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Evaluation of Obfuscations Present in Apple FairPlay

Evaluation of Obfuscations Present in Apple FairPlay

2023-08-29 08:32:45

FairPlay includes a set of algorithms created by Apple for digital rights administration (additionally known as DRM, digital rights administration). FairPlay is presently used to handle the decryption of iOS functions throughout their set up on Apple units. In truth, we all know that Apple distributes all functions within the Apple Retailer by way of the IPA file format. The IPA file format accommodates encrypted data that may then be utilized by the working system to put in an utility. The entire encrypted data is dealt with by way of FairPlay, which takes care of maintaining the decryption key and the entire course of safe to keep away from the opportunity of decrypting the contents of.ipa information to share the contents of an app (maybe paid for) within the mistaken palms.

On this article, we’re going to summarize some static safety measures that I used to be capable of finding inside user-space daemons operating FairPlay, the DRM system utilized by Apple. All data is believed to be present as of the date of the article; the working system from which the binaries had been extracted is macOS 13.5.1.

DRM programs and the safety of Apple functions

Defending mental property in digital kind has at all times been a objective on the a part of firms that distribute copyrighted materials. How can content material be distributed with out customers with the ability to copy, view, edit, and redistribute it? Most present programs use DRM expertise. Very briefly, DRM programs work within the following method: they obtain as enter a secret (it may be a movie, a picture, or an algorithm) that isn’t readable, for instance, uncooked information, course of the data, and transmit as output the unique content material. All that is carried out whereas making an attempt to maintain the method by which the unique data was obtained as hidden as potential. DRM programs are most generally used for viewing copyrighted content material: the consumer indicators a contract with a service supplier that guarantees to ship the requested data (films, TV collection, books). To forestall copying of the content material (the contract gives for just one consumer approved to view it with that contract), the data have to be accessible in order that no copying or exporting of the content material is anticipated.

From an Apple perspective, let’s assume that we wish to obtain a paid utility or new recreation by way of the Apple Retailer. We make the transaction, and our system will get a standalone archive, able to be put in. Technically talking, if there have been no safety measures, an individual might copy the app installer and cross it on to every other individual. End result? Lack of income on the a part of Apple and the developer who revealed the appliance because the archive copy is free. Paradoxically, by treating the IPA file format as a type of container, there could be no technical measure to cease utility sharing (archives are standalone). That is the place FairPlay™ expertise comes into play.

How can we shield the data contained throughout the archive? It’s clear that we should one way or the other cover the contents of the archive; by doing so, even when they had been to extract the IPA archive from the iPhone, attackers wouldn’t be capable to entry the contents. The data must be learn solely by the system processes which can be answerable for putting in the appliance. The following query could be: how can we cover the content material? Right here, an unparalleled Pandora’s field opens. One of many easiest and least costly concepts from an archival viewpoint is to encrypt the data utilizing a secret key. The data could be made readable once more by a easy name to decrypt(content material, key).

The selection of secret key, nevertheless, isn’t a banal downside. If we take into consideration the set up course of, we all know that the data will sooner or later need to be decrypted, that’s, made readable once more. Can we set a static key that applies to all iPhones offered? Definitely not. The selection of a static (i.e., hardcoded) key poses quite a lot of nuisances: attackers would solely need to discover a single key in an effort to acquire entry to all archives created by the Apple Retailer. Nevertheless, as soon as the important thing was found, the remainder would come naturally: installers could be decrypted very quickly, and the community would discover itself flooded with “cracked” archives. The selection of the static key might sound advantageous to use, maybe by splitting the “visibility” of a set of keys per system (iPhone 12 could have a sure key, iPhone 13 a distinct one) by exploiting {hardware}. However even this inventiveness doesn’t work; the important thing to sharing is throughout a number of units!

The answer truly contains the technology of “dynamic” keys, i.e., relying solely on the set up system, the account of the consumer who paid for the functions, and a set of metadata exchanged in the course of the transaction (to keep away from spoofing). Apple encrypts the contents of the IPA file the second it receives a brand new request from the Apple Retailer: encryption is finished with a public key related to the Apple account. As soon as obtained, the archive is decompressed and the person binary is decrypted utilizing the personal key throughout the system. The appliance is thus put in in plaintext and stays in plaintext throughout the Apple system.

The purpose at which the decryption takes place constitutes a big centralization level that may very well be helpful to research to attempt to get the IPA decrypted. If attackers might work out how the personal secret is generated from the system and related account, Apple’s safety system would fail immediately. With respect to the chance of spoofing, Apple contains inside its units sure stratagems to declare to cloud companies (Apple Retailer, iCloud, Apple Signing) that certainly the packet trade originates from an Apple system and never from an issued or simulated Apple system. Sadly as a consequence of numerous technical limitations it isn’t potential to show 100% “I’m not a simulated system, give me the binary, I’m [myaccount@icloud.com]”.

Static evaluation and obfuscation methods

FairPlay constitutes an important technological a part of Apple’s technical experience in mental property safety. A number of patents have been developed to explain this expertise and shield it: US8934624B2: Decoupling rights in a digital content unit from download, ES2373131T3: Safe distribution of content using descifrado keys. Defending how the decryption course of works is the principle objective of one other set of “anti-reverse engineering” applied sciences named software program obfuscation.

In truth, we all know that among the many big selection of methods that any business insider can devise to research software program, there’s a process known as static evaluation that enables for extra particular investigation of sure components of a bit of software program with out sending it into execution. The primary strategy of static evaluation is reverse engineering, which is the reconstruction of the unique code by inferring data from uncooked binary code.

