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1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis |
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2 * |
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3 * LibTomCrypt is a library that provides various cryptographic |
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4 * algorithms in a highly modular and flexible manner. |
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5 * |
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6 * The library is free for all purposes without any express |
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7 * guarantee it works. |
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8 * |
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9 * Tom St Denis, [email protected], http://libtomcrypt.org |
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10 */ |
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11 #include "tomcrypt.h" |
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12 |
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13 /** |
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14 @file der_encode_integer.c |
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15 ASN.1 DER, encode an integer, Tom St Denis |
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16 */ |
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17 |
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18 |
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19 #ifdef LTC_DER |
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20 |
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21 /* Exports a positive bignum as DER format (upto 2^32 bytes in size) */ |
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22 /** |
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23 Store a mp_int integer |
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24 @param num The first mp_int to encode |
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25 @param out [out] The destination for the DER encoded integers |
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26 @param outlen [in/out] The max size and resulting size of the DER encoded integers |
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27 @return CRYPT_OK if successful |
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28 */ |
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29 int der_encode_integer(mp_int *num, unsigned char *out, unsigned long *outlen) |
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30 { |
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31 unsigned long tmplen, y; |
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32 int err, leading_zero; |
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33 |
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34 LTC_ARGCHK(num != NULL); |
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35 LTC_ARGCHK(out != NULL); |
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36 LTC_ARGCHK(outlen != NULL); |
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37 |
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38 /* find out how big this will be */ |
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39 if ((err = der_length_integer(num, &tmplen)) != CRYPT_OK) { |
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40 return err; |
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41 } |
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42 |
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43 if (*outlen < tmplen) { |
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44 return CRYPT_BUFFER_OVERFLOW; |
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45 } |
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46 |
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47 if (mp_cmp_d(num, 0) != MP_LT) { |
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48 /* we only need a leading zero if the msb of the first byte is one */ |
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49 if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == MP_YES) { |
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50 leading_zero = 1; |
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51 } else { |
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52 leading_zero = 0; |
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53 } |
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54 |
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55 /* get length of num in bytes (plus 1 since we force the msbyte to zero) */ |
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56 y = mp_unsigned_bin_size(num) + leading_zero; |
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57 } else { |
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58 leading_zero = 0; |
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59 y = mp_count_bits(num); |
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60 y = y + (8 - (y & 7)); |
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61 y = y >> 3; |
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62 |
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63 } |
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64 |
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65 /* now store initial data */ |
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66 *out++ = 0x02; |
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67 if (y < 128) { |
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68 /* short form */ |
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69 *out++ = (unsigned char)y; |
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70 } else if (y < 256) { |
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71 *out++ = 0x81; |
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72 *out++ = y; |
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73 } else if (y < 65536UL) { |
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74 *out++ = 0x82; |
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75 *out++ = (y>>8)&255; |
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76 *out++ = y; |
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77 } else if (y < 16777216UL) { |
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78 *out++ = 0x83; |
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79 *out++ = (y>>16)&255; |
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80 *out++ = (y>>8)&255; |
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81 *out++ = y; |
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82 } else { |
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83 return CRYPT_INVALID_ARG; |
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84 } |
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85 |
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86 /* now store msbyte of zero if num is non-zero */ |
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87 if (leading_zero) { |
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88 *out++ = 0x00; |
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89 } |
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90 |
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91 /* if it's not zero store it as big endian */ |
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92 if (mp_cmp_d(num, 0) == MP_GT) { |
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93 /* now store the mpint */ |
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94 if ((err = mp_to_unsigned_bin(num, out)) != MP_OKAY) { |
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95 return mpi_to_ltc_error(err); |
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96 } |
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97 } else if (mp_iszero(num) != MP_YES) { |
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98 mp_int tmp; |
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99 /* negative */ |
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100 if (mp_init(&tmp) != MP_OKAY) { |
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101 return CRYPT_MEM; |
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102 } |
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103 |
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104 /* 2^roundup and subtract */ |
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105 y = mp_count_bits(num); |
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106 y = y + (8 - (y & 7)); |
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107 if (mp_2expt(&tmp, y) != MP_OKAY || mp_add(&tmp, num, &tmp) != MP_OKAY) { |
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108 mp_clear(&tmp); |
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109 return CRYPT_MEM; |
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110 } |
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111 |
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112 if ((err = mp_to_unsigned_bin(&tmp, out)) != MP_OKAY) { |
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113 mp_clear(&tmp); |
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114 return mpi_to_ltc_error(err); |
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115 } |
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116 mp_clear(&tmp); |
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117 } |
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118 |
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119 /* we good */ |
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120 *outlen = tmplen; |
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121 return CRYPT_OK; |
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122 } |
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123 |
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124 #endif |
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125 |
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126 /* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/integer/der_encode_integer.c,v $ */ |
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127 /* $Revision: 1.1 $ */ |
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128 /* $Date: 2005/05/16 15:08:11 $ */ |
