RSA 分段加解密【解决“不正确的长度”的异常】
RSA 是常用的非对称加密算法。最近使用时却出现了“不正确的长度”的异常，研究发现是由于待加密的数据超长所致。
.NET Framework 中提供的 RSA 算法规定：
待加密的字节数不能超过密钥的长度值除以 8 再减去 11（即：RSACryptoServiceProvider.KeySize / 8 - 11），而加密后得到密文的字节数，正好是密钥的长度值除以 8（即：RSACryptoServiceProvider.KeySize / 8）。
所以，如果要加密较长的数据，则可以采用分段加解密的方式，实现方式如下：
1 namespace Macroresolute.RSACryptoService 2 { 3 public static class RSACrypto 4 { 5 private static readonly Encoding Encoder = Encoding.UTF8; 6 7 public static String Encrypt(this String plaintext) 8 { 9 X509Certificate2 _X509Certificate2 = RSACrypto.RetrieveX509Certificate(); 10 using (RSACryptoServiceProvider RSACryptography = _X509Certificate2.PublicKey.Key as RSACryptoServiceProvider) 11 { 12 Byte[] PlaintextData = RSACrypto.Encoder.GetBytes(plaintext); 13 int MaxBlockSize = RSACryptography.KeySize / 8 - 11; //加密块最大长度限制 14 15 if (PlaintextData.Length <= MaxBlockSize) 16 return Convert.ToBase64String(RSACryptography.Encrypt(PlaintextData, false)); 17 18 using (MemoryStream PlaiStream = new MemoryStream(PlaintextData)) 19 using (MemoryStream CrypStream = new MemoryStream()) 20 { 21 Byte[] Buffer = new Byte[MaxBlockSize]; 22 int BlockSize = PlaiStream.Read(Buffer, 0, MaxBlockSize); 23 24 while (BlockSize > 0) 25 { 26 Byte[] ToEncrypt = new Byte[BlockSize]; 27 Array.Copy(Buffer, 0, ToEncrypt, 0, BlockSize); 28 29 Byte[] Cryptograph = RSACryptography.Encrypt(ToEncrypt, false); 30 CrypStream.Write(Cryptograph, 0, Cryptograph.Length); 31 32 BlockSize = PlaiStream.Read(Buffer, 0, MaxBlockSize); 33 } 34 35 return Convert.ToBase64String(CrypStream.ToArray(), Base64FormattingOptions.None); 36 } 37 } 38 } 39 40 public static String Decrypt(this String ciphertext) 41 { 42 X509Certificate2 _X509Certificate2 = RSACrypto.RetrieveX509Certificate(); 43 using (RSACryptoServiceProvider RSACryptography = _X509Certificate2.PrivateKey as RSACryptoServiceProvider) 44 { 45 Byte[] CiphertextData = Convert.FromBase64String(ciphertext); 46 int MaxBlockSize = RSACryptography.KeySize / 8; //解密块最大长度限制 47 48 if (CiphertextData.Length <= MaxBlockSize) 49 return RSACrypto.Encoder.GetString(RSACryptography.Decrypt(CiphertextData, false)); 50 51 using (MemoryStream CrypStream = new MemoryStream(CiphertextData)) 52 using (MemoryStream PlaiStream = new MemoryStream()) 53 { 54 Byte[] Buffer = new Byte[MaxBlockSize]; 55 int BlockSize = CrypStream.Read(Buffer, 0, MaxBlockSize); 56 57 while (BlockSize > 0) 58 { 59 Byte[] ToDecrypt = new Byte[BlockSize]; 60 Array.Copy(Buffer, 0, ToDecrypt, 0, BlockSize); 61 62 Byte[] Plaintext = RSACryptography.Decrypt(ToDecrypt, false); 63 PlaiStream.Write(Plaintext, 0, Plaintext.Length); 64 65 BlockSize = CrypStream.Read(Buffer, 0, MaxBlockSize); 66 } 67 68 return RSACrypto.Encoder.GetString(PlaiStream.ToArray()); 69 } 70 } 71 } 72 73 private static X509Certificate2 RetrieveX509Certificate() 74 { 75 return null; //检索用于 RSA 加密的 X509Certificate2 证书 76 } 77 } 78 }
注：以上加密方法返回的字符串类型为原始的 Base-64 ，若要用于 URL 传输，需另行处理！
