Endianness fun with SHA-2 algorithm
<p>I'm trying to write a SHA-2 implementation, but the result keeps coming back incorrect- I've tested on such things as the empty string. I implemented it in two steps, preprocessing and primary body.</p>

<pre><code>template&lt;typename T&gt; struct Output {
    std::array&lt;T, 8&gt; h;
};
template&lt;typename T&gt; struct Input {
    std::array&lt;T, 16&gt; c;
};
template&lt;typename T&gt; Output&lt;T&gt; sha2(Input&lt;T&gt; in) {
    T w[64];
    for(int i = 0; i &lt; 16; i++)
        w[i] = in.c[i];

    for(int i = 16; i &lt; 64; i++) {
        auto s0 = _rotr(w[i - 15], 7) ^ _rotr(w[i - 15], 18) ^ (w[i - 15] &gt;&gt; 3);

        auto s1 = _rotr(w[i - 2], 17) ^ _rotr(w[i - 2], 19) ^ (w[i - 2] &gt;&gt; 10);
        w[i] = w[i - 16] + s0 + w[i - 7] + s1;        
    }

    static const T k[] = {    
        0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 
        0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
        0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 
        0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
        0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 
        0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
        0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 
        0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
        0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 
        0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
        0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 
        0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
        0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 
        0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
        0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 
        0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
    };

    static const T h[] = {
        0x6a09e667,
        0xbb67ae85,
        0x3c6ef372,
        0xa54ff53a,
        0x510e527f,
        0x9b05688c,
        0x1f83d9ab,
        0x5be0cd19
    };

    T loopvars[8];
    for(int i = 0; i &lt; 8; i++)
        loopvars[i] = h[i];

    for(int i = 0; i &lt; 64; i++) {
        auto&amp;&amp; la = loopvars[0];
        auto&amp;&amp; lb = loopvars[1];
        auto&amp;&amp; lc = loopvars[2];
        auto&amp;&amp; ld = loopvars[3];
        auto&amp;&amp; le = loopvars[4];
        auto&amp;&amp; lf = loopvars[5];
        auto&amp;&amp; lg = loopvars[6];
        auto&amp;&amp; lh = loopvars[7];

        auto s0 = _rotr(la, 2) ^ _rotr(la, 13) ^ _rotr(la, 22);
        auto maj = (la &amp; lb) ^ (la &amp; lc) ^ (lb &amp; lc);
        auto t2 = s0 + maj;

        auto s1 = _rotr(le, 6) ^ _rotr(le, 11) ^ _rotr(le, 25);
        auto ch = (le &amp; lf) ^ ((~le) &amp; lg);
        auto t1 = lh + s1 + ch + k[i] + w[i];

        lh = lg;
        lg = lf;
        lf = le;
        le = ld + t1;
        ld = lc;
        lc = lb;
        lb = la;
        la = t1 + t2;
    }
    Output&lt;T&gt; output;
    for(int i = 0; i &lt; 8; i++) {
        output.h[i] = h[i] + loopvars[i];
    }
    return output;
}
Output&lt;unsigned int&gt; SHA2(std::vector&lt;char&gt; bytes) {
    auto bitlen = bytes.size() * 8;
    auto big_endian_bitlen = ::_byteswap_uint64(bitlen);
    if (bitlen &gt; 440)
        throw std::runtime_error("Epic fail!");
    Input&lt;unsigned int&gt; in;
    for(int i = 0; i &lt; 16; i++) {
        in.c[i] = 0;
    }
    memcpy(&amp;in.c[0], &amp;bytes.front(), bytes.size());
    in.c[bitlen / 32] |= (1 &lt;&lt; (bitlen % 32));
    // all zero by default, so no need to append the extra bits
    in.c[14] = (big_endian_bitlen &gt;&gt; 32);
    in.c[15] = big_endian_bitlen;
    return sha2(in);
}
</code></pre>

<p>I suspect endianness error. For example, when I listed the input of the primary body, it came back as <code>1 .. (511x)0</code>, which I'm pretty sure was correct. But when I tried swapping the values to respect endianness, I still did not get the correct output.</p>

<p>I'm <em>fairly</em> sure that the error is in the preprocessing step, as the primary body is endianness-independent, as far as I can tell.</p>

<p>Any suggestions as to where the implementation is incorrect?</p>

<p>Edit: Oh yes, <code>_byteswap_uint64</code> is an MSVC intrinsic for endianness conversion of 64bit unsigned integer, and <code>_rotr</code> right rotates 32bit unsigned integers. For GCC, you just use macro or define them as functions to the equivalent GCC intrinsics.</p>

<p>Just for reference, the incorrect output is</p>

<pre><code>de5c4195
c21e7e70
e6a365c2
77f6bc03
f651e23a
6fb9b88a
1decb688
d6fddf1f
</code></pre>

<p>whereas the correct output is</p>

<pre><code>e3b0c442
98fc1c14
9afbf4c8
996fb924
27ae41e4
649b934c
a495991b
7852b855
</code></pre>
