Implement Hashcash algorithm to fix 402 PaymentRequired errors on login

MegaApiClient.Tests/TestWebClient.cs

@@ -43,10 +43,15 @@ public int BufferSize
     }
 
     public string PostRequestJson(Uri url, string jsonData)
+    {
+      return PostRequestJson(url, jsonData, null);
+    }
+
+    public string PostRequestJson(Uri url, string jsonData, string hashcash)
     {
       return _policy.Execute(() =>
       {
-        var result = s_webClient.PostRequestJson(url, jsonData);
+        var result = s_webClient.PostRequestJson(url, jsonData, hashcash);
         OnCalled?.Invoke(CallType.PostRequestJson, url);
 
         return result;

MegaApiClient/Exceptions.cs

@@ -150,6 +150,11 @@ public enum ApiResultCode
     /// <summary>
     /// Login requires Two-Factor Authentication
     /// </summary>
-    TwoFactorAuthenticationError = -26
+    TwoFactorAuthenticationError = -26,
+
+    /// <summary>
+    /// Hashcash calculation required
+    /// </summary>
+    HashcashRequired = -27,
   }
 }

MegaApiClient/Interface/IWebClient.cs

@@ -8,6 +8,7 @@ public interface IWebClient
     int BufferSize { get; set; }
 
     string PostRequestJson(Uri url, string jsonData);
+    string PostRequestJson(Uri url, string jsonData, string hashcash);
 
     string PostRequestRaw(Uri url, Stream dataStream);
 

MegaApiClient/MegaApiClient.cs

@@ -1116,9 +1116,10 @@ private TResponse RequestCore<TResponse>(RequestBase request, byte[] key)
       object jsonData = null;
       var attempt = 0;
       var apiCode = ApiResultCode.Ok;
+      string hashcash = null;
       while (_options.ComputeApiRequestRetryWaitDelay(++attempt, out var retryDelay))
       {
-        var dataResult = _webClient.PostRequestJson(uri, dataRequest);
+        var dataResult = _webClient.PostRequestJson(uri, dataRequest, hashcash);
 
         if (string.IsNullOrEmpty(dataResult)
           || (jsonData = JsonConvert.DeserializeObject(dataResult)) == null
@@ -1136,6 +1137,16 @@ private TResponse RequestCore<TResponse>(RequestBase request, byte[] key)
             ApiRequestFailed?.Invoke(this, new ApiRequestFailedEventArgs(uri, attempt, retryDelay, apiCode, dataResult));
           }
 
+          if(apiCode == ApiResultCode.HashcashRequired)
+          {
+            var challenge = ((JArray)jsonData)[1].Value<string>();
+            hashcash = GenerateHashcashToken(challenge);
+            jsonData = null;
+            attempt = 0;
+            apiCode = ApiResultCode.Ok;
+            continue;
+          }
+
           if (apiCode == ApiResultCode.RequestFailedRetry)
           {
             Wait(retryDelay);
@@ -1311,6 +1322,115 @@ private IEnumerable<int> ComputeChunksSizesToUpload(long[] chunksPositions, long
       }
     }
 
+    private static string GenerateHashcashToken(string challenge)
+    {
+      var parts = challenge.Split(':');
+      if (parts.Length < 4)
+      {
+        throw new ArgumentException("Invalid challenge format");
+      }
+
+      var version = int.Parse(parts[0]);
+      if (version != 1)
+      {
+        throw new ArgumentException("Hashcash challenge is not version 1");
+      }
+
+      var easiness = int.Parse(parts[1]);
+      var tokenStr = parts[3];
+
+      var baseVal = ((easiness & 63) << 1) + 1;
+      var shifts = (easiness >> 6) * 7 + 3;
+      var threshold = baseVal << shifts;
+
+      var token = D64(tokenStr);
+
+      var iterations = 262144;
+      var chunkSize = 48;
+      var prefixSize = 4;
+      var totalSize = prefixSize + iterations * chunkSize;
+
+      var buffer = new byte[totalSize];
+
+      for (var i = 0; i < iterations; i++)
+      {
+        for (var j = 0; j < chunkSize; j++)
+        {
+          buffer[prefixSize + i * chunkSize + j] = token[j];
+        }
+      }
+
+      using (var sha256 = SHA256.Create())
+      {
+        while (true)
+        {
+          var hash = sha256.ComputeHash(buffer);
+          var hashPrefix = BitConverter.ToUInt32(hash, 0);
+
+          if (BitConverter.IsLittleEndian)
+          {
+            hashPrefix = ReverseBytes(hashPrefix);
+          }
+
+          if (hashPrefix <= threshold)
+          {
+            var prefix = E64(SubArray(buffer, 0, 4));
+            return $"1:{tokenStr}:{prefix}";
+          }
+
+          var j = 0;
+          while (true)
+          {
+            buffer[j]++;
+            if (buffer[j++] != 0)
+            {
+              break;
+            }
+          }
+        }
+      }
+    }
+
+    private static string E64(byte[] buffer)
+    {
+      return Convert.ToBase64String(buffer)
+          .Replace('+', '-')
+          .Replace('/', '_')
+          .TrimEnd('=');
+    }
+
+    private static byte[] D64(string s)
+    {
+      var padded = s
+          .Replace('-', '+')
+          .Replace('_', '/');
+      switch (padded.Length % 4)
+      {
+        case 2: padded += "=="; break;
+        case 3: padded += "="; break;
+      }
+      return Convert.FromBase64String(padded);
+    }
+
+    private static byte[] SubArray(byte[] data, int index, int length)
+    {
+      var result = new byte[length];
+      for (var i = 0; i < length; i++)
+      {
+        result[i] = data[index + i];
+      }
+      return result;
+    }
+
+    private static uint ReverseBytes(uint value)
+    {
+      return (value >> 24) |
+             ((value & 0x00FF0000) >> 8) |
+             ((value & 0x0000FF00) << 8) |
+             (value << 24);
+    }
+
+
     #endregion
 
