After being asleep for a long time, you wake up in an old prison cell. You turn around, what was that? Soon after, you are free, unexpectedly. You reach daylight and enter a beautiful, long forgotten and sometimes dangerous world. What awaits you at the end of your journey? Created during Wizard Jam 6 in December 2017. Mac OS X Server 10.5 – also marketed as Leopard Server; Mac OS X Server 10.6 – also marketed as Snow Leopard Server; Starting with Lion, there is no separate Mac OS X Server operating system. Instead the server components are a separate download from the Mac App Store. Mac OS X Lion Server – 10.7 – also marketed as OS X Lion Server. Mac OS, Microsoft Windows. You may redistribute it, verbatim or modified, providing that you comply with the terms of the CC-BY-SA.
These key combinations apply only to Mac computers with an Intel processor, not Mac computers with Apple silicon.
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To use any of these key combinations, press and hold the keys immediately after pressing the power button to turn on your Mac, or after your Mac begins to restart. Keep holding until the described behavior occurs.
- Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the Internet. macOS Recovery installs different versions of macOS, depending on the key combination you use while starting up. If your Mac is using a firmware password, you're prompted to enter the password.
- Option (⌥) or Alt: Start up to Startup Manager, which allows you to choose other available startup disks or volumes. If your Mac is using a firmware password, you're prompted to enter the password.
- Option-Command-P-R:Reset NVRAM or PRAM. If your Mac is using a firmware password, it ignores this key combination or starts up from macOS Recovery.
- Shift (⇧): Start up in safe mode. Disabled when using a firmware password.
- D: Start up to the Apple Diagnostics utility. Or use Option-Dto start up to this utility over the Internet. Disabled when using a firmware password.
- N: Start up from a NetBoot server, if your Mac supports network startup volumes. To use the default boot image on the server, hold down Option-N instead. Disabled when using a firmware password.
- Command-S: Start up in single-user mode. Disabled in macOS Mojave or later, or when using a firmware password.
- T: Start up in target disk mode. Disabled when using a firmware password.
- Command-V: Start up in verbose mode. Disabled when using a firmware password.
- Eject (⏏) or F12 or mouse button or trackpad button: Eject removable media, such as an optical disc. Disabled when using a firmware password.
If a key combination doesn't work
If a key combination doesn't work at startup, one of these solutions might help:
- Press and hold all keys in the combination together, not one at a time.
- Shut down your Mac. Then press the power button to turn on your Mac. Then press and hold the keys as your Mac starts up.
- Wait a few seconds before pressing the keys, to give your Mac more time to recognize the keyboard as it starts up. Some keyboards have a light that flashes briefly at startup, indicating that the keyboard is recognized and ready for use.
- If you're using a wireless keyboard, plug it into your Mac, if possible. Or use your built-in keyboard or a wired keyboard. If you're using a keyboard made for a PC, such as a keyboard with a Windows logo, try a keyboard made for Mac.
- If you're using Boot Camp to start up from Microsoft Windows, set Startup Disk preferences to start up from macOS instead. Then shut down or restart and try again.
Remember that some key combinations are disabled when your Mac is using a firmware password.
Learn more
- Keyboard shortcuts that you can use after your Mac has started up.
Reverse engineering is the direct opposite of building or engineering an application: you break things down bit by bit to see how they actually work. Developers incorporate reverse engineering techniques to solve tasks from investigating bugs in code to ensuring smooth and easy legacy code maintenance.
When reverse engineering software, the operating system it was created for should be one of the first things you pay attention to. In this article, we describe how to decompile macOS software and iOS apps. This tutorial will be useful for developers who want to know more about macOS software and iOS apps reverse engineering.
Contents:
Why do we need reverse engineering? The answer is rather simple.
When you build a piece of software, you usually have all of the source code available and can take a look at the source code at any time. So figuring out how a particular process or feature works shouldn't be too much of a challenge.
But what if you have an executable and you need to figure out how it works without access to any source code? The solution is obvious: you need to reverse engineer it.
