94 lines
2.2 KiB
C
94 lines
2.2 KiB
C
/* LibTomCrypt, modular cryptographic library -- Tom St Denis
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*
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* LibTomCrypt is a library that provides various cryptographic
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* algorithms in a highly modular and flexible manner.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://libtomcrypt.org
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*/
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#include "mycrypt.h"
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/* Exports a positive bignum as DER format (upto 2^32 bytes in size) */
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int der_encode_integer(mp_int *num, unsigned char *out, unsigned long *outlen)
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{
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unsigned long tmplen, x, y, z;
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int err, leading_zero;
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_ARGCHK(num != NULL);
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_ARGCHK(out != NULL);
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_ARGCHK(outlen != NULL);
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/* find out how big this will be */
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if ((err = der_length_integer(num, &tmplen)) != CRYPT_OK) {
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return err;
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}
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if (*outlen < tmplen) {
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return CRYPT_BUFFER_OVERFLOW;
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}
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/* we only need a leading zero if the msb of the first byte is one */
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if ((mp_count_bits(num) & 7) == 7 || mp_iszero(num) == MP_YES) {
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leading_zero = 1;
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} else {
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leading_zero = 0;
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}
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/* get length of num in bytes (plus 1 since we force the msbyte to zero) */
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y = mp_unsigned_bin_size(num) + leading_zero;
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/* now store initial data */
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*out++ = 0x02;
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if (y < 128) {
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/* short form */
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*out++ = (unsigned char)y;
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} else {
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/* long form (relies on y != 0) */
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/* get length of length... ;-) */
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x = y;
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z = 0;
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while (x) {
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++z;
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x >>= 8;
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}
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/* store length of length */
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*out++ = 0x80 | ((unsigned char)z);
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/* now store length */
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/* first shift length up so msbyte != 0 */
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x = y;
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while ((x & 0xFF000000) == 0) {
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x <<= 8;
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}
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/* now store length */
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while (z--) {
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*out++ = (unsigned char)((x >> 24) & 0xFF);
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x <<= 8;
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}
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}
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/* now store msbyte of zero if num is non-zero */
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if (leading_zero) {
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*out++ = 0x00;
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}
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/* if it's not zero store it as big endian */
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if (mp_iszero(num) == MP_NO) {
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/* now store the mpint */
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if ((err = mp_to_unsigned_bin(num, out)) != MP_OKAY) {
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return mpi_to_ltc_error(err);
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}
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}
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/* we good */
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*outlen = tmplen;
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return CRYPT_OK;
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}
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