RSA加密超出117字节的数据出错的解决方案

解密过程：
1、使用用户私钥解密网上传送的128位数据；
2、将结果使用ca公钥加密；
3、去掉用来混淆的随机数，提取出会话密钥

但.net中的rsa加密最多只能对117字节数据进行操作，导致128位数据不得不分两部分进行处理，于是加密数据不断膨胀。为了解决这个问题，并使得rsa加密、解密过程与ekey上的过程相一致，我只好编写自己的rsa加密算法。

经过查找了大量资料后，我决定利用现成的 biginteger 类。可以参考http://www.codeproject.com/csharp/biginteger.asp 得到更多的信息。利用 biginteger，我添加了两个方法rsaencrypt和rsadecrypt，实现rsa加密解密。这样就再也不用受117字节的限制了。

下面给出了两段程序，程序一是使用.net自带rsa加密算法实现加密解密，不过 textlength 属性一旦超过 117，系统将无法加密; 程序二是经过改造的系统，可以对128位的数据进行加密，没有了117的限制。程序二省略了biginteger类，需要的话可以从http://www.codeproject.com/csharp/biginteger.asp下载，不要忘了注释其中的main方法，否则在编译时会有一个编译错误，说有两个入口点（当然也可以在项目属性中指定一个入口点）。

程序一：

using system;
using system.security.cryptography;
using system.text;

class oldrsa
...{
   static void main()
   ...{
      int textlength = 117;
      byte[] encrypteddata;
      byte[] decrypteddata;
      string key1 = "<rsakeyvalue><modulus>4n6ejsx4qnfpp6h+wcpdjz8ssmmrjevjabqegsoobhknepo/v3m94nf89+zl5llh7/lurgcufnizvieth/z9+h/ydum0f3fjimn3utk1tk0ioff0cvc9lnerbeoejmkeqivujuc4c+bmqttn6urhfcy3r3zgp3feegqorljekvc=</modulus><exponent>aqab</exponent><p>7w2qsvrbn168ehc4v/fipml+7wukorrij9i8i21fs5glvyrja2czbzplkrahumloclgd/qkj0iapf17471nfkw==</p><q>8oztalinrk1vduvlhnppcnqsehbp9if5p+kwru07sfgwahnyewurg0epebvbgoe/1kzpkqb/wu8vsn4oeauohq==</q><dp>dih+5ouww5av7zlifvqdtents8b9uzbhcbvxry2vddaxbdr+swbse/gvmrg/9fmwk6zbhbopnj8tchkmqozhuq==</dp><dq>6g96q/gxeug3qk+dbp8hil9vsex5wd8ueigicv9/as/7iwqljgbama1xi8txrbo6mdbil2pgkf4uqeg5qeqzrq==</dq><inverseq>nyx28u1freiigxgx2s5+pxbb8wq0xvxne2g2mt0vq9xqdhbaxefpfznjnaga8ahvlunahqg5urgry3ogqono4g==</inverseq><d>pvkj1r1ntc3lhu+xgitvq9qe0tr9v6rcy7sfov9xbcm/ypf20q8sod3y0ad87u9ccssdwfjyagukai0wugjfgfalf8/4pfwqzrgblsn96klmklmy7c6oihlriw+myxmvaggsp3/r4se6wgk5ischjkikyv/pywoobdre6ordzie=</d></rsakeyvalue>";

      try
      ...{
         rsacryptoserviceprovider rsa = new rsacryptoserviceprovider();
         rsa.fromxmlstring(key1);

         byte[] datatoencrypt = generatebytes(textlength);
         console.writeline("original buff:  " + convert.tobase64string(datatoencrypt) + " ");

         encrypteddata = rsa.encrypt(datatoencrypt, false);
         console.writeline("encrypted buff:  " + convert.tobase64string(encrypteddata) + " ");

         decrypteddata = rsa.decrypt(encrypteddata,false);
         console.writeline("decrypted buff:  " + convert.tobase64string(decrypteddata) + " ");
      }
      catch
      ...{
         console.writeline("encryption failed.");
      }
   }

   //***********************************************************************
   // 随机生成一指定长度的字节数组
   //***********************************************************************
   public static byte[] generatebytes(int bytelength)
   ...{
      byte[] buff = new byte[bytelength];
      rngcryptoserviceprovider rng = new rngcryptoserviceprovider();

