Why Does My .NET 6 Decryption Cut Off Strings Encrypted in .NET 5?

Troubleshooting Decryption Cutoff Issue in .Net 6

When upgrading from .Net 5 to .Net 6, an issue arises where strings encrypted using an encryption class similar to the one provided in the question get cut off during decryption. This issue only occurs when using .Net 6, leading to the following behavior:

Input: "12345678901234567890"

.NET 5 Output: "12345678901234567890"

.NET 6 Output: "1234567890123456"

The difference in length is noticeable, and the cause lies in a breaking change in .Net 6 related to the behavior of streams like CryptoStream.

Breaking Change in .Net 6

Starting from .Net 6, reading from a stream using Read() or ReadAsync() now completes when one of the following conditions is met:

1. At least one byte is read from the stream.
2. The underlying stream returns 0 from a read call, indicating no more data is available.

Impact on Decryption Code

In the provided code, the following section in the Decrypt() method is unaffected:

var plainTextBytes = new byte[cipherTextBytes.Length];
var decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);

Prior to .Net 6, the behavior of CryptoStream ensured that the buffer was completely filled or the end of the stream was reached before the operation was considered complete. However, this is no longer the case in .Net 6, resulting in potential data loss when the decryption buffer is not fully populated.

Solution

To resolve this issue, you can modify the Decrypt() method to ensure that all bytes are properly read:

• Use CopyTo() Method:

  using (var plainTextStream = new MemoryStream())
  {
      cryptoStream.CopyTo(plainTextStream);
      var plainTextBytes = plainTextStream.ToArray();
      return Encoding.UTF8.GetString(plainTextBytes, 0, plainTextBytes.Length);
  } 

• Use StreamReader.ReadToEnd():

  using (var plainTextReader = new StreamReader(cryptoStream))
  {
      return plainTextReader.ReadToEnd();
  }  

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