作者：彭金龙 原文地址：http://www.jonllen.cn/jonllen/work/164.aspx
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。
SM2Cipher.cs
1
public
class
SM2Cipher
2
{ 3
private int ct = 1; 4 5private ECPoint p2; 6private SM3Digest sm3keybase; 7private SM3Digest sm3c3; 8 9private byte[] key = new byte[32]; 10private byte keyOff = 0; 11 12
public SM2Cipher() { } 13 14 15 16private void Reset() 17{ 18 sm3keybase = new SM3Digest(); 19 sm3c3 = new SM3Digest(); 20 21byte[] p; 22 23 p = p2.X.ToBigInteger().ToByteArrayUnsigned(); 24 sm3keybase.BlockUpdate(p, 0, p.Length); 25 sm3c3.BlockUpdate(p, 0, p.Length); 26 27 p = p2.Y.ToBigInteger().ToByteArrayUnsigned(); 28 sm3keybase.BlockUpdate(p, 0, p.Length); 29 30 ct = 1; 31 NextKey(); 32
} 33 34private void NextKey() 35{ 36 SM3Digest sm3keycur = new SM3Digest(sm3keybase); 37 sm3keycur.Update((byte)(ct >> 24 & 0x00ff)); 38 sm3keycur.Update((byte)(ct >> 16 & 0x00ff)); 39 sm3keycur.Update((byte)(ct >> 8 & 0x00ff)); 40 sm3keycur.Update((byte)(ct & 0x00ff)); 41 sm3keycur.DoFinal(key, 0); 42 keyOff = 0; 43 ct++; 44 } 45 46public virtual ECPoint InitEncipher(ECPoint userKey) 47{ 48 BigInteger k = null; 49 ECPoint c1 = null; 50 51if (1==1) 52{ 53 AsymmetricCipherKeyPair key = SM2CryptoServiceProvider.SM2KeyPairGenerator.GenerateKeyPair(); 54 ECPrivateKeyParameters ecpriv = (ECPrivateKeyParameters)key.Private; 55 ECPublicKeyParameters ecpub = (ECPublicKeyParameters)key.Public; 56 57 k = ecpriv.D; 58 c1 = ecpub.Q; 59 } 60 61 p2 = userKey.Multiply(k); 62 Reset(); 63 64return c1; 65 66 } 67 68public virtual void Encrypt(byte[] data) 69{ 70 sm3c3.BlockUpdate(data, 0, data.Length); 71for (int i = 0; i < data.Length; i++) 72{ 73if (keyOff == key.Length) 74 NextKey(); 75 76 data[i] ^= key[keyOff++]; 77 } 78 } 79 80public virtual void InitDecipher(BigInteger userD, ECPoint c1) 81{ 82 p2 = c1.Multiply(userD); 83 Reset(); 84 } 85 86public virtual void Decrypt(byte[] data) 87{ 88for (int i = 0; i < data.Length; i++) 89{ 90if (keyOff == key.Length) 91 NextKey(); 92 93 data[i] ^= key[keyOff++]; 94 } 95 sm3c3.BlockUpdate(data, 0, data.Length); 96 } 97 98public virtual void Dofinal(byte[] c3) 99{100byte[] p = p2.Y.ToBigInteger().ToByteArrayUnsigned();101 sm3c3.BlockUpdate(p, 0, p.Length);102 sm3c3.DoFinal(c3, 0);103 Reset();104 }105106107/// <summary>108/// 使用SM2公钥加密数据109/// </summary>110/// <param name="pubKey"></param>111/// <param name="plaintext"></param>112/// <returns></returns>113 public string Encrypt(ECPoint pubKey, byte[] plaintext)114{115116byte[] data = new byte[plaintext.Length];117 Array.Copy(plaintext, data, plaintext.Length);118119 ECPoint c1 = InitEncipher(pubKey);120 Encrypt(data);121122byte[] c3 = new byte[32];123 Dofinal(c3);124125string hexString = c1.X.ToBigInteger().ToString(16) + c1.Y.ToBigInteger().ToString(16)126+ Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(data)127+ Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(c3);128129return hexString;130131 }132133/// <summary>134/// 使用SM2解密数据135/// </summary>136/// <param