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83-708: For technical reasons, an encryption scheme usually uses a pseudo-random encryption key generated by an algorithm . It is possible to decrypt the message without possessing the key but, for a well-designed encryption scheme, considerable computational resources and skills are required. An authorized recipient can easily decrypt the message with the key provided by the originator to recipients but not to unauthorized users. Historically, various forms of encryption have been used to aid in cryptography. Early encryption techniques were often used in military messaging. Since then, new techniques have emerged and become commonplace in all areas of modern computing. Modern encryption schemes use

166-660: A RAND Corporation task force report published under DARPA sponsorship by J.P. Anderson and D.J. Edwards in 1970. While initially targeting the computer vision domain, backdoor attacks have expanded to encompass various other domains, including text, audio, ML-based computer-aided design, and ML-based wireless signal classification. Additionally, vulnerabilities in backdoors have been demonstrated in deep generative models , reinforcement learning (e.g., AI GO), and deep graph models. These broad-ranging potential risks have prompted concerns from national security agencies regarding their potentially disastrous consequences. A backdoor in

249-769: A cloud service for example. Homomorphic encryption and secure multi-party computation are emerging techniques to compute encrypted data; these techniques are general and Turing complete but incur high computational and/or communication costs. In response to encryption of data at rest, cyber-adversaries have developed new types of attacks. These more recent threats to encryption of data at rest include cryptographic attacks, stolen ciphertext attacks , attacks on encryption keys, insider attacks , data corruption or integrity attacks, data destruction attacks, and ransomware attacks. Data fragmentation and active defense data protection technologies attempt to counter some of these attacks, by distributing, moving, or mutating ciphertext so it

332-450: A 128-bit or higher key, like AES, will not be able to be brute-forced because the total amount of keys is 3.4028237e+38 possibilities. The most likely option for cracking ciphers with high key size is to find vulnerabilities in the cipher itself, like inherent biases and backdoors or by exploiting physical side effects through Side-channel attacks . For example, RC4 , a stream cipher, was cracked due to inherent biases and vulnerabilities in

415-667: A backdoor was discovered in certain Samsung Android products, like the Galaxy devices. The Samsung proprietary Android versions are fitted with a backdoor that provides remote access to the data stored on the device. In particular, the Samsung Android software that is in charge of handling the communications with the modem, using the Samsung IPC protocol, implements a class of requests known as remote file server (RFS) commands, that allows

498-423: A backdoor. Although some are secretly installed, other backdoors are deliberate and widely known. These kinds of backdoors have "legitimate" uses such as providing the manufacturer with a way to restore user passwords. Many systems that store information within the cloud fail to create accurate security measures. If many systems are connected within the cloud , hackers can gain access to all other platforms through

581-503: A challenge to today's encryption technology. For example, RSA encryption uses the multiplication of very large prime numbers to create a semiprime number for its public key. Decoding this key without its private key requires this semiprime number to be factored, which can take a very long time to do with modern computers. It would take a supercomputer anywhere between weeks to months to factor in this key. However, quantum computing can use quantum algorithms to factor this semiprime number in

664-409: A challenging problem. A single error in system design or execution can allow successful attacks. Sometimes an adversary can obtain unencrypted information without directly undoing the encryption. See for example traffic analysis , TEMPEST , or Trojan horse . Integrity protection mechanisms such as MACs and digital signatures must be applied to the ciphertext when it is first created, typically on

747-554: A combination of the two. The theory of asymmetric backdoors is part of a larger field now called cryptovirology . Notably, NSA inserted a kleptographic backdoor into the Dual EC DRBG standard. There exists an experimental asymmetric backdoor in RSA key generation. This OpenSSL RSA backdoor, designed by Young and Yung, utilizes a twisted pair of elliptic curves, and has been made available. A sophisticated form of black box backdoor

830-632: A completely deterministic and repeatable process. Pseudorandom number generators are often used in computer programming, as traditional sources of randomness available to humans (such as rolling dice) rely on physical processes not readily available to computer programs, although developments in hardware random number generator technology have challenged this. The generation of random numbers has many uses, such as for random sampling , Monte Carlo methods , board games , or gambling . In physics , however, most processes, such as gravitational acceleration, are deterministic, meaning that they always produce

