Malicious Hashing: Eve’s Variant of SHA-1

Malicious Hashing: Eve’s Variant of SHA-1

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  Malicious Hashing: Eve’sVariant of SHA-1 Ange Albertini1 , Jean-Philippe Aumasson2 , Maria Eichlseder3 , Florian Mendel3 , and Martin Schl¨affer3 1 Corkami, Germany 2 Kudelski Security, Switzerland 3 Graz University of Technology, Austria Abstract. We present collisions for a version of SHA-1 with modified constants, where the colliding payloads are valid binary files. Examples are given of colliding executables, archives, and images. Our malicious SHA-1 instances have round con- stants that differ from the original ones in only 40 bits (on average). Modified versions of cryptographic standards are typically used on closed systems (e.g., in pay-TV, me- dia and gaming platforms) and aim to differentiate cryptographic components across customers or services. Our proof-of-concept thus demonstrates the exploitability of custom SHA-1 versions for malicious purposes, such as the injection of user surveil- lance features. To encourage further research on such malicious hash functions, we propose definitions of malicious hash functions and of associated security notions. 1 Introduction In 2013, cryptography made the headlines following the revelation that NSA may not only have compromised cryptographic software and hardware, but also cryptographic algorithms. The most concrete example is the “key escrow” [11] or “master key” [5] property of the NSA- designed Dual EC DRBG [27]. The alleged backdoor is the number e such that eQ = P, where P and Q are two points on the elliptic curve specified as constants in Dual EC DRBG. Knowing e allows one to determine the internal state and thus to predict all future outputs. Despite other issues [7, 34] (see also [16, 17]), Dual EC DBRG was used as default pseudorandom number generator in EMC/RSA’s BSAFE library, allegedly following a $10M deal with NSA [24]. It is also speculated that NSA may have “manipulated constants” [33] of other algo- rithms, although no hard evidence has been published. This series of revelations prompted suspicions that NIST-standardized cryptography may be compromised by NSA. It also raised serious doubts on the security of commercial cryptography software, and even of open-source software. Several projects have been started to address those concerns, like #youbroketheinternet4 and the Open Crypto Audit Project5 . Research on cryptographic backdoors and malicious cryptography appears to have been the monopoly of intelligence agencies and of industry. Only a handful of peer-reviewed articles have been published in the open literature (see §§2.1), whereas research on related topics like covert-channel communication (e.g., [26,35]) or hardware trojans (e.g., [2,20]) is published regularly. Malicious ciphers have been investigated by Young and Yung [42] in their “cryptovirol- ogy” project, and to a lesser extent by Rijmen and Preneel [32] and Patarin and Goubin [29]. However we are unaware of any research about malicious hash functions—that is, hash func- tions designed such that the designer knows a property that allows her to compromise one or more security notions. Note that we distinguish backdoors (covert) from trapdoors (overt); for example VSH [9] is a trapdoor hash function, such that collisions can be found efficiently if the factorization of the RSA modulus is known. 4 http://youbroketheinternet.org. 5 http://opencryptoaudit.org
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  This paper thusinvestigates malicious hash functions: first their definition and potential applications, then a proof-of-concept by constructing a malicious version of SHA-1 with modified constants, for which two binary files (e.g., executables) collide. We have chosen SHA-1 as a target because it is (allegedly) the most deployed hash function and because of its background as an NSA/NIST design. We exploit the freedom of the four 32-bit round constants of SHA-1 to efficiently construct 1-block collisions such that two valid executables collide for this malicious SHA-1. Such a backdoor could be trivially added if a new constant is used in every step of the hash function. However, in SHA-1 only four different 32-bit round constants are used within its 80 steps, which significantly reduces the freedom of adding a backdoor. Actually our attack only modifies at most 80 (or, on average, 40) of the 128 bits of the constants. Our malicious SHA-1 can readily be exploited in applications that use custom hash functions (e.g., for customers’ segmentation) to ensure that a legitimate application can surreptitously be replaced with a malicious one, while still passing the integrity checks such as secure boot or application code signing. Outline. §2 attempts to formalize intuitive notions of malicious hash functions. We first define a malicious hash function as a pair of algorithms: a malicious generator (creating the function and its backdoor) and an exploit algorithm. §3 then presents a novel type of colli- sion attack, exploiting the freedom degrees of the SHA-1 constants to efficiently construct collisions. We describe the selection of a dedicated disturbance vector that minimizes the complexity and show examples of collisions. §4 discusses the application to structured file formats, and whether the constraints imposed by the attack can be satisfied with common file formats. We present examples of valid binary files that collide for our malicious SHA-1: executables (e.g., master boot records or shell scripts), archives (e.g., rar), and images (jpg). 2 Malicious Hashing We start with an overview of previous work related to malicious cryptography. Then we formalize intuitive notions of malicious hashing, first with a general definition of a malicious hash function, and then with specific security notions. 2.1 Malicious Cryptography and Backdoors The open cryptographic literature includes only a handful of works related to malicious applications of cryptography: – In 1997, Rijmen and Preneel [32] proposed to hide linear relations in S-boxes and presented “backdoor versions” of CAST and LOKI. These were broken in [40] as well as the general strategy proposed. Rijmen and Preneel noted that “[besides] the obvious use by government agencies to catch dangerous terrorists and drug dealers, trapdoor block ciphers can also be used for public key cryptography.” [32]. Indeed, [5] previously argued that a backdoor block cipher is equivalent to a public key cryptosystem. – In 1997, Patarin and Goubin [29] proposed an S-box-based asymmetric scheme con- structed as a 2-round SPN but publicly represented as the corresponding equations – keeping the S-boxes and linear transforms secret. This was broken independently by Ye et al. and Biham [3, 41]. This can be seen as an ancestor of white-box encryption schemes. – In 1998 and later, Young and Yung designed backdoor blackbox malicious ciphers, which assume that the algorithm is not known to an adversary. Such ciphers exploit low- entropy plaintexts to embed information about the key in ciphertexts through a covert channel [43,44]. Young and Yung coined the term cryptovirology [42] and cited various malicious applications of cryptography: ransomware, deniable data stealing, etc. 2
