Wrong usage of AES library? - c

Question

I want to use a memory-saving AES-128 implementation. I found the implementation of Karl Malbrain.
I am using it with the code below:
void encryptUboot(void){
//uint8_t key[AES_KEY_LENGTH] = {0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff, 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99};
uint8_t key[AES_KEY_LENGTH] = {0x54, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x61, 0x20, 0x74, 0x65, 0x73, 0x74, 0x21, 0x21};
uint8_t keyschedule[AES_KEY_LENGTH * AES_ROUNDS] = {0x00};
uint8_t message[5] = "test";
uint8_t cipher[16] = {0x00};
uint8_t i;
if(debug) printf("\n[D] Running AES-128 encryption\n");
aes_expand_key(key, keyschedule);
aes_encrypt(message, keyschedule, cipher);
printf("message: %s | cipher: ", message);
for(i = 0; i<AES_KEY_LENGTH; i++){
printf("%02x ", cipher[i]);
}
}
This outputs:
[D] Running AES-128 encryption
message: test | cipher: 2d 58 45 71 24 43 f5 cd 69 6d 07 b3 a3 29 de 8f
However, using the code from here (zip file) with the code below ...
// AES usage example
// compile as: gcc main.c aes.h aes.c
#include <stdlib.h>
#include "aes.h"
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[])
{
unsigned char key[KEY_128] = {0x54, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x61, 0x20, 0x74, 0x65, 0x73, 0x74, 0x21, 0x21};
unsigned char ptext[16] = "test";
unsigned char ctext[16];
unsigned char decptext[16];
unsigned int i = 0;
aes_ctx_t *ctx;
init_aes();
ctx = aes_alloc_ctx(key, sizeof(key));
if(!ctx) {
perror("aes_alloc_ctx");
return EXIT_FAILURE;
}
aes_encrypt(ctx, ptext, ctext);
for(i=0;i<KEY_128;i++) printf("%02x ", ctext[i]);
puts("");
aes_decrypt(ctx, ctext, decptext);
puts(decptext);
aes_free_ctx(ctx);
return EXIT_SUCCESS;
}
.. it outputs a different cipher:
1f 53 3f 60 15 d5 ab 16 69 b6 c6 3b 9e 77 2f 0c
test
Do you see my mistake? Obviously, I am instrumenting these libraries in a wrong way.
Thanks,
-P

Answer 1

Although I couldn't find the exact function you use in Malbrains code, I believe your problem lies in the difference in array length for message. The algorithm encrypts blocks of 128 bit (16 bytes), but you only allocated 5 bytes.
uint8_t message[5] = "test";
vs
unsigned char ptext[16] = "test";
Try initialising it with exactly the same data.
uint8_t message[16];
memset(message, 0, sizeof(message));
memcpy(message, "test", 5);