add base64 encoding implementation from Wikibooks

Source <https://en.wikibooks.org/wiki/Algorithm_Implementation/Miscellaneous/Base64>.

Licensed probably under CC-BY-SA. Compatibility with mdb-tools GPL
license should be checked.

src/util/base64.h

@@ -0,0 +1,80 @@
+// https://en.wikibooks.org/wiki/Algorithm_Implementation/Miscellaneous/Base64
+
+#include <inttypes.h>
+#include <string.h>
+ 
+// TODO: split on header and implementation
+static int base64encode(const void* data_buf, size_t dataLength, char* result, size_t resultSize)
+{
+   const char base64chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+   const uint8_t *data = (const uint8_t *)data_buf;
+   size_t resultIndex = 0;
+   size_t x;
+   uint32_t n = 0;
+   int padCount = dataLength % 3;
+   uint8_t n0, n1, n2, n3;
+ 
+   /* increment over the length of the string, three characters at a time */
+   for (x = 0; x < dataLength; x += 3) 
+   {
+      /* these three 8-bit (ASCII) characters become one 24-bit number */
+      n = ((uint32_t)data[x]) << 16; //parenthesis needed, compiler depending on flags can do the shifting before conversion to uint32_t, resulting to 0
+ 
+      if((x+1) < dataLength)
+         n += ((uint32_t)data[x+1]) << 8;//parenthesis needed, compiler depending on flags can do the shifting before conversion to uint32_t, resulting to 0
+ 
+      if((x+2) < dataLength)
+         n += data[x+2];
+ 
+      /* this 24-bit number gets separated into four 6-bit numbers */
+      n0 = (uint8_t)(n >> 18) & 63;
+      n1 = (uint8_t)(n >> 12) & 63;
+      n2 = (uint8_t)(n >> 6) & 63;
+      n3 = (uint8_t)n & 63;
+ 
+      /*
+       * if we have one byte available, then its encoding is spread
+       * out over two characters
+       */
+      if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
+      result[resultIndex++] = base64chars[n0];
+      if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
+      result[resultIndex++] = base64chars[n1];
+ 
+      /*
+       * if we have only two bytes available, then their encoding is
+       * spread out over three chars
+       */
+      if((x+1) < dataLength)
+      {
+         if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
+         result[resultIndex++] = base64chars[n2];
+      }
+ 
+      /*
+       * if we have all three bytes available, then their encoding is spread
+       * out over four characters
+       */
+      if((x+2) < dataLength)
+      {
+         if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
+         result[resultIndex++] = base64chars[n3];
+      }
+   }  
+ 
+   /*
+    * create and add padding that is required if we did not have a multiple of 3
+    * number of characters available
+    */
+   if (padCount > 0) 
+   { 
+      for (; padCount < 3; padCount++) 
+      { 
+         if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
+         result[resultIndex++] = '=';
+      } 
+   }
+   if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
+   result[resultIndex] = 0;
+   return 0;   /* indicate success */
+}