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  – In 2010,Filiol [15] proposed to use malicious pseudorandom generators to assist in the creation of executable code difficult to reverse-engineer. Typical applications are the design of malware that resist detection methods that search for what looks like obfuscated code (suggesting the hiding of malicious instructions). Note that we are concerned with backdoors in algorithms, regardless of its representation (pseudocode, assembly, circuit, etc.), as opposed to backdoors in software implementations (like Wagner and Biondi’s sabotaged RC4 [37]) or in hardware implementations (like bug attacks [4] and other hardware trojans). 2.2 Definitions We propose definitions of malicious hash functions as adversaries composed of a pair of algorithms: a (probabilistic) malicious generator and an exploit algorithm. Based on this formalism, we define intuitive notions of undetectability and undiscoverability. Malicious Hash Function. Contrary to typical security definitions, our adversary is not the attacker, so to speak: instead, the adversary Eve creates the primitive and knows the secret (i.e. the backdoor and how to exploit it), whereas honest parties (victims) attempt to cryptanalyze Eve’s design. We thus define a malicious hash function (or adversary) as a pair of efficient algorithms, modeling the ability to create malicious primitives and to exploit them: – A malicious generator, i.e. a probabilistic algorithm returning a hash function and a backdoor; – An exploit algorithm, i.e. a deterministic or probabilistic algorithm that uses the knowl- edge of the backdoor to bypass some security property of the hash function. We distinguish two types of backdoors: static, which have a deterministic exploit algorithm, and dynamic, which have a probabilistic one. Below, the hash algorithms and backdoors returned as outputs of a malicious generator are assumed to be encoded as bitstring in some normal form (algorithm program, etc.), and to be of reasonable length. The generator and exploit algorithms, as well as the backdoor string, are kept secret by the malicious designer. Static backdoors adversaries. Eve is a static collision adversary (SCA) if she designs a hash function for which she knows one pair of colliding messages. Definition 1 (SCA). A static collision adversary is a pair (GenSC, ExpSC) such that – The malicious generator GenSC is a probabilistic algorithm that returns a pair (H, b), where H is a hash function and b is a backdoor. – The exploit algorithm ExpSC is a deterministic algorithm that takes a hash function H and a backdoor b and that returns distinct m and m such that H(m) = H(m ). This definition can be generalized to an adversary producing a small number of collisions, through the definition of several ExpSC algorithms (e.g. ExpSC1, . . . , ExpSCn). As a static second-preimage adversary would not differ from that of static collision, our next definition relates to (first) preimages: Definition 2 (SPA). A static preimage adversary is a pair (GenSP, ExpSP) such that – The malicious generator GenSP is a probabilistic algorithm that returns a pair (H, b), where H is a hash function and b is a backdoor. – The exploit algorithm ExpSP is a deterministic algorithm that takes a hash function H and a backdoor b and that returns m such that H(m) has low entropy. In the above definition “low entropy” is informally defined as digest having a pattern that will convince a third party that “something is wrong” with the hash function; for example, the all-zero digest, a digest with all bytes identical, etc. 3
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  Dynamic Backdoors. Dynamicbackdoors extend static backdoors from one or a few successful attacks to an arbitrary number. In some sense, dynamic backdoors are to static backdoors what universal forgery is to existential and selective forgery for MACs. Definition 3 (DCA). A dynamic collision adversary is a pair (GenDC, ExpDC) such that – The malicious generator GenDC is a probabilistic algorithm that returns a pair (H, b), where H is a hash function and b is a backdoor. – The exploit algorithm ExpDC is a probabilistic algorithm that takes a hash function H and a backdoor b and that returns distinct m and m such that H(m) = H(m ). In this definition, ExpDC should be seen as an efficient sampling algorithm choosing the pair (m, m ) within a large set of colliding pairs, as implicitly defined by GenDC. The latter may be created in such a way that sampled messages satisfy a particular property, e.g. have a common prefix. The definitions of dynamic second-preimage and preimage adversaries follow naturally: Definition 4 (DSPA). A dynamic second-preimage adversary is a pair (GenDSP, ExpDSP) such that – The malicious generator GenDSP is a probabilistic algorithm that returns a pair (H, b), where H is a hash function and b is a backdoor. – The exploit algorithm ExpDSP is a probabilistic algorithm that takes a hash function H, a backdoor b, and a message m and that returns an m distinct from m such that H(m) = H(m ). Definition 5 (DPA). A dynamic preimage adversary is a pair (GenDP, ExpDP) such that – The malicious generator GenDP is a probabilistic algorithm that returns a pair (H, b), where H is a hash function and b is a backdoor. – The exploit algorithm ExpDP is a probabilistic algorithm that takes a hash function H, a backdoor b, and a digest d and that returns m such that H(m) = d. In the definitions of DSPA and DPA, the challenge values m and d are assumed sampled at random (unrestricted to uniform distributions). One may consider “subset” versions of (second) preimage backdoors, i.e. where the backdoor only helps if the challenge value belongs to a specific subset. For example, one may design a hash for which only preimages of short strings—as passwords—can be found by the exploit algorithm. Our last definition is that of a key-recovery backdoor, for some keyed hash function (e.g. HMAC): Definition 6 (KRA). A dynamic key-recovery adversary is a pair (GenKR, ExpKR) such that – The malicious generator GenKR is a probabilistic algorithm that returns a pair (H, b), where H is a hash function and b is a backdoor. – The exploit algorithm ExpKR is a probabilistic algorithm that takes a hash function H and a backdoor b and that has oracle-access to HK(·) for some key K and that returns K. The definition of KRA assumes K to be secret, and may be relaxed to subsets of “weak keys”. This definition may also be relaxed to model forgery backdoors, i.e. adversaries that can forge MAC’s (existentially, selectively, or universally) without recovering K. 4
