I use BASE64 to encode GUID value and add them to cookie. For example, an ecoded guid value is vClFwpDbWE6JPUlnlBXMWg==. When the server sends response, it will add this cookie. I check with Chrome, this value is correctly received by the browser. But when the browser sends another request, the cookie value is changed to "vClFwpDbWE6JPUlnlBXMWg" from HttpRequestMessage's cookies, why some characters are removed?
I use WebAPI2, MVC5 with IIS7.5.
ASP.NET sees the '=' character in the cookie and assumes it's a multi-value cookie (see related question Storing multiple values in cookies).
Your best bet is to store the GUID in the cookie as-is, e.g., by using Guid.ToString() to turn the GUID into a hex string and new Guid(string) to turn the hex string back into a GUID. Alternatively, if you really need to condense it down to BASE64, consider using HttpServerUtility's UrlTokenEncode and UrlTokenDecode methods. Those methods use an encoding which is very similar to BASE64 but which doesn't use characters like '+' and '=' which are treated specially by ASP.NET.
