Reverse Engineering Unknown File Formats with ImHex

# Reverse Engineering Unknown File Formats with ImHex | WerWolv Source: [https://werwolv.net/posts/file_format_reverse_engineering/](https://werwolv.net/posts/file_format_reverse_engineering/) ## [Introduction](https://werwolv.net/posts/file_format_reverse_engineering/#introduction) Over the years I’ve been asked the same question countless times: Person on Discord asking for help reverse engineering a file formatI usually couldn’t really give them a good answer except, “Look at the decompiled code of whatever program reads/writes these files and work backwards from there\.” This post is meant to change that\. We’ll go from a completely custom binary save file for the game[FEZ](https://fezgame.com/)to a full definition written in the Pattern Language, which is part of[ImHex](https://imhex.werwolv.net/), the hex editor I’ve been developing for the past few years\. It is free, open source and available on any operating system \(or even through the browser if you prefer that:[ImHex Web](https://web.imhex.werwolv.net/nightly)\)\. #### ImHex Version At the time of writing, some features used here are not in a release yet but only available in the Nightly build \(that can also be downloaded above from the same link\)\. If you’re on ImHex v1\.38\.1 or below and experiencing issues, consider upgrading to the Nightly build ## [Getting Started](https://werwolv.net/posts/file_format_reverse_engineering/#getting-started) #### Spoiler Warning FEZ was released all the way back in 2012\. Still, if you haven’t played it yet and want to get the full experience, I highly recommend playing it before you continue reading\. Some of the code shown here will contain heavy spoilers for secrets and endgame content that might ruin your experience\. You have been warned\. The first thing we need is the save file\. I downloaded the game from Steam \(the latest full release currently available, released 2\. December 2016\), started it and played for a little bit until it saved\. Then I went looking through my filesystem and found the save file under`/home/werwolv/\.local/share/FEZ/SaveSlot2`\. On Windows, it will be elsewhere\. Opening the file in ImHex shows this: ``` Hex View 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F00000000 3E 74 E1 41 6B BA DA 01 06 00 00 00 00 00 00 00 >t.Ak...........00000010 3A AC 78 49 C9 B4 CF 01 01 00 01 00 00 01 12 00 :.xI............00000020 00 00 01 11 44 4F 54 5F 4C 4F 43 4B 45 44 5F 44 ....DOT_LOCKED_D00000030 4F 4F 52 5F 41 00 01 10 44 4F 54 5F 4E 55 54 5F OOR_A...DOT_NUT_00000040 4E 5F 42 4F 4C 54 5F 41 00 01 0B 44 4F 54 5F 50 N_BOLT_A...DOT_P00000050 49 56 4F 54 5F 41 01 01 11 44 4F 54 5F 54 49 4D IVOT_A...DOT_TIM00000060 45 5F 53 57 49 54 43 48 5F 41 00 01 0F 44 4F 54 E_SWITCH_A...DOT00000070 5F 54 4F 4D 42 53 54 4F 4E 45 5F 41 01 01 0C 44 _TOMBSTONE_A...D00000080 4F 54 5F 54 52 45 41 53 55 52 45 00 01 0B 44 4F OT_TREASURE...DO00000090 54 5F 56 41 4C 56 45 5F 41 01 01 13 44 4F 54 5F T_VALVE_A...DOT_ ``` This already reveals a few things\. The file seems to be uncompressed and unencrypted, as seen by the plain\-text strings and other patterns in the file that can be easily spotted by just looking at the bytes and characters\. The data also doesn’t have a file magic \(some readable text at the start of the file to make it more easily identifiable\), and it doesn’t look like anything standard, as ImHex can’t identify its type directly either\. Magic file information from ImHexWithout any more information, we’re basically stuck here\. The data can mean anything, and only the program generating and parsing it can make sense of it\. ## [Decompiling the Game](https://werwolv.net/posts/file_format_reverse_engineering/#decompiling-the-game) ### [Finding the right files](https://werwolv.net/posts/file_format_reverse_engineering/#finding-the-right-files) Clicking on the gear icon on the Steam page and selecting`Manage \-\> Browse local files`brings us to the game’s binary location\. What immediately sticks out are files like`System\.Core\.dll`or`mscorlib\.dll`\. The game is written in the C\# programming language, which is generally really easy to