Looking for debugging help: Running tiny AES on an embedded dsp

[dconz13]

I'm trying to use the tinyAES library on a TMS320VC5510 DSP chip with a 192bit key. The code compiles fine but when I try to run the test.c, none of the test cases pass. I'm not sure what to change to fix this so that I am able to use the library for encryption.

My current suspicion is that maybe the lowest space the chip can allocate is 16bits so the uint8_t allocations and bit manipulations get messed up? Any ideas / leads would be much appreciated.

[kokke]

Hi @dconz13 and thanks for your interest in the project :)

I would start by checking sizeof(uint8_t) and verifying that it's 1.

You can usually verify by either printf("%u", sizeof(uint8_t)); or trying to compile something like this:

#include <stdint.h>

typedef int static_assertion [sizeof(uint8_t) == 1 ? 1 : -1];

Can you post your code ?

EDIT:

My current suspicion is that maybe the lowest space the chip can allocate is 16bits so the uint8_t allocations and bit manipulations get messed up?

I think I've seen something similar before with a TI DSP, so yeah - definitely verify that the CPU handles byte-wise arithmetic well, by checking the size of uint8_t.

[dconz13]

Hey thanks for the quick response. I ran printf("%u", sizeof(unsigned char)); and did get 1. So that weeds out the uint8_t from being the issue. One other notable thing is that I couldn't get the library to compile using the stdint.h for this compiler. To get around it I defined the following typedefs in aes.h and removed the stdint.h include:

typedef unsigned char uint8_t;
typedef unsigned int uint32_t;
typedef unsigned int uintptr_t;

Other than that, the aes.c, aes.h and test.c are unmodified. I'm currently trying to run the main function in the test.c file that runs the various test functions.

[kokke]

Okay so the size of uint8_t is correct.

Next I would ensure that test.c and aes.h + aes.c are compiled with AES192=1 defined, since it's different from the default, which is 128 bit keys.

You could check that by inserting this piece of code into test.c, aes.h and aes.c:

#if !defined(AES192) || (AES192 != 1)
  #error AES192 is not defined == 1
#endif

If you compile for instance aes.c with AES192=1 but compile test.c with AES128=1, there will be trouble.

[dconz13]

I added that code and confirmed that AES192 is set to 1. I decided to try checking the size of uint32_t and uintptr_t on the chip and noticed that printf("%u", sizeof(unsigned int)); returns 1 and prinf("%u", sizeof(unsigned long int)); returns 2. This doesn't match up with the values returned when I run it on my intel cpu or the values listed on tutorialspoint. Maybe I need to figure out how to allocate larger blocks of memory for these typedefs?

[C47D]

Maybe you can try checking the size of unsigned long long.

[dconz13]

unsigned long long returns 4. I tried changing uint32_t and uintptr_t to unsigned long long but still no luck. I also realized that the ecb mode (what I'm trying to run) doesn't use these types.

[dconz13]

I'm going to close the issue since it has to do with my particular architecture and not the library itself.

For my chip, since the uint8_t is using 16bits of memory, there is a carry bit that is propogating during the MixColumns function. Doing ... (*state)[i][x] ^= (Tm ^ Tmp) & 0xFF; drops the carry bit and produces the correct encryption result based off the nist aes expected values. Thanks for the help guys!

[kokke]

@C47D thanks for the help :)

@dconz13 I re-read your post and noticed that you didn't actually test the sizeof(uint8_t) but of unsigned char which is slightly different, in that uint8_t must be EXACTLY 1 byte wide, if the type exists.

For my chip, since the uint8_t is using 16bits of memory

Do I understand correctly that you're saying sizeof(unsigned char) is actually 2 on this chip?

I thought about adding assertions to the test-code, to make it fail faster, when assumptions aren't met - like so:

assert(sizeof(uint8_t) == 1);
assert(sizeof(uint32_t) == 4);

... which might have given you a hint faster.

I know that the compile didn't work with stdint.h out of the box, but then you made your own typedef and THAT would have been caught by the assertion.