There are plenty of encryption libraries out there for .NET and plenty for Flash/Flex. Getting them to communicate together on the other hand... well, just google .NET/Flex encryption and you'll soon realize, the two do not play very friendly.

We spent the better part of 5 days attempting every known encryption method out there via the AS3Crypto library (http://code.google.com/p/as3crypto/) (and 3 others freely available) on the Flex side and the same thing on the .NET side. Unfortunately, every test was a failure. 3 days into our attempts to decrypt a .NET encrypted string, we had some success as I was able to successfully pull out the first character of the encrypted string but nothing beyond that. We eventually settled on using an MD5 hash "system" to protect service calls. Not really wanting to settle on security, the .NET developer suggested one more attempt. Still unsuccessful until I altered one minor detail: encoding.

It turns out, on the .NET side, we were encoding the text in Unicode format before encrypting the string. Once it was passed over to Flex, I was never able to decode successfully because this. Once we updated the .NET to encode the string using UTF-8, I was able to successfully decode in Flex.

The one key from all the successful .NET to Flex encryption/decryption that I never saw: ensure your .NET encodes anything you want to encrypt in UTF-8 prior to encrypting.

.NET code:
public static string Encrypt(string plainText)
{
byte[] keyArray;
// path of file to encrypt
byte[] toEncryptArray = UTF8Encoding.UTF8.GetBytes(plainText);
string key = "RANDOMKEY";

keyArray = UTF8Encoding.UTF8.GetBytes(key);

System.Security.Cryptography.RijndaelManaged aes = new RijndaelManaged();
aes.Key = keyArray;
aes.Mode = CipherMode.ECB;
ICryptoTransform cTransform = aes.CreateEncryptor();
byte[] resultArray = cTransform.TransformFinalBlock
(toEncryptArray, 0, toEncryptArray.Length);
aes.Clear();
return Convert.ToBase64String(resultArray);
}

Flex code (using as3crypto):
private function decrypt(value:String, key:String):String
{
var keyByteArray:ByteArray = Hex.toArray(Hex.fromString(key));
var valueByteArray:ByteArray = Base64.decodeToByteArray(value));
var aes:ICipher = Crypto.getCipher("aes-ecb", keyByteArray);
aes.decrypt(valueByteArray);
return valueByteArray.toString();
}