Disclaimer: In practice, it's usually best to use higher-level open standards designed for these scenarios (such as Cryptographic Message Syntax provided by Rebex.Common assembly) instead of encrypting/decrypting data directly using low-level cryptographic primitives.
However, if you have a string encrypted directly using one of the supported Rijndael modes, it should be possible to decrypt the input data using Rebex SymmetricKeyAlgorithm class, and convert the resulting byte array to a string using .NET System.Text.Encoding class.
For example, let's say the string has been encrypted using Rijndael encryption in CBC mode, with PKCS #7 padding and with a 128-bit key and IV. So overall, we would have something like this on input:
// encrypted string
var encryptedData = new byte[]
{
0xF6, 0x94, 0x51, 0xFD, 0x34, 0x67, 0xAD, 0xC0,
0x76, 0x54, 0xE3, 0x4A, 0x21, 0x83, 0xA9, 0xF8,
0x53, 0xF4, 0x71, 0x97, 0x67, 0xD7, 0xC0, 0xAB,
0x59, 0xDC, 0xF6, 0x2B, 0xD0, 0xE9, 0xEE, 0x7F,
};
// charset used by the encrypted string
var encoding = Encoding.UTF8;
// 128-bit key
var key = new byte[]
{
0xA4, 0x6C, 0xDC, 0x10, 0x15, 0xD6, 0x94, 0xFF,
0x53, 0xDC, 0x35, 0x68, 0x4C, 0x98, 0x03, 0xEB,
};
// 128-bit initialization vector
var iv = new byte[]
{
0xBD, 0x1B, 0x28, 0xCE, 0x3E, 0x26, 0x24, 0x3C,
0xD0, 0x5C, 0x6E, 0x68, 0x69, 0x18, 0xE2, 0x41,
};
// cipher and padding mode
var cipherMode = CipherMode.CBC;
var paddingMode = PaddingMode.PKCS7;
Then, decryption itself will look like this:
// decrypt using AES (a subset of Rijndael)
string decryptedString;
using (var alg = new SymmetricKeyAlgorithm(SymmetricKeyAlgorithmId.AES))
{
// configure the algorithm
alg.Mode = cipherMode;
alg.Padding = paddingMode;
alg.SetKey(key);
alg.SetIV(iv);
// decrypt encrypted data and convert it to a string
using (ICryptoTransform transform = alg.CreateDecryptor())
{
byte[] decryptedData = transform.TransformFinalBlock(encryptedData, 0, encryptedData.Length);
decryptedString = encoding.GetString(decryptedData);
}
}
// show the decrypted string
Console.WriteLine("Decrypted string: {0}", decryptedString);