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  Stealth Definitions. Weattempt to formalize the intuitive notions of undetectability (“Is there a backdoor?”) and of undiscoverability (“What is the backdoor?”). It is tempting to define undetectability in terms of indistinguishability between a malicious algorithm and a legit one. However, such a definition does not lend itself to a practical evaluation of hash algorithms. We thus relax the notion to define undetectablity as the inability to determine the exploit algorithm (that is, how the backdoor works, regardless of whether one knows the necessary information, b). In other words, it should be difficult to reverse-engineer the backdoor. We thus have the following definition, applying to both collision and preimage backdoors: Definition 7. The backdoor in a malicious hash (Gen, Exp) is undetectable if given a H returned by Gen it is difficult to find Exp. Subtleties may lie in the specification of H: one can imagine a canonical-form description that directly reveals the presence of the backdoor, while another description or implemen- tation would make detection much more difficult. This issue is directly related to the notion of obfuscation (be it at the level of the algorithm, source code, intermediate representation, etc.). For example, malware (such as ransomware, or jailbreak kits) may use obfuscation to dissimulate malicious features, such as cryptographic components of 0-day exploits. Furthermore, backdoors may be introduced as sabotaged versions of legitimate designs. In that case, undetectability can take another sense, namely distinguishability from the original design. For example, in our malicious SHA-1, it is obvious that the function differs from the original SHA-1, and one may naturally suspect the existence of “poisonous” inputs, although those should be hard to determine. Undiscoverability is more easily defined than undetectability: it is the inability to find the backdoor b given the exploit algorithm. A general definition is as follows: Definition 8. The backdoor in a malicious hash (Gen, Exp) is undiscoverable if given Exp and H returned by Gen it is difficult to find b. In our proof-of-concept of a malicious SHA-1, undiscoverability is the hardness to recover the colliding pair, given the knowledge that a pair collides (and even the differential used). 3 Eve’s Variant of SHA-1 As a demonstration of the above concepts, we present an example of a static collision backdoor: Eve constructs a custom variant of SHA-1 that differs from the standardized specification only in the values of some round constants (up to 80 bits). Eve can use the additional freedom gained from choosing only four 32-bit constants to find a practical collision for the full modified SHA-1 function during its design. We show that Eve even has enough freedom to construct a meaningful collision block pair which she can, at a later point, use to build multiple colliding file pairs of a particular format (e.g., executable or archive format) with almost arbitrary content. The backdoor does not exploit any particular “weaknesses” of specific round constants, nor does it weaken the logical structure of the hash function. Instead, it only relies on the designer’s freedom to choose the constants during the attack. This freedom can be used to improve the complexity of previous attacks [36, 39] and thus makes it feasible to find collisions for the full hash function. For an attacker who only knows the modified constants but cannot choose them, colli- sions are as hard to find as for the original SHA-1.Thus, in terms of the definitions of the previous section, this backdoor is undiscoverable. It is, however, detectable since constants in hash functions are normally expected to be identifiable as nothing-up-your-sleeve numbers. This is hardly achievable in our attack. Below, we first give a short description of SHA-1 in §3.1 and briefly review previous dif- ferential collision attacks on SHA-1 in §3.2. Then, we build upon these previous differential attacks and describe how the freedom of choosing constants can be used to improve the attack complexity in §3.3. 5
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  3.1 Short Descriptionof SHA-1 SHA-1 is a hash function designed by the NSA and standardized by NIST in 1995. It is an iterative hash function based on the Merkle-Damg˚ard design principle [10,25], processes 512- bit message blocks and produces a 160-bit hash value by iterating a compression function f. For a detailed description of SHA-1 we refer to [28]. The compression function f uses the Davies-Meyer construction which consists of two main parts: the message expansion and the state update transformation. The message expansion of SHA-1 is a linear expansion of the 16 message words (denoted by Mi) to 80 expanded message words Wi, Wi = Mi for 0 ≤ i ≤ 15, (Wi−3 ⊕ Wi−8 ⊕ Wi−14 ⊕ Wi−16) 1 for 16 ≤ i ≤ 79 . The state update transformation of SHA-1 consists of 4 rounds of 20 steps each. In each step, the expanded message word Wi is used to update the 5 chaining variables as depicted in Fig. 1. In each round, the step update uses different Boolean functions fr and additive constants Kr, which are shown in Table 1. Ai Ai−1 Bi Bi−1 Ci Ci−1 Di Di−1 Ei Ei−1 Kr Wi fr 2 5 Fig. 1: The step function of SHA-1. Table 1: The round constants Kr and Boolean functions fr used in each step of SHA-1. round r step i Kr fr 1 0 ≤ i ≤ 19 5a827999 fIF(B, C, D) = B ∧ C ⊕ ¬B ∧ D 2 20 ≤ i ≤ 39 6ed9eba1 fXOR(B, C, D) = B ⊕ C ⊕ D 3 40 ≤ i ≤ 59 8f1bbcdc fMAJ(B, C, D) = B ∧ C ⊕ B ∧ D ⊕ C ∧ D 4 60 ≤ i ≤ 79 ca62c1d6 fXOR(B, C, D) = B ⊕ C ⊕ D For Eve’s modified, malicious hash function, we only change the values of K2, K3 and K4. The remaining definition is left unchanged. Note that the original SHA-1 constants are chosen as the square roots of 2, 3, 5 and 10: K1 = √ 2 · 230 , K2 = √ 3 · 230 , K3 = √ 5 · 230 , K4 = √ 10 · 230 . 3.2 Differential Attack Strategy for SHA-1 At CRYPTO 2005, Wang et al. presented the first collision attack on full SHA-1 with a theoretical complexity of about 269 [39]. This was later improved to 263 by the same 6
