StreamReader returning empty string when decrypting using Aes

Question

public static byte[] TestEncrypt(string plainText, byte[] key, byte[] iv)
{
    using Aes aes = Aes.Create();
    
    aes.Key = key;
    aes.IV = iv;
  
    // Create a new MemoryStream object to contain the encrypted bytes.
    using MemoryStream memoryStream = new MemoryStream();
    // Create a CryptoStream object to perform the encryption.
    using CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateEncryptor(), CryptoStreamMode.Write);
    // Encrypt the plaintext.
    using StreamWriter streamWriter = new StreamWriter(cryptoStream);
    streamWriter.Write(plainText);
    cryptoStream.FlushFinalBlock();

    byte[] cipherText = memoryStream.ToArray();

    return cipherText;
}

 public static string TestDecrypt(byte[] encryptedText, byte[] key, byte[] iv)
 {
     using Aes aes = Aes.Create();

     aes.Key = key;
     aes.IV = iv;
     using MemoryStream memoryStream = new MemoryStream(encryptedText);

     memoryStream.Position = 0;
     using CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateDecryptor(), CryptoStreamMode.Read);
         
     using StreamReader streamReader = new StreamReader(cryptoStream);
     cryptoStream.Flush();
     var data = streamReader.ReadToEnd() ;
     return data;
 }

I am using above two methods to encrypt and decrypt a string. I find that the streamreader always returns an empty string. I have verified multiple times that both the methods receive the same key and iv values including that the encrypted text is not modified/tampered when passed to TestDecrypt method.

I am still unable to figure out why an empty string is being returned ?

Please test the methods with any random key and IV value. I just want to know if the encrypted text is decrypted successfully.

I expect to see the encrypted text being successfully decrypted back to the original text. The result is an empty string which is incorrect.

Could you try removing cryptoStream.Flush() from the TestDecrypt method? It looks suspicious for a read-only stream. — Dmitry
– Dmitry, Commented Apr 16 at 19:00

Wyck · Accepted Answer · 2025-04-16 19:16:49Z

3

You're calling cryptoStream.FlushFinalBlock() outside the StreamWriter's disposal scope. Just do the write inside the using block and let the using block handle flushing the stream.

Make the following change to TestEncrypt:

    using (StreamWriter streamWriter = new StreamWriter(cryptoStream)) {
        streamWriter.Write(plainText);
    }
    // cryptoStream.FlushFinalBlock(); // Not necessary
    byte[] cipherText = memoryStream.ToArray();

as well this change in your TestDecrypt function:

    // cryptoStream.Flush(); // Not necessary

answered Apr 16 at 19:16

Wyck

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1 Comment

Gouri Apr 16 at 19:54

Thank you so much. Finally it worked after I made changes to TestEncrypt function. I had added cryptoStream.Flush() in TestDescrypt function just for trial and error purpose and I removed that as well.

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