* yarrow_key_event.c: New file.

* Makefile.am (libnettle_a_SOURCES): Added yarrow_key_event.c.

Rev: src/nettle/yarrow_key_event.c:1.1

yarrow_key_event.c

+/* yarrow_key_event.c
+ *
+ * Exampel entropy estimator for key-like input events. */
+
+/* nettle, low-level cryptographics library
+ *
+ * Copyright (C) 2001 Niels Mller
+ *  
+ * The nettle library is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or (at your
+ * option) any later version.
+ * 
+ * The nettle library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+ * License for more details.
+ * 
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with the nettle library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+#include "yarrow.h"
+
+void
+yarrow_key_event_init(struct yarrow_key_event_ctx *ctx)
+{
+  unsigned i;
+  
+  ctx->index = 0;
+  for (i = 0; i < YARROW_KEY_EVENT_BUFFER; i++)
+    ctx->chars[i] = 0;  
+}
+
+unsigned
+yarrow_key_event_estimate(struct yarrow_key_event_ctx *ctx,
+			  unsigned key, unsigned time)
+{
+  unsigned entropy = 0;
+  unsigned i;
+
+  /* Look at timing first. */
+  if (ctx->index)
+    {
+      if ( (time - ctx->previous) < 256)
+        entropy++;
+    }
+  ctx->previous = time;
+
+  if (!key)
+    return entropy;
+  
+  for (i = 0; i < YARROW_KEY_EVENT_BUFFER; i++)
+    if (key == ctx->chars[i])
+      /* This is a recent character. Ignore it. */
+      return entropy;
+
+  /* Count one bit of entropy, unless this was one of the initial 16
+   * characters. */
+  if (ctx->chars[ctx->index])
+    entropy++;
+  
+  /* Remember the character. */
+  
+  ctx->chars[ctx->index] = key;
+  ctx->index = (ctx->index + 1) % YARROW_KEY_EVENT_BUFFER;
+
+  return entropy;
+}
+