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  authors [38]. Sincethen, several papers on the cryptanalysis of SHA-1 have been published [1,13,14,19,36]. Nevertheless, no practical collision has been shown for full SHA-1 to date. Our practical and meaningful collision attacks on malicious SHA-1 are based on the differential attacks by Wang et al. in [39] and its improvements. In a differential collision attack, we first construct a high-probability differential characteristic that yields a zero output difference, i.e., a collision. In the second stage, we probabilistically try to find a confirming message pair for this differential characteristic. By using a differential characteristic with a lower probability at the beginning of the hash function (first round of SHA-1), the probability of the remaining characteristic can be further improved. Since the message can be chosen freely in a hash function attack, we can significantly improve the complexity of finding confirming message pairs at the beginning of the hash function using message modification techniques [39]. A high-level overview of such a differential attack on SHA-1 is given as follows: 1. Find a differential characteristic (a) Construct the high-probability part (b) Determine the low-probability part 2. Find a confirming message pair (a) Use message modification in low-probability part (b) Perform random trials in high-probability part The high-probability part of the differential characteristic for SHA-1 covers round 2 to round 4. It has been shown in [8, 21, 39] that for SHA-1, the best way to construct these high-probability characteristics is to interleave so-called local collisions (one disturbing and a set of correcting differences). These characteristics can be easily constructed by using a linearized variant of the hash function and tools from coding theory [30,31]. The probability of this characteristic determines the complexity of the attack on SHA-1. The low-probability part and message modification take place in round 1 and are typ- ically performed using automated non-linear equation solving tools [14, 22, 23]. We stress that the total complexity is still above 260 for all published collision attacks so far, which is only practical for attackers with large computing power (NSA, Google, etc.). 3.3 Malicious Collision Attack In SHA-1, a new 32-bit constant K1, . . . , K4 is used in each of the four rounds. In our mali- cious collision attack, we use the freedom of these four constants to reduce the complexity of the attack. Similar to message modification, we choose the constants during the search for a confirming message pair. We modify the constants in a round-by-round strategy, al- ways selecting a round constant such that the differential characteristic for the steps of the current round can be satisfied. Since we have to choose the constants when processing the first block, we can only improve the complexity of this block. Hence, we need to use a differential characteristic that results in a single-block collision. Note that all the collisions attacks on SHA-1 so far use a 2-block characteristic. To find the high-probability part of a differential characteristic for round 2–4 resulting in a 1-block collision, a linearized variant of the hash function can be used. However, using algorithms from coding theory, we only find differential characteristics that maximize the overall probability and do not take the additional freedom we have in the choice of the constants in SHA-1 into account. Therefore, to minimize the overall attack complexity, we did not use these differential characteristics. Instead, we are interested in a differential characteristic such that the minimum of the three probabilities for round 2, 3 and 4 is maximized. To find such a characteristic, we start with the best overall characteristic and modify it to suit our needs. In previous attacks on SHA-1, the best differential characteristics for rounds 2–4 have differences only at bit position 2 for some 16 consecutive state words Ai [21]. We assume that the best differential characteristic has the same property in our case. Hence, we only need 7
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  to determine all216 possible differential characteristics with differences only at bit position 2 in 16 consecutive state words Ai and linearly expand them backward and forward. A similar approach has also been used to attack SHA-0 [8] and SHA-1 [21,39] . For each of these 216 differential characteristics, we estimate the cost of finding a mali- cious single-block collision. These costs are roughly determined by the number of differences (disturbances) in Ai in each round. For details on the cost computations, we refer to [30]. The estimated costs for the best differential characteristics suited for our attack are given in Table 2, and the correspond message differences are given in Table 3. Table 2: Probabilities for rounds 2–4 of the differential characteristics suitable for our attack. candidate r = 2 r = 3 r = 4 total MD1 2−40 2−40 2−15 2−95 MD2 2−39 2−42 2−13 2−94 MD3 2−39 2−42 2−11 2−92 Table 3: List of message differences suitable for our attack MD1 00000003 20000074 88000000 e8000062 c8000043 28000004 40000042 48000046 88000002 00000014 08000002 a0000054 88000002 80000000 a8000003 a8000060 MD2 20000074 88000000 e8000062 c8000043 28000004 40000042 48000046 88000002 00000014 08000002 a0000054 88000002 80000000 a8000003 a8000060 00000003 MD3 88000000 e8000062 c8000043 28000004 40000042 48000046 88000002 00000014 08000002 a0000054 88000002 80000000 a8000003 a8000060 00000003 c0000002 The high-probability differential characteristic with message difference MD1 is best suit- able for our intended file formats (see §4) and used as the starting point to search for a low-probability differential characteristic for the first round of SHA-1. We use an automatic search tool [22,23] to find the low-probability part of the characteristic. The result is shown in Table 5 in the appendix. Overall, the complexity of finding a colliding message pair using malicious constants for this differential characteristic in our attack is approximately 248 , which is feasible in practice as demonstrated below and in §4. After the differential characteristic is fixed, we probabilistically search for a confirming message pair. We start with only the first constant K1 fixed (e.g., to the standard value) and search for a message pair that confirms at least the first round (20 steps) of the char- acteristic, and is also suitable for our file format. This is easier than finding a message pair that works for all four rounds (with fixed constants), since fewer constraints need to be satisfied. The complexity of this step is negligible. Now, we can exhaustively search through all 232 options for K2 until we find one that confirms round 2. Only if no such constant is found, we backtrack and modify the message words. Since the differential characteristic for message difference MD1 holds with probability 2−40 in round 2 and we can test 232 options for K2, this step of the attack will only succeed with a probability of 2−8 . Hence, completing this step alone has a complexity of approximately 240 . Once we have found a candidate for K2 such that the differential characteristic holds in round 2, we proceed in the same way with K3. Again, the differential characteristic will hold with only a probability of 2−40 in round 3 and we can test only 232 options for K3. 8
