1、第一步先用openssl将pem的key转换为der的key //bin>openssl.exe rsa -in rsakeydec.pem -outform der -out pri.der 

2、调用下面的程序直接读取der转换为c#所需要的xml Key，之后进行密文解密 

 

using System;using System.IO;using System.Security.Cryptography;using System.Text;namespace ConsoleApplication3 {    class Program {        private static int GetIntegerSize(BinaryReader binr) {            int count;            var bt = binr.ReadByte();            if(bt != 0x02)        //expect integer                return 0;            bt = binr.ReadByte();            if(bt == 0x81)                count = binr.ReadByte();    // data size in next byte            else                if(bt == 0x82) {                var highbyte = binr.ReadByte();                var lowbyte = binr.ReadByte();                byte[] modint = { lowbyte,highbyte,0x00,0x00 };                count = BitConverter.ToInt32(modint,0);            } else {                count = bt;        // we already have the data size            }            while(binr.ReadByte() == 0x00) {    //remove high order zeros in data                count -= 1;            }            binr.BaseStream.Seek(-1,SeekOrigin.Current);        //last ReadByte wasn't a removed zero, so back up a byte            return count;        }        /// 
        /// e.g:"D:\\pri.der";        /// 
        ///         /// 
             public static RSACryptoServiceProvider DecodeRsaPrivateKey(string filePath) {            byte[] MODULUS, E, D, P, Q, DP, DQ, IQ;            FileStream fs = new FileStream(filePath,FileMode.Open,FileAccess.Read);            BinaryReader binr = new BinaryReader(fs);    //wrap Memory Stream with BinaryReader for easy reading            try {                var twobytes = binr.ReadUInt16();                if(twobytes == 0x8130) //data read as little endian order (actual data order for Sequence is 30 81)                    binr.ReadByte();        //advance 1 byte                else if(twobytes == 0x8230)                    binr.ReadInt16();       //advance 2 bytes                else                    return null;                twobytes = binr.ReadUInt16();                if(twobytes != 0x0102) //version number                    return null;                var bt = binr.ReadByte();                if(bt != 0x00)                    return null;                var elems = GetIntegerSize(binr);                MODULUS = binr.ReadBytes(elems);                elems = GetIntegerSize(binr);                E = binr.ReadBytes(elems);                elems = GetIntegerSize(binr);                D = binr.ReadBytes(elems);                elems = GetIntegerSize(binr);                P = binr.ReadBytes(elems);                elems = GetIntegerSize(binr);                Q = binr.ReadBytes(elems);                elems = GetIntegerSize(binr);                DP = binr.ReadBytes(elems);                elems = GetIntegerSize(binr);                DQ = binr.ReadBytes(elems);                elems = GetIntegerSize(binr);                IQ = binr.ReadBytes(elems);                // ------- create RSACryptoServiceProvider instance and initialize with public key -----                RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();                RSAParameters rsAparams = new RSAParameters();                rsAparams.Modulus = MODULUS;                rsAparams.Exponent = E;                rsAparams.D = D;                rsAparams.P = P;                rsAparams.Q = Q;                rsAparams.DP = DP;                rsAparams.DQ = DQ;                rsAparams.InverseQ = IQ;                rsa.ImportParameters(rsAparams);                return rsa;            } catch(Exception e) {                Console.WriteLine(e.Message + e.StackTrace);                return null;            } finally {                binr.Close();            }        }        /// 
        /// 导出私钥XML解密格式        /// 
        /// 
             public static string  PrivateKeyDecXml() {            RSACryptoServiceProvider rsaProvider = DecodeRsaPrivateKey(@"D:\\pri.der");            var privateKey = rsaProvider.ToXmlString(true);            return privateKey;        }        /// 
        /// RSA解密        /// 
        ///         ///         /// 
             public static string RsaDecrypt(string privatekey,string content) {            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();            byte[] cipherbytes;            rsa.FromXmlString(privatekey);            cipherbytes = rsa.Decrypt(Convert.FromBase64String(content),false);            return Encoding.UTF8.GetString(cipherbytes);        }            static void Main(string[] args) {                var str= PrivateKeyDecXml();            var s = RsaDecrypt(str,                "jB6EKo9Oakc8MYMlKorZmj415+s8Qf1sHr1MIPjZcybaFhRQDVb+MBZ734p45lc2RlEJUDsy5HRH8I4LvcGSNE0kJu+ge2yy9E8xOD3dWUTU9/30vv7cEbQ5WSHtjl0MyvhTX51x3vrW/oqubH0H3p827GF3c+ALPlxrvG1gHTc=");        }    }}