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 }