     #region AuthInfos

MegaApiClient/MegaApiClient.csproj

@@ -47,12 +47,6 @@
     <Reference Include="System.Net.Http" />
   </ItemGroup>
 
-  <ItemGroup Condition="$(TargetFramework.StartsWith('netstandard')) == 'true'">
-    <PackageReference Include="System.Net.Http">
-      <Version>4.3.4</Version>
-    </PackageReference>
-  </ItemGroup>
-
   <ItemGroup Condition="$(TargetFramework) == 'netstandard1.3'">
     <PackageReference Include="Microsoft.AspNetCore.WebUtilities" Version="1.1.2" />
   </ItemGroup>
@@ -81,7 +75,12 @@
       <IncludeAssets>runtime;build;native;contentfiles;analyzers</IncludeAssets>
     </PackageReference>
   </ItemGroup>
-
+  <ItemGroup Condition="$(TargetFramework.StartsWith('netstandard')) == 'true'">
+    <PackageReference Include="System.Net.Http">
+      <Version>4.3.4</Version>
+    </PackageReference>
+  </ItemGroup>
+
   <!-- Deterministic Builds -->
   <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">
     <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>

MegaApiClient/WebClient_HttpClient.cs

@@ -51,7 +51,18 @@ public string PostRequestJson(Uri url, string jsonData)
     {
       using (var jsonStream = new MemoryStream(jsonData.ToBytes()))
       {
-        using (var responseStream = PostRequest(url, jsonStream, "application/json"))
+        using (var responseStream = PostRequest(url, jsonStream, "application/json", null))
+        {
+          return StreamToString(responseStream);
+        }
+      }
+    }
+
+    public string PostRequestJson(Uri url, string jsonData, string hashcash)
+    {
+      using (var jsonStream = new MemoryStream(jsonData.ToBytes()))
+      {
+        using (var responseStream = PostRequest(url, jsonStream, "application/json", hashcash))
         {
           return StreamToString(responseStream);
         }
@@ -76,7 +87,11 @@ public Stream GetRequestRaw(Uri url)
       return _httpClient.GetStreamAsync(url).Result;
     }
 
-    private Stream PostRequest(Uri url, Stream dataStream, string contentType)
+    private Stream PostRequest(Uri url, Stream dataStream, string contentType) {
+      return PostRequest(url, dataStream, contentType, null);
+    }
+
+    private Stream PostRequest(Uri url, Stream dataStream, string contentType, string hashcash)
     {
       using (var content = new StreamContent(dataStream, BufferSize))
       {
@@ -87,6 +102,11 @@ private Stream PostRequest(Uri url, Stream dataStream, string contentType)
           Content = content
         };
 
+        if(hashcash != null)
+        {
+          requestMessage.Headers.Add("X-Hashcash", hashcash);
+        }
+
         var response = _httpClient.SendAsync(requestMessage, HttpCompletionOption.ResponseHeadersRead).Result;
         if (!response.IsSuccessStatusCode
             && response.StatusCode == HttpStatusCode.InternalServerError
@@ -95,6 +115,20 @@ private Stream PostRequest(Uri url, Stream dataStream, string contentType)
           return new MemoryStream(Encoding.UTF8.GetBytes(((long)ApiResultCode.RequestFailedRetry).ToString()));
         }
 
+        if(!response.IsSuccessStatusCode
+           && response.StatusCode == HttpStatusCode.PaymentRequired
+           && response.Headers.TryGetValues("X-Hashcash", out var hashcashValues))
+         {
+           foreach(var value in hashcashValues)
+           {
+             hashcash = value;
+             break;
+           }
+
+          var json = $"[{(long)ApiResultCode.HashcashRequired}, '{hashcash}']"; 
+          return new MemoryStream(Encoding.UTF8.GetBytes(json));
+         }
+
         response.EnsureSuccessStatusCode();
 
         return response.Content.ReadAsStreamAsync().Result;

MegaApiClient/WebClient_HttpWebRequest.cs

@@ -30,10 +30,15 @@ public WebClient(int responseTimeout = DefaultResponseTimeout, string userAgent
     public int BufferSize { get; set; }
 
     public string PostRequestJson(Uri url, string jsonData)
+    {
+      return PostRequestJson(url, jsonData, null);
+    }
+
+    public string PostRequestJson(Uri url, string jsonData, string hashcash)
     {
       using (var jsonStream = new MemoryStream(jsonData.ToBytes()))
       {
-        using (var responseStream = PostRequest(url, jsonStream, "application/json"))
+        using (var responseStream = PostRequest(url, jsonStream, "application/json", hashcash))
         {
           return StreamToString(responseStream);
         }
@@ -62,11 +67,20 @@ public Stream GetRequestRaw(Uri url)
     }
 
     private Stream PostRequest(Uri url, Stream dataStream, string contentType)
+    {
+      return PostRequest(url, dataStream, contentType, null);
+    }
+
+    private Stream PostRequest(Uri url, Stream dataStream, string contentType, string hashcash)
     {
       var request = CreateRequest(url);
       request.ContentLength = dataStream.Length;
       request.Method = "POST";
       request.ContentType = contentType;
+      if(hashcash != null)
+      {
+        request.Headers.Add("X-Hashcash", hashcash);
+      }
 
       using (var requestStream = request.GetRequestStream())
       {