name="privateKey"></param>137/// <param name="ciphertext"></param>138/// <returns></returns>139 public byte[] Decrypt(BigInteger privateKey, string ciphertext)140{141142string hexString = ciphertext;143string c1X = hexString.Substring(0, 64);144string c1Y = hexString.Substring(0 + c1X.Length, 64);145string encrypData = hexString.Substring(c1X.Length + c1Y.Length, hexString.Length - c1X.Length - c1Y.Length - 64);146string c3 = hexString.Substring(hexString.Length - 64);147148byte[] data = SM2CryptoServiceProvider.StrToToHexByte(encrypData);149150 ECPoint c1 = CreatePoint(c1X, c1Y);151152 InitDecipher(privateKey, c1);153 Decrypt(data);154155byte[] c3_ = new byte[32];156 Dofinal(c3_);157158string decryptData = Encoding.Default.GetString(data);159bool isDecrypt = Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(c3_) == c3;160161return (isDecrypt ? data : new byte[0]);162 }163164165/// <summary>166/// 创建坐标点167/// </summary>168/// <param name="x"></param>169/// <param name="y"></param>170/// <returns></returns>171 public static ECPoint CreatePoint(string x, string y)172{173174 BigInteger biX = new BigInteger(x, 16);175 BigInteger biY = new BigInteger(y, 16);176 ECFieldElement fx = new FpFieldElement(SM2CryptoServiceProvider.ecc_p, biX);177 ECFieldElement fy = new FpFieldElement(SM2CryptoServiceProvider.ecc_p, biY);178 ECPoint point = new FpPoint(SM2CryptoServiceProvider.ecc_curve, fx,fy);179return point;180 }181182183184
}
public
class
SM2Cipher
2
{ 3private int ct = 1; 4 5private ECPoint p2; 6private SM3Digest sm3keybase; 7private SM3Digest sm3c3; 8 9private byte[] key = new byte[32]; 10private byte keyOff = 0; 11 12public SM2Cipher() { } 13 14 15 16private void Reset() 17{ 18 sm3keybase = new SM3Digest(); 19 sm3c3 = new SM3Digest(); 20 21byte[] p; 22 23 p = p2.X.ToBigInteger().ToByteArrayUnsigned(); 24 sm3keybase.BlockUpdate(p, 0, p.Length); 25 sm3c3.BlockUpdate(p, 0, p.Length); 26 27 p = p2.Y.ToBigInteger().ToByteArrayUnsigned(); 28 sm3keybase.BlockUpdate(p, 0, p.Length); 29 30 ct = 1; 31 NextKey(); 32 } 33 34private void NextKey() 35{ 36 SM3Digest sm3keycur = new SM3Digest(sm3keybase); 37 sm3keycur.Update((byte)(ct >> 24 & 0x00ff)); 38 sm3keycur.Update((byte)(ct >> 16 & 0x00ff)); 39 sm3keycur.Update((byte)(ct >> 8 & 0x00ff)); 40 sm3keycur.Update((byte)(ct & 0x00ff)); 41 sm3keycur.DoFinal(key, 0); 42 keyOff = 0; 43 ct++; 44 } 45 46public virtual ECPoint InitEncipher(ECPoint userKey) 47{ 48 BigInteger k = null; 49 ECPoint c1 = null; 50 51if (1==1) 52{ 53 AsymmetricCipherKeyPair key = SM2CryptoServiceProvider.SM2KeyPairGenerator.GenerateKeyPair(); 54 ECPrivateKeyParameters ecpriv = (ECPrivateKeyParameters)key.Private; 55 ECPublicKeyParameters ecpub = (ECPublicKeyParameters)key.Public; 56 57 k = ecpriv.D; 58 c1 = ecpub.Q; 59 } 60 61 p2 = userKey.Multiply(k); 62 Reset(); 63 64return c1; 65 66 } 67 68public virtual void Encrypt(byte[] data) 69{ 70 sm3c3.BlockUpdate(data, 0, data.Length); 71for (int i = 0; i < data.Length; i++) 72{ 73if (keyOff == key.Length) 74 