public override SecurityToken CreateToken(SecurityTokenDescriptor tokenDescriptor)
{
    if (tokenDescriptor == null)
    {
        throw new ArgumentNullException("tokenDescriptor");
    }

    NameValueCollection properties = new NameValueCollection();
    properties.Add(SimpleWebTokenConstants.Id, Guid.NewGuid().ToString());
    properties.Add(SimpleWebTokenConstants.Issuer, tokenDescriptor.TokenIssuerName);
    properties.Add(SimpleWebTokenConstants.Audience, tokenDescriptor.AppliesToAddress);
    properties.Add(SimpleWebTokenConstants.ExpiresOn, SecondsFromSwtBaseTime(tokenDescriptor.Lifetime.Expires));
    properties.Add(SimpleWebTokenConstants.ValidFrom, SecondsFromSwtBaseTime(tokenDescriptor.Lifetime.Created));

    foreach (Claim claim in tokenDescriptor.Subject.Claims)
    {
        properties.Add(claim.Type, claim.Value);
    }

    SimpleWebToken token = new SimpleWebToken(properties);
    return token;
}