/*
 * PKCS#5 padding mechanism for udp tunnel
 *
 * mechanism: pkcs5
 * arguments: block size to pad to, must be power of 2 and <=128
 *
 * restrictions: none
 * encoding: append between 1 and n bytes, all of the same value being
 *           the number of bytes appended
 */
/*
 * This file is part of ipif, part of userv-utils
 *
 * Copyright 1996-2013 Ian Jackson <ijackson@chiark.greenend.org.uk>
 * Copyright 1998 David Damerell <damerell@chiark.greenend.org.uk>
 * Copyright 1999,2003
 *    Chancellor Masters and Scholars of the University of Cambridge
 * Copyright 2010 Tony Finch <fanf@dotat.at>
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program 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
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with userv-utils; if not, see http://www.gnu.org/licenses/.
 */

#include "forwarder.h"

struct mechdata {
  unsigned mask;
};

static unsigned long setup(struct mechdata **md_r) {
  struct mechdata *md;
  unsigned long blocksize;

  XMALLOC(md);

  blocksize= getarg_ulong();
  md->mask= blocksize - 1;
  arg_assert(!(md->mask & blocksize));
  arg_assert(blocksize <= 255);

  *md_r= md;
  return blocksize;
}

static void mes_pkcs5(struct mechdata **md_r, int *maxprefix_io, int *maxsuffix_io) {
  unsigned long blocksize;
  
  blocksize= setup(md_r);
  *maxsuffix_io += blocksize + 1;
}

static void mds_pkcs5(struct mechdata **md_r) {
  setup(md_r);
}

static void menc_pkcs5(struct mechdata *md, struct buffer *buf) {
  unsigned char *pad;
  int padlen;

                                  /* eg with blocksize=4 mask=3 mask+2=5   */
                                  /* msgsize    20    21    22    23   24  */
  padlen= md->mask - buf->size;   /*           -17   -18   -19   -16  -17  */
  padlen &= md->mask;             /*             3     2     1     0    3  */
  padlen++;                       /*             4     3     2     1    4  */

  pad= buf_append(buf,padlen);
  memset(pad,padlen,padlen);
}

static const char *mdec_pkcs5(struct mechdata *md, struct buffer *buf) {
  unsigned char *padp;
  unsigned padlen;
  int i;

  BUF_UNAPPEND(padp,buf,1);
  padlen= *padp;
  if (!padlen || (padlen > md->mask+1)) return "invalid length";

  BUF_UNAPPEND(padp,buf,padlen-1);
  for (i=0; i<padlen-1; i++)
    if (*++padp != padlen) return "corrupted padding";

  return 0;
}

STANDARD_MECHANISMLIST("pkcs5",pkcs5)