Simple class to get a a Gravatar URI in WinRT

GravatarWinRT.cs

using System;
using Windows.Security.Cryptography;
using Windows.Security.Cryptography.Core;

namespace IF.Utilities.Common
{
    public class Gravatar
    {
        private const string GravatarFormatString = "http://www.gravatar.com/avatar/{0}.jpg?d=retro&r=g&s={1}";

        public static Uri GetGravatarUri(string email, int width = 80)
        {
            var hash = email.HashMD5();

            var uri = string.Format(GravatarFormatString, hash, width);

            return new Uri(uri);
        }
    }

    public static class Md5Ex
    {
        public static string HashMD5(this string str)
        {
            var alg = HashAlgorithmProvider.OpenAlgorithm("MD5");
            var buff = CryptographicBuffer.ConvertStringToBinary(str, BinaryStringEncoding.Utf8);
            var hashed = alg.HashData(buff);
            var res = CryptographicBuffer.EncodeToHexString(hashed);
            return res;
        }
    }
}