Replaced Classless.Hasher's Adler32 with a simple, valve-compatible implementation. Added Steam2 file checksum verification to DepotDownloader, so existing files on disk can be used instead of downloading new copies.

DepotDownloader/ContentDownloader.cs

@@ -655,6 +655,12 @@ namespace DepotDownloader
 
                 Console.WriteLine( " Done!" );
 
+                Console.Write("Downloading depot checksums...");
+
+                Steam2ChecksumData checksums = session.DownloadChecksums();
+
+                Console.WriteLine(" Done!");
+
                 StringBuilder manifestBuilder = new StringBuilder();
 
                 byte[] cryptKey = CDRManager.GetDepotEncryptionKey( depotId, depotVersion );
@@ -703,10 +709,16 @@ namespace DepotDownloader
                     float perc = ( ( float )x / ( float )manifest.Nodes.Count ) * 100.0f;
                     Console.WriteLine("{0,6:#00.00}%\t{1}", perc, downloadPath);
 
-                    // TODO: non-checksum validation
                     FileInfo fi = new FileInfo( downloadPath );
-                    if ( fi.Exists && fi.Length == dirEntry.SizeOrCount )
-                        continue;
+                    if (fi.Exists && fi.Length == dirEntry.SizeOrCount)
+                    {
+                        // Similar file, let's check checksums
+                        if(Util.ValidateFileChecksums(fi, checksums.GetFileChecksums(dirEntry.FileID)))
+                        {
+                            // checksums OK
+                            continue;
+                        }
+                    }
 
                     var file = session.DownloadFile( dirEntry, ContentServerClient.StorageSession.DownloadPriority.High, cryptKey );
 

DepotDownloader/DepotDownloader.csproj

@@ -55,6 +55,10 @@
     <PlatformTarget>x86</PlatformTarget>
   </PropertyGroup>
   <ItemGroup>
+    <Reference Include="Classless.Hasher, Version=0.9.4179.24334, Culture=neutral, PublicKeyToken=25b6bb7f72693b18, processorArchitecture=MSIL">
+      <SpecificVersion>False</SpecificVersion>
+      <HintPath>..\..\..\SteamKit2\Classless.Hasher.dll</HintPath>
+    </Reference>
     <Reference Include="System" />
     <Reference Include="System.Core">
       <RequiredTargetFramework>3.5</RequiredTargetFramework>

DepotDownloader/Util.cs

@@ -2,7 +2,9 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
+using System.IO;
 using System.Runtime.InteropServices;
+using Classless.Hasher;
 
 namespace DepotDownloader
 {
@@ -75,5 +77,85 @@ namespace DepotDownloader
 
             return password.ToString();
         }
+
+        // Validate a file against Steam2 Checksums
+        public static bool ValidateFileChecksums( FileInfo file, int [] checksums )
+        {
+            byte[] chunk = new byte[0x8000]; // checksums are for 32KB at a time
+
+            FileStream fs = file.OpenRead();
+            int read, cnt=0;
+            while ((read = fs.Read(chunk, 0, 0x8000)) > 0)
+            {
+                byte[] tempchunk;
+                if(read < 0x8000)
+                {
+                    tempchunk = new byte[read];
+                    Array.Copy(chunk, 0, tempchunk, 0, read);
+                }
+                else
+                {
+                    tempchunk = chunk;
+                }
+                int adler = BitConverter.ToInt32(AdlerHash(tempchunk), 0);
+                int crc32 = BitConverter.ToInt32(CRCHash(tempchunk), 0);
+                if((adler ^ crc32) != checksums[cnt])
+                {
+                    fs.Close();
+                    return false;
+                }
+                ++cnt;   
+            }
+            return (cnt == checksums.Length);
+        }
+
+        // Generate Steam2 Checksums for a file
+        public static int [] CalculateFileChecksums( FileInfo file )
+        {
+            byte[] chunk = new byte[0x8000]; // checksums are for 32KB at a time
+            int [] checksums = new int[((file.Length-1)/0x8000)+1];
+            FileStream fs = file.OpenRead();
+            int read, cnt=0;
+            while ((read = fs.Read(chunk, 0, 0x8000)) > 0)
+            {
+                byte[] tempchunk;
+                if(read < 0x8000)
+                {
+                    tempchunk = new byte[read];
+                    Array.Copy(chunk, 0, tempchunk, 0, read);
+                }
+                else
+                {
+                    tempchunk = chunk;
+                }
+                int adler = BitConverter.ToInt32(AdlerHash(tempchunk), 0);
+                int crc32 = BitConverter.ToInt32(CRCHash(tempchunk), 0);
+                checksums[cnt++] = adler ^ crc32;
+            }
+            fs.Close();
+            return checksums;
+        }
+
+        public static byte[] CRCHash( byte[] input )
+        {
+            using ( Crc crc = new Crc( CrcParameters.GetParameters( CrcStandard.Crc32Bit ) ) )
+            {
+                byte[] hash = crc.ComputeHash( input );
+                Array.Reverse( hash );
+
+                return hash;
+            }
+        }
+
+        public static byte[] AdlerHash(byte[] input)
+        {
+            uint a = 0, b = 0;
+            for (int i = 0; i < input.Length; i++)
+            {
+                a = (a + input[i]) % 65521;
+                b = (b + a) % 65521;
+            }
+            return BitConverter.GetBytes(a | (b << 16));
+        }
     }
 }