Harden ValidateSignature against invalid input

src/Stripe.net/Services/Events/EventUtility.cs

@@ -166,10 +166,22 @@ public static void ValidateSignature(string json, string stripeSignatureHeader,
 
         private static ILookup<string, string> ParseStripeSignature(string stripeSignatureHeader)
         {
+            (string Key, string Value) ParseItem(string item)
+            {
+                string[] parts = item.Trim().Split(new[] { '=' }, 2);
+                if (parts.Length != 2)
+                {
+                    throw new StripeException(
+                        "The signature header format is unexpected.");
+                }
+
+                return (parts[0], parts[1]);
+            }
+
             return stripeSignatureHeader.Trim()
                 .Split(',')
-                .Select(item => item.Trim().Split(new[] { '=' }, 2))
-                .ToLookup(item => item[0], item => item[1]);
+                .Select(item => ParseItem(item))
+                .ToLookup(item => item.Key, item => item.Value);
         }
 
         private static bool IsSignaturePresent(string signature, IEnumerable<string> signatures)

src/StripeTests/Services/Events/EventUtilityTest.cs

@@ -101,5 +101,19 @@ public void CanDisableThrowOnApiVersionMismatch()
             var evt = EventUtility.ParseEvent(this.json, false);
             Assert.Equal("2017-05-25", evt.ApiVersion);
         }
+
+        [Theory]
+        [InlineData("t=,v1=2220f87ef101a04665f11cdf770523143f875572008577fa0f20882ddb9cc3c7,v0=63f3a72374a733066c4be69ed7f8e5ac85c22c9f0a6a612ab9a025a9e4ee7eef")]
+        [InlineData("t,v1=,v0=")]
+        [InlineData("t,v1=,v0")]
+        [InlineData("t,v1=,")]
+        [InlineData("t,,")]
+        [InlineData(",,")]
+        [InlineData("t")]
+        public void ValidateSignatureHandlesIncorrectHeaderValues(string headerValue)
+        {
+            Assert.Throws<StripeException>(() =>
+                EventUtility.ValidateSignature("{}", headerValue, string.Empty));
+        }
     }
 }