不限长度RAS加密解密

RSA 是常用的非对称加密算法。最近使用时却出现了提示“不正确的长度”，排查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：

加密部分代码如下:

public static String Encrypt(string xmlPublicKey, string EncryptString)
{
    using (RSACryptoServiceProvider RSACryptography = new RSACryptoServiceProvider())
    {
        RSACryptography.FromXmlString(xmlPublicKey);
        Byte[] PlaintextData = Encoding.UTF8.GetBytes(EncryptString);
        int MaxBlockSize = RSACryptography.KeySize / 8 - 11;    //加密块最大长度限制

        if (PlaintextData.Length <= MaxBlockSize)
            return Convert.ToBase64String(RSACryptography.Encrypt(PlaintextData, false));

        using (MemoryStream PlaiStream = new MemoryStream(PlaintextData))
        using (MemoryStream CrypStream = new MemoryStream())
        {
            Byte[] Buffer = new Byte[MaxBlockSize];
            int BlockSize = PlaiStream.Read(Buffer, 0, MaxBlockSize);

            while (BlockSize > 0)
            {
                Byte[] ToEncrypt = new Byte[BlockSize];
                Array.Copy(Buffer, 0, ToEncrypt, 0, BlockSize);

                Byte[] Cryptograph = RSACryptography.Encrypt(ToEncrypt, false);
                CrypStream.Write(Cryptograph, 0, Cryptograph.Length);

                BlockSize = PlaiStream.Read(Buffer, 0, MaxBlockSize);
            }

            return Convert.ToBase64String(CrypStream.ToArray(), Base64FormattingOptions.None);
        }
    }
}

//解密部分代码如下

public static String Decrypt(string xmlPrivateKey, string EncryptString)
{
    using (RSACryptoServiceProvider RSACryptography = new RSACryptoServiceProvider())
    {

        try
        {
            RSACryptography.FromXmlString(xmlPrivateKey);
            Byte[] CiphertextData = Convert.FromBase64String(EncryptString);
            int MaxBlockSize = RSACryptography.KeySize / 8;    //解密块最大长度限制

            if (CiphertextData.Length <= MaxBlockSize)
                return Encoding.UTF8.GetString(RSACryptography.Decrypt(CiphertextData, false));

            using (MemoryStream CrypStream = new MemoryStream(CiphertextData))
            using (MemoryStream PlaiStream = new MemoryStream())
            {
                Byte[] Buffer = new Byte[MaxBlockSize];
                int BlockSize = CrypStream.Read(Buffer, 0, MaxBlockSize);

                while (BlockSize > 0)
                {
                    Byte[] ToDecrypt = new Byte[BlockSize];
                    Array.Copy(Buffer, 0, ToDecrypt, 0, BlockSize);

                    Byte[] Plaintext = RSACryptography.Decrypt(ToDecrypt, false);
                    PlaiStream.Write(Plaintext, 0, Plaintext.Length);

                    BlockSize = CrypStream.Read(Buffer, 0, MaxBlockSize);
                }

                return Encoding.UTF8.GetString(PlaiStream.ToArray());
            }
        }
        catch (Exception)
        {
            return "解密失败,请检查传输数据";
        }
    }
}