There are several reasons why you might need to use reverse engineering:
- To research complicated software issues
- To improve software compatibility with third-party solutions and formats
- To improve interactions between software and the platform
- To provide easy maintenance of legacy code
- And more
Below, we take a closer look at the basic structure of an executable, briefly cover reversing Objective-C and Swift code, list several of the most popular tools for reverse engineering macOS and iOS apps, and give some reverse engineering tips for a number of use cases.
Let's start with some basics that you need to know before you try to reverse engineer your first executable.
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Professional Reverse Engineering ServicesBefore you start reversing
If you've finally decided to reverse engineer binary, then you should understand that some parts of it probably contain executable code. Therefore, before you even start reversing a piece of software, you need to learn the executable binary structure.
Executable binary format
In the world of Mach kernel-based operating systems, it's common to use the Mach-O executable format. These executables can be inside thin or fat binary files. Here's how these two types of binaries differ:
- A thin binary contains a single Mach-O executable
- A fat binary may contain many Mach-O executables
We use fat binaries to merge executable code in one single file for different CPU instruction sets.
Here's the basic structure of a Mach-O executable:
Let's take a closer look at each component.
Every binary begins with a header. This is a key part of every executable for macOS and iOS. It's the first part of the executable read by the loader during image loading.
A fat binary begins with a fat header, while a thin binary begins with a mach header. Every header starts with a magicnumber used to identify it.
A fat header describes the locations of mach headers for executables in a binary. A mach header describes general information about the current executable file.
A mach header contains load commands that represent several things crucial for image loading:
- Segments and sections of the executable and its mapping to virtual memory
- Paths to the linked dynamic libraries
- Location of tables of symbols
- Code signature
Segments are typically large pieces of an executable file mapped by a loader to some location in the virtual address space.
In the image above, you can see a lot of information about the chosen segment:
- Offset in the current executable
- Size
- Address
- Size of the region appointed for segment mapping
- Segment attributes
All segments consist of sections. A section is part of the segment that's intended to store some specific type of content. For example, the __text section of the __TEXT segment contains executable code, and the __la_symbol_ptr section of the DATA segment contains a table of pointers to so-called lazy external symbols.
Every dynamic library dependency is described by a load command containing the path to the dynamic library binary file and its version.
In addition, load commands contain the following information critical for the operation of executable code: Backflip studios layoffs.
- Location of symbol tables
- Location of import and stub tables
- Location of the table with information for the dynamic loader
The main symbol table contains all symbols used in the current executable. Every locally or externally defined symbol or even stub (which can be generated for an external call that executes through an import table) is mentioned here. This table is divided into three parts, showing whether the symbol is debug, local, or external. Every entry in the main symbol table represents a particular part of the executable code by specifying the offset of its name in the string table, type, section ordinal, and other type-specific information.
There's a string table that contains names of symbols defined in the main symbol table. There's also a dynamic symbol table that links import table entries to the appropriate symbol. In addition, there's one more table that contains information used by the dynamic loader for every external symbol.
Read also:
How to Reverse Engineer (Windows) Software the Right Way
Code signature data
A code signature can also be rather helpful when reverse engineering a binary. While a code signature is one of the poorly documented (but still open-source) parts of an executable, its content can be displayed by means of the codesign tool (see the image below).
Code signature data contains a number of important elements:
- Code directory
- Сode signing requirements
- Description of sealed resources
- Entitlements
- Code signature
Let's take a closer look at each element.
The code directory is a structure that contains miscellaneous information (hash algorithm, table size, size of code pages, etc.) and a table of hashes. The table itself consists of two parts: positive and negative.
The positive part of the table of hashes contains hashes of executable code pages.
The negative part optionally contains hashes of such code signature parts as code signing requirements, resources, and entitlements, as well as a hash of the Info.plist file.
Code signing requirements, resources, and entitlements are just bytestreams of the appropriate files located inside a bundle.
The code signature is an encrypted code directory represented in CMS format.
