Why Are My Decrypted Strings Truncated After Upgrading to .NET 6?

Problem with Encrypting Strings After Updating to .NET 6

Problem Summary:

When updating a .NET project to version 6, it was discovered that the decryption of encrypted strings results in a partially truncated output compared to the original input. Specifically, a portion of the decrypted string is cut off, the length difference being consistent.

Cause:

The issue stems from a breaking change in .NET 6 affecting streams like CryptoStream. Previously, these streams behaved uniquely in not completing read operations until all provided buffer space was filled or the end of the stream was reached.

With .NET 6, these streams now behave more consistently with other streams. If a read operation is performed with a buffer of length N, it completes when either:

• At least one byte is read from the stream, or
• The underlying stream returns 0 from its own read operation.

Code Impact:

In the provided encryption/decryption code, the CryptoStream being used is not properly accounting for this change. Specifically, the code fails to check whether all bytes were read and returned during the decryption process.

Solution:

To resolve the issue, the code needs to be modified to ensure it reads all available bytes during decryption. This can be achieved by using one of the following approaches:

• Using CopyTo: This method directly copies all data from the CryptoStream to a new buffer, guaranteeing that all bytes are read.

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

• Using StreamReader: Since the plaintext is UTF8-encoded, using a StreamReader to read the data from the CryptoStream directly is a convenient solution.

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

By implementing these fixes, the code will ensure that all decrypted bytes are captured and properly returned as part of the output string.

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