_ Git - BearSSL/blob - src/ec/ecdsa_atr.c

   1 /*
   2  * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining
   5  * a copy of this software and associated documentation files (the
   6  * "Software"), to deal in the Software without restriction, including
   7  * without limitation the rights to use, copy, modify, merge, publish,
   8  * distribute, sublicense, and/or sell copies of the Software, and to
   9  * permit persons to whom the Software is furnished to do so, subject to
  10  * the following conditions:
  11  *
  12  * The above copyright notice and this permission notice shall be
  13  * included in all copies or substantial portions of the Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  16  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  17  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  18  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  19  * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  20  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  21  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22  * SOFTWARE.
  23  */
  24
  25 #include "inner.h"
  26
  27 /* see bearssl_ec.h */
  28 size_t
  29 br_ecdsa_asn1_to_raw(void *sig, size_t sig_len)
  30 {
  31         /*
  32          * Note: this code is a bit lenient in that it accepts a few
  33          * deviations to DER with regards to minimality of encoding of
  34          * lengths and integer values. These deviations are still
  35          * unambiguous.
  36          *
  37          * Signature format is a SEQUENCE of two INTEGER values. We
  38          * support only integers of less than 127 bytes each (signed
  39          * encoding) so the resulting raw signature will have length
  40          * at most 254 bytes.
  41          */
  42
  43         unsigned char *buf, *r, *s;
  44         size_t zlen, rlen, slen, off;
  45         unsigned char tmp[254];
  46
  47         buf = sig;
  48         if (sig_len < 8) {
  49                 return 0;
  50         }
  51
  52         /*
  53          * First byte is SEQUENCE tag.
  54          */
  55         if (buf[0] != 0x30) {
  56                 return 0;
  57         }
  58
  59         /*
  60          * The SEQUENCE length will be encoded over one or two bytes. We
  61          * limit the total SEQUENCE contents to 255 bytes, because it
  62          * makes things simpler; this is enough for subgroup orders up
  63          * to 999 bits.
  64          */
  65         zlen = buf[1];
  66         if (zlen > 0x80) {
  67                 if (zlen != 0x81) {
  68                         return 0;
  69                 }
  70                 zlen = buf[2];
  71                 if (zlen != sig_len - 3) {
  72                         return 0;
  73                 }
  74                 off = 3;
  75         } else {
  76                 if (zlen != sig_len - 2) {
  77                         return 0;
  78                 }
  79                 off = 2;
  80         }
  81
  82         /*
  83          * First INTEGER (r).
  84          */
  85         if (buf[off ++] != 0x02) {
  86                 return 0;
  87         }
  88         rlen = buf[off ++];
  89         if (rlen >= 0x80) {
  90                 return 0;
  91         }
  92         r = buf + off;
  93         off += rlen;
  94
  95         /*
  96          * Second INTEGER (s).
  97          */
  98         if (off + 2 > sig_len) {
  99                 return 0;
 100         }
 101         if (buf[off ++] != 0x02) {
 102                 return 0;
 103         }
 104         slen = buf[off ++];
 105         if (slen >= 0x80 || slen != sig_len - off) {
 106                 return 0;
 107         }
 108         s = buf + off;
 109
 110         /*
 111          * Removing leading zeros from r and s.
 112          */
 113         while (rlen > 0 && *r == 0) {
 114                 rlen --;
 115                 r ++;
 116         }
 117         while (slen > 0 && *s == 0) {
 118                 slen --;
 119                 s ++;
 120         }
 121
 122         /*
 123          * Compute common length for the two integers, then copy integers
 124          * into the temporary buffer, and finally copy it back over the
 125          * signature buffer.
 126          */
 127         zlen = rlen > slen ? rlen : slen;
 128         sig_len = zlen << 1;
 129         memset(tmp, 0, sig_len);
 130         memcpy(tmp + zlen - rlen, r, rlen);
 131         memcpy(tmp + sig_len - slen, s, slen);
 132         memcpy(sig, tmp, sig_len);
 133         return sig_len;
 134 }