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POaesimc instruction gives incorrect result
 

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  1. POaesimc instruction gives incorrect result
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    <p>I'm trying to implement AES cryptography using the AES machine instructions (basing it on Intel's <a href="http://software.intel.com/file/24917" rel="nofollow noreferrer">white paper</a>) available on my Sandy Bridge. Unfortunately, I've come to a halt in the phase of generating the round keys for decryption. Specifically, the instruction <code>aesimc</code> (applying the <em>Inverse Mix Columns</em> operation) returns an incorrect result. </p> <p>In their paper they have an example: <img src="https://i.stack.imgur.com/da3LE.png" alt="enter image description here"></p> <p>So with input:</p> <pre><code>48 69 28 53 68 61 79 29 5B 47 75 65 72 6F 6E 5D </code></pre> <p>I get the following using <code>_mm_aesimc_si128()</code>:</p> <pre><code>2D BF F9 31 99 CD 3A 37 B7 C7 81 FD 7D E0 3D 8E </code></pre> <p>It should have returned:</p> <pre><code>62 7A 6F 66 44 B1 09 C8 2B 18 33 0A 81 C3 B3 E5 </code></pre> <p>Not the same result. <strong>Why is this the case?</strong></p> <p>If you want to reproduce it, I tested it with the code below (remember the arguments <code>-maes -msse4</code> when compiling):</p> <pre><code>#include &lt;wmmintrin.h&gt; #include &lt;iostream&gt; using namespace std; void print_m128i(__m128i data) { unsigned char *ptr = (unsigned char*) &amp;data; for (int i = 0; i &lt; 16; i++) { int val = (int) ptr[i]; if (val &lt; 0xF) { cout &lt;&lt; "0"; } cout &lt;&lt; uppercase &lt;&lt; hex &lt;&lt; val &lt;&lt; " "; } cout &lt;&lt; endl; } int main() { unsigned char *data = (unsigned char*) "\x48\x69\x28\x53\x68\x61\x79\x29\x5B\x47\x75\x65\x72\x6F\x6E\x5D"; __m128i num = _mm_loadu_si128((__m128i*) data); __m128i num2 = _mm_aesimc_si128(num); print_m128i(num2); return 0; } </code></pre> <p><strong>EDIT:</strong> The example in Intel's white paper was wrong. As Hans suggested, my chip is little-endian so byte-swapping is necessary - to and fro.</p>
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    1. USMorten Kristensen
    1. USMorten Kristensen
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    1. COBecause that looks like a C-string, I assume the compiler includes a trailing `'\0'` ascii NUL in `data`. Does `__mm_loadu_si128()` read exactly the correct length? (I expect it does, but I've been wrong when making assumptions before. ;) Furthermore, the string pointed to by `data` may be 4-aligned or 8-aligned, but these instructions _feel_ like they require 16-aligned memory.
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      1. USsarnold
    2. COGood point. :) `_mm_loadu_si128()` doesn't require a 16-aligned address (hence the 'u'), `_mm_load_si128` does however. It reads exactly 128 bits. But I tried aligning it anyway with `unsigned char *data __attribute__((aligned(16))) = ..` and it didn't change the result unfortunately.
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      1. USMorten Kristensen
    3. COdarn -- thanks for the quick feedback. :)
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      1. USsarnold
 

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