913-608: A computer, product, embedded device (e.g. a home router ), or its embodiment (e.g. part of a cryptosystem , algorithm , chipset , or even a "homunculus computer"—a tiny computer-within-a-computer such as that found in Intel's AMT technology ). Backdoors are most often used for securing remote access to a computer, or obtaining access to plaintext in cryptosystems. From there it may be used to gain access to privileged information like passwords, corrupt or delete data on hard drives, or transfer information within autoschediastic networks. In

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996-444: A covert backdoor becomes unveiled. Even direct admissions of responsibility must be scrutinized carefully if the confessing party is beholden to other powerful interests. Many computer worms , such as Sobig and Mydoom , install a backdoor on the affected computer (generally a PC on broadband running Microsoft Windows and Microsoft Outlook ). Such backdoors appear to be installed so that spammers can send junk e-mail from

1079-435: A covert rootkit running in the photomask etching equipment could enact this discrepancy unbeknown to the photomask manufacturer, either, and by such means, one backdoor potentially leads to another. In general terms, the long dependency-chains in the modern, highly specialized technological economy and innumerable human-elements process control-points make it difficult to conclusively pinpoint responsibility at such time as

1162-409: A large number of messages. Padding a message's payload before encrypting it can help obscure the cleartext's true length, at the cost of increasing the ciphertext's size and introducing or increasing bandwidth overhead . Messages may be padded randomly or deterministically , with each approach having different tradeoffs. Encrypting and padding messages to form padded uniform random blobs or PURBs

1245-428: A level of security that will be able to counter the threat of quantum computing. Encryption is an important tool but is not sufficient alone to ensure the security or privacy of sensitive information throughout its lifetime. Most applications of encryption protect information only at rest or in transit, leaving sensitive data in clear text and potentially vulnerable to improper disclosure during processing, such as by

1328-411: A login system might take the form of a hard coded user and password combination which gives access to the system. An example of this sort of backdoor was used as a plot device in the 1983 film WarGames , in which the architect of the " WOPR " computer system had inserted a hardcoded password-less account which gave the user access to the system, and to undocumented parts of the system (in particular,

1411-524: A message's content and it cannot be tampered with at rest or in transit, a message's length is a form of metadata that can still leak sensitive information about the message. For example, the well-known CRIME and BREACH attacks against HTTPS were side-channel attacks that relied on information leakage via the length of encrypted content. Traffic analysis is a broad class of techniques that often employs message lengths to infer sensitive implementation about traffic flows by aggregating information about

1494-528: A modified version of the Unix C compiler that would put an invisible backdoor in the Unix login command when it noticed that the login program was being compiled, and would also add this feature undetectably to future compiler versions upon their compilation as well. As the compiler itself was a compiled program, users would be extremely unlikely to notice the machine code instructions that performed these tasks. (Because of

1577-468: A number of cloak and dagger considerations that come into play when apportioning responsibility. Covert backdoors sometimes masquerade as inadvertent defects (bugs) for reasons of plausible deniability . In some cases, these might begin life as an actual bug (inadvertent error), which, once discovered are then deliberately left unfixed and undisclosed, whether by a rogue employee for personal advantage, or with C-level executive awareness and oversight. It

1660-406: A part of the standard library and compiles it. After that, every program compiled by that Delphi installation will contain the virus. An attack that propagates by building its own Trojan horse can be especially hard to discover. It resulted in many software vendors releasing infected executables without realizing it, sometimes claiming false positives. After all, the executable was not tampered with,

1743-654: A potential limitation of today's encryption methods. The length of the encryption key is an indicator of the strength of the encryption method. For example, the original encryption key, DES (Data Encryption Standard), was 56 bits, meaning it had 2^56 combination possibilities. With today's computing power, a 56-bit key is no longer secure, being vulnerable to brute force attacks . Quantum computing uses properties of quantum mechanics in order to process large amounts of data simultaneously. Quantum computing has been found to achieve computing speeds thousands of times faster than today's supercomputers. This computing power presents

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1826-534: A ready supply of random digits was in 1927, when the Cambridge University Press published a table of 41,600 digits developed by L.H.C. Tippett . In 1947, the RAND Corporation generated numbers by the electronic simulation of a roulette wheel; the results were eventually published in 1955 as A Million Random Digits with 100,000 Normal Deviates . In theoretical computer science , a distribution

1909-404: A storage device involve overwriting the device's whole content with zeros, ones, or other patterns – a process which can take a significant amount of time, depending on the capacity and the type of storage medium. Cryptography offers a way of making the erasure almost instantaneous. This method is called crypto-shredding . An example implementation of this method can be found on iOS devices, where

1992-482: A system has been compromised with a backdoor or Trojan horse, such as the Trusting Trust compiler, it is very hard for the "rightful" user to regain control of the system – typically one should rebuild a clean system and transfer data (but not executables) over. However, several practical weaknesses in the Trusting Trust scheme have been suggested. For example, a sufficiently motivated user could painstakingly review

2075-416: A variant where the system initialization code is modified to insert a backdoor during booting , as this is complex and poorly understood, and call it an "initialization trapdoor"; this is now known as a boot sector virus . This attack was then actually implemented by Ken Thompson , and popularized in his Turing Award acceptance speech in 1983, "Reflections on Trusting Trust", which points out that trust

2158-497: A video game-like simulation mode and direct interaction with the artificial intelligence ). Although the number of backdoors in systems using proprietary software (software whose source code is not publicly available) is not widely credited, they are nevertheless frequently exposed. Programmers have even succeeded in secretly installing large amounts of benign code as Easter eggs in programs, although such cases may involve official forbearance, if not actual permission. There are

2241-406: Is pseudorandom against a class of adversaries if no adversary from the class can distinguish it from the uniform distribution with significant advantage. This notion of pseudorandomness is studied in computational complexity theory and has applications to cryptography . Formally, let S and T be finite sets and let F = { f : S → T } be a class of functions. A distribution D over S

2324-415: Is a compiler backdoor , where not only is a compiler subverted—to insert a backdoor in some other program, such as a login program—but it is further modified to detect when it is compiling itself and then inserts both the backdoor insertion code (targeting the other program) and the code-modifying self-compilation, like the mechanism through which retroviruses infect their host. This can be done by modifying

2407-423: Is a practice guaranteeing that the cipher text leaks no metadata about its cleartext's content, and leaks asymptotically minimal O ( log ⁡ log ⁡ M ) {\displaystyle O(\log \log M)} information via its length. Pseudo-random A pseudorandom sequence of numbers is one that appears to be statistically random , despite having been produced by

2490-456: Is also possible for an entirely above-board corporation's technology base to be covertly and untraceably tainted by external agents (hackers), though this level of sophistication is thought to exist mainly at the level of nation state actors. For example, if a photomask obtained from a photomask supplier differs in a few gates from its photomask specification, a chip manufacturer would be hard-pressed to detect this if otherwise functionally silent;

2573-690: Is another somewhat different example of using encryption on data at rest. Encryption is also used to protect data in transit, for example data being transferred via networks (e.g. the Internet, e-commerce ), mobile telephones , wireless microphones , wireless intercom systems, Bluetooth devices and bank automatic teller machines . There have been numerous reports of data in transit being intercepted in recent years. Data should also be encrypted when transmitted across networks in order to protect against eavesdropping of network traffic by unauthorized users. Conventional methods for permanently deleting data from

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2656-437: Is at most ε. In typical applications, the class F describes a model of computation with bounded resources and one is interested in designing distributions D with certain properties that are pseudorandom against F . The distribution D is often specified as the output of a pseudorandom generator . Backdoor (computing) A backdoor is a typically covert method of bypassing normal authentication or encryption in

2739-460: Is constantly evolving to prevent eavesdropping attacks. One of the first "modern" cipher suites, DES , used a 56-bit key with 72,057,594,037,927,936 possibilities; it was cracked in 1999 by EFF's brute-force DES cracker , which required 22 hours and 15 minutes to do so. Modern encryption standards often use stronger key sizes, such as AES (256-bit mode), TwoFish , ChaCha20-Poly1305 , Serpent (configurable up to 512-bit). Cipher suites that use

2822-664: Is important for the sequence to be demonstrably unpredictable, physical sources of random numbers have been used, such as radioactive decay, atmospheric electromagnetic noise harvested from a radio tuned between stations, or intermixed timings of keystrokes . The time investment needed to obtain these numbers leads to a compromise: using some of these physics readings as a seed for a pseudorandom number generator. Before modern computing, researchers requiring random numbers would either generate them through various means ( dice , cards , roulette wheels , etc.) or use existing random number tables. The first attempt to provide researchers with