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  Therefore, we needto repeat the previous steps of the attack 28 times to find a solution. Including the expected 28 tries for the previous step to reach the current one, completing this step has an expected complexity of roughly 248 . Finally, we need to find K4. Since the last round of the characteristic has a high proba- bility, such a constant is very likely to exist and this step of the attack only adds negligible cost to the final attack complexity of about 248 . Normally, with fixed constants, an attacker would have to backtrack in the case of a contradiction in the later steps. Eve as the designer, on the other hand, has a chance that choosing a different constant might repair the contradictions for another round. This significantly improves the complexity of the differential attack. For predefined constants, the complexity of the attack for this particular disturbance vector would be roughly 295 . Note that we do not need the whole freedom of all 4 constants. The first constant in round 1 can be chosen arbitrarily (e.g., we keep it as in the original SHA-1 specification). For the last constant in round 4, we can fix approximately 16 bits of the constant. That is, 80 bits of the constants need to be changed compared to the original values. More freedom in choosing the constants is possible if we increase the attack complexity. An example of a colliding message pair for our malicious SHA-1 variant with modified constants is given in Table 4. The constants differ from the original values by 45 (of 128) bits. Another example is given in Table 6 in the appendix. In the following section, we will show how this pair can be used to construct meaningful collisions. Table 4: Example of a collision for malicious SHA-1 with modified constants K1...4. K1...4 5a827999 4eb9d7f7 bad18e2f d79e5877 IV 67452301 efcdab89 98badcfe 10325476 c3d2e1f0 m ffd8ffe1 e2001250 b6cef608 34f4fe83 ffae884f afe56e6f fc50fae6 28c40f81 1b1d3283 b48c11bc b1d4b511 a976cb20 a7a929f0 2327f9bb ecde01c0 7dc00852 m∗ ffd8ffe2 c2001224 3ecef608 dcf4fee1 37ae880c 87e56e6b bc50faa4 60c40fc7 931d3281 b48c11a8 b9d4b513 0976cb74 2fa929f2 a327f9bb 44de01c3 d5c00832 ∆m 00000003 20000074 88000000 e8000062 c8000043 28000004 40000042 48000046 88000002 00000014 08000002 a0000054 88000002 80000000 a8000003 a8000060 h(m) 1896b202 394b0aae 54526cfa e72ec5f2 42b1837e 4 Building Meaningful Collisions To exploit the malicious SHA-1 described in §3, we propose several types of executable, archive and image file formats for which two colliding files can be created, and such that the behavior of the two files can be fully controlled by the attacker. Below, we first discuss the constraints that the files have to satisfy, in order to collide with our malicious SHA-1. We then investigate common binary file formats to determine whether they could allow us to construct a malicious SHA-1 for which two valid files collide. Finally, we present actual collisions, and characterize the associated instances of malicious SHA-1. 4.1 Constraints The attack strategy and the available message differences impose several constraints for possible applications. Most importantly, the exact hash function definition with the final constants is only fixed during the attack. This implies that the differences between the two 9
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  final files willbe limited to a single block. In addition, this block must correspond to the first 512 bits of the final files. After this initial collision block, the file pair can be extended arbitrarily with a common suffix. Due to this limitation, for example, the method that was used to find colliding PostScript files for MD5 [12] cannot be applied here. For the exact values of the first block, the attack allows a certain freedom. The attacker can fix the values of a few bits in advance. However, fixing too many bits will increase the attack complexity. Additionally, choosing the bits is constrained by the fixed message difference. In all our example files, we use message difference MD1 from Table 3, which offers a slightly better expected attack complexity than MD2 and MD3. All of the available message differences have a difference in the first word, as well as the last byte. 4.2 Binary File Format Overview Binary file formats typically have a predefined structure and in particular a “magic signa- ture” in their first bytes, which is used to identify the type of binary and to define diverse metadata. As a preliminary to the construction of colliding binaries, we provide basic in- formation on binary files so as to understand the obstacles posed for the construction of colliding files. We also discuss both our failed and successful attempts to build colliding binary ex- ecutables. Note that once a collision can be created—that is, if the block difference can be introduced without fatally altering the file structure—the programs executed in each of the two colliding files can be fully controlled. In practice, both programs may execute a legitimate application, but one of the two colliding files prepends the execution of a trojan that will persistenly compromise the machine. Magic Signatures. Most binary file formats enforce a “magic signature” at offset 0, to enable programs to recognize the type of file, its version, etc., in order to process it according to its specific format. For example, the utilities file and binwalk rely mostly on magic signatures to identify files and their type. Some formats, notably most archive formats, also allow the signature to start later in the file, at a higher offset. Signatures are typically 4 bytes long. Some are longer, such as that of the PNG format (89504e470d0a1a0a), or the RAR archive format (526172211a0700), and some are smaller (PE’s 2-byte “MZ”, TIFF’s 2-byte “MM” and “II”). Note that none of our colliding blocks offer four unmodified consecutive bytes. This implies that collisions for our malicious SHA-1 cannot be files with a fixed 4-byte signature at offset 0. Executables: PE. The PE (Portable Executable) format is the standard format for Win- dows executables (.exe files). The PE format, as defined in 1993, is based on the older DOS EXE format (from 1981). PE thus retains the MZ signature (4d5a) from DOS, however in PE it is mostly useless: the only components of the header used are the MZ signature and the last component, which is a pointer to the more modern PE header. This leaves an entirely controllable buffer of 58 bytes near the top of the file, which is tempting to use to build colliding PEs. PE thus seems an interesting candidate for malicious collisions: it is very commonly used, and its header provides freedom degrees to introduce differences. The only restrictions in the header are in the first two bytes (which must be set to the MZ string) and in the four bytes at offset 60, where the 4-byte pointer to the PE header is encoded. Unfortunately, the structure of the differential attack forces the most significant byte of the PE header to be (at least) 40. This gives a minimal pointer of 40000000, that is, 1 GiB. Such a file, even if syntaxically correct, is not supported by Windows: it is correctly parsed, but then the OS fails to load it (In practice, the biggest accepted value for this pointer in a working PE is around 9000000) 10