Architectures
One more thing you should pay special attention to before you learn how to reverse engineer a macOS or iOS app is the architecture it was designed for. Modern desktop devices usually use x86-64 CPUs. Mobile devices use ARMv7, ARMv7s, ARMv8-A, ARMv8.2-A, ARMv8.3-A, and ARM64 CPUs.
Knowledge of instruction sets is important when reverse engineering algorithms. In addition, it's good to be familiar with calling conventions and some things specific to ARM-based systems on a chip (SoC), like thumb mode and opcodes format.
Caches
Nowadays, all system frameworks and dynamic libraries are merged into a single file called the shared cache. This file is located at the following address: /System/Library/Caches/com.apple.dyld/.
These are the basic things you need to know about before doing any reverse engineering. Now let's talk about the macOS and iOS reverse engineering tools that can help you on this journey.
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Restoring Classes – Useful Tips for Software Reverse Engineers https://sdirectsoft.mystrikingly.com/blog/pogo-the-fridge-mac-os.
Software reverse engineering tools
Below are standard command-line tools for reverse engineering iOS and macOS apps. These tools are available out of the box on Mac:
- lldb is a powerful debugger used in Xcode. You can use this tool to reverse engineer and debug code written in C++, Objective-C, and C. lldb allows you to debug code on both actual iOS devices and simulators.
- otool is a console tool for browsing and editing in mach-o executables. It displays specified parts of libraries and object files.
- nm is a console tool for browsing names and symbols in mach-o executables.
- codesign is a useful tool for working with code signatures. It provides comprehensive information on code signatures and allows for creating and manipulating them.
In addition, there are several third-party reverse engineering utilities:
- IDA
- MachOView
- Class-dump
- Hopper
- Dsc_extractor
- Ghidra
Let's look closer at each of these utilities.
IDA (Interactive DisAssembler) is one of the most famous and widely used reverse engineering tools. IDA is a disassembler and debugger that's suitable for performing complex research of executables. It's a cross-platform tool that runs on macOS, Windows, and Linux.
IDA can be used for disassembling software designed for macOS, Windows, and Linux platforms. The program has a free evaluation version with limited functionality. There's also a paid version, IDA Pro, which supports a wider range of processors and plugins.
MachOView is a utility that works similarly to the otool and nm console tools. The key difference is that MachOView does have a GUI, so you can browse the structure of mach-o files in a more comfortable way. In fact, MachOView was used to make most of the screenshots you see in this article. MachOView is free to use, but unfortunately, it isn't always stable.
Class-dump is a free command-line utility for analyzing the Objective-C segment of mach-o files. With class-dump, you can get pretty much the same information as from otool but in the form of standard Objective-C declarations. In particular, class-dump creates declarations for classes, categories, and protocols.
Hopper is an interactive tool for disassembling, decompiling, and debugging software and applications. Similarly to IDA, Hopper has a free version with a limited set of features in addition to a paid version. Hopper was designed for Linux and macOS and works best for retrieving Objective-C specific information from the analyzed binary.
Dsc_extractor is Apple's own open-source tool for extracting libraries and frameworks from dyld_shared_cache. When extracting data, the utility saves the locations and original names of all extracted objects.
Ghidra is an open-source reverse engineering framework provided by the NSA. It supports macOS, Windows, and Linux. Ghidra can be used as a decompiler, as well as a tool for performing such tasks as assembling/disassembling, graphing, and scripting code. It can be customized with the help of scripts and plugins written in Java or Python.
Read also:
9 Best Reverse Engineering Tools for 2019
Specifics of programming languages
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Now, let's look at some of the specifics of reverse engineering code written in particular programming languages. Within this article, we focus on the peculiarities of reverse engineering solutions written in Objective-C and Swift.
How to reverse engineer Objective-C code
Objective-C is commonly used for developing applications for macOS and iOS. It relies on a specific C runtime, which somewhat simplifies the process of reverse engineering.
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Let's consider a simple code from an actual application:
