/// <summary>
/// Encrypts a string with the server's public key and convert it to base64.
/// </summary>
/// <param name="s">The string to be encrypted.</param>
/// <returns>The encrypted string in base64 format.</returns>
private String PpEncryptAndBase64(String s)
{
Contract.Requires(!ReferenceEquals(s, null));
RSACrypto rsa = new RSACrypto();
rsa.ImportCspBlob(_publicKey);
byte[] sEncrypted = rsa.Encrypt(_utf8.GetBytes(s));
return Convert.ToBase64String(sEncrypted);
}