      // 该数组已使用密码增强的随机字节进行填充
      rng.getbytes(buff);
      return buff;
   }
}

程序二：

using system;
using system.security.cryptography;
using system.text;

class newrsa
...{
   public static void main()
   ...{
      int textlength = 128;
      byte[] encrypteddata;
      byte[] decrypteddata;
      string key1 = "<rsakeyvalue><modulus>4n6ejsx4qnfpp6h+wcpdjz8ssmmrjevjabqegsoobhknepo/v3m94nf89+zl5llh7/lurgcufnizvieth/z9+h/ydum0f3fjimn3utk1tk0ioff0cvc9lnerbeoejmkeqivujuc4c+bmqttn6urhfcy3r3zgp3feegqorljekvc=</modulus><exponent>aqab</exponent><p>7w2qsvrbn168ehc4v/fipml+7wukorrij9i8i21fs5glvyrja2czbzplkrahumloclgd/qkj0iapf17471nfkw==</p><q>8oztalinrk1vduvlhnppcnqsehbp9if5p+kwru07sfgwahnyewurg0epebvbgoe/1kzpkqb/wu8vsn4oeauohq==</q><dp>dih+5ouww5av7zlifvqdtents8b9uzbhcbvxry2vddaxbdr+swbse/gvmrg/9fmwk6zbhbopnj8tchkmqozhuq==</dp><dq>6g96q/gxeug3qk+dbp8hil9vsex5wd8ueigicv9/as/7iwqljgbama1xi8txrbo6mdbil2pgkf4uqeg5qeqzrq==</dq><inverseq>nyx28u1freiigxgx2s5+pxbb8wq0xvxne2g2mt0vq9xqdhbaxefpfznjnaga8ahvlunahqg5urgry3ogqono4g==</inverseq><d>pvkj1r1ntc3lhu+xgitvq9qe0tr9v6rcy7sfov9xbcm/ypf20q8sod3y0ad87u9ccssdwfjyagukai0wugjfgfalf8/4pfwqzrgblsn96klmklmy7c6oihlriw+myxmvaggsp3/r4se6wgk5ischjkikyv/pywoobdre6ordzie=</d></rsakeyvalue>";

      try
      ...{
         rsacryptoserviceprovider rsa = new rsacryptoserviceprovider();
         rsa.fromxmlstring(key1);
         rsaparameters rsakeyinfo = rsa.exportparameters(true);

         byte[] datatoencrypt = generatebytes(textlength);
         console.writeline("original buff:  " + convert.tobase64string(datatoencrypt) + " ");

         encrypteddata = rsaencrypt(datatoencrypt, rsakeyinfo.exponent, rsakeyinfo.modulus);
         console.writeline("encrypted buff:  " + convert.tobase64string(encrypteddata) + " ");

         decrypteddata = rsadecrypt(encrypteddata, rsakeyinfo.d, rsakeyinfo.modulus);
         console.writeline("decrypted buff:  " + convert.tobase64string(decrypteddata) + " ");
      }
      catch
      ...{
         console.writeline("encryption failed.");
      }
   }

   //***********************************************************************
   // rsa encrypt
   //***********************************************************************
   static public byte[] rsaencrypt(byte[] datatoencrypt, byte[] exponent, byte[] modulus)
   ...{
      biginteger original = new biginteger(datatoencrypt);
      biginteger e = new biginteger(exponent);
      biginteger n = new biginteger(modulus);

      biginteger encrypted = original.modpow(e,n);
      return hexstringtobyte(encrypted.tohexstring());
   }

   //***********************************************************************
   // rsa decrypt
   //***********************************************************************
   static public byte[] rsadecrypt(byte[] encrypteddata, byte[] d, byte[] modulus)
   ...{
      biginteger encrypted = new biginteger(encrypteddata);
      biginteger d = new biginteger(d);
      biginteger n = new biginteger(modulus);

      biginteger decrypted = encrypted.modpow(d,n);
      return hexstringtobyte(decrypted.tohexstring());
   }

   //***********************************************************************
   // 将 hexstring 转换为 byte[] 数组
   //***********************************************************************
   static public byte[] hexstringtobyte(string hexstring)
   ...{
      byte[] byteresult = new byte[hexstring.length/2];

      for(int i = 0; i < hexstring.length/2; i++)
         byteresult[i] = convert.tobyte(hexstring.substring(i*2,2),16);

      return byteresult;
   }

   //***********************************************************************
   // 随机生成一指定长度的字节数组
   //***********************************************************************
   public static byte[] generatebytes(int bytelength)
   ...{
      byte[] buff = new byte[bytelength];
      rngcryptoserviceprovider rng = new rngcryptoserviceprovider();

      // 该数组已使用密码增强的随机字节进行填充
      rng.getbytes(buff);
      return buff;
   }
}