enhancing so user doesn't have to generate their own signature string

Nexmo.Api/Cryptography/SmsSignatureGenerator.cs

@@ -20,20 +20,18 @@ public enum Method
         }
         public static string GenerateSignature(string query, string securitySecret, Method method)
         {
-            var queryToSign = "&" + query;
-            queryToSign = queryToSign.Remove(queryToSign.Length - 1);
             // security secret provided, sort and sign request
             if (method == Method.md5hash)
             {
-                queryToSign += securitySecret;
+                query += securitySecret;
                 var hashgen = MD5.Create();
-                var hash = hashgen.ComputeHash(Encoding.UTF8.GetBytes(queryToSign));
+                var hash = hashgen.ComputeHash(Encoding.UTF8.GetBytes(query));
                 return ByteArrayToHexHelper.ByteArrayToHex(hash).ToLower();
             }
             else
             {
                 var securityBytes = Encoding.UTF8.GetBytes(securitySecret);
-                var input = Encoding.UTF8.GetBytes(queryToSign);
+                var input = Encoding.UTF8.GetBytes(query);
                 HMAC hmacGen = new HMACMD5(securityBytes);
                 switch (method)
                 {

Nexmo.Api/Request/ApiRequest.cs

@@ -68,7 +68,9 @@ private static StringBuilder BuildQueryString(IDictionary<string, string> parame
             var sortedParams = new SortedDictionary<string, string>(parameters);
             buildStringFromParams(sortedParams, sb);
             buildSignatureStringFromParams(sortedParams, signature_sb);
-            var signature = SmsSignatureGenerator.GenerateSignature(signature_sb.ToString(), securitySecret, method);
+            var queryToSign = "&" + signature_sb.ToString();
+            queryToSign = queryToSign.Remove(queryToSign.Length - 1);
+            var signature = SmsSignatureGenerator.GenerateSignature(queryToSign, securitySecret, method);
             sb.AppendFormat("sig={0}", signature);
             return sb;
         }

Nexmo.Api/SMS.cs

@@ -1,4 +1,9 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Text;
+using Microsoft.AspNetCore.Http;
 using Newtonsoft.Json;
 using Nexmo.Api.Request;
 
@@ -301,7 +306,7 @@ public class SMSInbound
             /// The time at UTC±00:00  that Nexmo started to push this inbound message to your webhook endpoint. The message-timestamp is in the following format YYYY-MM-DD HH:MM:SS. For example, 2020-01-01 12:00:00.
             /// </summary>
             [JsonProperty("message-timestamp")]
-            public DateTime message_timestamp { get; set; }
+            public string message_timestamp { get; set; }
             /// <summary>
             /// A unix timestamp representation of message-timestamp.
             /// </summary>
@@ -359,6 +364,37 @@ public class SMSInbound
             /// Signature if Applicable
             /// </summary>
             public string sig { get; set; }
+
+            /// <summary>
+            /// converts dictionary into properly formatted signature string
+            /// </summary>
+            /// <param name="query"></param>
+            /// <returns></returns>
+            public static string ConstructSignatureStringFromDictionary(Dictionary<string,string> query)
+            {
+                try
+                {
+                    var sig_sb = new StringBuilder();
+                    var sorted_dict = new SortedDictionary<string, string>(StringComparer.Ordinal);
+                    foreach (var key in query.Keys)
+                    {
+                        sorted_dict.Add(key, query[key].ToString());
+                    }
+                    foreach (var key in sorted_dict.Keys)
+                    {
+                        if (key == "sig")
+                        {
+                            continue;
+                        }
+                        sig_sb.AppendFormat("&{0}={1}", key.Replace('=', '_').Replace('&', '_'), sorted_dict[key].ToString().Replace('=', '_').Replace('&', '_'));
+                    }
+                    return sig_sb.ToString();
+                }
+                catch
+                {
+                    return "";
+                }
+            }
         }
 
         /// <summary>