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      1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
      2  *
      3  * LibTomCrypt is a library that provides various cryptographic
      4  * algorithms in a highly modular and flexible manner.
      5  *
      6  * The library is free for all purposes without any express
      7  * guarantee it works.
      8  *
      9  * Tom St Denis, tomstdenis (at) gmail.com, http://libtomcrypt.com
     10  */
     11 #include "tomcrypt.h"
     12 
     13 /*! \file pkcs_1_v1_5_encode.c
     14  *
     15  *  PKCS #1 v1.5 Padding (Andreas Lange)
     16  */
     17 
     18 #ifdef PKCS_1
     19 
     20 /*! \brief PKCS #1 v1.5 encode.
     21  *
     22  *  \param msg              The data to encode
     23  *  \param msglen           The length of the data to encode (octets)
     24  *  \param block_type       Block type to use in padding (\sa ltc_pkcs_1_v1_5_blocks)
     25  *  \param modulus_bitlen   The bit length of the RSA modulus
     26  *  \param prng             An active PRNG state (only for LTC_PKCS_1_EME)
     27  *  \param prng_idx         The index of the PRNG desired (only for LTC_PKCS_1_EME)
     28  *  \param out              [out] The destination for the encoded data
     29  *  \param outlen           [in/out] The max size and resulting size of the encoded data
     30  *
     31  *  \return CRYPT_OK if successful
     32  */
     33 int pkcs_1_v1_5_encode(const unsigned char *msg,
     34                              unsigned long  msglen,
     35                                        int  block_type,
     36                              unsigned long  modulus_bitlen,
     37                                 prng_state *prng,
     38                                        int  prng_idx,
     39                              unsigned char *out,
     40                              unsigned long *outlen)
     41 {
     42   unsigned long modulus_len, ps_len, i;
     43   unsigned char *ps;
     44   int result;
     45 
     46   /* valid block_type? */
     47   if ((block_type != LTC_PKCS_1_EMSA) &&
     48       (block_type != LTC_PKCS_1_EME)) {
     49      return CRYPT_PK_INVALID_PADDING;
     50   }
     51 
     52   if (block_type == LTC_PKCS_1_EME) {    /* encryption padding, we need a valid PRNG */
     53     if ((result = prng_is_valid(prng_idx)) != CRYPT_OK) {
     54        return result;
     55     }
     56   }
     57 
     58   modulus_len = (modulus_bitlen >> 3) + (modulus_bitlen & 7 ? 1 : 0);
     59 
     60   /* test message size */
     61   if ((msglen + 11) > modulus_len) {
     62     return CRYPT_PK_INVALID_SIZE;
     63   }
     64 
     65   if (*outlen < modulus_len) {
     66     *outlen = modulus_len;
     67     result = CRYPT_BUFFER_OVERFLOW;
     68     goto bail;
     69   }
     70 
     71   /* generate an octets string PS */
     72   ps = &out[2];
     73   ps_len = modulus_len - msglen - 3;
     74 
     75   if (block_type == LTC_PKCS_1_EME) {
     76     /* now choose a random ps */
     77     if (prng_descriptor[prng_idx].read(ps, ps_len, prng) != ps_len) {
     78       result = CRYPT_ERROR_READPRNG;
     79       goto bail;
     80     }
     81 
     82     /* transform zero bytes (if any) to non-zero random bytes */
     83     for (i = 0; i < ps_len; i++) {
     84       while (ps[i] == 0) {
     85         if (prng_descriptor[prng_idx].read(&ps[i], 1, prng) != 1) {
     86           result = CRYPT_ERROR_READPRNG;
     87           goto bail;
     88         }
     89       }
     90     }
     91   } else {
     92     XMEMSET(ps, 0xFF, ps_len);
     93   }
     94 
     95   /* create string of length modulus_len */
     96   out[0]          = 0x00;
     97   out[1]          = (unsigned char)block_type;  /* block_type 1 or 2 */
     98   out[2 + ps_len] = 0x00;
     99   XMEMCPY(&out[2 + ps_len + 1], msg, msglen);
    100   *outlen = modulus_len;
    101 
    102   result  = CRYPT_OK;
    103 bail:
    104   return result;
    105 } /* pkcs_1_v1_5_encode */
    106 
    107 #endif /* #ifdef PKCS_1 */
    108 
    109 /* $Source: /cvs/libtom/libtomcrypt/src/pk/pkcs1/pkcs_1_v1_5_encode.c,v $ */
    110 /* $Revision: 1.2 $ */
    111 /* $Date: 2006/11/01 09:12:06 $ */
    112