Hi, I want to produce a shared secret key between a Smartphone and a NRF52 in order to encrypt data later with AES-128 (example of key sharing: here).
To do that i need to implement a ECDH (Elliptic Curve Diffie Hellman) in application level (not in protocol stack) because BLE 4.0 and 4.2 don't implement this key sharing on the same way.
I have read in infocenter that the nrf_crypto library supports only NIST p-256 curve, and i can already generate private and public key with my android application and derivate the shared key using this type of curve (example here)
On NRF side, i've installed all the libraries (nrf_crypto.h, ecc.h...) and micro_ecc_lib_nrf52.lib and i'am using these differents functions :
nrf_crypto_init (void)
nrf_crypto_public_key_compute (uint32_t curve, nrf_crypto_key_t const *p_sk, nrf_crypto_key_t *p_pk)
So, I have try to declare the curve and differents types of keys :
#define SECP256R1 0x04
uint8_t privatekey[32] = { 0x11, 0x11, 0x24, 0x50, 0x38, 0xb6, 0xed, 0x60, 0xc3, 0x60, 0x12, 0x50, 0x00, 0xd2, 0x25, 0x0f, 0xee, 0x37, 0xbf, 0x00, 0x02, 0x6b, 0x50, 0x92, 0x80, 0x32, 0x42, 0x2e, 0xd6, 0x68, 0xde, 0xdf }; // random private key NRF
uint8_t publicNRF[64] = { 0x00, 0x00...} ; //public key NRF initilalized at 0x00
then the nrf_crypto_key_t:
static const nrf_crypto_key_t crypto_key_privatekey =
{
.p_le_data = (uint8_t *) privatekey,
.len = sizeof(privatekey)
};
static nrf_crypto_key_t crypto_key_publicNRF =
{
.p_le_data = (uint8_t *) publicNRF,
.len = sizeof(publicNRF)
};
and then the functions crypto in main:
int main (void)
{
...
nrf_crypto_init ();
nrf_crypto_public_key_compute(SECP256R1, &crypto_key_privatekey, &crypto_key_publicNRF); //generate a 64 bytes public key from a 32 bytes private key
...
}
the function nrf_crypto_key_compute return me NRF_SUCCESS and fill public key with these values :
X: 02700CA6E1285C640EBA51B34724C6ED7647CEBDCA6828C987913361350DA017
Y: 59D31A28EB7D14A10237CA6468C852F70CFD8753565136FF6FFB40FA25EE01D7
using the private key: 1111245038b6ed60c360125000d2250fee37bf00026b50928032422ed668dedf (dec:7719605451653719210803119225407308115048817886466044753560789111057171406559)
But when i'm computing the public key using this website and curve secp256r1 It generate these public keys values:
X: D104BAEF260CCB567EEEE50B67ADB544F8D607034532CF57A44AFFA526034602
Y: B07B48B83E940C94AC3FE11C398BE0307A518EFE59B3F9443852E02787226067
So, my question is why these keys are differents, is there a problem while computing public key with ECC ?
Thanks you in advance
