deriving keys with HKDF

hkdf.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <mbedtls/hkdf.h>
#include <stdint.h>
#include <string.h>

// salt is shared between server and client
uint8_t salt[32];

void server()
{
    // input key .. must be setup during one time provisioning or OTP
    uint8_t input[] = {
        0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08
    };

    // info can be a device identifier / some stored ids within device  from EEPROM etc
    uint8_t info[] = {
        0xde, 0xad, 0xbe, 0xef
    };

    // generate a salt
    memset(salt, 0, sizeof(salt));
    {
        int fd;

        fd = open("/dev/urandom", O_RDONLY);
        if (fd < 0) {
            printf("failed to generate a random\n");
            return;
        }

        read(fd, salt, sizeof(salt));
        close(fd);
    }

    // below series of steps derive the key

    uint8_t okm[32];

    memset(okm, 0, sizeof(okm));

    // choose SHA256 for MD
    const mbedtls_md_info_t *mdinfo = mbedtls_md_info_from_type(MBEDTLS_MD_SHA256);
    if (!mdinfo) {
        return;
    }

    int ret;

    ret = mbedtls_hkdf(mdinfo, salt, sizeof(salt), input, sizeof(input), info, sizeof(info), okm, sizeof(okm));
    if (ret != 0) {
        printf("failed to hkdf\n");
        return;
    }

    printf("HKDF derived at server: ");

    int i;

    for (i = 0; i < sizeof(okm); i ++) {
        printf("%02x ", okm[i]);
    }
    printf("\n");
}

void client()
{

    // input key .. must be setup during manufacturing
    uint8_t input[] = {
        0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08
    };


    // info can be a device identifier / some stored ids within device  from EEPROM etc
    uint8_t info[] = {
        0xde, 0xad, 0xbe, 0xef
    };

    uint8_t okm[32];

    memset(okm, 0, sizeof(okm));

    // choose SHA256 as MD
    const mbedtls_md_info_t *mdinfo = mbedtls_md_info_from_type(MBEDTLS_MD_SHA256);
    if (!mdinfo) {
        return;
    }

    int ret;

    ret = mbedtls_hkdf(mdinfo, salt, sizeof(salt), input, sizeof(input), info, sizeof(info), okm, sizeof(okm));
    if (ret != 0) {
        printf("failed to hkdf\n");
        return;
    }

    printf("HKDF derived at client: ");

    int i;

    for (i = 0; i < sizeof(okm); i ++) {
        printf("%02x ", okm[i]);
    }
    printf("\n");
}

int main()
{
    // server creates a salt and shares it with client
    //
    // server assumes the input key material is commonly known and setup
    // between server and client within their hardware / read only memory
    //
    // salt is shared
    //
    // device identifier is used as info field for a device specific key generation
    server();
    client();
}