Added support for entropy calculation.

 * Currently used for encryption on function level.

Signed-off-by: Toni Uhlig <matzeton@googlemail.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/utils.h b/utils.h
new file mode 100644
index 0000000..fa0e2e0
--- /dev/null
+++ b/utils.h
@@ -0,0 +1,51 @@
+#ifndef UTILS_H
+#define UTILS_H 1
+
+#include <math.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#define MAXLEN 961 // maximum string length
+
+static inline size_t makehist(unsigned char const * const buf, ssize_t * const hist, size_t len) {
+  ssize_t wherechar[256];
+  size_t i, histlen;
+  histlen = 0;
+  for (i = 0; i < 256; i++)
+    wherechar[i] = -1;
+  for (i = 0; i < len; i++) {
+    if (wherechar[buf[i]] == -1) {
+      wherechar[(int)buf[i]] = histlen;
+      histlen++;
+    }
+    hist[wherechar[(int)buf[i]]]++;
+  }
+  return histlen;
+}
+
+static inline double entropy(ssize_t * const hist, size_t histlen, size_t len) {
+  size_t i;
+  double H;
+  H = 0.0;
+  for (i = 0; i < histlen; i++) {
+    H -= (double)hist[i] / len * log2((double)hist[i] / len);
+  }
+  return H;
+}
+
+static inline double entropy_from_buffer(unsigned char const * const buffer, size_t size)
+{
+  ssize_t * const hist_array = malloc(size * sizeof(*hist_array));
+
+  if (!hist_array) {
+    return -1.0;
+  }
+
+  size_t hist_length = makehist(buffer, hist_array, size);
+  double entr = entropy(hist_array, hist_length, size);
+
+  free(hist_array);
+  return entr;
+}
+
+#endif