.NET提供常用的加密算法类，支持RSA的类是RSACryptoServiceProvider（命名空 间：System.Security.Cryptography），但只支持公钥加密，私钥解密。RSACryptoServiceProvider类包 括：Modulus、Exponent、P、Q、DP、DQ、InverseQ、D等8个属性，其中Modulus和Exponent就是公 钥，Modulus和D就是私钥，RSACryptoServiceProvider类提供导出公钥的方法，也提供导出私钥的方法，但导出的私钥包含上面 8个属性，显然要用RSACryptoServiceProvider实现私钥加密公钥是不可行的。

    从RSA的原理来看，公钥加密私钥解密和私钥加密公钥解密应该是等价的，在某些情况下，比如共享软件加密，我们需要用私钥加密注册码或注册文件，发给用户，用户用公钥解密注册码或注册文件进行合法性验证。

    本 人利用网上找的一个C#版的大整数类BigInteger（本人认为这是偶发现的效率最高的一个C#版大整数类）来实现私钥加密公钥加密（事实上也完全支 持公租加密私钥解密），但没有使用类BigInteger的大素数生成函数，而是直接使用类RSACryptoServiceProvider来生成大素 数。其中加密函数和解密函数的实现如下：

                /**//*
                 功能：用指定的私钥(n,d)加密指定字符串source
                */
                private string EncryptString(string source, BigInteger d, BigInteger n)
                ...{
                        int len = source.Length;
                        int len1 = 0;
                        int blockLen = 0;
                        if ((len % 128) == 0)
                                len1 = len / 128;
                        else
                                len1 = len / 128 + 1;
                        string block = "";
                        string temp = "";
                        for (int i = 0; i < len1; i++)
                        ...{
                                if (len >= 128)
                                        blockLen = 128;
                                else
                                        blockLen = len;
                                block = source.Substring(i * 128, blockLen);
                                byte[] oText = System.Text.Encoding.Default.GetBytes(block);
                                BigInteger biText = new BigInteger(oText);
                                BigInteger biEnText = biText.modPow(d, n);
                                string temp1 = biEnText.ToHexString();
                                temp += temp1;
                                len -= blockLen;
                        }
                        return temp;
                }

                /**//*
                 功能：用指定的公钥(n,e)解密指定字符串source
                */
                private string DecryptString(string source, BigInteger e, BigInteger n)
                ...{
                        int len = source.Length;
                        int len1 = 0;
                        int blockLen = 0;
                        if ((len % 256) == 0)
                                len1 = len / 256;
                        else
                                len1 = len / 256 + 1;
                        string block = "";
                        string temp = "";
                        for (int i = 0; i < len1; i++)
                        ...{
                                if (len >= 256)
                                        blockLen = 256;
                                else
                                        blockLen = len;
                                block = source.Substring(i * 256, blockLen);
                                BigInteger biText = new BigInteger(block, 16);
                                BigInteger biEnText = biText.modPow(e, n);
                                string temp1 = System.Text.Encoding.Default.GetString(biEnText.getBytes());
                                temp += temp1;
                                len -= blockLen;
                        }
                        return temp;
                }

     加密过程和解密过程代码如下所示：

                /**//*
                 加密过程,其中d、n是RSACryptoServiceProvider生成的D、Modulus
                */
                private string EncryptProcess(string source, string d, string n)
                ...{
                        byte[] N = Convert.FromBase64String(n);
                        byte[] D = Convert.FromBase64String(d);
                        BigInteger biN = new BigInteger(N);
                        BigInteger biD = new BigInteger(D);
                        return EncryptString(source, biD, biN);
                }

                /**//*
                 解密过程,其中e、n是RSACryptoServiceProvider生成的Exponent、Modulus
                */
                private string DecryptProcess(string source, string e, string n)
                ...{
                        byte[] N = Convert.FromBase64String(n);
                        byte[] E = Convert.FromBase64String(e);
                        BigInteger biN = new BigInteger(N);
                        BigInteger biE = new BigInteger(E);
                        return DecryptString(source, biE, biN);
                }

     以上方法经本人实际使用，效果良好，希望对朋友们有帮助。

     PS：文中所用大整数类下载地址：http://www.hugesoft.net/ContentPage.aspx?p1=010001&p2=201（PS：原文此链接失效应使用原地址http://www.codeproject.com/KB/cs/biginteger.aspx）

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