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| In [[cryptography]], a [[cryptosystem]] is '''semantically secure''' if any [[PP (complexity)|probabilistic, polynomial-time algorithm]] (PPTA) that is given the ciphertext of a certain message <math>m</math> (taken from any distribution of messages), and the message's length, cannot determine any partial information on the message with probability [[negligible function|non-negligibly]] higher than all other PPTA's that only have access to the message length (and not the ciphertext).<ref name="goldwasser-micali 1982">[[Shafi Goldwasser|S. Goldwasser]] and [[Silvio Micali|S. Micali]], [http://portal.acm.org/citation.cfm?id=802212 Probabilistic encryption & how to play mental poker keeping secret all partial information], Annual ACM Symposium on Theory of Computing, 1982.</ref> In other words, knowledge of the ciphertext (and length) of some unknown message does not reveal any additional information on the message that can be feasibly extracted. This concept is the computational complexity analogue to [[Claude Shannon|Shannon's]] concept of [[perfect secrecy]]. Perfect secrecy means that the ciphertext reveals no information at all about the plaintext, whereas semantic security implies that any information revealed cannot be feasibly extracted<ref name="shannon">{{cite journal| last=Shannon| first=Claude| title=Communication Theory of Secrecy Systems| journal=Bell System Technical Journal| volume=28| issue=4| pages=656–715| year=1949}}</ref><ref name=Goldreich>[[Oded Goldreich|Goldreich, Oded.]] Foundations of Cryptography: Volume 2, Basic Applications. Vol. 2. Cambridge university press, 2004.</ref>{{rp|378-381}}.
| | Are you constantly having issues with your PC? Are you always searching for methods to strengthen PC performance? Then this is the post you have been searching for. Here we'll discuss some of the many asked issues whenever it comes to having you PC serve you well; how can I create my computer quicker for free? How to make my computer run faster?<br><br>We all know which the registry is the critical component of the Windows operating program as it stores all information about the Dll files, programs found on the computer plus program settings. However, because days by, it really is unavoidable that we could encounter registry matter due to a huge amount of invalid, useless plus unwelcome entries.<br><br>The Windows registry is a program database of information. Windows and alternative software shop a great deal of settings plus alternative information inside it, and retrieve such info within the registry all the time. The registry is equally a bottleneck inside that because it happens to be the heart of the operating program, any problems with it could result errors plus bring the running system down.<br><br>There is several reasons why a computer could lose speed. Normal computer employ, such as surfing the Internet could get your operating system inside a condition where it has no choice nevertheless to slow down. The constant entering and deleting of temporary files that occur whenever you surf the Web leave the registries with thousands of false indicators inside our running system's registry.<br><br>Use a [http://bestregistrycleanerfix.com/tune-up-utilities tuneup utilities]. This will look a Windows registry for 3 kinds of keys that will really hurt PC performance. These are: duplicate, missing, plus corrupted.<br><br>The leading reason why I can not make my PC run quicker was the program registry plus it being fragmented. So software to defragment or clean the registry are needed. Such software are called registry products. Like all alternative software, there are paid ones plus free ones with their advantages plus disadvantages. To choose between the two is the user's choice.<br><br>Your registry is the spot all a significant configurations for hardware, software plus consumer profile configurations and needs are stored. Every time one of these factors is changed, the database then begins to expand. Over time, the registry will become bloated with unnecessary files. This causes a general slow down nevertheless inside extreme instances may result significant jobs and programs to stop working all together.<br><br>By changing the technique you employ the web we can have access more of the valuable bandwidth. This might ultimately give we a quicker surfing experience. Here is a link to 3 methods to customize the PC speed online. |
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| ==History==
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| The notion of semantic security was first put forward by [[Shafi Goldwasser|Goldwasser]] and [[Silvio Micali|Micali]] in 1982.<ref name="goldwasser-micali 1982" /> However, the definition they initially proposed offered no straightforward means to prove the security of practical cryptosystems. Goldwasser/Micali subsequently demonstrated that semantic security is equivalent to another definition of security called [[ciphertext indistinguishability]].<ref name="goldwasser-micali 1984">[[Shafi Goldwasser|S. Goldwasser]] and [[Silvio Micali|S. Micali]], [http://dx.doi.org/10.1016/0022-0000(84)90070-9 Probabilistic encryption]. Journal of Computer and System Sciences, 28:270-299, 1984.</ref> This latter definition is more common than the original definition of semantic security because it better facilitates proving the security of practical cryptosystems. | |
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| ==Symmetric-key cryptography==
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| In the case of [[symmetric-key algorithm]] cryptosystems, an adversary must not be able to compute any information about a plaintext from its ciphertext. This may be posited as an adversary, given two plaintexts of equal length and their two respective ciphertexts, cannot determine which ciphertext belongs to which plaintext.