reverse engineer\. Tools like[JetBrains Rider](https://www.jetbrains.com/rider/)can decompile the binaries back to what looks like the original source code\. For that, we can open the game’s folder as a project and then simply`Right Click \-\> View in Assembly Explorer`for all the`\.dll`files that look interesting\. To me, particularly interesting were`FEZ\.exe`,`FezEngine\.dll`,`Common\.dll`,`ContentSerialization\.dll`and`EasyStorage\.dll`\. The rest are system libraries or external dependencies that look unrelated to what we’re trying to do here\. ### [Finding the right functions](https://werwolv.net/posts/file_format_reverse_engineering/#finding-the-right-functions) Just clicking through the namespaces quickly reveals an interesting\-looking file:`EasyStorage \-\> PCSaveDevice`\. In the constructor of that class, we can also immediately see`string str = "SaveSlot" \+ \(object\) index;`, which looks like it’s building the name of our file,`SaveSlot2`, so we found the right place for sure\. Scrolling down a bit, we can find a function called`Save`that creates a byte buffer and starts filling it in using a`BinaryWriter`stream before saving it to our save file location\. Bingo\! ``` public virtual bool Save(string fileName, SaveAction saveAction){ // ... byte[] buffer = new byte[40960 /*0xA000*/]; using (MemoryStream output = new MemoryStream(buffer)) { using (BinaryWriter writer = new BinaryWriter((Stream) output)) { writer.Write(DateTime.Now.ToFileTime()); saveAction(writer); if (output.Length < 40960L /*0xA000*/) { long length = 40960L /*0xA000*/ - output.Length; writer.Write(new byte[length]); } else if (output.Length > 40960L /*0xA000*/) throw new InvalidOperationException( "Save file greater than the imposed limit!" ); } } // ...} ``` ## [Writing the ImHex Pattern](https://werwolv.net/posts/file_format_reverse_engineering/#writing-the-imhex-pattern) ### [Humble Beginnings](https://werwolv.net/posts/file_format_reverse_engineering/#humble-beginnings) Now that we’ve found where the save file is being generated, we can start writing a Pattern file in ImHex to decode the data\. Open the`Pattern Editor`tab to reveal a text editor where we can write our source code\. We can start simply by creating a`struct FezSaveFile`and placing it at the start of the file using the`@`placement operator\. ``` struct FezSaveFile { // Struct Definition};FezSaveFile saveFile @ 0x00; ``` This instantiates the`FezSaveFile`pattern object at address`0x00`of our file\. Next, in the save file generation code, we see`writer\.Write\(DateTime\.Now\.ToFileTime\(\)\);`, which writes the current timestamp as a[Windows File Time](https://learn.microsoft.com/en-us/dotnet/api/system.datetime.tofiletime?view=net-10.0#remarks)to the output\. As seen in the Remarks section of the docs, this is simply a little endian, 64\-bit value \(a`long`in C\#\) that represents the number of 100 ns intervals that have passed since the year of our lord 1601 A\.D\. We could, of course, properly decode this value and everything but to get started we can simply place a`s64`in its place in the Pattern to read it\. Alternatively, we can also write type aliases with the`using`keyword to make the code in our pattern resemble the types used in the real code even more closely\. These simply define a new type that has the exact properties of the type on the right hand side but with a potentially more descriptive name\. ``` using int = s32;using long = s64;struct FezSaveFile { long fileTime;};FezSaveFile saveFile @ 0x00; ``` After clicking thebutton at the bottom of the Pattern Editor \(or pressing the`F5`key\), the region of that value is now highlighted in the Hex Editor View, and it also appears in the pattern tree in the Pattern Data View\. Highlighted Bytes in the Hex Editor and decoded value in the Pattern Data ViewFor this particular case though, we’re in luck and the standard library already implements a type for decoding a Windows`FILETIME`value\. To get access to it, we can`import`the`type\.time`library which defines that type and then use it like any other type in our code: ``` import type.time;struct FezSaveFile { type::FILETIME fileTime;};FezSaveFile saveFile @ 0x00; ``` This simple change now turns that unreadable number from before into a nice, human readable representation of the actual time value: Decoding the FILETIME value using the \`type::FILETIME\` type from the standard libraryAnd that’s it for the start, congrats\! You wrote your first pattern\! #### \[\[fixed\_size\]\] attribute One thing we can also see in the code is that the`Save\(\)`function ensures that the save file is always`0xA000`bytes long\. If it’s shorter, it will pad it out with zeros, and if it is longer, an`InvalidOperationException`will be thrown\. This maps incredibly well to the`\[\[fixed\_size\(0xA000\)\]\]`attribute that can be attached to`FezSaveFile`to ensure that\. This is entirely optional but helps document the official behavior\. ### [The Actual Save Data](https://werwolv.net/posts/file_format_reverse_engineering/#the-actual-save-data) Back to the C\# code, the next thing that’s done is to call out to the`saveAction`callback, which is implemented elsewhere\. Thankfully, Rider helps here, as you can just Ctrl\-click on the name of the`Save`function to find definitions\. There we see a few places it’s called from, but the interesting one is in`GameStateManager\.cs SaveInternal\(\)`\. Find usages in JetBrains Rider``` private void SaveInternal(bool ngpBackup){ // ... this.ActiveSaveDevice.Save( "SaveSlot" + (object) this.SaveSlot, new SaveAction(this.DoSave) ); // ...} ``` There we can see that the actual dumping of the save data is delegated to the`DoSave\(\)`function, which calls`SaveFileOperations\.Write\(\)`\. This is the juicy stuff now\. Here we can see aaaaaaalll the different fields that are being written out to the binary\. ``` public static void Write(CrcWriter w, SaveData sd){ w.Write(6L); w.Write(sd.CreationTime); w.Write(sd.Finished32); w.Write(sd.Finished64); w.Write(sd.HasFPView); w.Write(sd.HasStereo3D); w.Write(sd.CanNewGamePlus); w.Write(sd.IsNewGamePlus); // ...} ``` Looking at the types of those values allows them to be easily converted to the ImHex Pattern: ``` struct FezSaveFile { // From PCSaveDevice.cs type::FILETIME fileTime; // From SaveFileOperations.cs long version; // Checked in the `Read()` function to be 6 long creationTime; bool finished32; bool finished64; bool hasFpView; bool hasStereo3d; bool canNewGamePlus; bool isNewGamePlus;}; ``` The first field seems to be a save file version as can be seen in the`Read\(\)`function below which reads that field, makes sure it is also 6 and throws an exception if it’s not\. We can simply parse that field but if we want to be extra fancy and make sure that we only load files that are actually compatible with our pattern, we can easily assert on this field\. In the Pattern Language, we can have conditions and function calls intertwined with our type definitions which makes things like this possible: ``` import std.sys;struct FezSaveFile { // From PCSaveDevice.cs type::FILETIME fileTime; // From SaveFileOperations.cs long version; // Checked in the `Read()` function to be 6 std::assert(version == 6, "Unsupported Save File Version. Only Version 6 is supported"); ``` ### [Objects and Strings](https://werwolv.net/posts/file_format_reverse_engineering/#objects-and-strings) The next part is interesting\. Here, a list of String \-\> Bool key\-value pairs is being serialized\. First, the number of pairs is stored, followed by that number of serialized pairs\. ``` w.Write(sd.OneTimeTutorials.Count);foreach (KeyValuePair<string, bool> oneTimeTutorial in sd.OneTimeTutorials){ w.WriteObject(oneTimeTutorial.Key); w.Write(oneTimeTutorial.Value);} ``` `w\.WriteObject`is a bit more involved and deserves a closer look\. ``` public static void WriteObject(this CrcWriter writer, string s){ writer.Write(s != null); if (s == null) return; writer.Write(s);} ``` Objects seem to be defined as something that may or may not exist\. First, a`bool`is written to the file that represents whether or not the object is`null`\. If it is`null`, that’s the end of it, and we don’t write down anything more\. If it’s not`null`, though, we serialize the value\. In the Pattern Language, this looks like this: ``` struct Object<T> { bool isValid; if (isValid) T value;}; ``` This code defines a new template struct called`Object`\. It places a`bool`in the output file, then checks if that`bool`is`true`\. Only if it is does it place a value of the template parameter’s type\. Next we need to see how`string`types are serialized\. There’s another function of the`BinaryWriter`that does this: ``` public virtual void Write(string value){ if (this.disposed) throw new ObjectDisposedException(nameof (BinaryWriter), "Cannot write to a closed BinaryWriter"); this.Write7BitEncodedInt(this.m_encoding.GetByteCount(value)); if (this.stringBuffer == null) { this.stringBuffer = new byte[512 /*0x0200*/]; this.maxCharsPerRound = 512 /*0x0200*/ / this.m_encoding.GetMaxByteCount(1); } int charIndex = 0; int charCount; for (int length = value.Length; length > 0; length -= charCount) { charCount = length <= this.maxCharsPerRound ? length : this.maxCharsPerRound; this.OutStream.Write(this.stringBuffer, 0, this.m_encoding.GetBytes(value, charIndex, charCount, this.stringBuffer, 0)); charIndex += charCount; }} ``` This seems to first be writing out the length of the string in bytes in some`7BitEncodedInt`format followed by the actual string data\. ``` protected void Write7BitEncodedInt(int value){ do { int num1 = value >> 7 & 33554431 /*0x01FFFFFF*/; byte num2 = (byte) (value & (int) sbyte.MaxValue); if (num1 != 0) num2 |= (byte) 128 /*0x80*/; this.Write(num2); value = num1; } while (value != 0);} ``` `Write7BitEncodedInt`looks a bit daunting, but after playing through it with some values, all this does is use theMSBof each byte as a flag to tell the parser if there’s another byte still coming\. The other 7 bits are the actual encoded value\. In the Pattern Language this can be implemented like this: ``` struct SevenBitEncodedIntByte { // Read a byte u8 byte; // If that byte doesn't have bit 7 set, // this is the last one and we can stop here. if ((byte & 0x80) == 0x00) break;};struct SevenBitEncodedInt { // Keep decoding bytes until the `break` above is run SevenBitEncodedIntByte bytes[while(true)];}; ``` Additionally, to make this type a bit easier to work with, we can use the`\[\[format\]\]`attribute to display the decoded integer value in the Pattern Data View and the`\[\[transform\]\]`attribute so the rest of our code can simply read from a variable of this type and get back the decoded integer value instead: ``` struct SevenBitEncodedInt { SevenBitEncodedIntByte bytes[while(true)];} [[format("transformSevenBitEncodedInt"), transform("transformSevenBitEncodedInt")]];fn transformSevenBitEncodedInt(ref auto encodedInt) { u64 result = 0; // Loop over all the bytes we placed before for (u32 i = 0, i < std::core::member_count(encodedInt.bytes), i += 1) { // Each byte contains the next more-significant group of 7 bits result |= (encodedInt.bytes[i].byte & 0x7F) << (i * 7); } return result;}; ``` Now that all of this is done, we can finally define our String type\. Again with a nice`\[\[format\]\]`function so we can see the string directly in the UI\. ``` struct String { // Read the string's size SevenBitEncodedInt size; char string[size];} [[format("formatString")]];fn formatString(ref auto string) { return string.string;}; ``` The final Object<String\> type being parsed by ImHex### [Lists](https://werwolv.net/posts/file_format_reverse_engineering/#lists) ``` w.Write(sd.OneTimeTutorials.Count);foreach (KeyValuePair<string, bool> oneTimeTutorial in sd.OneTimeTutorials){ w.WriteObject(oneTimeTutorial.Key); w.Write(oneTimeTutorial.Value);} ``` Back to the code from before, we can define a few more types now to finally parse this construct\. This time using a`s32`\(or our`int`type alias\) because`Count`is a`int`in C\#\. ``` struct List<T> { // Read the number of items int count; // Place an array of `count` items down T items[count];};struct KeyValuePair<Key, Value> { Key key; Value value;}; ``` All of this together now lets us finally decode the list\. Take a step back for a bit and see how all of this came together and how it maps to the C\# code\. ``` struct FezSaveFile { // ... List< // A size-prefixed list containing... KeyValuePair< // pairs of... Object<String>, // an optional, length