Implement presigned policy

pkg/signature/errors.go

@@ -0,0 +1,32 @@
+package signature
+
+// MissingDateHeader date header missing
+type MissingDateHeader struct{}
+
+func (e MissingDateHeader) Error() string {
+	return "Missing date header"
+}
+
+// MissingExpiresQuery expires query string missing
+type MissingExpiresQuery struct{}
+
+func (e MissingExpiresQuery) Error() string {
+	return "Missing expires query string"
+}
+
+// ExpiredPresignedRequest request already expired
+type ExpiredPresignedRequest struct{}
+
+func (e ExpiredPresignedRequest) Error() string {
+	return "Presigned request already expired"
+}
+
+// DoesNotMatch invalid signature
+type DoesNotMatch struct {
+	SignatureSent       string
+	SignatureCalculated string
+}
+
+func (e DoesNotMatch) Error() string {
+	return "The request signature we calculated does not match the signature you provided"
+}