hello-libsodium.c

#include <stdio.h>
#include <sodium.h>

#define MESSAGE ((const unsigned char *) "test")
#define MESSAGE_LEN 4
#define CIPHERTEXT_LEN (crypto_secretbox_MACBYTES + MESSAGE_LEN)

void receiving_end(unsigned char *ciphertext, unsigned char *nonce, unsigned char *key) {
    unsigned char decrypted[MESSAGE_LEN];

    printf("ciphertext: %s\n", ciphertext);

    if (crypto_secretbox_open_easy(decrypted, ciphertext, CIPHERTEXT_LEN, nonce, key) != 0) {
        printf("message forged!\n");
    }
    else {
        printf("message fine!: %s\n", decrypted);
    }
}


int main(void)
{
    unsigned char nonce[crypto_secretbox_NONCEBYTES];
    unsigned char key[crypto_secretbox_KEYBYTES];
    unsigned char ciphertext[CIPHERTEXT_LEN];

    randombytes_buf(nonce, sizeof nonce);
    randombytes_buf(key, sizeof key);
    crypto_secretbox_easy(ciphertext, MESSAGE, MESSAGE_LEN, nonce, key);


    printf("ciphertext sent: %s\n", ciphertext);
    receiving_end(ciphertext, nonce, key);

    ciphertext[3] += 1;
    printf("ciphertext sent: %s\n", ciphertext);
    receiving_end(ciphertext, nonce, key);
    return 0;