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  Due to thislimitation, we could not construct valid compact PE executables that collide for a malicious SHA-1. Note that the Unix and OS X counterpart of PEs (ELF and Mach-O files, respectively) fix at least the first 4 bytes, and thus cannot be exploited for malicious collisions either. Headerless Executables: MBR and COM. Some older formats like master boot records (MBR) and DOS executables (COM) do not include a magic signature or any header. Instead, code execution starts directly at offset 0. By introducing a jump instruc- tion to the subsequent block, we can have total control of the first block and thus create collisions (as long as the difference allows for the jump instruction with distinct reasonable addresses). Running in 16-bit x86 code, the block can start with a jump, encoded as eb XX, where XX is a signed char that should be positive. Both blocks will immediately jump to different pieces of code of colliding MBR or colliding COM. To demonstrate the feasibility of this approach, example files are given in the appendix. Compressed Archives: RAR and 7z. Like any other archive file format, the RAR archive allows to start at any offset. However, unlike the ZIP, it is parsed top-down. So if a block creates a valid Rar signature that is broken by its twin, then both files can be valid Rars yet different. We could thus create two colliding archives, which can each contain arbitrary content. A very similar method can be used to build colliding 7z archives (and probably other types of compressed archives). Images: JPEG. The JPEG file format is organized in a chunk-based manner: chunks are called segments, and each segment starts with a 2 bytes marker. The first byte of the marker is always ff, the second is anything but 00 or ff. A JPEG file must start with a “Start Of Image” (SOI) marker, ffd8. Segments have a variable size, encoded directly after the marker, on 2 bytes, in little endian format. Typically, right after the SOI segment starts the APP0 segment, with marker ffe0. This segment contains the familiar ”JFIF” string. However, most JPEG viewers do not require the second segment to start right after SOI. Adding megabytes of garbage data between the SOI marker and the APP0 segment of a JPEG file will still make it valid for most tools – as long as this data does not contain any valid marker, ff(01-fe). Not only we can insert almost-random data before the first segment, but we can insert any dummy segment that has an encoded length – this will enable us to control the parser, to give it the data we want. If each of our colliding files contains a valid segment marker with a different size and offset, each of them can have a valid APP0 segment at a different offset (provided that the sum of segment size and segment offset differs). To make the bruteforcing phase easier, we can use any of the following segments: 1. the APPx segments, with marker ffe(0-f) 2. the COM segment, with marker fffe So, after getting 2 colliding blocks, creating 2 JPEG headers with a suitable dummy segment, we can start the actual data of file J1 after the second dummy segment (with larger sum of segment size and offset). Right after the first dummy segment, we start another dummy segment to cover the actual data of file J1. After this second dummy segment, the data of the file J2 can start. If the length of any of the JPEG file cannot fit on 2 bytes, then several dummy segments need to be written consecutively. Thus, we are able to get a colliding pair of valid files, on a modern format, still used daily by most computers. Combining Formats: Polyglots. Since the formats discussed above require their magical signatures at different positions in the file, it is possible to construct a first block (of 64 bytes) that suits multiple file formats. For instance, JPEG requires fixed values in the first 11
• 12.
  few words, whilearchives like RAR can start their signature at a higher offset. Thus, we can construct colliding block pairs for a fixed selection of constants that can later be used to construct colliding files of multiple types with almost arbitrary content. Examples are given in §4.3 (JPEG-RAR) and in the appendix (MBR-RAR-Script). 4.3 Example Files We use the attack strategy from §3 to build a colliding pair of JPEG images and one of RAR archives, both for the same set of malicious SHA-1 constants. Since JPEG requires the file to start with ffd8, MD1 is the only one of the message differences given in Table 3 that is suitable for a collision between two JPEG files. The following bytes are set to ffe?, where ? differs between the two files and can be any value. Additionally, the last byte of this first 64-byte-block is fixed to also allow the block to be used for RAR collisions: It is set to the first byte of the RAR signature, 52, in one of the blocks, and to a different value as determined by MD1 in the other block. Using these constraints as a starting point, we search for a differential characteristic. The result is given in Table 5 in the appendix. Note that at this point, the first round constant K1 is fixed to an arbitrary value (we use the original constant), while K2, K3, K4 are still free. They are determined together with the full first 64-byte block in the next phase. The result is the message pair already given in Table 4. The malicious SHA-1 variant differs from the standardized SHA-1 in the values of 45 bits of the round constants. Now, we can append suitable followup blocks to create valid JPEG or RAR file pairs, both with arbitrary content. As an example, both images in Figure 2 hash to the colliding digest h(m) = 1896b202 394b0aae 54526cfa e72ec5f2 42b1837e using the malicious round constants K1 = 5a827999, K2 = 4eb9d7f7, K3 = bad18e2f, K4 = d79e5877. jpgrar0.jpg jpgrar1.jpg good.txt “good” jpgrar0.rar evil.txt “evil” jpgrar1.rar collision collision identicalidentical Fig. 2: A colliding JPEG/RAR polyglot file pair for malicious SHA-1 (see Table 4). 12
• 13.
  The appendices containanother example block for a different set of SHA-1 constants that is suitable for MBRs, shell scripts and RAR archives, including a complete hexdump of an example file pair. 13
• 14.
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• 16.
