I'm using the library OpenSSL for encrypting through AES. Since AES is a block cipher, I need to split the data in chunks of 16 bytes. Thus, if I want to recover the message, I need to unite the chunks at the end of the program.

This is my code:

#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <openssl/aes.h>

char key[] = "secretkey123";

int main() {
    unsigned char text_slice[128];
    unsigned char enc_slice[128];
    unsigned char dec_slice[128];
    
    unsigned char in[160];
    
    unsigned char enc_out[160] = "";
    unsigned char dec_out[160] = "";
    
    int i;
    int k = 10;
    for (i = 0; i < 16 * k; i  ) {
        in[i] = 'A'   (rand() % 26);
    }
    in[160] = '\0';
    printf("IN:%s\n", in);
    
    AES_KEY enc_key, dec_key;
    
    AES_set_encrypt_key(key, 128, &enc_key);
    
    AES_set_decrypt_key(key, 128, &dec_key);
    
    for (i = 0; i < k; i  ) {
        text_slice[0] = '\0';
        enc_slice[0] = "\0";
        dec_slice[0] = "\0";
        memcpy(&text_slice[0], &in[15 * i], 15);
        text_slice[16] = "\0";
        printf("TEXT SLICE: %s \n", text_slice);
        AES_encrypt(text_slice, enc_slice, &enc_key);
        memcpy(&enc_out[16 * i], &enc_slice[0], 16);
    }
    printf("ENC:%s\n", enc_out);
    for (i = 0; i < k; i  ) {
        text_slice[0] = '\0';
        enc_slice[0] = "\0";
        dec_slice[0] = "\0";
        memcpy(enc_slice, &enc_out[16 * i], 16);
        enc_slice[16] = "\0";
    
        AES_decrypt(enc_slice, dec_slice, &dec_key);
        printf("Dec slice:%s \n", dec_slice);
        memcpy(&dec_out[16 * i], &dec_slice[0], 16);
    }
    printf("DEC OUT:%s\n", dec_out);
    
    return 0;
}

The output of the program is the following:

IN:NWLRBBMQBHCDARZOWKKYHIDDQSCDXRJMOWFRXSJYBLDBEFSARCBYNECDYGGXXPKLORELLNMPAPQFWKHOPKMCOQHNWNKUEWHSQMGBBUQCLJJIVSWMDKQTBXIXMVTRRBLJPTNSNFWZQFJMAFADRRWSOFSBCNUVQHFF
TEXT SLICE: NWLRBBMQBHCDARZ 
TEXT SLICE: OWKKYHIDDQSCDXR 
TEXT SLICE: JMOWFRXSJYBLDBE 
TEXT SLICE: FSARCBYNECDYGGX 
TEXT SLICE: XPKLORELLNMPAPQ 
TEXT SLICE: FWKHOPKMCOQHNWN 
TEXT SLICE: KUEWHSQMGBBUQCL 
TEXT SLICE: JJIVSWMDKQTBXIX 
TEXT SLICE: MVTRRBLJPTNSNFW 
TEXT SLICE: ZQFJMAFADRRWSOF 
ENC:j�Q���