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| ==Public-key cryptography==
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| {{Refimprove section|date=September 2012}}
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| For an [[asymmetric key encryption algorithm]] cryptosystem to be semantically secure, it must be infeasible for a [[computational complexity|computationally bounded]] adversary to derive significant information about a message (plaintext) when given only its [[ciphertext]] and the corresponding public encryption key. Semantic security considers only the case of a "passive" attacker, i.e., one who generates and observes ciphertexts using the public key and plaintexts of her choice. Unlike other security definitions, semantic security does not consider the case of [[chosen ciphertext attack]] ([[CCA]]), where an attacker is able to request the decryption of chosen ciphertexts, and many semantically secure encryption schemes are demonstrably insecure against chosen ciphertext attack. Consequently, semantic security is now considered an insufficient condition for securing a general-purpose encryption scheme.
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| Indistinguishability under [[chosen plaintext attack|Chosen Plaintext Attack]] ([[IND-CPA]]) is commonly defined by the following [[game]]:
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| # A probabilistic polynomial time-bounded adversary is given a public key, which it may use to generate any number of ciphertexts (within polynomial bounds).
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| # The adversary generates two equal-length messages <math>m_0</math> and <math>m_1</math>, and transmits them to a challenge oracle along with the public key.
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| # The challenge oracle selects one of the messages by flipping a fair coin, encrypts the message under the public key, and returns the resulting ciphertext <math>c</math> to the adversary.
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| The underlying [[cryptosystem]] is IND-CPA (and thus semantically secure under chosen plaintext attack) if the adversary cannot determine which of the two messages was chosen by the oracle, with probability significantly greater than <math>1/2</math> (the success rate of random guessing). Variants of this definition define indistinguishability under [[chosen ciphertext attack]] and [[adaptive chosen ciphertext attack]] ([[IND-CCA]], [[IND-CCA2]]).
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| Because the adversary possesses the public encryption key in the above game, a semantically secure encryption scheme must by definition be [[probabilistic encryption|probabilistic]], possessing a component of [[randomness]]; if this were not the case, the adversary could simply compute the deterministic encryption of <math>m_0</math> and <math>m_1</math> and compare these encryptions with the returned ciphertext <math>c</math> to successfully guess the oracle's choice.
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| Semantically secure encryption algorithms include [[Goldwasser-Micali cryptosystem|Goldwasser-Micali]], [[El Gamal]] and [[Paillier]]. These schemes are considered [[provable security|provably secure]], as their semantic security can be reduced to solving some hard mathematical problem (e.g., [[Decisional Diffie-Hellman assumption|Decisional Diffie-Hellman]] or the [[Quadratic Residuosity Problem]]). Other, semantically insecure algorithms such as [[RSA (algorithm)|RSA]], can be made semantically secure (under stronger assumptions) through the use of random encryption padding schemes such as [[Optimal Asymmetric Encryption Padding]] (OAEP).
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| ==References==
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| <references />
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| [[Category:Theory of cryptography]]
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Are you constantly having issues with your PC? Are you always searching for methods to strengthen PC performance? Then this is the post you have been searching for. Here we'll discuss some of the many asked issues whenever it comes to having you PC serve you well; how can I create my computer quicker for free? How to make my computer run faster?
We all know which the registry is the critical component of the Windows operating program as it stores all information about the Dll files, programs found on the computer plus program settings. However, because days by, it really is unavoidable that we could encounter registry matter due to a huge amount of invalid, useless plus unwelcome entries.
The Windows registry is a program database of information. Windows and alternative software shop a great deal of settings plus alternative information inside it, and retrieve such info within the registry all the time. The registry is equally a bottleneck inside that because it happens to be the heart of the operating program, any problems with it could result errors plus bring the running system down.
There is several reasons why a computer could lose speed. Normal computer employ, such as surfing the Internet could get your operating system inside a condition where it has no choice nevertheless to slow down. The constant entering and deleting of temporary files that occur whenever you surf the Web leave the registries with thousands of false indicators inside our running system's registry.
Use a tuneup utilities. This will look a Windows registry for 3 kinds of keys that will really hurt PC performance. These are: duplicate, missing, plus corrupted.
The leading reason why I can not make my PC run quicker was the program registry plus it being fragmented. So software to defragment or clean the registry are needed. Such software are called registry products. Like all alternative software, there are paid ones plus free ones with their advantages plus disadvantages. To choose between the two is the user's choice.
Your registry is the spot all a significant configurations for hardware, software plus consumer profile configurations and needs are stored. Every time one of these factors is changed, the database then begins to expand. Over time, the registry will become bloated with unnecessary files. This causes a general slow down nevertheless inside extreme instances may result significant jobs and programs to stop working all together.
By changing the technique you employ the web we can have access more of the valuable bandwidth. This might ultimately give we a quicker surfing experience. Here is a link to 3 methods to customize the PC speed online.