Why is my .NET 6 string decryption truncated after upgrading from .NET 5?

Problem when upgrading to .Net 6 - Encrypting string

When upgrading a project from .Net 5 to .Net 6, a problem occurs problem decrypting strings using an encryption/decryption class. The result is a decrypted string cut off at a certain point depending on the length of the input string.

Cause

A change in the behavior of stream-related classes such as CryptoStream, in .Net 6. In previous versions, these classes read all bytes from the buffer or reached the end of the stream before completing the read operation. In .Net 6, they complete after reading at least one byte or after returning 0 from a read call from the underlying stream.

Affected Code

The problem arises in the Decrypt method of the encryption class, specifically in reading decrypted bytes from CryptoStream using a buffer fixed:

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

Solution

To resolve the problem, it is necessary to check the number of bytes actually read by CryptoStream and adjust the reading accordingly. This can be done by checking the Length property of the MemoryStream after the read operation.

Alternatively, a better approach is to use the CryptoStream's CopyTo method or create a StreamReader directly on top of the CryptoStream. These approaches ensure that all decrypted bytes are read and returned:

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

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

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