在前面文章我们已经可以制作SM2证书了，主要应用了SM2签名验证算法和SM3摘要算法，在本文中主要介绍SM2公钥加密算法。这里我们使用SM2数字证书来做SM2非对称加密，然后使用硬件加密设备做解密，比如加密文件只能由指定的智能密码钥匙UKey才能解开。
SM2加密算法
SM2加密同样使用接收方公钥加密，公钥由一个曲线坐标点组成，在X.509证书中的共钥表示为04标记开始的2个32byte的BigInteger，即曲线点P（x,y）。SM2公钥加密算法比RSA相对复杂，加密结果由3个部分组成，SM2加密过程中使用了随机数，因此同样的明文数据每一次加密结果都不一样。SM2加密算法流程如下图所示。
根据国密推荐的SM2椭圆曲线公钥密码算法，首先产生随机数计算出曲线点C1，2个32byte的BigInteger大数，即为SM2加密结果的第1部分。第2部分则是真正的密文，是对明文的加密结果，长度和明文一样。第3部分是杂凑值，用来效验数据。按国密推荐的256位椭圆曲线，明文加密结果比原长度会大96byte。
SM2加密算法同样也可以基于使用BouncyCastle库实现。一般使用数字证书来标识身份，同时使用证书中公钥加密数据。如下SM2Cipher类是C#下SM2软算法实现。
1publicclass SM2Cipher
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privateint ct =1;
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private ECPoint p2;
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private SM3Digest sm3keybase;
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private SM3Digest sm3c3;
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privatebyte[] key =newbyte[32];
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privatebyte keyOff =0;
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public SM2Cipher() ...{ }
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privatevoid Reset()
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...{
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sm3keybase =new SM3Digest();
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sm3c3 =new SM3Digest();
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byte[] p;
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p = p2.X.ToBigInteger().ToByteArrayUnsigned();
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sm3keybase.BlockUpdate(p, 0, p.Length);
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sm3c3.BlockUpdate(p, 0, p.Length);
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p = p2.Y.ToBigInteger().ToByteArrayUnsigned();
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sm3keybase.BlockUpdate(p, 0, p.Length);
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ct =1;
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NextKey();
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}
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privatevoid NextKey()
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...{
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SM3Digest sm3keycur =new SM3Digest(sm3keybase);
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sm3keycur.Update((byte)(ct >>24&0x00ff));
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sm3keycur.Update((byte)(ct >>16&0x00ff));
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sm3keycur.Update((byte)(ct >>8&0x00ff));
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sm3keycur.Update((byte)(ct &0x00ff));
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sm3keycur.DoFinal(key, 0);
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keyOff =0;
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ct++;
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}
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publicvirtual ECPoint InitEncipher(ECPoint userKey)
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...{
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BigInteger k =null;
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ECPoint c1 =null;
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if (1==1)
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...{
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AsymmetricCipherKeyPair key = SM2CryptoServiceProvider.SM2KeyPairGenerator.GenerateKeyPair();
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ECPrivateKeyParameters ecpriv = (ECPrivateKeyParameters)key.Private;
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ECPublicKeyParameters ecpub = (ECPublicKeyParameters)key.Public;
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k = ecpriv.D;
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c1 = ecpub.Q;
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}
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p2 = userKey.Multiply(k);
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Reset();
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return c1;
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}
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publicvirtualvoid Encrypt(byte[] data)
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...{
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sm3c3.BlockUpdate(data, 0, data.Length);
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for (int i =0; i < data.Length; i++)
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...{
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if (keyOff == key.Length)
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NextKey();
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data[i] ^= key[keyOff++];
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}
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}
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publicvirtualvoid InitDecipher(BigInteger userD, ECPoint c1)
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...{
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p2 = c1.Multiply(userD);
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Reset();
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}
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publicvirtualvoid Decrypt(byte[] data)
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...{
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for (int i =0; i < data.Length; i++)
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...{
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if (keyOff == key.Length)
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NextKey();
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data[i] ^= key[keyOff++];
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}
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sm3c3.BlockUpdate(data, 0, data.Length);
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}
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publicvirtualvoid Dofinal(byte[] c3)
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...{
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byte[] p = p2.Y.ToBigInteger().ToByteArrayUnsigned();
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sm3c3.BlockUpdate(p, 0, p.Length);
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sm3c3.DoFinal(c3, 0);
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Reset();
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}
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/**////<summary>
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/// 使用SM2公钥加密数据
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///</summary>
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///<param></param>
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///<param></param>
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///<returns></returns>
113publicstring Encrypt(ECPoint pubKey, byte[] plaintext)
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...{
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byte[] data =newbyte[plaintext.Length];
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Array.Copy(plaintext, data, plaintext.Length);
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ECPoint c1 = InitEncipher(pubKey);
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Encrypt(data);
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byte[] c3 =newbyte[32];
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Dofinal(c3);
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string hexString = c1.X.ToBigInteger().ToString(16) + c1.Y.ToBigInteger().ToString(16)
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+ Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(data)
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+ Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(c3);
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return hexString;
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}
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/**////<summary>
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/// 使用SM2解密数据
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///</summary>
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///<param></param>
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///<param></param>
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///<returns></returns>
139publicbyte[] Decrypt(BigInteger privateKey, string ciphertext)
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...{
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string hexString = ciphertext;
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string c1X = hexString.Substring(0, 64);
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string c1Y = hexString.Substring(0+ c1X.Length, 64);
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string encrypData = hexString.Substring(c1X.Length + c1Y.Length, hexString.Length - c1X.Length - c1Y.Length -64);
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string c3 = hexString.Substring(hexString.Length -64);
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byte[] data = SM2CryptoServiceProvider.StrToToHexByte(encrypData);
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ECPoint c1 = CreatePoint(c1X, c1Y);
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InitDecipher(privateKey, c1);
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Decrypt(data);
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byte[] c3_ =newbyte[32];
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Dofinal(c3_);
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string decryptData = Encoding.Default.GetString(data);
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bool isDecrypt = Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(c3_) == c3;
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return (isDecrypt ? data : newbyte[0]);
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}
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/**////<summary>
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/// 创建坐标点
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///</summary>
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///<param></param>
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///<param></param>
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///<returns></returns>
171publicstatic ECPoint CreatePoint(string x, string y)
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...{
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BigInteger biX =new BigInteger(x, 16);
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BigInteger biY =new BigInteger(y, 16);
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ECFieldElement fx =new FpFieldElement(SM2CryptoServiceProvider.ecc_p, biX);
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ECFieldElement fy =new FpFieldElement(SM2CryptoServiceProvider.ecc_p, biY);
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ECPoint point =new FpPoint(SM2CryptoServiceProvider.ecc_curve, fx,fy);
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return point;
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}
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SM2解密算法