How to split an array in c? - c

Question

I need a logic to acheive my target. I've buffer array size of 38400. This array data can be filled by a controller. Here i have to obtain an AES algorithm. In that i've to read 16 bytes of data from the buffer then encrypt, upto end of buffer. How to spilt an array into 16 bytes and encrypt? . I used following logic but i can't get it right now ?
unsigned char ptext[16] = "Attack at dawn!";
unsigned char ctext[16];
unsigned char decptext[16];
unsigned char buffer[120*160*2];
for (int count = 0; count < 120*160*2; count ++)
buffer[count] = count + 1;
for (i = 0; i < 120*160*2; i ++)
{
ptext[i]= buffer[i];
if(i%15 == 0)
{
aes_encrypt(ctx, ptext, ctext);
for(k = 0; k<=i; k++)
{
ptext[k]='\0';
}
}
}
void aes_encrypt(aes_ctx_t *ctx, unsigned char input[16], unsigned char output[16])
{
int i;
// copy input to state
for(i = 0; i < 16; i++)
ctx->state[i & 0x03][i >> 2] = input[i];
aes_addroundkey(ctx, 0);
for(i = 1; i < ctx->rounds; i++) {
aes_subbytes(ctx);
aes_shiftrows(ctx);
aes_mixcolumns(ctx);
aes_addroundkey(ctx, i);
}
aes_subbytes(ctx);
aes_shiftrows(ctx);
aes_addroundkey(ctx, ctx->rounds);
// copy state to output
for(i = 0; i < 16; i++)
{
output[i] = ctx->state[i & 0x03][i >> 2];
printf("%c",output[i]);
}
}
Note: I've filled buffer[] with random numbers.
Me only have to know how to split an array.
Thanks in Advance.

Answer 1

You don't need to "split" the array (whatever "split" means for you.) Just operate on every 16-byte segment of it:
void process_segment(unsigned char segment[])
{
// Work on the first 16 bytes of 'segment'.
}
// ...
unsigned char buffer[120*160*2];
for (size_t i = 0; i < 120*160*2; i += 16) {
process_segment(buffer + i);
}
The above is just an example. If you want a nested for loop instead, you'd do something like this:
unsigned char buffer[120*160*2];
for (size_t i = 0; i < 120*160*2; i += 16) {
unsigned char* segment = buffer + i;
// Work on the first 16 bytes of 'segment'.
for (size_t j = 0; j < 16; ++j) {
// Work on segment[j].
}
}
You should probably change your aes_encrypt() function to take an unsigned char input[] instead of an unsigned char input[16] so that you can pass segment to it.
The code you posted would then become something like this:
unsigned char ptext[16] = "Attack at dawn!";
unsigned char ctext[16];
unsigned char decptext[16];
unsigned char buffer[120*160*2];
for (int count = 0; count < 120*160*2; count++)
buffer[count] = count + 1;
for (i = 0; i < 120*160*2; i += 16) {
unsigned char *segment = buffer + i;
aes_encrypt(ctx, segment, ctext);
// Clear the current 16-byte segment.
memset(segment, '\0', 16);
// ctext now contains the encrypted data of the current
// 16-byte segment. I assume you want to save it somewhere
// now since it will be overridden in the next iteration of
// the loop.
}
And the signature of your aes_encrypt() function would become:
void aes_encrypt(aes_ctx_t *ctx, unsigned char input[],
unsigned char output[16])