#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <err.h>
#include <inttypes.h>
#include <arpa/inet.h>
#include <unistd.h>

#include <mbedtls/pk.h>
#include <mbedtls/entropy.h>
#include "mbedtls/ctr_drbg.h"
#include "mbedtls/gcm.h"

const unsigned short PORT = 58132;

static const unsigned char pubkey[] =
"-----BEGIN PUBLIC KEY-----\n"
"MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAwWlNmLHOOzZpdrfp+EAA\n"
"Nwqab0FhQwCyZ/u+ySBW5XxPf6mobySvtJrLdsWzdwnup/UfwZiEhJk/4wpD4Qf/\n"
"2+syuJi3Rf7L+Jfh//Qf9uXAS80+LYad7dW0c1r5nt+F9Can5fBn7futnd8n672T\n"
"+y8QpHRwX9GtaILvYQe5GQac8cfq2rGUd5iYj5KSdcaIZnJ4YgnjLHg2PMbtEJwq\n"
"cV+2oAkcOPzTAJoNE7XjLZTwXmLlFgL/2cN4uJZBDnwv3RZSAhpdYF4KOJmE2GFs\n"
"2jdvRUrYO7WSl8fM16vRH4vz5MNNcaprg2MlXheVTPQa+WMdcz7dyQx8s9kRVPPU\n"
"SwIDAQAB \n"
"-----END PUBLIC KEY----- \n";

static const unsigned char pp[] = "IJUHZGFDXTZKHJKHGFDHZLUÖDRTFGHHJGHH";

enum { SESSION_ID_LENGTH = 8, AES_KEY_LENGTH = 16, GCM_IV_LENGTH = 16, GCM_TAG_LENGTH = 16 };
static uint64_t session_id = 0x0123456789abcdef;
static uint8_t aes_key[16] = { 0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef, 0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef };

void send_udp(int sock, mbedtls_ctr_drbg_context *ctr_drbg, const uint8_t *text, size_t len) {
    uint8_t iv[GCM_IV_LENGTH];
    mbedtls_ctr_drbg_random(ctr_drbg, iv, sizeof(GCM_IV_LENGTH));

    const size_t total_length = 1 + SESSION_ID_LENGTH + GCM_IV_LENGTH + GCM_TAG_LENGTH + len;
    uint8_t *output = alloca(total_length);
    output[0] = 1;
    memcpy(output + 1, (uint8_t*)&session_id, SESSION_ID_LENGTH);
    memcpy(output + 1 + SESSION_ID_LENGTH, iv, GCM_IV_LENGTH);

    mbedtls_gcm_context ctx;
    mbedtls_gcm_init(&ctx);
    mbedtls_gcm_setkey(&ctx, MBEDTLS_CIPHER_ID_AES, aes_key, 8 * AES_KEY_LENGTH);

    if (!mbedtls_gcm_crypt_and_tag(&ctx, MBEDTLS_GCM_ENCRYPT, len, iv, GCM_IV_LENGTH, (uint8_t*)&session_id, SESSION_ID_LENGTH,
                                  text, output + 1 + SESSION_ID_LENGTH + GCM_IV_LENGTH + GCM_TAG_LENGTH, GCM_TAG_LENGTH,
                                  output + 1 + SESSION_ID_LENGTH + GCM_IV_LENGTH)) {
        struct sockaddr_in to;
        memset(&to, 0, sizeof(to));
        to.sin_family      = AF_INET;
        to.sin_addr.s_addr = inet_addr("127.0.0.1");
        to.sin_port        = htons(PORT);

        sendto(sock, output, total_length, 0, (struct sockaddr*)&to, sizeof(to));
    }

    mbedtls_gcm_free(&ctx);
}

void new_session(int sock, mbedtls_ctr_drbg_context *ctr_drbg) {
    mbedtls_ctr_drbg_random(ctr_drbg, (uint8_t*)&session_id, sizeof(session_id));
    mbedtls_ctr_drbg_random(ctr_drbg, aes_key, sizeof(aes_key));

    unsigned char output[512];
    output[0] = 0;
    memcpy(output + 1, (uint8_t*)&session_id, SESSION_ID_LENGTH);

    mbedtls_pk_context pk;
    mbedtls_pk_init(&pk);
    int ret = 0;
    if ((ret = mbedtls_pk_parse_public_key(&pk, pubkey, sizeof(pubkey)) ) != 0 )
        errx(-1, "mbedtls_pk_parse_public_keyfile returned -0x%04x\n", -ret );

    size_t olen = 0;
    if ((ret = mbedtls_pk_encrypt(&pk, aes_key, AES_KEY_LENGTH, output + 1 + SESSION_ID_LENGTH, &olen,
                                  sizeof(output) - 1 - SESSION_ID_LENGTH, mbedtls_ctr_drbg_random, ctr_drbg)) != 0)
        errx(-1, "mbedtls_pk_encrypt returned -0x%04x\n", -ret );

    mbedtls_pk_free(&pk);

    struct sockaddr_in to;
    memset(&to, 0, sizeof(to));
    to.sin_family      = AF_INET;
    to.sin_addr.s_addr = inet_addr("127.0.0.1");
    to.sin_port        = htons(PORT);

    sendto(sock, output, olen + 1 + SESSION_ID_LENGTH, 0, (struct sockaddr*)&to, sizeof(to));
}

int main() {
    mbedtls_ctr_drbg_context ctr_drbg;
    mbedtls_entropy_context entropy;

    mbedtls_entropy_init(&entropy);
    mbedtls_ctr_drbg_init(&ctr_drbg);
    mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func, &entropy, pp, sizeof(pp));

    int sock = socket(AF_INET, SOCK_DGRAM, 0);

    new_session(sock, &ctr_drbg);

    sleep(3);

    const unsigned char *logline = (const unsigned char*)"Juchuuu, es klappt!\n";
    send_udp(sock, &ctr_drbg, logline, strlen((char*)logline));
    send_udp(sock, &ctr_drbg, logline, strlen((char*)logline));
    send_udp(sock, &ctr_drbg, logline, strlen((char*)logline));

    close(sock);
}