  Table 5: Characteristiccorresponding to message difference MD1. i Ai Wi -5 00001111010010111000011111000011 -4 01000000110010010101000111011000 -3 01100010111010110111001111111010 -2 11101111110011011010101110001001 -1 01100111010001010010001100000001 0 1001111110001101100110001001010n 111111111101100011111111111000nu 1 00u1101001100--------001111u0n00 11u0001000000000-----0100unu0n00 2 n111n00---1------------uu000un00 u011n1-----------------000001000 3 0uuuu111--0---0---uu---0-0un11nn nnu-n--------------------nn000u1 4 1n01u1110---u-n-------u0011001n0 uu1-u------------------00u0011uu 5 0011011n1n00--0-un0101-10n1u0n00 10u0u--------------------1101u11 6 n1n1n1n01000--1-100101-00n000011 1u111--------------------u-001u0 7 nu1nnnnnnnnnnnnnnnnnnnnnnnn000n1 0n10u00------------------n000nn1 8 101111-10011000000010000111nu0u1 n001u----------------------000u1 9 0-1010101000000000000000001un001 10110100-----------------01u1u00 10 u1n00------------------------01u 1011n---------------------0-00n1 11 -00-0--------------------1100001 u0u----------------------n1n0n00 12 -0010-----------------------00-1 u01-n----------------------100n0 13 ----1-----------------------1100 n0100----------------------11011 14 -------------------------------n u1u-u---------------------0000nn 15 -1--------------------------0--- n1u-u-------------------0un10010 16 -0--------------------------1--- 1110----------------------1000un 17 ------------------------------u- nn01---------------------10111u1 18 -------------------------------n nn001--------------------n-010nu 19 -------------------------------- un1----------------------un111n1 20 -------------------------------- n11-----------------------0011nu 21 ------------------------------n- 0u01---------------------01110n1 22 -------------------------------n 0u0----------------------u1100nu 23 -------------------------------- nn1----------------------un001n1 24 -------------------------------- n010-----------------------100un 25 ------------------------------n- 1n1---------------------011001n0 26 -n-----------------------------u 011----------------------u-110un 27 -------------------------------- nn00---------------------nu0u0n0 28 ------------------------------u- nn101---------------------11001u 29 n-----------------------------n- uu1n---------------------n0101n0 30 -------------------------------- 1n1u---------------------u1u01u0 31 -------------------------------- uu0u---------------------11101u0 32 -------------------------------u 01n----------------------10110un 33 -------------------------------- 01n--------------------10nu00001 34 u------------------------------- 10u----------------------10100nu 35 ------------------------------n- nu0--------------------1001n11n1 36 ------------------------------n- 1n0----------------------u0111n0 37 -------------------------------- nun--------------------00u1100u0 38 ------------------------------u- n0u-----------------------110001 39 -------------------------------- 10n0-------------------10n010111 40 n------------------------------- u01----------------------10000n0 41 -------------------------------- 101----------------------01u1001 42 u------------------------------u u10-0---------------------0111un 43 -------------------------------- n0u--------------------01nun0010 44 -------------------------------- n1u----------------------10000nu 45 ------------------------------n- nu0----------------------00111n1 46 ------------------------------u- uuu----------------------u1111u1 47 -------------------------------- uun--------------------10n0000u0 48 ------------------------------u- n00---------------------10101100 49 -------------------------------- 100--------------------11n010100 50 ------------------------------u- 010--------------------1-0100010 51 -------------------------------- 010--------------------01n100010 52 -------------------------------- u01----------------------11100n0 53 -------------------------------- 101--------------------010110101 54 -------------------------------n n11----------------------101100n 55 -------------------------------- u00--------------------110u11000 56 ------------------------------u- 100----------------------00011uu 57 ------------------------------u- 1u1--------------------11n1011u0 58 -------------------------------u 1u1----------------------n00101n 59 -------------------------------- nu0--------------------10nu001n1 60 -------------------------------- 101---------------------110000nu 61 ------------------------------n- un1--------------------0010000n1 62 ------------------------------n- 1n--1------------------10u0111n1 63 -------------------------------- uu1--------------------11u0111u0 64 -------------------------------- u10-1------------------0100000n0 65 -------------------------------- 0--------------------11110001011 66 ------------------------------n- 111-------------------0-000011n1 67 -------------------------------- u1-------------------0110u100100 68 ------------------------------u- --0--------------------011000100 69 -------------------------------- u1---------------------01n000010 70 ------------------------------u- n-1--------------------111011101 71 -------------------------------- -0-------------------1100n01100- 72 ------------------------------u- u----------------------000001110 73 -------------------------------- --1------------------0011n111011 74 ------------------------------u- n-----------------------101111-- 75 -------------------------------- ---------------------0000n011101 76 -------------------------------- u----------------------1110001u- 77 -------------------------------- -----------------------101000--1 78 -------------------------------- n---------------------000011101- 79 -------------------------------- n--------------------111000101-- 16
• 17.
  B More MeaningfulCollision Examples: MBR, RAR, Shell Script Similar to §4.3, we give another polyglot block and file pair. The block pair allows to create MBR (master boot record), RAR, and shell script collisions, all with virtually arbitrary semantic content. Table 6 specifies the malicious constants and the basic colliding block which can serve as a first block (header) for the mentioned file formats. The constants differ from the original values by 41 (of 128) bits. Fig. 4 shows the hexdump of a com- plete example file pair, while Fig. 3 illustrates the behaviour of the executed files. Both files can be booted as a master boot record (or emulated using qemu-system-i386 -fda shmbrarX.mbr), extracted as a RAR archive, and executed as a shell script. . Table 6: Example of a collision for malicious SHA-1 with modified constants K1...4. K1...4 5a827999 82b1c71a 5141963a a664c1d6 m 23deeb7c 8a8a34ee 8d603f3e 389c313c 12697fa2 e9adb8d4 dc8f9fb2 b79d1dc8 d0015f70 87cfe8fa 99683eca 46a1ccd8 1fa8f02b f87e061a b5820745 c5bc7d52 m∗ 23deeb7f aa8a349a 05603f3e d09c315e da697fe1 c1adb8d0 9c8f9ff0 ff9d1d8e 58015f72 87cfe8ee 91683ec8 e6a1cc8c 97a8f029 787e061a 1d820746 6dbc7d32 ∆m 00000003 20000074 88000000 e8000062 c8000043 28000004 40000042 48000046 88000002 00000014 08000002 a0000054 88000002 80000000 a8000003 a8000060 h(m) 0c426a87 6b6e8e42 f0558066 2ddd5a85 939db8b9 good! shmbrar0.mbr evil! shmbrar1.mbr good. shmbrar0.sh evil. shmbrar1.sh good 0090 90. . . shmbrar0.rar evil “evil .txt” shmbrar1.rar collision collision collision identicalidentical Fig. 3: A colliding MBR/RAR/script polyglot file pair for malicious SHA-1. 17
• 18.
