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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@gmail.com, http://libtomcrypt.org
*/
#include "tomcrypt.h"
/**
@file der_encode_integer.c
ASN.1 DER, encode an integer, Tom St Denis
*/
#ifdef LTC_DER
/* Exports a positive bignum as DER format (upto 2^32 bytes in size) */
/**
Store a mp_int integer
@param num The first mp_int to encode
@param out [out] The destination for the DER encoded integers
@param outlen [in/out] The max size and resulting size of the DER encoded integers
@return CRYPT_OK if successful
*/
int der_encode_integer(mp_int *num, unsigned char *out, unsigned long *outlen)
{
unsigned long tmplen, y;
int err, leading_zero;
LTC_ARGCHK(num != NULL);
LTC_ARGCHK(out != NULL);
LTC_ARGCHK(outlen != NULL);
/* find out how big this will be */
if ((err = der_length_integer(num, &tmplen)) != CRYPT_OK) {
return err;
}
if (*outlen < tmplen) {
return CRYPT_BUFFER_OVERFLOW;
}
if (mp_cmp_d(num, 0) != MP_LT) {
/* we only need a leading zero if the msb of the first byte is one */
if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == MP_YES) {
leading_zero = 1;
} else {
leading_zero = 0;
}
/* get length of num in bytes (plus 1 since we force the msbyte to zero) */
y = mp_unsigned_bin_size(num) + leading_zero;
} else {
leading_zero = 0;
y = mp_count_bits(num);
y = y + (8 - (y & 7));
y = y >> 3;
}
/* now store initial data */
*out++ = 0x02;
if (y < 128) {
/* short form */
*out++ = (unsigned char)y;
} else if (y < 256) {
*out++ = 0x81;
*out++ = y;
} else if (y < 65536UL) {
*out++ = 0x82;
*out++ = (y>>8)&255;
*out++ = y;
} else if (y < 16777216UL) {
*out++ = 0x83;
*out++ = (y>>16)&255;
*out++ = (y>>8)&255;
*out++ = y;
} else {
return CRYPT_INVALID_ARG;
}
/* now store msbyte of zero if num is non-zero */
if (leading_zero) {
*out++ = 0x00;
}
/* if it's not zero store it as big endian */
if (mp_cmp_d(num, 0) == MP_GT) {
/* now store the mpint */
if ((err = mp_to_unsigned_bin(num, out)) != MP_OKAY) {
return mpi_to_ltc_error(err);
}
} else if (mp_iszero(num) != MP_YES) {
mp_int tmp;
/* negative */
if (mp_init(&tmp) != MP_OKAY) {
return CRYPT_MEM;
}
/* 2^roundup and subtract */
y = mp_count_bits(num);
y = y + (8 - (y & 7));
if (mp_2expt(&tmp, y) != MP_OKAY || mp_add(&tmp, num, &tmp) != MP_OKAY) {
mp_clear(&tmp);
return CRYPT_MEM;
}
if ((err = mp_to_unsigned_bin(&tmp, out)) != MP_OKAY) {
mp_clear(&tmp);
return mpi_to_ltc_error(err);
}
mp_clear(&tmp);
}
/* we good */
*outlen = tmplen;
return CRYPT_OK;
}
#endif
/* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/integer/der_encode_integer.c,v $ */
/* $Revision: 1.1 $ */
/* $Date: 2005/05/16 15:08:11 $ */
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