/// <summary>
        /// parsing pem file private key
        /// </summary>
        /// <param name="pemstr">pkcs8 private key</param>
        /// <returns></returns>
        public static RSACryptoServiceProvider DecodePemPrivateKey(String pemstr)
        {
            byte[] pkcs8privatekey;
            pkcs8privatekey = Convert.FromBase64String(pemstr);
            if (pkcs8privatekey != null)
            {
                RSACryptoServiceProvider rsa = DecodePrivateKeyInfo(pkcs8privatekey);
                return rsa;
            }
            else
                return null;
        }

        /// <summary>
        /// 打印出XML格式的私鑰
        /// </summary>
        /// <param name="pkcs8"></param>
        /// <returns></returns>
        private static RSACryptoServiceProvider DecodePrivateKeyInfo(byte[] pkcs8)
        {

            byte[] SeqOID = { 0x30, 0x0D, 0x06, 0x09, 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01, 0x05, 0x00 };
            byte[] seq = new byte[15];

            MemoryStream mem = new MemoryStream(pkcs8);
            int lenstream = (int)mem.Length;
            BinaryReader binr = new BinaryReader(mem);
            byte bt = 0;
            ushort twobytes = 0;

            try
            {
                twobytes = binr.ReadUInt16();
                if (twobytes == 0x8130)
                    binr.ReadByte();
                else if (twobytes == 0x8230)
                    binr.ReadInt16();
                else
                    return null;

                bt = binr.ReadByte();
                if (bt != 0x02)
                    return null;

                twobytes = binr.ReadUInt16();

                if (twobytes != 0x0001)
                    return null;

                seq = binr.ReadBytes(15);
                if (!CompareBytearrays(seq, SeqOID))
                    return null;

                bt = binr.ReadByte();
                if (bt != 0x04)
                    return null;

                bt = binr.ReadByte();
                if (bt == 0x81)
                    binr.ReadByte();
                else
                    if (bt == 0x82)
                        binr.ReadUInt16();
                byte[] rsaprivkey = binr.ReadBytes((int)(lenstream - mem.Position));
                RSACryptoServiceProvider rsacsp = DecodeRSAPrivateKey(rsaprivkey);
                Console.WriteLine(rsacsp.ToXmlString(true));
                return rsacsp;
            }
            catch (Exception)
            {
                return null;
            }
            finally { binr.Close(); }
        }


        private static bool CompareBytearrays(byte[] a, byte[] b)
        {
            if (a.Length != b.Length)
                return false;
            int i = 0;
            foreach (byte c in a)
            {
                if (c != b[i])
                    return false;
                i++;
            }
            return true;
        }

        private static RSACryptoServiceProvider DecodeRSAPrivateKey(byte[] privkey)
        {
            byte[] MODULUS, E, D, P, Q, DP, DQ, IQ;
            MemoryStream mem = new MemoryStream(privkey);
            BinaryReader binr = new BinaryReader(mem);
            byte bt = 0;
            ushort twobytes = 0;
            int elems = 0;
            try
            {
                twobytes = binr.ReadUInt16();
                if (twobytes == 0x8130)
                    binr.ReadByte();
                else if (twobytes == 0x8230)
                    binr.ReadInt16();
                else
                    return null;

                twobytes = binr.ReadUInt16();
                if (twobytes != 0x0102)
                    return null;
                bt = binr.ReadByte();
                if (bt != 0x00)
                    return null;

                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);

                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)
            {
                return null;
            }
            finally { binr.Close(); }
        }

        private static int GetIntegerSize(BinaryReader binr)
        {
            byte bt = 0;
            byte lowbyte = 0x00;
            byte highbyte = 0x00;
            int count = 0;
            bt = binr.ReadByte();
            if (bt != 0x02)
                return 0;
            bt = binr.ReadByte();

            if (bt == 0x81)
                count = binr.ReadByte();
            else
                if (bt == 0x82)
                {
                    highbyte = binr.ReadByte();
                    lowbyte = binr.ReadByte();
                    byte[] modint = { lowbyte, highbyte, 0x00, 0x00 };
                    count = BitConverter.ToInt32(modint, 0);
                }
                else
                {
                    count = bt;
                }
            while (binr.ReadByte() == 0x00)
            {
                count -= 1;
            }
            binr.BaseStream.Seek(-1, SeekOrigin.Current);
            return count;
        }
 