2905-410: Is more difficult to identify, steal, corrupt, or destroy. The question of balancing the need for national security with the right to privacy has been debated for years, since encryption has become critical in today's digital society. The modern encryption debate started around the '90s when US government tried to ban cryptography because, according to them, it would threaten national security. The debate

2988-557: Is much harder to inspect, as it is designed to be machine-readable, not human-readable. These backdoors can be inserted either directly in the on-disk object code, or inserted at some point during compilation, assembly linking, or loading—in the latter case the backdoor never appears on disk, only in memory. Object code backdoors are difficult to detect by inspection of the object code, but are easily detected by simply checking for changes (differences), notably in length or in checksum, and in some cases can be detected or analyzed by disassembling

3071-477: Is polarized around two opposing views. Those who see strong encryption as a problem making it easier for criminals to hide their illegal acts online and others who argue that encryption keep digital communications safe. The debate heated up in 2014, when Big Tech like Apple and Google set encryption by default in their devices. This was the start of a series of controversies that puts governments, companies and internet users at stake. Encryption, by itself, can protect

3154-431: Is relative, and the only software one can truly trust is code where every step of the bootstrapping has been inspected. This backdoor mechanism is based on the fact that people only review source (human-written) code, and not compiled machine code ( object code ). A program called a compiler is used to create the second from the first, and the compiler is usually trusted to do an honest job. Thompson's paper describes

3237-413: Is still very limited. Quantum computing currently is not commercially available, cannot handle large amounts of code, and only exists as computational devices, not computers. Furthermore, quantum computing advancements will be able to be used in favor of encryption as well. The National Security Agency (NSA) is currently preparing post-quantum encryption standards for the future. Quantum encryption promises

3320-415: Is ε- pseudorandom against F if for every f in F , the statistical distance between the distributions f ( X ) {\displaystyle f(X)} and f ( Y ) {\displaystyle f(Y)} , where X {\displaystyle X} is sampled from D and Y {\displaystyle Y} is sampled from the uniform distribution on S ,

3403-846: The Computer Security Institute reported that in 2007, 71% of companies surveyed used encryption for some of their data in transit, and 53% used encryption for some of their data in storage. Encryption can be used to protect data "at rest", such as information stored on computers and storage devices (e.g. USB flash drives ). In recent years, there have been numerous reports of confidential data, such as customers' personal records, being exposed through loss or theft of laptops or backup drives; encrypting such files at rest helps protect them if physical security measures fail. Digital rights management systems, which prevent unauthorized use or reproduction of copyrighted material and protect software against reverse engineering (see also copy protection ),

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3486-437: The revision control system . In this case, a two-line change appeared to check root access permissions of a caller to the sys_wait4 function, but because it used assignment = instead of equality checking == , it actually granted permissions to the system. This difference is easily overlooked, and could even be interpreted as an accidental typographical error, rather than an intentional attack. In January 2014,

3569-481: The United States, the 1994 Communications Assistance for Law Enforcement Act forces internet providers to provide backdoors for government authorities. In 2024, the U.S. government realized that China had been tapping communications in the U.S. using that infrastructure for months, or perhaps longer; China recorded presidential candidate campaign office phone calls —including employees of the then-vice president of

3652-613: The Wheel Cipher or the Jefferson Disk , although never actually built, was theorized as a spool that could jumble an English message up to 36 characters. The message could be decrypted by plugging in the jumbled message to a receiver with an identical cipher. A similar device to the Jefferson Disk, the M-94 , was developed in 1917 independently by US Army Major Joseph Mauborne. This device

3735-413: The attacker can both inspect and tamper with encrypted data by performing a man-in-the-middle attack anywhere along the message's path. The common practice of TLS interception by network operators represents a controlled and institutionally sanctioned form of such an attack, but countries have also attempted to employ such attacks as a form of control and censorship. Even when encryption correctly hides

3818-423: The attacker who plants it, even if the full implementation of the backdoor becomes public (e.g. via publishing, being discovered and disclosed by reverse engineering , etc.). Also, it is computationally intractable to detect the presence of an asymmetric backdoor under black-box queries. This class of attacks have been termed kleptography ; they can be carried out in software, hardware (for example, smartcards ), or

