为什么 AesManaged.FeedbackSizeValue = 8 当 AesManaged.FeedbackSize = 128

Question

我希望明确设置加密参数（从 .NET 4.7 默认值读取），以避免在未来的框架版本实现不同的默认值时出现解密困难。

为什么手动设置反馈FeedbackSize后，FeedbackSizeValue没有变化？

AesManaged aes = new AesManaged();
aes.Mode = CipherMode.CBC;
aes.KeySize = 256;
aes.Padding = PaddingMode.PKCS7;
aes.BlockSize = 128;
aes.FeedbackSize = aes.BlockSize;

这里是aes对象，看FeedbackSizeValue 8

-       aes {System.Security.Cryptography.AesManaged}   System.Security.Cryptography.AesManaged

-       aes {System.Security.Cryptography.AesManaged}   System.Security.Cryptography.AesManaged
        BlockSize   128 int
        BlockSizeValue  128 int
        FeedbackSize    128 int
        FeedbackSizeValue   8   int
+       IV  {byte[16]}  byte[]
        IVValue null    byte[]
+       Key {byte[32]}  byte[]
        KeySize 256 int
        KeySizeValue    256 int
        KeyValue    null    byte[]
+       LegalBlockSizes {System.Security.Cryptography.KeySizes[1]}  System.Security.Cryptography.KeySizes[]
+       LegalBlockSizesValue    {System.Security.Cryptography.KeySizes[1]}  System.Security.Cryptography.KeySizes[]
+       LegalKeySizes   {System.Security.Cryptography.KeySizes[1]}  System.Security.Cryptography.KeySizes[]
+       LegalKeySizesValue  {System.Security.Cryptography.KeySizes[1]}  System.Security.Cryptography.KeySizes[]
        Mode    CBC System.Security.Cryptography.CipherMode
        ModeValue   CBC System.Security.Cryptography.CipherMode
        Padding PKCS7   System.Security.Cryptography.PaddingMode
        PaddingValue    PKCS7   System.Security.Cryptography.PaddingMode
+       m_rijndael  {System.Security.Cryptography.RijndaelManaged}  System.Security.Cryptography.RijndaelManaged
+       Static members      

Answer 1

CBC 模式没有反馈大小。见 CBC mode:

图片来自维基百科

也不需要指定块大小(aes.BlockSize = 128;)，AES只有一种块大小。 Rijndael 确实支持多种块大小，并且必须指定 128 位的块大小才能本质上成为 AES。虽然许多 Rijndael 实现默认为 128 位的块大小，但最好完全指定参数。