title: speeding up md5 program
body: <p>This is a example of md5 in C, but the program is very slow it takes a little over a second to encode a simple string, What is slowing the program down? </p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  #include &lt;string.h&gt;
  #include &lt;stdint.h&gt;
  
  // Constants are the integer part of the sines of integers (in radians) * 2^32.
  const uint32_t k[64] = {
      0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee ,
      0xf57c0faf, 0x4787c62a, 0xa8304613, 0xfd469501 ,
      0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be ,
      0x6b901122, 0xfd987193, 0xa679438e, 0x49b40821 ,
      0xf61e2562, 0xc040b340, 0x265e5a51, 0xe9b6c7aa ,
      0xd62f105d, 0x02441453, 0xd8a1e681, 0xe7d3fbc8 ,
      0x21e1cde6, 0xc33707d6, 0xf4d50d87, 0x455a14ed ,
      0xa9e3e905, 0xfcefa3f8, 0x676f02d9, 0x8d2a4c8a ,
      0xfffa3942, 0x8771f681, 0x6d9d6122, 0xfde5380c ,
      0xa4beea44, 0x4bdecfa9, 0xf6bb4b60, 0xbebfbc70 ,
      0x289b7ec6, 0xeaa127fa, 0xd4ef3085, 0x04881d05 ,
      0xd9d4d039, 0xe6db99e5, 0x1fa27cf8, 0xc4ac5665 ,
      0xf4292244, 0x432aff97, 0xab9423a7, 0xfc93a039 ,
      0x655b59c3, 0x8f0ccc92, 0xffeff47d, 0x85845dd1 ,
      0x6fa87e4f, 0xfe2ce6e0, 0xa3014314, 0x4e0811a1 ,
      0xf7537e82, 0xbd3af235, 0x2ad7d2bb, 0xeb86d391 };
  
  // r specifies the per-round shift amounts
  const uint32_t r[] = {7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22,
      5,  9, 14, 20, 5,  9, 14, 20, 5,  9, 14, 20, 5,  9, 14, 20,
      4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23,
      6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21};
  
  // leftrotate function definition
  #define LEFTROTATE(x, c) (((x) &lt;&lt; (c)) | ((x) &gt;&gt; (32 - (c))))
  
  void to_bytes(uint32_t val, uint8_t *bytes)
  {
      bytes[0] = (uint8_t) val;
      bytes[1] = (uint8_t) (val &gt;&gt; 8);
      bytes[2] = (uint8_t) (val &gt;&gt; 16);
      bytes[3] = (uint8_t) (val &gt;&gt; 24);
  }
  
  uint32_t to_int32(const uint8_t *bytes)
  {
      return (uint32_t) bytes[0]
      | ((uint32_t) bytes[1] &lt;&lt; 8)
      | ((uint32_t) bytes[2] &lt;&lt; 16)
      | ((uint32_t) bytes[3] &lt;&lt; 24);
  }
  
  void md5(const uint8_t *initial_msg, size_t initial_len, uint8_t *digest) {
  
      // These vars will contain the hash
      uint32_t h0, h1, h2, h3;
  
      // Message (to prepare)
      uint8_t *msg = NULL;
  
      size_t new_len, offset;
      uint32_t w[16];
      uint32_t a, b, c, d, i, f, g, temp;
  
      // Initialize variables - simple count in nibbles:
      h0 = 0x67452301;
      h1 = 0xefcdab89;
      h2 = 0x98badcfe;
      h3 = 0x10325476;
  
      //Pre-processing:
      //append "1" bit to message
      //append "0" bits until message length in bits ≡ 448 (mod 512)
      //append length mod (2^64) to message
  
      for (new_len = initial_len + 1; new_len % (512/8) != 448/8; new_len++)
          ;
  
      msg = (uint8_t*)malloc(new_len + 8);
      memcpy(msg, initial_msg, initial_len);
      msg[initial_len] = 0x80; // append the "1" bit; most significant bit is "first"
      for (offset = initial_len + 1; offset &lt; new_len; offset++)
          msg[offset] = 0; // append "0" bits
  
      // append the len in bits at the end of the buffer.
      to_bytes(initial_len*8, msg + new_len);
      // initial_len&gt;&gt;29 == initial_len*8&gt;&gt;32, but avoids overflow.
      to_bytes(initial_len&gt;&gt;29, msg + new_len + 4);
  
      // Process the message in successive 512-bit chunks:
      //for each 512-bit chunk of message:
      for(offset=0; offset&lt;new_len; offset += (512/8)) {
  
          // break chunk into sixteen 32-bit words w[j], 0 ≤ j ≤ 15
          for (i = 0; i &lt; 16; i++)
              w[i] = to_int32(msg + offset + i*4);
  
          // Initialize hash value for this chunk:
          a = h0;
          b = h1;
          c = h2;
          d = h3;
  
          // Main loop:
          for(i = 0; i&lt;64; i++) {
  
              if (i &lt; 16) {
                  f = (b &amp; c) | ((~b) &amp; d);
                  g = i;
              } else if (i &lt; 32) {
                  f = (d &amp; b) | ((~d) &amp; c);
                  g = (5*i + 1) % 16;
              } else if (i &lt; 48) {
                  f = b ^ c ^ d;
                  g = (3*i + 5) % 16;
              } else {
                  f = c ^ (b | (~d));
                  g = (7*i) % 16;
              }
  
              temp = d;
              d = c;
              c = b;
              b = b + LEFTROTATE((a + f + k[i] + w[g]), r[i]);
              a = temp;
  
          }
  
          // Add this chunk's hash to result so far:
          h0 += a;
          h1 += b;
          h2 += c;
          h3 += d;
  
      }
  
      // cleanup
      free(msg);
  
      //var char digest[16] := h0 append h1 append h2 append h3 //(Output is in little-endian)
      to_bytes(h0, digest);
      to_bytes(h1, digest + 4);
      to_bytes(h2, digest + 8);
      to_bytes(h3, digest + 12);
  }
  
  int main(int argc, char **argv) {
      char *msg = argv[1];
      size_t len;
      int i;
      uint8_t result[16];
  
      if (argc &lt; 2) {
          printf("usage: %s 'string'\n", argv[0]);
          return 1;
      }
  
      len = strlen(msg);
  
      // benchmark
      for (i = 0; i &lt; 1000000; i++) {
          md5((uint8_t*)msg, len, result);
      }
  
      // display result
      for (i = 0; i &lt; 16; i++)
          printf("%2.2x", result[i]);
      puts("");
  
      return 0;
  }
  </code></pre>