Uncooked binaries the truth is encompass two principal items of knowledge to operate: information and directions. By a large number of steps, packages equivalent to Ghidra, IDA or Binary Ninja can reconstruct a lot of the unique supply code. Though not completely, the software program analyst is ready to infer a lot of the semantics of the software program: the way it works, what strategies it calls, and what data it makes use of from the working system are a few of the examples of questions we are able to reply by way of reverse engineering.

Reverse engineering permits one to derive to a very good diploma of approximation the algorithms that FairPlay makes use of in an effort to decrypt the content material. By the information it’s then potential to strive to determine how the key secret is constructed, and with sufficient effort an attacker might work out methods to copy the decryption methodology to develop a decryption software. Deriving the algorithm could be an issue for Apple and its traders. The answer to guard the directions and information is to use some types of obfuscation that make the method of reverse engineering evaluation tougher.

Types of obfuscation are a part of a self-discipline of laptop science known as Software program Safety that goals to guard the code and knowledge contained inside a program. Examples of functions of obfuscation embody: defending mental property, making reverse engineering tougher to stop the invention of vulnerabilities, and mitigating exploits. These methods are utilized to the syntax of a program (i.e., the uncooked directions) and make it potential to cover a lot of the unique semantics. Manipulating uncooked code may be very complicated, so it’s essential to have a stable basis to have the ability to modify the code with out unintended effects.

As we are going to see within the following paragraphs, FairPlay was constructed in a method that hides a lot of the instruction and information. Are these methods, nevertheless, actually efficient? We’ll try to reply this on the finish of the article. Recall, the truth is, that obfuscation and code safety are methods that should at all times be modified from launch to launch since there isn’t any definitive resolution to guard a bit of software program. By obfuscation we’re in a position to complicate all makes an attempt at reverse engineering, however we actually can not forestall or forestall the evaluation of the binary.

The fairplayd daemon

Within the subsequent few paragraphs we are going to try to research intimately a user-side daemon that may be discovered inside Apple’s household of working programs. The usage of FairPlay isn’t restricted solely to the iOS cellular platform: FairPlay can be utilized by macOS to handle a safe channel by way of which to convey the digital content material to be protected (films, TV collection..). This sub-technology is named FairPlay Streaming and permits copyrighted content material to be distributed by encrypting the content material. A doc that summarizes at a excessive stage the way it works is FairPlay Streaming Overview.

We then wish to discover out extra about using FairPlay inside macOS and see if the algorithms have been protected by way of obfuscations. We discover that a lot of the administration of FairPlay is assigned to a framework known as CoreFP.Framework (Core Fair Play) discovered throughout the /System/Library/PrivateFrameworks folder. Non-public frameworks are a set of libraries devoted to sure particular macOS options which can be thought of personal, which means that they haven’t been launched for public use and all strategies contained inside are to be thought of legitimate just for Apple functions.

CoreFP.Framework is presently utilized by some functions and daemons equivalent to Safari and AMPLibraryAgent. Aspect notice earlier than persevering with: if in case you have a non-public framework and are curious the place it’s presently getting used, you possibly can run the command lsof | grep -i [name_framework] the place name_framework is the identify of the framework library. In consequence we are going to see numerous energetic processes which have opened the personal framework; on this case, we briefly dwell on AMPLibraryAgent. AMPLibraryAgent is a user-space daemon that’s used to handle the consumer’s media (TV.app and Music.app). AMPLibraryAgent is a type of intermediate course of between the encrypted content material coming from Apple’s servers and the tip consumer interacting with the decrypted content material by way of the TV.app and Music.app shoppers.

FairPlayd is the daemon that’s invoked by AMPLibraryAgent and is utilized in follow to decrypt the contents. We then start to research the contents of the CoreFP.Framework folder. Though the privateframeworks are contained inside a dynamic cache known as dyld, the binaries of CoreFP.Framework can be found with none particular preparations that the consumer has to make on the dynamic cache. Inside CoreFP, we are able to discover: CoreFP and fairplayd. CoreFP is the binary that’s utilized by system processes and constitutes the library, whereas fairplayd is the consumer house daemon. The consumer house daemon makes use of the kernel element known as FairPlayIOKit.

We extract the x86_64 model of fairplayd and reserve it to a folder of our selection through the command lipo -extract x86_64 fairplayd ~/fairplayd. The fairplayd file is a traditional executable Mach-O file, it has no explicit fields throughout the header value mentioning. So let`s import it inside one among our reverse engineering instruments (at will we are able to use Ghidra, IDA, Binary Ninja or Hopper). We use IDA for comfort on this article, though we have to be particularly cautious when importing the binary into different instruments (we are going to clarify why on the finish of the article). We let IDA work to reconstruct the data throughout the binary (by way of part parsing, disassembling, decompiling).

