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/* yarrow_key_event.c
*
* Exampel entropy estimator for key-like input events. */
/* nettle, low-level cryptographics library
*
* Copyright (C) 2001 Niels Möller
*
* 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;
ctx->previous = 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->previous && (time > ctx->previous) )
{
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;
}
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