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26-404: The vulnerability exploited is a combination of chosen plaintext attack and inadvertent information leakage through data compression, similar to that described in 2002 by the cryptographer John Kelsey . It relies on the attacker being able to observe the size of the ciphertext sent by the browser while at the same time inducing the browser to make multiple carefully crafted web connections to

52-444: A chosen-plaintext attack the adversary can (possibly adaptively ) ask for the ciphertexts of arbitrary plaintext messages. This is formalized by allowing the adversary to interact with an encryption oracle , viewed as a black box . The attacker’s goal is to reveal all or a part of the secret encryption key. It may seem infeasible in practice that an attacker could obtain ciphertexts for given plaintexts. However, modern cryptography

78-486: A chosen-plaintext attack. The following attack on the Caesar cipher allows full recovery of the secret key: With more intricate or complex encryption methodologies the decryption method becomes more resource-intensive, however, the core concept is still relatively the same. The following attack on a one-time pad allows full recovery of the secret key. Suppose the message length and key length are equal to n . While

104-443: A small part of the plaintext may need to be chosen by the attacker; such attacks are known as plaintext injection attacks. A chosen-plaintext attack is more powerful than known-plaintext attack , because the attacker can directly target specific terms or patterns without having to wait for these to appear naturally, allowing faster gathering of data relevant to cryptanalysis. Therefore, any cipher that prevents chosen-plaintext attacks

130-419: Is adaptive sort , whose behavior changes upon the presortedness of its input. An example of an adaptive algorithm in radar systems is the constant false alarm rate (CFAR) detector. In machine learning and optimization , many algorithms are adaptive or have adaptive variants, which usually means that the algorithm parameters such as learning rate are automatically adjusted according to statistics about

156-432: Is unconditionally malleable . Adaptive algorithm An adaptive algorithm is an algorithm that changes its behavior at the time it is run, based on information available and on a priori defined reward mechanism (or criterion). Such information could be the story of recently received data, information on the available computational resources, or other run-time acquired (or a priori known) information related to

182-477: Is also secure against known-plaintext and ciphertext-only attacks. However, a chosen-plaintext attack is less powerful than a chosen-ciphertext attack , where the attacker can obtain the plaintexts of arbitrary ciphertexts. A CCA-attacker can sometimes break a CPA-secure system. For example, the El Gamal cipher is secure against chosen plaintext attacks, but vulnerable to chosen ciphertext attacks because it

208-473: Is an attack model for cryptanalysis which presumes that the attacker can obtain the ciphertexts for arbitrary plaintexts . The goal of the attack is to gain information that reduces the security of the encryption scheme. Modern ciphers aim to provide semantic security, also known as ciphertext indistinguishability under chosen-plaintext attack , and they are therefore, by design, generally immune to chosen-plaintext attacks if correctly implemented. In

234-439: Is carried out as follows : Consider the following extension of the above situation. After the last step, A cipher has indistinguishable encryptions under a chosen-plaintext attack if after running the above experiment the adversary can't guess correctly ( b = b' ) with probability non- negligibly better than 1/2. The following examples demonstrate how some ciphers that meet other security definitions may be broken with

260-458: Is implemented in software or hardware and is used for a diverse range of applications; for many cases, a chosen-plaintext attack is often very feasible (see also In practice ). Chosen-plaintext attacks become extremely important in the context of public key cryptography where the encryption key is public and so attackers can encrypt any plaintext they choose. There are two forms of chosen-plaintext attacks: A general batch chosen-plaintext attack

286-459: The BEAST exploit. The exploit was due to be revealed in full at the 2012 ekoparty security conference. Rizzo and Duong presented CRIME as a general attack that works effectively against a large number of protocols, including but not limited to SPDY (which always compresses request headers), TLS (which may compress records) and HTTP (which may compress responses). CRIME can be defeated by preventing

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312-549: The August 2013 Black Hat conference, researchers Gluck, Harris and Prado announced a variant of the CRIME exploit against HTTP compression called BREACH (short for Browser Reconnaissance and Exfiltration via Adaptive Compression of Hypertext). It uncovers HTTPS secrets by attacking the inbuilt HTTP data compression used by webservers to reduce network traffic. Chosen plaintext attack A chosen-plaintext attack ( CPA )

338-487: The codebreakers decrypt the code used on the second leg, having supplied the original text . In modern day, chosen-plaintext attacks (CPAs) are often used to break symmetric ciphers . To be considered CPA-secure, the symmetric cipher must not be vulnerable to chosen-plaintext attacks. Thus, it is important for symmetric cipher implementors to understand how an attacker would attempt to break their cipher and make relevant improvements. For some chosen-plaintext attacks, only

