The value in Digest header included in the client's request or the server's response is invalid.
Possible issue might be that the digest is calculated before the content is fully encoded. The specification says: "Many frameworks or proxies might try to automatically modify your response after you sign it. For example, they may try to add additional gzip coding to your response's Content-Encodings if they detect that the client supports it. In many cases, this would be a good thing, but in this case, such changes could break your HTTP Signature (because we sign the content after it has been encoded). Make sure that you disable such automatic modifications when you use HTTP Signatures for signing."