  00000000 23 deeb 7c 8a 8a 34 ee 8d 60 3f 3e 38 9c 31 3c |#..|..4..‘?>8.1<| 00000010 12 69 7f a2 e9 ad b8 d4 dc 8f 9f b2 b7 9d 1d c8 |.i..............| 00000020 d0 01 5f 70 87 cf e8 fa 99 68 3e ca 46 a1 cc d8 |.._p.....h>.F...| 00000030 1f a8 f0 2b f8 7e 06 1a b5 82 07 45 c5 bc 7d 52 |...+.~.....E..}R| 00000040 61 72 21 1a 07 00 cf 90 73 00 00 0d 00 00 00 00 |ar!.....s.......| 00000050 00 00 00 74 40 74 20 80 24 00 89 01 00 00 89 01 |...t@t .$.......| 00000060 00 00 02 2c 36 db 31 da 4b 2a 44 14 30 04 00 20 |...,6.1.K*D.0.. | 00000070 00 00 00 67 6f 6f 64 00 90 90 90 90 90 90 90 90 |...good.........| 00000080 90 eb 03 90 eb 08 be a3 7c e8 0b 00 eb fe be a9 |........|.......| 00000090 7c e8 03 00 eb fe 90 ac 3c 00 74 06 b4 0e cd 10 ||.......<.t.....| 000000a0 eb f5 c3 67 6f 6f 64 21 00 65 76 69 6c 21 00 cc |...good!.evil!..| 000000b0 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc |................| * 000001f0 cc cc cc cc cc cc cc cc cc cc cc cc cc cc 55 aa |..............U.| 00000200 c4 3d 7b 00 40 07 00 52 61 72 21 1a 07 00 cf 90 |.={[email protected]!.....| 00000210 73 00 00 0d 00 00 00 00 00 00 00 cf 40 74 20 80 |s...........@t .| 00000220 24 00 08 00 00 00 08 00 00 00 02 23 0e 33 a2 dc |$..........#.3..| 00000230 45 2a 44 1d 30 04 00 20 00 00 00 65 76 69 6c 65 |E*D.0.. ...evile| 00000240 76 69 6c 2e 74 78 74 c4 3d 7b 00 40 07 00 0a 0a |vil.txt.={.@....| 00000250 69 66 20 5b 20 60 6f 64 20 2d 74 20 64 49 20 2d |if [ ‘od -t dI -| 00000260 6a 33 20 2d 4e 31 20 2d 41 6e 20 22 24 7b 30 7d |j3 -N1 -An "${0}| 00000270 22 60 20 2d 65 71 20 22 31 32 34 22 20 5d 3b 20 |"‘ -eq "124" ]; | 00000280 74 68 65 6e 20 0a 20 20 65 63 68 6f 20 22 67 6f |then . echo "go| 00000290 6f 64 2e 22 3b 0a 65 6c 73 65 0a 20 20 65 63 68 |od.";.else. ech| 000002a0 6f 20 22 65 76 69 6c 2e 22 3b 0a 66 69 0a |o "evil.";.fi.| 000002ae (a) First file shmbrar0 00000000 23 de eb 7f aa 8a 34 9a 05 60 3f 3e d0 9c 31 5e |#.....4..‘?>..1^| 00000010 da 69 7f e1 c1 ad b8 d0 9c 8f 9f f0 ff 9d 1d 8e |.i..............| 00000020 58 01 5f 72 87 cf e8 ee 91 68 3e c8 e6 a1 cc 8c |X._r.....h>.....| 00000030 97 a8 f0 29 78 7e 06 1a 1d 82 07 46 6d bc 7d 32 |...)x~.....Fm.}2| 00000040 61 72 21 1a 07 00 cf 90 73 00 00 0d 00 00 00 00 |ar!.....s.......| 00000050 00 00 00 74 40 74 20 80 24 00 89 01 00 00 89 01 |...t@t .$.......| 00000060 00 00 02 2c 36 db 31 da 4b 2a 44 14 30 04 00 20 |...,6.1.K*D.0.. | 00000070 00 00 00 67 6f 6f 64 00 90 90 90 90 90 90 90 90 |...good.........| 00000080 90 eb 03 90 eb 08 be a3 7c e8 0b 00 eb fe be a9 |........|.......| 00000090 7c e8 03 00 eb fe 90 ac 3c 00 74 06 b4 0e cd 10 ||.......<.t.....| 000000a0 eb f5 c3 67 6f 6f 64 21 00 65 76 69 6c 21 00 cc |...good!.evil!..| 000000b0 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc |................| * 000001f0 cc cc cc cc cc cc cc cc cc cc cc cc cc cc 55 aa |..............U.| 00000200 c4 3d 7b 00 40 07 00 52 61 72 21 1a 07 00 cf 90 |.={[email protected]!.....| 00000210 73 00 00 0d 00 00 00 00 00 00 00 cf 40 74 20 80 |s...........@t .| 00000220 24 00 08 00 00 00 08 00 00 00 02 23 0e 33 a2 dc |$..........#.3..| 00000230 45 2a 44 1d 30 04 00 20 00 00 00 65 76 69 6c 65 |E*D.0.. ...evile| 00000240 76 69 6c 2e 74 78 74 c4 3d 7b 00 40 07 00 0a 0a |vil.txt.={.@....| 00000250 69 66 20 5b 20 60 6f 64 20 2d 74 20 64 49 20 2d |if [ ‘od -t dI -| 00000260 6a 33 20 2d 4e 31 20 2d 41 6e 20 22 24 7b 30 7d |j3 -N1 -An "${0}| 00000270 22 60 20 2d 65 71 20 22 31 32 34 22 20 5d 3b 20 |"‘ -eq "124" ]; | 00000280 74 68 65 6e 20 0a 20 20 65 63 68 6f 20 22 67 6f |then . echo "go| 00000290 6f 64 2e 22 3b 0a 65 6c 73 65 0a 20 20 65 63 68 |od.";.else. ech| 000002a0 6f 20 22 65 76 69 6c 2e 22 3b 0a 66 69 0a |o "evil.";.fi.| 000002ae (b) Second file shmbrar1 Fig. 4: Hexdumps of a colliding MBR/RAR/script polyglot file pair for malicious SHA-1. 18