AesCryptoServiceProvider in Xamarin - xamarin

Question

I have to use Aes encryption in our Xamarin PCL project. Our project uses the portable framework and not the standard so I can't use the new built in classes. I tried changing to the .NET Standard but ended up with countless errors and since I've never used it before, I figured I probably shouldn't make such a change now.
I thought this could be done using the PCLCrypto package but haven't been able to get it to work.
We currently support iOS and Android.
I'm trying to figure out how to convert the following code from our server side.
private static readonly byte[] CipherKey = ConvertHexStringToByteArray("some key");
static public string EncryptString(string originalPayload)
{
return Encrypt(originalPayload, CipherKey);
}
static private string Encrypt(string originalPayload, byte[] privateKey)
{
string encryptedPayload = "";
using (var aes = new AesCryptoServiceProvider()
{
Key = privateKey,
Mode = CipherMode.CBC,
Padding = PaddingMode.PKCS7
})
{
var input = Encoding.UTF8.GetBytes(originalPayload);
aes.GenerateIV();
var iv = aes.IV;
using (var encrypter = aes.CreateEncryptor(aes.Key, iv))
using (var cipherStream = new MemoryStream())
{
using (var tCryptoStream = new CryptoStream(cipherStream, encrypter, CryptoStreamMode.Write))
using (var tBinaryWriter = new BinaryWriter(tCryptoStream))
{
//Prepend IV to data
cipherStream.Write(iv, 0, iv.Length); //Write iv to the plain stream (not tested though)
tBinaryWriter.Write(input);
tCryptoStream.FlushFinalBlock();
}
encryptedPayload = Convert.ToBase64String(cipherStream.ToArray());
}
}
return encryptedPayload;
}
static public string DecryptString(string data)
{
return DecryptString(Convert.FromBase64String(data), CipherKey);
}
static private string DecryptString(byte[] encryptedString, byte[] encryptionKey)
{
using (var provider = new AesCryptoServiceProvider())
{
provider.Key = encryptionKey;
provider.Mode = CipherMode.CBC;
using (var ms = new MemoryStream(encryptedString))
{
// Read the first 16 bytes which is the IV.
byte[] iv = new byte[16];
ms.Read(iv, 0, 16);
provider.IV = iv;
using (var decryptor = provider.CreateDecryptor())
{
using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
{
using (var sr = new StreamReader(cs))
{
return sr.ReadToEnd();
}
}
}
}
}
}
Update
I found some code using PCLCrypto but can't get it working right. The decrypted string turns out to be squares.
Here's what I'm trying to use to decrypt the string from the server.
public static string DecryptAes(byte[] data, byte[] key)
{
ISymmetricKeyAlgorithmProvider aes = WinRTCrypto.SymmetricKeyAlgorithmProvider.OpenAlgorithm(SymmetricAlgorithmName.Aes,
SymmetricAlgorithmMode.Cbc, SymmetricAlgorithmPadding.None);
ICryptographicKey symetricKey = aes.CreateSymmetricKey(key);
string returnValue = "";
using (var ms = new MemoryStream(data))
{
// Read the first 16 bytes which is the IV.
byte[] iv = new byte[16];
ms.Read(iv, 0, 16);
using (var decryptor = WinRTCrypto.CryptographicEngine.CreateDecryptor(symetricKey, iv))
{
using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
{
using (var sr = new StreamReader(cs))
{
returnValue = sr.ReadToEnd();
}
}
}
}
return returnValue;
}

Answer 1

I was able to get it working. Here's the code in case someone runs into the same issue.
static public string Encrypt(string originalPayload, byte[] privateKey)
{
string encryptedPayload = "";
ISymmetricKeyAlgorithmProvider aes = WinRTCrypto.SymmetricKeyAlgorithmProvider.OpenAlgorithm(SymmetricAlgorithm.AesCbcPkcs7);
ICryptographicKey symetricKey = aes.CreateSymmetricKey(privateKey);
var iv = WinRTCrypto.CryptographicBuffer.GenerateRandom(aes.BlockLength);
var input = Encoding.UTF8.GetBytes(originalPayload);
using (var encrypter = WinRTCrypto.CryptographicEngine.CreateEncryptor(symetricKey, iv))
{
using (var cipherStream = new MemoryStream())
{
using (var tCryptoStream = new CryptoStream(cipherStream, encrypter, CryptoStreamMode.Write))
using (var tBinaryWriter = new BinaryWriter(tCryptoStream))
{
//Prepend IV to data
cipherStream.Write(iv, 0, iv.Length); //Write iv to the plain stream (not tested though)
tBinaryWriter.Write(input);
tCryptoStream.FlushFinalBlock();
}
encryptedPayload = Convert.ToBase64String(cipherStream.ToArray());
}
}
return encryptedPayload;
}
public static string DecryptAes(byte[] data, byte[] key)
{
ISymmetricKeyAlgorithmProvider aes = WinRTCrypto.SymmetricKeyAlgorithmProvider.OpenAlgorithm(SymmetricAlgorithm.AesCbcPkcs7);
ICryptographicKey symetricKey = aes.CreateSymmetricKey(key);
string returnValue = "";
using (var ms = new MemoryStream(data))
{
// Read the first 16 bytes which is the IV.
byte[] iv = new byte[16];
ms.Read(iv, 0, 16);
using (var decryptor = WinRTCrypto.CryptographicEngine.CreateDecryptor(symetricKey, iv))
{
using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
{
using (var sr = new StreamReader(cs))
{
returnValue = sr.ReadToEnd();
}
}
}
}
return returnValue;
}