(byteString.c_str(), NULL, 16);
bytes.push_back(byte);
}
return bytes;
}
int main(int argc, char* argv[])
{
// Set up the public key....
std::string sPubKeyString = "E3A7E51FE102286D071026111088F680761FDCD7031E3D56244BBE07451601E78AD08AD40EADCF380900985A1FAB94DE6D02DB91920F1144E9EBC4E248444969";
std::string sKeyInAscii = HexToBytes(sPubKeyString);
std::string sPub64(base64_encode(sKeyInAscii));
std::string sKeyInPem = std::string("-----BEGIN PUBLIC KEY-----\n") + sPub64 + std::string("\n-----END PUBLIC KEY-----");
const char* pzKey = sKeyInPem.c_str();
std::unique_ptr< BIO, std::function<void(BIO*)>> bo(BIO_new(BIO_s_mem()), [](BIO* b) { BIO_free(b); });
BIO_write(bo.get(), pzKey, strlen(pzKey));
std::unique_ptr< EC_KEY, std::function<void(EC_KEY*)>> zPublicKey(EC_KEY_new_by_curve_name(NID_X9_62_prime256v1), [](EC_KEY* b) { EC_KEY_free(b); });
EC_KEY* pPubKey = zPublicKey.get();
PEM_read_bio_EC_PUBKEY(bo.get(), &pPubKey, NULL, NULL);
if (EC_KEY_check_key(pPubKey) == 1) {
printf("EC Key valid.\n");
}
else {
printf("EC Key Invalid!\n");
}
}
This code prints EC Key Invalid. I dont know what wrong I am doing. Thanks in advance