prefixed string... bool // and a bool > > oneTimeTutorials;}; ``` The final List< type being parsed by ImHex### [Enums](https://werwolv.net/posts/file_format_reverse_engineering/#enums) Following the same pattern, we can keep going down the`Write`function and decoding the data in ImHex\. The next interesting bit is this code: ``` foreach (ActorType artifact in sd.Artifacts) w.Write((int) artifact); ``` This writes down not integers directly, but an enumeration instead\. We could just treat it as an`int`like the serializer code does, but it would be nicer to keep the names available in ImHex as well\. The definition of the`enum`in C\# can be copy\-pasted over almost 1:1: ``` public enum ActorType{ None, Ladder, Bouncer, Sign, GoldenCube, // ...} ``` ``` enum ActorType : int{ None, Ladder, Bouncer, Sign, GoldenCube, // ...}; ``` This type can then simply be used in place of a`int`and gives us a nicer display: ActorType enum as seen in ImHex### [More Subtypes](https://werwolv.net/posts/file_format_reverse_engineering/#more-subtypes) The same pattern as above keeps on going for a while longer until we reach the end where we have some nested serialization: ``` foreach (KeyValuePair<string, LevelSaveData> keyValuePair in sd.World){ w.WriteObject(keyValuePair.Key); SaveFileOperations.Write(w, keyValuePair.Value);} ``` This maps really nicely to a new struct that we can call`LevelSaveData`and just keep going in there as before: ``` struct LevelSaveData { // ...};struct FezSaveFile { // ... List<KeyValuePair<Object<String>, LevelSaveData>> world;}; ``` You should now have everything that’s needed to decode the rest of the format yourself\. Give it a try\! ## [The Fruits of our Labour](https://werwolv.net/posts/file_format_reverse_engineering/#the-fruits-of-our-labour) At this point, you should be able to look at the Hex Editor View and see every single byte \(except the large padding at the end\) highlighted with some color\. You can now browse through the Pattern Data View and inspect what all these different values mean and even modify them by double\-clicking the value\! Fully decoded Save File in ImHexIf you’re interested in how I did it, you can take a look at[My Pattern](https://gist.github.com/WerWolv/b0b616853b03a86fb0214c2e4fbfcbdb)\. ## [Wrapping things up](https://werwolv.net/posts/file_format_reverse_engineering/#wrapping-things-up) After reading this post you should have a basic understanding of how to reverse engineer a binary file format\. Of course, not all programs will be as easy to decompile and analyze as this one but the general workflow remains the same: **Check if the file is in a known format** This can be done in various ways, ImHex magic detection and tools like binwalk can help a lot\. If it is an existing format, there might be tools available already to parse that format\. Otherwise you may be able to look at the specification **Find the piece of code that parses or generates that file** This usually means looking at the program’s Source Code if available or to decompile the program first using a tool that fits the language used\. Rider works great for \.NET, Ghidra, IDA or Binary Ninja for native\-compiled programs, Recaf for JVM languages\. What helps me is to look for library calls for doing File I/O or for strings mentioning \(parts of\) the generated or loaded file name\. **Analyze the code and find the building blocks used in the file** Most file formats want the same thing: To store integers, booleans, strings and other data structures\. Identifying them is the first step to understanding the file step by step **Write a Pattern File to document your findings and verify their validity** Patterns are great not only for decoding the file once you know how it works but also for documenting and verifying your findings along the way\. If you have any questions about the process, ImHex or the Pattern Language itself, feel free to reach out on the[ImHex Discord Server](https://discord.gg/X63jZ36xBY), via[Discord DMs @werwolv](https://discord.com/users/181738643008782346)or by[email at`\[email protected\]`](https://werwolv.net/cdn-cgi/l/email-protection#4f272a360f382a3d3820233961212a3b)\. ---