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/* LibTomCrypt, modular cryptographic library -- Tom St Denis
*
* LibTomCrypt is a library that provides various cryptographic
* algorithms in a highly modular and flexible manner.
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, tomstdenis@iahu.ca, http://libtomcrypt.org
*/
#include "mycrypt.h"
/* decodes a DER INTEGER in [in]. You have to tell this function
* how many bytes are available [inlen]. It will then attempt to
* read the INTEGER. If all goes well it stores the number of bytes
* read in [inlen] and the number in [num].
*/
int der_decode_integer(const unsigned char *in, unsigned long *inlen, mp_int *num)
{
unsigned long tmplen, y, z;
_ARGCHK(num != NULL);
_ARGCHK(in != NULL);
_ARGCHK(inlen != NULL);
/* save copy of max output size */
tmplen = *inlen;
*inlen = 0;
/* min DER INTEGER is 0x02 01 00 == 0 */
if (tmplen < (1 + 1 + 1)) {
return CRYPT_INVALID_PACKET;
}
/* ok expect 0x02 when we AND with 0011 1111 [3F] */
if ((*in++ & 0x3F) != 0x02) {
return CRYPT_INVALID_PACKET;
}
++(*inlen);
/* now decode the len stuff */
z = *in++;
++(*inlen);
if ((z & 0x80) == 0x00) {
/* short form */
/* will it overflow? */
if (*inlen + z > tmplen) {
return CRYPT_INVALID_PACKET;
}
/* no so read it */
(*inlen) += z;
return mpi_to_ltc_error(mp_read_unsigned_bin(num, (unsigned char *)in, z));
} else {
/* long form */
z &= 0x7F;
/* will number of length bytes overflow? (or > 4) */
if (((*inlen + z) > tmplen) || (z > 4)) {
return CRYPT_INVALID_PACKET;
}
/* now read it in */
y = 0;
while (z--) {
y = ((unsigned long)(*in++)) | (y << 8);
++(*inlen);
}
/* now will reading y bytes overrun? */
if ((*inlen + y) > tmplen) {
return CRYPT_INVALID_PACKET;
}
/* no so read it */
(*inlen) += y;
return mpi_to_ltc_error(mp_read_unsigned_bin(num, (unsigned char *)in, y));
}
}
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