IDA Screenshot

By default IDA opens the binary by inserting the reader on the principle image, specifically _main, the entry level of the fairplayd daemon. As we are able to see, we instantly contact on the facility of obfuscation, and after some time we understand that the whole binary was truly constructed in such a method as to obfuscate all directions. Affirmation of that is additionally given by the decompiler, beneath is a short excerpt:

v298 = &v297;
v297 = (((unsigned int) v298 &0x52491520 | (2 *(_DWORD) v298) &0x80120A40) ^ 0x40090520) + ((-1704077140 - ((unsigned int) v298 &0x8924A850)) &0x8924A854) + ((((unsigned int) v298 &0x24924288) + 689062540) &0x24924288 | (2 *(_DWORD) v298) &0x506D9510);
v302 = 62;
((void(__fastcall*)(__int64, _QWORD, _QWORD, _QWORD))((char*) *(&off_1002B0BF0 + (unsigned int)(unsigned __int8)((unsigned __int8) v298 ^ byte_100237430[byte_1002AEEF0[(unsigned __int8) v298] ^ 0x3A]) +944) -790860942))(31 LL,0 LL,0 LL,0 LL);
LODWORD(v303) = 1312628203 *((unsigned int) &v303 ^ 0x4EBE92AB) + 8;
((void(__fastcall*)(unsigned __int64 *))((char*) *(&off_1002B0BF0 +(unsigned int)(unsigned __int8)(byte_100237330[byte_1002AEDF0[(unsigned__int8) &v297] ^ 0x18] ^ (unsigned __int8) &v297) +	532) -1051853286))(&v303);
LODWORD(v303) = 2064956458 - 1106503637 *(((unsigned int) &v303 - 2 *((unsigned int) &v303 &0x6F01D10) + 116399381) ^ 0x92F4EBC6);
sub_10015D450(&v303);
v21 = HIDWORD(v303);
v22 = (HIDWORD(v303) == 1923298241) | 2;

Panic! The analyst confronted with such code has few decisions: go away reverse engineering or attempt to examine additional. Most individuals making use of obfuscation methods hope that more often than not the analyst will select the primary path. Whether it is too sophisticated to research the habits of an utility by reverse engineering, it isn’t value it. Nevertheless, it’s mandatory for obfuscation to be properly carried out earlier than there will be any main slips. On this article we are going to attempt to discover out that it truly solely takes a easy have a look at the uncooked code to establish some widespread obfuscation patterns. Within the subsequent few paragraphs we are going to introduce some obfuscation methods and the way they’re carried out.

Earlier than persevering with, we must always point out that there are completely different obfuscation methods relying on the useful resource to be protected. As we are going to see later, completely different obfuscation methods have a distinct price: the opaque predicate approach has a completely completely different price than management movement flattening. That stated, let’s get began!

Useful Tip: With the presence of obfuscations utilized to directions and information, it’s a good suggestion to attempt to use the decompiler view as little as potential. In truth, the decompiler deduces some high-level data from how the machine directions had been positioned throughout the binary and what information they work on. Since a lot of the directions are supposed to complicate the decompiler end result, abandoning the decompiler result’s at all times a good suggestion.

Combined Boolean Arithmetic Expression

The primary obfuscation approach we deal with is information obfuscation utilizing combined expressions with algebraic (sum, subtraction, multiplication, division) and boolean (logical operations) operators. A lot of these expressions are used when there’s a want to cover a given numeric fixed inside a program. A continuing datum might characterize a price utilized in an encryption algorithm (“Nothing-up-my-sleeve-number”), but additionally a string, a reminiscence deal with, and extra. However not solely that! If throughout processing, equivalent to throughout decryption, a cryptographic algorithm performs a easy addition, it’s potential to make the arithmetic expression extra complicated.

Examples of arithmetic-boolean expressions are: v298 & 0x52491520 | (2 * (_DWORD) v298) & 0x80120A40) ^ 0x40090520) + ((-1704077140 - (v298 & 0x8924A850)) & 0x8924A854) + ((((unsigned int) v298 & 0x24924288) + 689062540) &0x24924288 | (2 * v298) & 0x506D9510). I’ve already written in a earlier publish how to be able to create these expressions by making use of transformation guidelines. The method Apple used is identical: take a relentless from the code, rewrite the fixed utilizing arithmetic operators, after which apply the transformations. Will we have already got an expression? We proceed to use the foundations of transformations. Notice that just some transformations will be utilized since they don’t change the semantics of the unique expression. On the finish of the method, the expression is translated again into machine language in order that it may be reinserted throughout the binary.

Within the case of fairplayd, numeric constants most frequently characterize addresses at which to leap from one primary block to a different. Addresses discuss with different primary blocks or to stubs used to name strategies from different libraries. We’ll focus on this selection in additional element in obfuscating the management movement.

We are able to truly see two forms of obfuscation of Boolean expressions relying on the kind of meeting instruction used. We are able to establish a traditional instance of obfuscation by a Boolean expression:

*(_BYTE *)(a1 + v2) = -13 * (-71 * (59 * *(_BYTE *)(a1 + v2) - 107) + 71 * (v2 & 0xF ^ 0x9C)) - 111;

This in meeting is translated as:

cdqe
movzx   ecx, byte ptr [rdi+rax]
imul    ecx, 0x3B
add     cl, 0x95
movzx   ecx, cl
imul    ecx, 0xB9
mov     edx, eax
and     edx, 0x0F
xor     edx, 0x9C
imul    edx, 0x47
add     edx, ecx
imul    ecx, edx, 0x0D
add     cl, 0x91
mov     [rdi+rax], cl

We notice that these meeting directions are fairly widespread to search out throughout the Intel x86_64 instruction set, and with a little bit effort it’s potential to scale back the expression to a simplified expression by way of some framework, equivalent to Msynth. These directions are additionally labeled traditionally as SISD: a single expression appearing on a single information merchandise (the sum between the cl and 0x95 registers has no unintended effects on the opposite registers).

