Question

I would like to encrypt and decrypt strings with a password. I use C# and WinRT (MetroStyle). Have somebody a class for encryption/decryption?

Answer 1

The normal .Net System.Security.Cryptography namespace does not exist in Metro. You use the CryptographicEngine class in Windows.Security.Cryptography.Core namespace instead.

If the password is only being verified/authenticated, do not encrypt it. Instead, use the following:

using Windows.Security.Cryptography.Core;
using Windows.Security.Cryptography;
using Windows.Storage.Streams;

...
// Use Password Based Key Derivation Function 2 (PBKDF2 or RFC2898)
KeyDerivationAlgorithmProvider pbkdf2 = 
    KeyDerivationAlgorithmProvider.OpenAlgorithm(
        KeyDerivationAlgorithmNames.Pbkdf2Sha256);

// Do not store passwords in strings if you can avoid them. The
// password may be retained in memory until it is garbage collected.
// Crashing the application and looking at the memory dump may 
// reveal it.
IBuffer passwordBuffer = 
     CryptographicBuffer.ConvertStringToBinary("password", 
         BinaryStringEncoding.Utf8);
CryptographicKey key = pbkdf2.CreateKey(passwordBuffer);

// Use random salt and 10,000 iterations. Store the salt along with 
// the derviedBytes (see below).
IBuffer salt = CryptographicBuffer.GenerateRandom(32);
KeyDerivationParameters parameters = 
    KeyDerivationParameters.BuildForPbkdf2(salt, 10000);

// Store the returned 32 bytes along with the salt for later verification
byte[] derviedBytes = 
    CryptographicEngine.DeriveKeyMaterial(key, parameters, 32).ToArray();

When a password is supplied run through the same process using the same salt and compare derivedBytes. Store the secret as you would an encryption key.

If the password will be used, such as to connect to another service:

// Use AES, CBC mode with PKCS#7 padding (good default choice)
SymmetricKeyAlgorithmProvider aesCbcPkcs7 = 
    SymmetricKeyAlgorithmProvider.OpenAlgorithm(SymmetricAlgorithmNames.AesCbcPkcs7);

// Create an AES 128-bit (16 byte) key
CryptographicKey key = 
    aesCbcPkcs7.CreateSymmetricKey(CryptographicBuffer.GenerateRandom(16));

// Creata a 16 byte initialization vector
IBuffer iv = CryptographicBuffer.GenerateRandom(aesCbcPkcs7.BlockLength);

// Encrypt the data
byte[] plainText = Encoding.UTF8.GetBytes("Hello, world!"); // Data to encrypt
byte[] cipherText = CryptographicEngine.Encrypt(
    key, plainText.AsBuffer(), iv).ToArray();

// Decrypt the data
string newPlainText = new string(
    Encoding.UTF8.GetChars(CryptographicEngine.Decrypt(
        key, cipherText.AsBuffer(), iv).ToArray()));

// newPlainText contains "Hello, world!"

As with any cryptography, make sure to protect your keys appropriately and follow best practise. The linked documentation also provides examples.