需要引用的命名空间

using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Paddings;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;
using Org.BouncyCastle.Utilities.Encoders;

关键代码

public class SM4Helper
    {
     
       public static void GenerateKeyHex(out string key, out string iv)
        {
            //生成对称加密算法
            var keyGenerator = new CipherKeyGenerator();
            keyGenerator.Init(new KeyGenerationParameters(new SecureRandom(), 128));
            var a = keyGenerator.GenerateKey();
            key = Hex.ToHexString(a);

            SecureRandom random = new SecureRandom();
            byte[] b = new byte[16]; // IV长度为16字节
            random.NextBytes(b);
            iv = Hex.ToHexString(b);
        }

        public static byte[] ECBEncrypt(byte[] input, string key)
        {
            var engine = new SM4Engine();
            var cipher = new PaddedBufferedBlockCipher(engine, new Pkcs7Padding());
            KeyParameter keyParam = new KeyParameter(Decode(key));
            cipher.Init(true, keyParam);

            var output = new byte[cipher.GetOutputSize(input.Length)];
            int length = cipher.ProcessBytes(input, 0, input.Length, output, 0);
            cipher.DoFinal(output, length);

            return output;
        }
        public static byte[] ECBDecrypt(byte[] input, string key)
        {
            var engine = new SM4Engine();
            var cipher = new PaddedBufferedBlockCipher(engine, new Pkcs7Padding());
            KeyParameter keyParam = new KeyParameter(Decode(key));
            cipher.Init(false, keyParam);

            var output = new byte[cipher.GetOutputSize(input.Length)];
            int length = cipher.ProcessBytes(input, 0, input.Length, output, 0);
            cipher.DoFinal(output, length);
            return RemovePadding(output);
        }

        public static byte[] CBCEncrypt(byte[] plaintext, string key, string iv)
        {
            IBufferedCipher cipher = new PaddedBufferedBlockCipher(new CbcBlockCipher(new SM4Engine()), new Pkcs7Padding());
            cipher.Init(true, new ParametersWithIV(new KeyParameter(Decode(key)), Decode(iv)));
            byte[] output = new byte[cipher.GetOutputSize(plaintext.Length)];
            int bytesProcessed = cipher.ProcessBytes(plaintext, 0, plaintext.Length, output, 0);
            cipher.DoFinal(output, bytesProcessed);
            return output;
        }
        public static byte[] CBCDecrypt(byte[] ciphertext, string key, string iv)
        {
            IBufferedCipher cipher = new PaddedBufferedBlockCipher(new CbcBlockCipher(new SM4Engine()), new Pkcs7Padding());
            cipher.Init(false, new ParametersWithIV(new KeyParameter(Decode(key)), Decode(iv)));
            byte[] output = new byte[cipher.GetOutputSize(ciphertext.Length)];
            int bytesProcessed = cipher.ProcessBytes(ciphertext, 0, ciphertext.Length, output, 0);
            cipher.DoFinal(output, bytesProcessed);
            return RemovePadding(output);
        }

       
        public static byte[] RemovePadding(byte[] data)
        {
            int lastNonZeroByte = data.Length - 1;
            while (data[lastNonZeroByte] == 0)
            {
                lastNonZeroByte--;
            }
            
            byte[] result = new byte[lastNonZeroByte + 1];
            Array.Copy(data, result, lastNonZeroByte + 1);
            return result;
        }

       public static byte[] Decode(string key)
        {
            return Regex.IsMatch(key, "^[0-9a-f]+$", RegexOptions.IgnoreCase) ? Hex.Decode(key) : Convert.FromBase64String(key);
        }
    }

代码调用：

 var encrypted = SM4Helper.ECBEncrypt(Encoding.UTF8.GetBytes(plainText), this.secret);
 Convert.ToBase64String(encrypted);