One other kind of logical arithmetic expression that I’ve presently discovered entails one other kind of instruction. The Intel x86_64 instruction set makes use of a subsystem known as MMX that’s a part of the SIMD (Single Instruction, A number of Information) household of directions and permits a single instruction to function on a number of information. For instance, a primary block of those directions are:

movdqu  	xmm0, xmmword ptr [rdi+rdx]
pmovzxbw 	xmm3, xmm0
punpckhbw 	xmm0, xmm0
movdqa  	xmm1, cs:xmmword_100216450
pmullw  	xmm0, xmm1
pand    	xmm0, xmm6
pmullw  	xmm3, xmm1
pand    	xmm3, xmm6
packuswb 	xmm3, xmm0
paddb   	xmm3, cs:xmmword_100216470
pshufd  	xmm0, xmm3, 0EEh
pmovzxbd 	xmm8, xmm0
pshufd  	xmm0, xmm3, 0FFh
pmovzxbd 	xmm13, xmm0
pshufd  	xmm0, xmm3, 55h ; 'U'
pmovzxbd 	xmm9, xmm0
pmovzxbd 	xmm10, xmm3
movdqa  	xmm0, cs:xmmword_100216600
pmulld  	xmm10, xmm0
pmulld  	xmm9, xmm0
pmulld  	xmm13, xmm0
pmulld  	xmm8, xmm0
movdqa  	xmm0, xmm14
movdqa  	xmm4, cs:xmmword_100216480
pand    	xmm0, xmm4

These sorts of operations are complicated to translate into high-level code as a result of they’re depending on the structure and on whether or not one instruction will be transformed into a number of high-level directions (for instance: paddb permits 128-bit addition between the xmm3 register and the information situated at deal with xmmword_100216470). IDA solves this downside by defining features equivalent to mmu_addb(xmm3, xmmword_100216470) however we have no idea, nevertheless, what occurs inside.

Concept: SIMD computation opens numerous alternatives for obfuscation primarily based on MBA expressions: is it potential to make use of expertise for matrix computation/parallel to obfuscate a sure fixed? The concept could be to have a collection of MBA expressions to compute in parallel to search out the results of a relentless. This may decrease the overhead given by introducing the complication of an expression (and since I’ve a decrease overhead, I can improve the diploma of illegibility of the expression). Plus it might be depending on the Apple Silicon structure, making it tougher to emulate the computation. Attainable issues with this method embody: discovering equations that may be executed in parallel, checking methods to work together with parallel cores (through software program abstraction?), checking the semantics of expressions.

The strategy of obfuscation by combined boolean arithmetic expressions has been extensively used within the fairplayd binary. A superb instance of how this obfuscation has been used will be discovered within the operate sub_1001F5D32: the biggest operate inside which Boolean arithmetic expressions are discovered. There are presently a number of strategies for making an attempt to summarize arithmetic Boolean expressions, however most depend on a single approach known as symbolic execution and contain using Proof Solvers equivalent to z3 to confirm the semantics between the obfuscated expression and the synthesized expression.

One of the best software you possibly can presently use to resolve arithmetic Boolean expressions is goomba, developed by HexRays and accessible by default in IDA Professional and IDA Groups variations 8.3. A superb different is the Msynth framework developed by Tim Blazytko, which succeeds properly. In the end, whereas MBA obfuscation is an effective different for making expressions extra sophisticated, there are numerous instruments for fixing and rewriting expressions.

Opaque Predicates

Opaque predicates are one other very “low-cost” approach for introducing obfuscation inside directions. This method consists of introducing some at all times true or at all times false situations that trigger the decompiler to discover blocks of directions with zero utility. The at all times true or at all times false situations embody a direct or oblique bounce to primary blocks that may by no means be executed: they don’t current further performance, they solely add complexity to the features being analyzed.

Assume, for instance, that now we have the next supply code:

int sum(int a, int b){
	int end result = a + b + c;
	return end result;
}

To guard the given a part of the end result we are able to rewrite the code like this:

int sum(int a, int b){
	int end result = a + b;

	if(a == 0 && a == 1 && a - 4 >= 55 && (a * 4 - 36 * 0xff - 0xc) < 2){
		end result += 1 * 4 << 2 - 0x5c;
	} else if (a == 5 && a != 5){
		if(b > 4 && b < 4 && b != 4 && 352610 == 122){
			end result += 50 * 0xf5 * 352610;
			end result += decrypt(key);
		}
	} else {
		return end result + decrypt(key);
	}
}

To our amazement we are going to discover that the decompiler would wrestle to reconstruct the unique verify, so it might miss some at all times true or at all times false checks. The protection given by these opaque predicates is proportional to the diploma of illegibility and complexity given by the verify made between the (high-level) ifs or the varied algebraic arithmetic statements given earlier than the cmp assertion. Code throughout the ifs is introduced as useless code i.e., code that may by no means be executed: throughout decompilation, nevertheless, it isn’t potential to acknowledge between code that may run and useless code. Opaque predicates subsequently add levels of “confusion” to binary evaluation.

A typical instance of this transformation is the process sub_100005FC0: after the classical prologue, we are able to discover a operate name after which two completely different execution branches. The situation to leap right into a department or proceed is given by the check al, 2 instruction: at excessive stage the situation could be equal to if (al % 4 == 0), i.e. if the quantity saved inside register al is divisible by 4 proceed with the execution, in any other case bounce to loc_10000505F. The check instruction truly performs AND between the register al and the fast worth 0x2: performing an AND with a right away worth means checking the rest of the register division with the fast worth al & 2 == al % 4.

