Static Interface Method

I have a class to encrypt data. But it will be used as a DLL, and for this I need to create an interface to show the methods (I tested without the interface and it did not work). However, it contains 2 static methods, and I'm getting an error for this. Is there a way to do this interface or convert my class to no longer use static?

Follow my code:

using System;
using System.Security.Cryptography;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;

namespace PortalRH.DLL
{
[ComVisible(true), ClassInterface(ClassInterfaceType.None),
Guid("00AC4F7E-71B0-4BC7-AD8E-1175CD88457A")]
public class Criptografia 
{
private string chave = "chave";
public Criptografia(){}


// Essa seqüência constante é usada como um valor "salt" para as chamadas de função PasswordDeriveBytes .
// Este tamanho da IV (em bytes) devem = (KeySize / 8). KeySize padrão é 256, portanto, a IV deve ser
// 32 bytes de comprimento. Usando uma seqüência de 16 caracteres aqui nos dá 32 bytes quando convertido para um array de bytes.
private static readonly byte[] initVectorBytes = Encoding.ASCII.GetBytes("tu89geji340t89u2");

// Esta constante é utilizado para determinar o tamanho da chave do algoritmo de encriptação.
private const int keysize = 256;

public static string Encrypt(string plainText, string passPhrase)
{
    byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);
    using (PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null))
    {
        byte[] keyBytes = password.GetBytes(keysize / 8);
        using (RijndaelManaged symmetricKey = new RijndaelManaged())
        {
            symmetricKey.Mode = CipherMode.CBC;
            using (ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, initVectorBytes))
            {
                using (MemoryStream memoryStream = new MemoryStream())
                {
                    using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                    {
                        cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
                        cryptoStream.FlushFinalBlock();
                        byte[] cipherTextBytes = memoryStream.ToArray();
                        return Convert.ToBase64String(cipherTextBytes);
                    }
                }
            }
        }
    }
}

public static string Decrypt(string cipherText, string passPhrase)
{
    byte[] cipherTextBytes = Convert.FromBase64String(cipherText);
    using (PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null))
    {
        byte[] keyBytes = password.GetBytes(keysize / 8);
        using (RijndaelManaged symmetricKey = new RijndaelManaged())
        {
            symmetricKey.Mode = CipherMode.CBC;
            using (ICryptoTransform decryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes))
            {
                using (MemoryStream memoryStream = new MemoryStream(cipherTextBytes))
                {
                    using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                    {
                        byte[] plainTextBytes = new byte[cipherTextBytes.Length];
                        int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);
                        return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
                    }
                }
            }
        }
    }
}

}

Interface:

using System.Runtime.InteropServices;

namespace PortalRH.DLL
{
[Guid("16E2B6C1-1CC2-4B71-BE4E-9F6DF103AA3E")]
public interface ICriptografia
{


    string Encrypt(string str);
    string Decrypt(string str);

}

}

The error returned is the following:

> An object reference is required for the non-static field, method, or property 
> 'PortalRH.DLL.Criptografia.Decrypt(string, string)'