3901-469: The backdoor operator to perform via modem remote I/O operations on the device hard disk or other storage. As the modem is running Samsung proprietary Android software, it is likely that it offers over-the-air remote control that could then be used to issue the RFS commands and thus to access the file system on the device. Harder to detect backdoors involve modifying object code , rather than source code—object code

3984-400: The backdoor, for example detecting that the subverted binary is being checksummed and returning the expected value, not the actual value. To conceal these further subversions, the tools must also conceal the changes in themselves—for example, a subverted checksummer must also detect if it is checksumming itself (or other subverted tools) and return false values. This leads to extensive changes in

4067-581: The cipher. In the context of cryptography, encryption serves as a mechanism to ensure confidentiality . Since data may be visible on the Internet, sensitive information such as passwords and personal communication may be exposed to potential interceptors . The process of encrypting and decrypting messages involves keys . The two main types of keys in cryptographic systems are symmetric-key and public-key (also known as asymmetric-key). Many complex cryptographic algorithms often use simple modular arithmetic in their implementations. In symmetric-key schemes,

4150-510: The code would be to try over 17,000 combinations within 24 hours. The Allies used computing power to severely limit the number of reasonable combinations they needed to check every day, leading to the breaking of the Enigma Machine. Today, encryption is used in the transfer of communication over the Internet for security and commerce. As computing power continues to increase, computer encryption

4233-475: The compiler recompiled from original source with the compromised compiler executable: the backdoor has been bootstrapped. This attack dates to a 1974 paper by Karger and Schell, and was popularized in Thompson's 1984 article, entitled "Reflections on Trusting Trust"; it is hence colloquially known as the "Trusting Trust" attack. See compiler backdoors , below, for details. Analogous attacks can target lower levels of

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4316-588: The compiler was. It is believed that the Induc-A virus had been propagating for at least a year before it was discovered. In 2015, a malicious copy of Xcode, XcodeGhost , also performed a similar attack and infected iOS apps from a dozen of software companies in China. Globally, 4,000 apps were found to be affected. It was not a true Thompson Trojan, as it does not infect development tools themselves, but it did prove that toolchain poisoning can cause substantial damages. Once

4399-443: The compiler, this in turn can be fixed by recompiling the compiler, removing the backdoor insertion code. This defense can in turn be subverted by putting a source meta-backdoor in the compiler, so that when it detects that it is compiling itself it then inserts this meta-backdoor generator, together with the original backdoor generator for the original program under attack. After this is done, the source meta-backdoor can be removed, and

4482-450: The compiler-under-test. That source, compiled with both compilers, results in two different stage-1 compilers, which however should have the same behavior. Thus the same source compiled with both stage-1 compilers must then result in two identical stage-2 compilers. A formal proof is given that the latter comparison guarantees that the purported source code and executable of the compiler-under-test correspond, under some assumptions. This method

4565-457: The concepts of public-key and symmetric-key . Modern encryption techniques ensure security because modern computers are inefficient at cracking the encryption. One of the earliest forms of encryption is symbol replacement, which was first found in the tomb of Khnumhotep II , who lived in 1900 BC Egypt. Symbol replacement encryption is “non-standard,” which means that the symbols require a cipher or key to understand. This type of early encryption

4648-549: The confidentiality of messages, but other techniques are still needed to protect the integrity and authenticity of a message; for example, verification of a message authentication code (MAC) or a digital signature usually done by a hashing algorithm or a PGP signature . Authenticated encryption algorithms are designed to provide both encryption and integrity protection together. Standards for cryptographic software and hardware to perform encryption are widely available, but successfully using encryption to ensure security may be

4731-506: The cryptographic key is kept in a dedicated ' effaceable storage'. Because the key is stored on the same device, this setup on its own does not offer full privacy or security protection if an unauthorized person gains physical access to the device. Encryption is used in the 21st century to protect digital data and information systems. As computing power increased over the years, encryption technology has only become more advanced and secure. However, this advancement in technology has also exposed

4814-409: The deterministic process is a computer algorithm called a pseudorandom number generator , which must first be provided with a number called a random seed . Since the same seed will yield the same sequence every time, it is important that the seed be well chosen and kept hidden, especially in security applications, where the pattern's unpredictability is a critical feature. In some cases where it