Visualizing the graph tree of the operate we are able to see how the essential block just under the situation is a really massive block.

Fuction Graph tree on IDA

Intuitively one would suppose {that a} very massive primary block could be an integral a part of the processing. If this system weren’t obfuscated, it is a very actual assumption. Nevertheless, now we have to think about that Apple’s builders have sophisticated the directions exactly in an effort to problem the automated binary evaluation instruments. On this case, the biggest primary block (TRUE department of the situation at % 4 == 0) represents a transparent instance of opaque predicate. In truth, if we noticed probably the most substantial elementary block, we might see numerous ineffective meeting operations.

Graph tree on IDA

Notice that the screenshot has been cropped to keep away from filling the whole article with the picture of the essential block. We ask: Is it precisely a primary block that’s ineffective for computation functions? Is the situation al % 4 == 0 at all times true or at all times false? The job of obfuscation could be to stop any type of inference throughout the situation. And it’s: we can not predict whether or not the instruction will at all times have a relentless end result or not.

What we are able to do, nevertheless, is to see the essential block current beneath the essential block of the TRUE situation al % 4 == 0. The essential block we’re then represents the FALSE situation (i.e. following jnz loc_100005F2F, we come to research the situation loc_100005F2F). A great way to verify whether or not the biggest primary block is definitely helpful or not is to see if there are any dependencies between the information used within the suspect primary block and the opposite primary blocks.

On this case, we are able to see that the essential block hundreds the deal with of xmmword_1002B6460, which was beforehand used within the “suspect” primary block.

Basic block, false condition of if instruction

Within the suspect primary block, the reminiscence location xmmword_1002B6460 is rewritten by an data current in one other reminiscence location. It’s subsequently tough to confirm the complicated dependency between the directions! Nevertheless on this case, it’s straightforward to verify the dependency because the most “necessary” code for this operate will be translated like this:

xmmword_1002B5460 = 0x0EC0C7C941423B0F77D59F9E25CFAC016;
xmmword_1002142F0 = 0x6432B8A1D491C1746754EB00EF0F9478;

// prologo
[... omissis ...]
// eax = 
if (al % 4 == 0){
	// useless code
	xmmword_1002B5460 = xmmword_1002142F0;
}
memcpy(v6, &xmmword_1002B5460, 0x1000);

// epilogo

What’s the worth of the variable xmmword_1002B5460? A lot relies on the situation of the if. Nevertheless, even in uncertainty, the obfuscation given by the opaque predicate seems to be not very highly effective: now we have two decisions, which means we are able to propagate a lot of the modifications in a parallel method (verifying what occurs if we enter the TRUE situation block or inside to dam the FALSE situation). Different questions that may come to thoughts:

  • how are we positive that the meeting code of the biggest primary block consists of “useless code”? The directions presently present that different information is overwritten, however that information is rarely used. The query stays open on different procedures that would use this information (even when the evaluation of the references has not highlighted this risk, we can not belief 100% of the binary evaluation instruments).

  • how can we ensure that xmmword_1002142F0 isn’t overwritten by different features? As common, we use the highly effective XREF software: there aren’t any different procedures that learn/write the reminiscence of xmmword_1002142F0. However keep in mind that XREF checks primarily based on data gleaned from automated binary evaluation instruments. Even only one piece of knowledge misplaced in the course of the preliminary parsing could cause main complications on the finish of the parse (ie throughout reference scanning).

  • how can we make sure that the situation al % 4 == 0 isn’t at all times happy? We might solely reply this query when now we have invented the crystal ball. What issues to us isn’t a lot verifying whether or not the situation is true or false: we truly wish to perform an evaluation of the information to know the results of the “suspect” primary block of being useless code. To confirm this, we carried out an evaluation on the essential block instantly following the suspected block. If we’re unsure, as on this case, we are able to additionally propagate the modifications to the next blocks to see if in a method or one other we are able to infer another information.

To reply the query “so what does sub_100005FC0 do in follow?”, simply have a look at the essential block simply after the memcpy operate name. Allow us to keep in mind that the signature (that’s the declaration of the operate) is the next: sub_100005FC0(int64 a1, int a2).

for ( i = 0; i != a2; ++i ){
	v4 = i + 15;
	if ( i >= 0 )
		v4 = i;

	*(_BYTE *)(a1 + i) = -13 * v6[256 * (__int64)(int)(i - (v4 & 0xFFFFFFF0))
	 	+ (unsigned __int8)(59 * *(_BYTE *)(a1 + i) - 107)] - 111;
}

The sub_100005FC0 process then proceeds to iterate over the v6 buffer to deobfuscate the contents of the phrase xmmword_1002B5460. The most costly instruction, computationally talking, is an MBA expression to be deobfuscated. Please discuss with the MBA subsection to know methods to deobfuscate the expression and rewrite it. You probably have issues with pointers, I can rewrite the expression as:

a1[i] = -13 * v6[256 * (i - (v4 & 0xFFFFFFF0)) + 59 * a1[i] - 107] - 111;

The obfuscation measure on this case is not efficient and lets you get well a lot of the unique data. There are quite a few instances of the tendency to incorporate “ineffective” opaque predicates (like this one mentioned) throughout the fairplayd program. The opaque predicates launched by Apple don’t provide you with a headache: a very good software program analyst would be capable to distinguish between useless branches and directions that can’t be executed. Apple must overview the development of the opaque predicates to make the essential blocks much more complicated.