NextKey(); 75 76 data[i] ^= key[keyOff++]; 77 } 78 } 79 80public virtual void InitDecipher(BigInteger userD, ECPoint c1) 81{ 82 p2 = c1.Multiply(userD); 83 Reset(); 84 } 85 86public virtual void Decrypt(byte[] data) 87{ 88for (int i = 0; i < data.Length; i++) 89{ 90if (keyOff == key.Length) 91 NextKey(); 92 93 data[i] ^= key[keyOff++]; 94 } 95 sm3c3.BlockUpdate(data, 0, data.Length); 96 } 97 98public virtual void Dofinal(byte[] c3) 99{100byte[] p = p2.Y.ToBigInteger().ToByteArrayUnsigned();101 sm3c3.BlockUpdate(p, 0, p.Length);102 sm3c3.DoFinal(c3, 0);103 Reset();104 }105106107/// <summary>108/// 使用SM2公钥加密数据109/// </summary>110/// <param name="pubKey"></param>111/// <param name="plaintext"></param>112/// <returns></returns>113 public string Encrypt(ECPoint pubKey, byte[] plaintext)114{115116byte[] data = new byte[plaintext.Length];117 Array.Copy(plaintext, data, plaintext.Length);118119 ECPoint c1 = InitEncipher(pubKey);120 Encrypt(data);121122byte[] c3 = new byte[32];123 Dofinal(c3);124125string hexString = c1.X.ToBigInteger().ToString(16) + c1.Y.ToBigInteger().ToString(16)126+ Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(data)127+ Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(c3);128129return hexString;130131 }132133/// <summary>134/// 使用SM2解密数据135/// </summary>136/// <param name="privateKey"></param>137/// <param name="ciphertext"></param>138/// <returns></returns>139 public byte[] Decrypt(BigInteger privateKey, string ciphertext)140{141142string hexString = ciphertext;143string c1X = hexString.Substring(0, 64);144string c1Y = hexString.Substring(0 + c1X.Length, 64);145string encrypData = hexString.Substring(c1X.Length + c1Y.Length, hexString.Length - c1X.Length - c1Y.Length - 64);146string c3 = hexString.Substring(hexString.Length - 64);147148byte[] data = SM2CryptoServiceProvider.StrToToHexByte(encrypData);149150 ECPoint c1 = CreatePoint(c1X, c1Y);151152 InitDecipher(privateKey, c1);153 Decrypt(data);154155byte[] c3_ = new byte[32];156 Dofinal(c3_);157158string decryptData = Encoding.Default.GetString(data);159bool isDecrypt = Org.BouncyCastle.Utilities.Encoders.Hex.ToHexString(c3_) == c3;160161return (isDecrypt ? data : new byte[0]);162 }163164165/// <summary>166/// 创建坐标点167/// </summary>168/// <param name="x"></param>169/// <param name="y"></param>170/// <returns></returns>171 public static ECPoint CreatePoint(string x, string y)172{173174 BigInteger biX = new BigInteger(x, 16);175 BigInteger biY = new BigInteger(y, 16);176 ECFieldElement fx = new FpFieldElement(SM2CryptoServiceProvider.ecc_p, biX);177 ECFieldElement fy = new FpFieldElement(SM2CryptoServiceProvider.ecc_p, biY);178 ECPoint point = new FpPoint(SM2CryptoServiceProvider.ecc_curve, fx,fy);179return point;180 }181182183184}
SM2解密算法
SM2解密算法是加密逆运算。首先需要从密文中取出加密结果的3部分值，然后通过私钥计算出 M'明文值，最后效验数据。SM2解密算法流程如下图所示。
SM2解密同样也可以使用软算法实现。但因为涉及到私钥运算，为保护私钥安全，建议在硬件设备中运行，例如UKey等存储介质这样可以更好的保护密钥安全。拿文件加密来说，首先拿UKey里面的加密证书加密，这部分可在应用系统内完成。解密的话则需要加密证书对应UKey才能做解密，由应用系统调用UKey解密接口，在物理硬件内完成数据解密，同时可以受设备Ping码保护。