4897-451: The encryption and decryption keys are the same. Communicating parties must have the same key in order to achieve secure communication. The German Enigma Machine used a new symmetric-key each day for encoding and decoding messages. In addition to traditional encryption types, individuals can enhance their security by using VPNs or specific browser settings to encrypt their internet connection, providing additional privacy protection while browsing

4980-488: The encryption and decryption keys. A publicly available public-key encryption application called Pretty Good Privacy (PGP) was written in 1991 by Phil Zimmermann , and distributed free of charge with source code. PGP was purchased by Symantec in 2010 and is regularly updated. Encryption has long been used by militaries and governments to facilitate secret communication. It is now commonly used in protecting information within many kinds of civilian systems. For example,

5063-580: The infected machines. Others, such as the Sony/BMG rootkit , placed secretly on millions of music CDs through late 2005, are intended as DRM measures—and, in that case, as data-gathering agents , since both surreptitious programs they installed routinely contacted central servers. A sophisticated attempt to plant a backdoor in the Linux kernel , exposed in November 2003, added a small and subtle code change by subverting

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5146-456: The machine code of the untrusted compiler before using it. As mentioned above, there are ways to hide the Trojan horse, such as subverting the disassembler; but there are ways to counter that defense, too, such as writing a disassembler from scratch. A generic method to counter trusting trust attacks is called diverse double-compiling . The method requires a different compiler and the source code of

5229-479: The most vulnerable system. Default passwords (or other default credentials) can function as backdoors if they are not changed by the user. Some debugging features can also act as backdoors if they are not removed in the release version. In 1993, the United States government attempted to deploy an encryption system, the Clipper chip , with an explicit backdoor for law enforcement and national security access. The chip

5312-482: The nation– and of the candidates themselves. A backdoor may take the form of a hidden part of a program, a separate program (e.g. Back Orifice may subvert the system through a rootkit ), code in the firmware of the hardware, or parts of an operating system such as Windows . Trojan horses can be used to create vulnerabilities in a device. A Trojan horse may appear to be an entirely legitimate program, but when executed, it triggers an activity that may install

5395-486: The object code. Further, object code backdoors can be removed (assuming source code is available) by simply recompiling from source on a trusted system. Thus for such backdoors to avoid detection, all extant copies of a binary must be subverted, and any validation checksums must also be compromised, and source must be unavailable, to prevent recompilation. Alternatively, these other tools (length checks, diff, checksumming, disassemblers) can themselves be compromised to conceal

5478-493: The original exploit in 2002, and, in 2009, Wheeler wrote a historical overview and survey of the literature. In 2023, Cox published an annotated version of Thompson's backdoor source code. Thompson's version was, officially, never released into the wild. However, it is believed that a version was distributed to BBN and at least one use of the backdoor was recorded. There are scattered anecdotal reports of such backdoors in subsequent years. In August 2009, an attack of this kind

5561-435: The same amount of time it takes for normal computers to generate it. This would make all data protected by current public-key encryption vulnerable to quantum computing attacks. Other encryption techniques like elliptic curve cryptography and symmetric key encryption are also vulnerable to quantum computing. While quantum computing could be a threat to encryption security in the future, quantum computing as it currently stands

5644-460: The same device used to compose the message, to protect a message end-to-end along its full transmission path; otherwise, any node between the sender and the encryption agent could potentially tamper with it. Encrypting at the time of creation is only secure if the encryption device itself has correct keys and has not been tampered with. If an endpoint device has been configured to trust a root certificate that an attacker controls, for example, then

5727-448: The same outcome from the same starting point. Some notable exceptions are radioactive decay and quantum measurement , which are both modeled as being truly random processes in the underlying physics. Since these processes are not practical sources of random numbers, pseudorandom numbers are used, which ideally have the unpredictability of a truly random sequence, despite being generated by a deterministic process. In many applications,

5810-399: The second task, the compiler's source code would appear "clean".) What's worse, in Thompson's proof of concept implementation, the subverted compiler also subverted the analysis program (the disassembler ), so that anyone who examined the binaries in the usual way would not actually see the real code that was running, but something else instead. Karger and Schell gave an updated analysis of

5893-476: The source code, and the resulting compromised compiler (object code) can compile the original (unmodified) source code and insert itself: the exploit has been boot-strapped. This attack was originally presented in Karger & Schell (1974), which was a United States Air Force security analysis of Multics , where they described such an attack on a PL/I compiler, and call it a "compiler trap door". They also mention