Concept: To make obfuscation much more efficient, you would wish to make every elementary block extra complicated (maybe making an attempt to make the management movement extra sophisticated). A transparent instance of how arduous the decompiler’s job will be made is introduced within the subsection management movement flattening.

Transferring the stack

Earlier than persevering with with our dialogue, let’s persist with the evaluation of the sub_100005FC0 operate and show the prologue of the process earlier than the check al, 0x2 assertion.

Screen of prologue of sub_100005FC0

Will we discover something unusual? I confess that it was not straightforward to research, however after the evaluation of IDA we perceive one other “obfuscation” approach that Apple has utilized to guard fairplayd. Very subtly the stack is moved up (or down, relying on the way you wish to construct the stack). Why would a program wish to transfer the stack?

All instruction evaluation software program gives, among the many many evaluation steps, a specific kind of perception known as “stack evaluation”. This method lets you get well how the stack and native variables are composed: it’s important to have the ability to appropriately deduce how the stack consists, in any other case the evaluation software program is not in a position to perceive the movement management and the varied operate calls.

For this process, the kind of error that IDA experiences is sp-analysis failed i.e. it fails to hint how the stack pointer is modified. In our case, IDA fails to comprehend that the shift is finished exactly to confuse the evaluation. Certainly with the instruction mov eax, 1010h and sub rsp, rax, IDA is led to suppose that there’s a native buffer known as var_1020 of measurement 0x1010 bytes. This is only one of many examples the place IDA fails to rebuild the unique stack.

Different examples will be present in features with a number of statements: the process sub_10000F620 has a parameter on the stack that occupies 0x2CE7AB73 bytes (753380211 bytes = about 753 MB). Such a big parameter on the stack we all know can not exist: the stack will be elevated in measurement, however normally reaches most 65520kb (larger values trigger errors in macOS). The worth 753 MB is given by an primary block by which the sub esp, 0x2CE7AB73 instruction is used, an unreachable primary block, subsequently thought of useless code. Nevertheless, IDA doesn’t have the instruments to comprehend that such a price is simply too massive to exist: IDA doesn’t acknowledge that its evaluation is inaccurate, quite the opposite it refuses to decompile the operate as a result of the dimensions of the stack body is simply too massive. With a single instruction, Apple manages to place a serious barrier to anybody who desires to decompile the characteristic.

Is it so tough to repair this case? For a newbie, sure. For a motivated reverse engineering scholar, no. Luckily, IDA permits the analyst to have the ability to modify a part of the data that he has deduced. By redefining the stack, a lot of the 753MB of variable will be ignored. An alternate method is to report a sure path as useless code (simply choose the bytes of that block and never outline it through the choice known as Undefine). Within the case of transferring the stack pointer up, after we attempt to make software program evaluation complicated, we’re probably not speaking about obfuscation: we’re not affecting the readability of the code. Quite, let’s act on some limitations that these instruments need to get well data. article grouping some methods to confuse the evaluation methods utilized by IDA was written by Markus Gaasedelen: Dangers of the Decompiler.

Management Move Flattening

Management Move Flattening is one other obfuscation approach discovered throughout the FairPlay binary and is probably probably the most highly effective utilized by Apple within the area of code safety for fairplayd. Move management signifies how this system evolves over time by way of instruction movement. What features a process calls, what sort of jumps the verify makes (if conditional or unconditional), the hyperlink between the varied primary blocks, the situations for which the execution strikes and lots of different data are inferred from the movement management.

The high-level directions of movement management are the everyday directions that will let you divide the execution of a program into a number of instances: if, if else, if else if else, swap, do whereas, for and lots of others. Most of those situations translate into machine language directions which for Intel develop into cmp, check, jmp. If the high-level movement management change is dictated by “logical” directions, the low-level movement management change is to alter the deal with of the following instruction, or relatively, bounce to a sure instruction.

Among the many data that we are able to deduce from the movement management, we are able to derive the varied connections between the essential blocks and proceed to hold out the intra-procedural evaluation, i.e. attempt to reconstruct this system execution movement between the varied procedures. The movement of management additionally permits to get well necessary data (equivalent to loops) to subsequently proceed to the interpretation from machine code to pseudo C code.

To extra clearly present the facility of obfuscation utilizing management movement flattening, let’s take an instance operate: sub_10003FE60. From IDA, we choose the instance operate, proper click on and select the tree view. The tree view is a characteristic of IDA which lets you perceive: how the essential blocks are linked, the dependencies between the blocks and the execution branches. Usually by way of this view it’s potential to get a greater concept of what the pseudo code translated by the decompiler will seem like.

The objective of the management movement flattening approach is to flatten the management movement, i.e. to rework the management movement by way of some conditional and unconditional jumps. The approach was developed by Chenxi Wang in his doctoral thesis entitled A Safety Structure for Survivability Mechanisms. Usually the management movement of a process is sort of developed vertically, as you possibly can see within the picture beneath.