通過將pkcs8金鑰生成XML格式的金鑰然後通過FromXmlString方法匯入再進行解密。

  public static String decryptByPrivateKey(String inputStr, string privateKey, Encoding encoding)
        {
            String result = null;
            byte[] input = Convert.FromBase64String(inputStr);
            RSAParameters rasParams = ConvertFromPrivateKey(privateKey);
            RSACryptoServiceProvider rsaService = new RSACryptoServiceProvider();
            //rsaService.ImportParameters(rasParams);
            rsaService.FromXmlString(@"<RSAKeyValue><Modulus>pEDZ+Bd97XPLbRdYZqkSHWgwU5vgGtUXIiTO+VOwjKL6UQFmGb2Uk6D5fttCYuwDNT59ECrHtqbKWF5G4MtQD03Ss8WLobxwpjR8XiQ8JbIgc1IQoUPFxS8Ls7LTOzMH2DTDPHLySSGKtrHrNnxmhDofhseRsue9LQAVlt6Qj3M=</Modulus><Exponent>AQAB</Exponent><P>0IcsAdPe2QcGnXjDc1ABJGOYpUEJnBGzmAC0dRRCBGJFSwUBkyTG6VNboPtcKk504lsoZyF8oUlxMUJuYznJrw==</P><Q>yaVjzQquigXHbBfXu9xbdILDDacuybb0IEk0tH/AV6MouHRm6nCtdeF9L0ZGdEdoJreeRHmH95+jjRRxti57fQ==</Q><DP>a6Q7HT5nHJS533PFQLMdw9D9rOBju4KPuHk0rfMlGOo5Zr5Qz8n+O9dhxzm5m8UgJPB1bLcmhINKwOjP3ryn1Q==</DP><DQ>grpu0WezcFf+yd1LR8e8ovSDcD6K3st1ydisCrkoAakNpUgR0LvsORhPCG8PDiOdKjywpsQDmk4swjIW5n2MIQ==</DQ><InverseQ>lKFa6ityR0OLoOu9qhTTdZfFO3hr+ZETelUBd6hMlTPVHu6AHH1eeQ0Q6ELG8UGkgJH3UlEG/9btzoFpw2LQzQ==</InverseQ><D>h0p3fzbyXYycSX4KLX0jU5+G9scUn2VXiaMh7BH4AM8ZOXHahGc4IyVbfBbjppFZzxhqcM+b8qGNV55H6U7t4GtR9JtxUqqSZ/JqgQnTd8PUqrJI2Z/HpeY0lzvXHF1VOtzgP7sBPKnZikoeoJH7FEt9ghtGY6rAXj6A9RCOMCk=</D></RSAKeyValue>");
            /*
            rsaService.FromXmlString(@"<RSAKeyValue>
                                        <Modulus>4hH5e3UWRo9TSzHcM/DPvTzHhRKaV/28TUCFBx5jg/ErfSJiMQh3plp6h9BkupMrO0gNq198hzMpy/0rt9w3KC7Mm4ZYLNqv8GoynQE1XZnDdJu3JXu5wwAvDg7PYsXhtGikG6kzERsNAQ4enyOCGuWyRj28WsitrKEpIlhAtDE=</Modulus>
                                        <Exponent>AQAB</Exponent>
	                                    <P>+plx7dXt1GKvoE+4G4aeLKohyp6eglOU1NhvfYlJYFtKL8HdtYqb1dt4El+LZ8s+teP9O1+wEDS/LFHV8e8lFw==</P>
	                                    <Q>5vEzwxHIEwZU2mbrqWSp8tP1oH/Xb53Kvlhanub+wjR1kuobKCeBiffgAF92blMJKl1RFxRuwFiT4w//2ARN9w==</Q>
	                                    <DP>0WTwDqRn8sR3LcQO+WkVuAp869p4ZjvCq8sh9cqGNm5xYdBjGnN0XVDiVq/hQ1xPzRpxGXCOJCx+5WCIHX0PBw==</DP>
	                                    <DQ>eTL4sVUxpRpbicsdvmZ/8lpeu/6bt44eCssfs+kSpiUV1yZZXjdtelTGECwJYVjepIcYuX4VqYhTBIUvBdUsfQ==</DQ>
	                                    <InverseQ>ti+ID24mDqOdPQOCR5RYCKadoTYm844R9XtRzQdTXgnnXHOURcl/+Ovr1X+PFLb2Aw/yV+qSaIky3reyhdfhRw==</InverseQ>
	                                    <D>Ph9EWDmXknImKF6eLSO6Yj0Gcrnm8dEG4o+dPpSKMiKembdOFj1Ga88nJpUMaPyx/4tQPkxx8tELZkO+tLpt6TeHrZ8mVXmKfqEDhzya+wXrc60799i72R6FTHSYz8AbcEf/RMh5OgCz2oyg/J/ck4tGPMpJwjjv4s0dYBsmfYU=</D>
                                    </RSAKeyValue>");
            */
            
            RSAParameters rsapars = rsaService.ExportParameters(false);
            byte[] output = rsaService.Decrypt(input, false);
            result = encoding.GetString(output);
            return result;
        }