5976-452: The system and tools being needed to conceal a single change. As object code can be regenerated by recompiling (reassembling, relinking) the original source code, making a persistent object code backdoor (without modifying source code) requires subverting the compiler itself—so that when it detects that it is compiling the program under attack it inserts the backdoor—or alternatively the assembler, linker, or loader. As this requires subverting

6059-467: The system to bypass security facilities and permit direct access to data. The use of the word trapdoor here clearly coincides with more recent definitions of a backdoor. However, since the advent of public key cryptography the term trapdoor has acquired a different meaning (see trapdoor function ), and thus the term "backdoor" is now preferred, only after the term trapdoor went out of use. More generally, such security breaches were discussed at length in

6142-600: The system, such as the operating system, and can be inserted during the system booting process; these are also mentioned by Karger and Schell in 1974, and now exist in the form of boot sector viruses . A traditional backdoor is a symmetric backdoor: anyone that finds the backdoor can in turn use it. The notion of an asymmetric backdoor was introduced by Adam Young and Moti Yung in the Proceedings of Advances in Cryptology – Crypto '96 . An asymmetric backdoor can only be used by

6225-506: The technique of frequency analysis – which was an attempt to crack ciphers systematically, including the Caesar cipher. This technique looked at the frequency of letters in the encrypted message to determine the appropriate shift: for example, the most common letter in English text is E and is therefore likely to be represented by the letter that appears most commonly in the ciphertext. This technique

6308-405: The web. In public-key encryption schemes, the encryption key is published for anyone to use and encrypt messages. However, only the receiving party has access to the decryption key that enables messages to be read. Public-key encryption was first described in a secret document in 1973; beforehand, all encryption schemes were symmetric-key (also called private-key). Although published subsequently,

6391-567: The work of Diffie and Hellman was published in a journal with a large readership, and the value of the methodology was explicitly described. The method became known as the Diffie-Hellman key exchange . RSA (Rivest–Shamir–Adleman) is another notable public-key cryptosystem . Created in 1978, it is still used today for applications involving digital signatures . Using number theory , the RSA algorithm selects two prime numbers , which help generate both

6474-492: Was applied by its author to verify that the C compiler of the GCC suite (v. 3.0.4) contained no trojan, using icc (v. 11.0) as the different compiler. In practice such verifications are not done by end users, except in extreme circumstances of intrusion detection and analysis, due to the rarity of such sophisticated attacks, and because programs are typically distributed in binary form. Removing backdoors (including compiler backdoors)

6557-462: Was discovered by Sophos labs. The W32/Induc-A virus infected the program compiler for Delphi , a Windows programming language. The virus introduced its own code to the compilation of new Delphi programs, allowing it to infect and propagate to many systems, without the knowledge of the software programmer. The virus looks for a Delphi installation, modifies the SysConst.pas file, which is the source code of

6640-403: Was rendered ineffective by the polyalphabetic cipher , described by Al-Qalqashandi (1355–1418) and Leon Battista Alberti (in 1465), which varied the substitution alphabet as encryption proceeded in order to confound such analysis. Around 1790, Thomas Jefferson theorized a cipher to encode and decode messages to provide a more secure way of military correspondence. The cipher, known today as

6723-471: Was unsuccessful. Recent proposals to counter backdoors include creating a database of backdoors' triggers and then using neural networks to detect them. The threat of backdoors surfaced when multiuser and networked operating systems became widely adopted. Petersen and Turn discussed computer subversion in a paper published in the proceedings of the 1967 AFIPS Conference. They noted a class of active infiltration attacks that use "trapdoor" entry points into

6806-612: Was used in U.S. military communications until 1942. In World War II, the Axis powers used a more advanced version of the M-94 called the Enigma Machine . The Enigma Machine was more complex because unlike the Jefferson Wheel and the M-94, each day the jumble of letters switched to a completely new combination. Each day's combination was only known by the Axis, so many thought the only way to break

6889-474: Was used throughout Ancient Greece and Rome for military purposes. One of the most famous military encryption developments was the Caesar cipher , in which a plaintext letter is shifted a fixed number of positions along the alphabet to get the encoded letter. A message encoded with this type of encryption could be decoded with a fixed number on the Caesar cipher. Around 800 AD, Arab mathematician Al-Kindi developed

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