#define PBKDF2_SALT_LEN 16
#define SHA256_OUTSIZE  32
hashlib_PBKDF2(const uint8_t* password, size_t plen, uint8_t* out, const uint8_t* salt, size_t iter_count, size_t keylen){

    struct { uint8_t salt[16]; uint32_t block; } saltbuffer;
    uint8_t sha_buffer[SHA256_OUTSIZE];
    size_t outlen;
    SHA256_CTX ctx;
    uint32_t mbuffer[64];
    memcpy(saltbuffer.salt, salt, sizeof saltbuffer.salt);
    size_t block = 0;

    //HASH = sha256
    // DK = T(i) for i in 0 => (keylen/HASH), concat

    for(outlen = 0; outlen < keylen; outlen+=32, block++){
        //T(i) = F(password, salt, c, i) = U1 xor U2 xor ... xor Uc
            //U1 = PRF1(Password, Salt + INT_32_BE(i))
            //U2 = PRF(Password, U1)
            //⋮
            //Uc = PRF(Password, Uc−1)
        size_t copylen = (keylen-outlen > SHA256_OUTSIZE) ? SHA256_OUTSIZE : keylen-outlen;
        saltbuffer.block = block;
        hashlib_Sha256Init(&ctx, mbuffer);
        hashlib_Sha256Update(&ctx, &saltbuffer, sizeof saltbuffer);
        hashlib_Sha256Final(&ctx, sha_buffer);
        for(int i=1; i<iter_count; i++){
            hashlib_Sha256Init(&ctx, NULL);
            hashlib_Sha256Update(&ctx, sha_buffer, SHA256_OUTSIZE);
            hashlib_Sha256Final(&ctx, sha_buffer);
        }
        memcpy(&out[outlen], sha_buffer, copylen);
    }

}