You may remember in one of my previous posts I recommended the Affirma ThreeSharp library for interacting with your Amazon S3 storage system. One little ‘gotcha’ I ran into having used the library for a while is that the library does not UrlEncode the file name prior to sending it to Amazon. So If your file has a non URL safe character such as a ‘#’ in it you will receive an exception like the title of this post stating there was a signature mismatch. This is in fact a bit of a red herring, but once I looked at the logs for the files that generated this exception it became pretty clear what the issue was. So don’t forget to call System.Web.HttpUtility.UrlEncode(keyName)) on your key before passing it to the Three Sharp library.