IDA vertical control flow

See Also

With the appliance of management movement flattening, the process graph is closely modified by making it “extra horizontal,” that’s, flattened or flatten. Here’s a typical instance of how a operate will be modified by making use of (easy) management movement flattening:

IDA control flow after applying the control flow flattening

We are able to then see how the essential blocks have all been delivered to the identical de facto stage by horizontally extending the graph of primary blocks. The case of management movement flattening taken to the acute drives the analyst loopy, equivalent to this operate depicted beneath:

IDA vertical control flow extreme

As will be seen from the picture simply above, management movement flattening will be utilized to assemble even bogus primary blocks that may confuse the decompiler. The top result’s a management movement that’s extraordinarily complicated to research. At a excessive stage, the transformation approach will be translated by a easy swap. Suppose now we have a process:

int sum(int a, int b){
	int s = 4;
	int c;
	if ( a > 500 ){
		c = 2;
	} else {
		c = 0;
	}
	int end result = s + a + b + c;
	return end result;
}

The management movement of this easy operate is given by the next graph:

Control flow graph

Now we wish this vertical graph to be as flattened as potential! To do that we use one other highly effective assemble of programming languages: the swap instruction. The swap instruction permits the execution to be divided into a number of branches, creating primary blocks organized horizontally. To change from one department to a different within the swap, nevertheless, we are able to use an auxiliary variable, known as state, which shops the present state we’re in. Does this sound acquainted? This method sounds so much just like the synthesis of a finite state machine! The auxiliary variable helps to alter state with out worrying about some unintended effects (or with out using labels and goto). Allow us to then transfer on to the precise transformation! For now, allow us to assume that we don’t add any pointless primary blocks.

The steps we carry out are as follows:

  1. Creation of the “begin” block: accommodates the initialization of the state variable and all of the variables that shall be used throughout the primary blocks of the swap. It’s potential to maneuver the declarations throughout the swap branches, nevertheless, we should take note of potential unintended effects of utilizing native variables inside to a assemble.

  2. Dispatcher creation: This primary block will verify what state to leap to. It’s a easy whereas with a swap assertion inside: that is to keep away from that after getting into one of many swap branches, the bounce out reaches the final assertion of the operate.

  3. Transfer primary blocks: I select a brand new quantity for a brand new state. I insert the code throughout the recognized primary block together with the state change. The state variable ought to level to the following primary block I wish to execute. I remind you that I may create new elementary blocks by subdividing bigger primary blocks or including directions that may by no means be executed. The restrict for obfuscation is creativeness :- )

  4. Transformation verify: I verify that certainly the operate can terminate, listening to the final block that’s executed. All transformations of obfuscations ought to truly be checked by way of some proof solvers and mathematical equations to stop the unique semantics from being modified in the course of the obfuscation course of.

Making use of management movement flattening:

int sum(int a, int b){
	int state = 0;
	int s, c, end result;
	whereas(1){
		swap(state){
			case 2:
				if (a > 500){
					state = 4;
				} else {
					state = -1;
				}
				break;
			case 0:
				s = 4;
				state = 2;
				break;
			case -1:
				c = 0;
				state = 5;
				break;
			case 5:
				end result = s + a + b + c;
			case 0x54292639:
				return end result;
			case 4:
				c = 2;
				state = 5;
				break;
		}
	}
	return -542926392;
}

The end result:

Control flow graph

Some notes we write per level:

  • the graph doesn’t include the whereas instruction to keep away from including too many arrows and making the graph extra complicated. When a department of a swap ends with the break instruction, execution resumes from the analysis of the state variable. The graph doesn’t include the final instruction return -542926392;, it needs to be inserted with an arrow after dispatcher execution.

  • primary block A and primary block B are two primary blocks that had been inserted to spotlight the distinction within the management movement whether or not the variable a leads to better than 500 or not. If certainly the situation is true, we alter state by going to state 4, in any other case we go to state -1. Notice that the quantity for every state is trivially a quantity chosen at random to establish every state with a label.

  • An attacker might nonetheless predict the instruction execution movement by following the evolution of the state variable. There are some gimmicks to obfuscate the worth of the state variable equivalent to utilizing MBA expressions and aliasing. Additionally it is potential to introduce pointless blocks, opaque predicates and dynamically assemble the swap to divert the analyst or hinder the decompiler’s evaluation.

  • the variable state is rarely assigned with the worth 0x54292639. So how can we make sure that the operate terminates by returning end result? Within the earlier code we used a small stratagem: we reap the benefits of not mentioning break throughout the swap department. On this case, execution goes instantly from state 5 to state 0x54292639 as a result of this system is executed sequentially (proven as “C Magic”). Generally it’s potential that the compiler applies optimizations to not create the department of 0x54292639, so remember to disable the optimizations if you’d like the graph to be the identical because the picture above.

Simply out of curiosity, let’s strive compiling a program that makes use of each sum features to see how efficient this type of approach is. This system the truth is has two features sum_1, the obfuscated operate, and sum_2, the unique operate. The sum_2 operate is decompiled appropriately:

__int64 __fastcall sum_2(int a1, int a2){
  int v3; // [rsp+4h] [rbp-10h]

  if ( a1 <= 500 )
    v3 = 0;
  else
    v3 = 2;
  return (unsigned int)(v3 + a2 + a1 + 4);
}

The operate is similar to the unique. As well as, the management movement graph constructed by IDA isn’t suspicious and appears the identical because the graph constructed by us manually. Totally different is the case for sum_1:

__int64 __fastcall sum_1(int a1, int a2){
  int v3; // [rsp+8h] [rbp-14h]
  int v4; // [rsp+Ch] [rbp-10h]
  int i; // [rsp+10h] [rbp-Ch]

  for ( i = 0; ; i = 5 )
  {
    whereas ( 1 )
    {
      whereas ( 1 )
      {
        whereas ( i == -1 )
        {
          v3 = 0;
          i = 5;
        }
        if ( i )
          break;
        v4 = 4;
        i = 2;
      }
      if ( i != 2 )
        break;
      if ( a1 <= 500 )
        i = -1;
      else
        i = 4;
    }
    if ( i != 4 )
      break;
    v3 = 2;
  }
  return (unsigned int)(v3 + a2 + a1 + v4);
}

In the meantime, we are able to say that now we have managed to confuse the IDA decompiler. Certainly, we discover {that a} for is constructed, then 3 whereas(1) loops one inside the opposite. Though the results of the decompiler will not be precisely appropriate and is completely different from the unique code, this system works. An attacker would then be capable to decide up this system logic. This can be a typical instance of weak obfuscation: the attacker might have some problem reconstructing the unique management movement, however there’s a good probability that the preliminary logic will be recovered. To make it much more complicated, we are able to use the opposite methods: insertion of opaque predicates, transformations of constants, transformations of arithmetic Boolean expressions.

Returning to fairplayd, the management movement flattening approach is used closely to attempt to obfuscate the management movement evolution that fairplay has throughout execution. We open with IDA the operate sub_10003FE60 and extract the logic of operation through the decompiler view. Specifically, we are able to see that the decompiler can perceive using a high-level switch-like construction:

swap ( (v7 == 0) + v2 ) 2u;
      ....

Nevertheless, by inspecting all instances throughout the swap, we could have nonsense directions! It’s because Apple’s builders have managed to closely obfuscate the state variable, specifically v2 and v7. However, the decompiler acknowledges the swap assemble solely by the presence of bounce tables, explicit tables whose entries encompass reminiscence addresses to leap to. Suppose you’ve gotten 5 branches of a swap, you possibly can retailer 5 completely different addresses in reminiscence, every related to a distinct department of the swap. When evaluating, you’d merely bounce to the deal with pointed to the proper cell within the desk. These tables are saved at particular places within the binary, and with a little bit luck it’s potential to revive all of them.

To boost the bar of research problem even larger, the builders determined that for some features the bounce desk could be recreated dynamically. Very briefly, this system allocates a reminiscence house the place addresses will be entered that may later be used as branches of a swap. The calculation of the addresses is finished by the same old MBA expressions, and as a consequence of numerous technical limitations of static evaluation it isn’t potential to find out the addresses precisely. This in a nutshell fully destroys any risk for IDA and the analyst to get well the unique program logic and construction.

Typical instance of deal with building comes from the final portion of code in every primary block (the truth that deal with building happens inline provides complexity to the code):

lea     r12, jpt_10003FF12								; deal with the place to fetch the following deal with
movsxd  rax, ds:(jpt_10003FF12 - 100218890h)[r12+rax*4] ; transfer the deal with referred by jpt_10003FF12 - 100218890h
lea     rcx, loc_10005A140 								; load the deal with of loc_10005A140 (base deal with of bounce desk)
add     rcx, rax										; add the rax worth
mov     r14, [rbp+var_58]								; save earlier variable on stack
jmp     rcx												; bounce to subsequent swap case

The management movement flattening approach thus seems to be a very good obfuscation approach by which Apple is ready to forestall static evaluation by attackers. Dynamic desk building, mixed with the set of methods talked about within the earlier subsections (opaque predicates, MBA) are the important thing to fairplayd obfuscation.

Are additional obfuscation measures potential?

All through this text now we have tried to establish some widespread obfuscation patterns that may be present in fairplayd, a userspace daemon utilized by Apple to guard copyrighted content material inside macOS. Some measures have proved efficient: they really complicate any try to precisely reverse engineer this system. The measure that’s most complicit on this complication is given by the management movement flattening which eliminates the possibilities for an attacker to get well the unique construction of this system. Attackers are confronted with dynamically constructed bounce tables, unrecoverable switches, and dynamically generated code.

Different obfuscation methods appear to be good concepts, however solely in idea: opaque predicates are easy sufficient to parse, and thru a extra thorough verify of the constructing blocks it’s potential to differentiate between the unique code and the modified code to introduce obfuscation.

Is there an ideal match to have the ability to obfuscate the code? Is it potential to plan a method able to producing unassailable safety? No. It’ll at all times be an ongoing evolution between instruments that attempt to deobfuscate hidden content material and new methods to have the ability to bypass these instruments and apply heavier obfuscation. That is given by the limitation of digital content material administration: the data should finally be capable to be seen and subsequently deobfuscate, versus different forms of data that may be hidden endlessly contained in the units. It’s doubtless that after yet one more article about obfuscation inside fairplayd the safety measures shall be up to date once more, inflicting the article content material to be outdated.

One potential avenue that Apple might pursue to use extra highly effective obfuscation is to maneuver a lot of the execution to an obfuscation approach known as “virtualization”. This method entails writing an interpreter and a totally ad-hoc instruction set structure to carry out the operations essential to decrypt the content material. For the time being the approach doesn’t appear to be used inside fairplayd, however there are some indicators (like switches with lots of branches) that appear to foretell using information analysis and dynamic code manufacturing. Actually, I haven’t had time to research this half intimately but.

As at all times, if in case you have any criticisms, options or every other feedback, you possibly can write an e-mail to seekbytes@protonmail.com. I by no means wish to be the voice of the reality, so experiences of errors or inaccuracies are at all times welcome.

Developed in some autumn afternoon – 2021-2023 – RSS feed

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