364-478: The desired and the actual signal). For example, stable partition , using no additional memory is O ( n lg n ) but given O ( n ) memory, it can be O ( n ) in time. As implemented by the C++ Standard Library , stable_partition is adaptive and so it acquires as much memory as it can get (up to what it would need at most) and applies the algorithm using that available memory. Another example

390-535: The environment in which it operates. Among the most used adaptive algorithms is the Widrow-Hoff’s least mean squares (LMS) , which represents a class of stochastic gradient-descent algorithms used in adaptive filtering and machine learning. In adaptive filtering the LMS is used to mimic a desired filter by finding the filter coefficients that relate to producing the least mean square of the error signal (difference between

416-593: The message and immediately reported to their superiors that "AF" was low on water, confirming the Navy's hypothesis and allowing them to position their force to win the battle . Also during World War II , Allied codebreakers at Bletchley Park would sometimes ask the Royal Air Force to lay mines at a position that didn't have any abbreviations or alternatives in the German naval system's grid reference. The hope

442-569: The name "one-time" pad). In World War II US Navy cryptanalysts discovered that Japan was planning to attack a location referred to as "AF". They believed that "AF" might be Midway Island , because other locations in the Hawaiian Islands had codewords that began with "A". To prove their hypothesis that "AF" corresponded to "Midway Island" they asked the US forces at Midway to send a plaintext message about low supplies. The Japanese intercepted

468-416: The one-time pad is used as an example of an information-theoretically secure cryptosystem, this security only holds under security definitions weaker than CPA security. This is because under the formal definition of CPA security the encryption oracle has no state. This vulnerability may not be applicable to all practical implementations – the one-time pad can still be made secure if key reuse is avoided (hence

494-427: The optimisation thus far (e.g. the rate of convergence ). Examples include adaptive simulated annealing , adaptive coordinate descent , adaptive quadrature , AdaBoost , Adagrad , Adadelta, RMSprop , and Adam . In data compression , adaptive coding algorithms such as Adaptive Huffman coding or Prediction by partial matching can take a stream of data as input, and adapt their compression technique based on

520-437: The server picks one of them and sends it back in its ServerHello message. The server can only choose a compression method the client has offered, so if the client only offers 'none' (no compression), the data will not be compressed. Similarly, since 'no compression' must be allowed by all TLS clients, a server can always refuse to use compression. As of September 2012, the CRIME exploit against SPDY and TLS-level compression

546-442: The source, which includes the secret content that the attacker desires to discover. Divide and conquer techniques can then be used to home in on the true secret content in a relatively small number of probe attempts that is a small multiple of the number of secret bytes to be recovered. The CRIME exploit was hypothesized by Adam Langley, and first demonstrated by the security researchers Juliano Rizzo and Thai Duong, who also created

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572-558: The symbols that they have already encountered. In signal processing , the Adaptive Transform Acoustic Coding (ATRAC) codec used in MiniDisc recorders is called "adaptive" because the window length (the size of an audio "chunk") can change according to the nature of the sound being compressed, to try to achieve the best-sounding compression strategy. This software-engineering -related article

598-421: The target site. The attacker then observes the change in size of the compressed request payload, which contains both the secret cookie that is sent by the browser only to the target site, and variable content created by the attacker, as the variable content is altered. When the size of the compressed content is reduced, it can be inferred that it is probable that some part of the injected content matches some part of

624-534: The use of compression, either at the client end, by the browser disabling the compression of SPDY requests, or by the website preventing the use of data compression on such transactions using the protocol negotiation features of the TLS protocol. As detailed in The Transport Layer Security (TLS) Protocol Version 1.2 , the client sends a list of compression algorithms in its ClientHello message, and

650-629: Was described as mitigated in the then-latest versions of the Chrome and Firefox web browsers. Some websites have applied countermeasures at their end. The nginx web-server was not vulnerable to CRIME since 1.0.9/1.1.6 (October/November 2011) using OpenSSL 1.0.0+, and since 1.2.2/1.3.2 (June / July 2012) using all versions of OpenSSL. Note that as of December 2013 the CRIME exploit against HTTP compression has not been mitigated at all. Rizzo and Duong have warned that this vulnerability might be even more widespread than SPDY and TLS compression combined. At

676-688: Was that the Germans, seeing the mines, would use an Enigma machine to encrypt a warning message about the mines and an "all clear" message after they were removed, giving the allies enough information about the message to break the German naval Enigma. This process of planting a known-plaintext was called gardening . Allied codebreakers also helped craft messages sent by double agent Juan Pujol García , whose encrypted radio reports were received in Madrid, manually decrypted, and then re-encrypted with an Enigma machine for transmission to Berlin. This helped

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