 <p>Ciphertext is very unlikely to be genuinely UTF-16-encoded text. Assuming that the <em>encryption</em> side had something like:</p> <pre><code>string encryptedText = Encoding.Unicode.GetString(encryptedBytes); </code></pre> <p>you've basically lost data. The result of encryption is <em>not</em> text - it's arbitrary binary data. If you want to convert that to text for some transport reason, you should use Base64, e.g.</p> <pre><code>string base64EncryptedText = Convert.ToBase64String(encryptedBytes); </code></pre> <p>Then use <code>Convert.FromBase64String</code> to recover the original encrypted binary data which is ready to decrypt.</p>