رمزنگاری جابجایی

*In cryptography, a transposition cipher is a method of encryption by which the positions held by units of plaintext (which are

*commonly characters or groups of characters) are shifted according to a regular system, so that the ciphertext constitutes a

*permutation of the plaintext. That is, the order of the units is changed (the plaintext is reordered). Mathematically a bijective function

*is used on the characters' positions to encrypt and ani nverse function to decrypt.

*Following are some implementations.

*Contents

*1 Rail Fence cipher

*2 Route cipher

*3 Columnar transposition

*4 Myszkowski transposition

*5 Detection and cryptanalysis

*6 References

*

*Rail Fence cipher

*The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it is encoded. In the rail fence cipher,

*the plaintext is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom. The

*message is then read off in rows. For example, using three "rails" and a message of 'WE ARE DISCOVERED. FLEE AT ONCE', the

*cipherer writes out:

*W . . . E . . . C . . . R . . . L . . . T . . . E

*. E . R . D . S . O . E . E . F . E . A . O . C .

*. . A . . . I . . . V . . . D . . . E . . . N . .

*Then reads off:

*WECRL TEERD SOEEF EAOCA IVDEN

*(The cipherer has broken this ciphertext up into blocks of five to help avoid errors. This is a common technique used to make the

*cipher more easily readable. The spacing is not related to spaces in the plaintext and so does not carry any information about the

*plaintext.)

*Route cipher

*In a route cipher, the plaintext is first written out in a grid of given dimensions, then read off in a pattern given in the key. For

*example, using the same plaintext that we used forr ail fence:

*W R I O R F E O E

*E E S V E L A N J

*A D C E D E T C X

*The key might specify "spiral inwards, clockwise, starting from the top right". That would give a cipher text of:

*EJXCTEDECDAEWRIORFEONALEVSE

*Route ciphers have many more keys than a rail fence. In fact, for messages of reasonable length, the number of possible keys is

*potentially too great to be enumerated even by modern machinery. However, not all keys are equally good. Badly chosen routes will

*leave excessive chunks of plaintext, or text simply reversed, and this will give cryptanalysts a clue as to the routes.

*Columnar transposition

*In a columnar transposition, the message is written out in rows of a fixed length, and then read out again column by column, and the

*columns are chosen in some scrambled order. Both the width of the rows and the permutation of the columns are usually defined by a

*keyword. For example, the keyword ZEBRAS is of length 6 (so the rows are of length 6), and the permutation is defined by the

*alphabetical order of the letters in the keyword. In this case, the order would be "6 3 2 4 1 5".

*6 3 2 4 1 5

*W E A R E D

*I S C O V E

*R E D F L E

*E A T O N C

*E Q K J E U

*providing five nulls (QKJEU), these letters can be randomly selected as they just fill out the incomplete columns and are not part of

*the message. The ciphertext is then read off as:

*EVLNE ACDTK ESEAQ ROFOJ DEECU WIREE

*In the irregular case, the columns are not completed by nulls:

*6 3 2 4 1 5

*W E A R E D

*I S C O V E

*R E D F L E

*E A T O N C

*E

*This results in the following ciphertext:

*EVLNA CDTES EAROF ODEEC WIREE

*To decipher it, the recipient has to work out the column lengths by dividing the message length by the key length. Then he can write

*the message out in columns again, then re-order the columns by reforming the key word.

*Myszkowski transposition

*A variant form of columnar transposition, proposed by Émile Victor Théodore Myszkowski in 1902, requires a keyword with

*recurrent letters. In usual practice, subsequent occurrences of a keyword letter are treated as if the next letter in alphabetical order,

*e.g., the keyword TOMATO yields a numeric keystring of "532164."

*In Myszkowski transposition, recurrent keyword letters are numbered identical,l yTOMATO yielding a keystring of "432143."

*4 3 2 1 4 3

*W E A R E D

*I S C O V E

*R E D F L E

*E A T O N C

*E

*Plaintext columns with unique numbers are transcribed downward; those with recurring numbers are transcribed left to right:

*ROFOA CDTED SEEEA CWEIV RLENE

*Detection and cryptanalysis

*Since transposition does not affect the frequency of individual

* symbols, simple transposition can be easily detected by the

*cryptanalyst by doing a frequency count. If the ciphertext exhibits a frequency distribution very similar to plaintext, it is most likely a

*transposition. This can then often be attacked by anagramming

رمزنگاری کوانتومی

*Quantum cryptography is the science of exploiting quantum mechanical properties to perform cryptographic tasks. The best known

*example of quantum cryptography is quantum key distribution which offers an information-theoretically secure solution to the key

*exchange problem. Currently used popular public-key encryption and signature schemes (e.g., RSA and ElGamal) can be broken by

*quantum adversaries. The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic

*Quantum key distribution

*The most well known and developed application of quantum cryptography is quantum key distribution (QKD), which is the process

*of using quantum communication to establish a shared key between two parties (Alice and Bob, for example) without a third party

*symmetric cryptography.

*(Eve) learning anything about that key, even if Eve can eavesdrop on all communication between Alice and Bob. If Eve tries to learn

*information about the key being established, key establishment will fail causing Alice and Bob to notice. Once the key is established,

*it is then typically used for encrypted communication using classical techniques. For instance, the exchanged key could be used as for

*.

*tasks that are proven or conjectured to be impossible using only classical (i.e. non-quantum) communication (see below for

*examples). For example, it is impossible to copy data encoded in a quantum state and the very act of reading data encoded in a

*quantum state changes the state. This is used to detect eavesdropping in quantum key distribution.

*Quantum commitment

*Following the discovery of quantum key distribution and its unconditional security, researchers tried to achieve other cryptographic

*tasks with unconditional security. One such task was commitment. A commitment scheme allows a party Alice to fix a certain value

*(to "commit") in such a way that Alice cannot change that value while at the same time ensuring that the recipient Bob cannot learn

*anything about that value until Alice reveals it. Such commitment schemes are commonly used in cryptographic protocols. In the

*quantum setting, they would be particularly useful: Crépeau and Kilian showed that from a commitment and a quantum channel, one

*can construct an unconditionally secure protocol for performing so-calledo blivious transfer.[13] Oblivious transfer, on the other hand,

*had been shown by Kilian to allow implementation of almost any distributed computation in a secure way (so-called secure multiparty

*computation).[14]

Bounded- and noisy-quantum-storage model

 

*One possibility to construct unconditionally secure quantum commitment and quantum oblivious transfer (OT) protocols is to use the

*bounded quantum storage model (BQSM). In this model, we assume that the amount of quantum data that an adversary can store is

*limited by some known constant Q. We do not, however, impose any limit on the amount of classical (i.e., non-quantum) data the

*adversary may store.

*In the BQSM, one can construct commitment and oblivious transfer protocols.[18] The underlying idea is the following: The protocol

*parties exchange more than Q quantum bits (qubits). Since even a dishonest party cannot store all that information (the quantum

*memory of the adversary is limited to Q qubits), a large part of the data will have to be either measured or discarded. Forcing

*dishonest parties to measure a large part of the data allows to circumvent the impossibility result by Mayers;[16] commitment and

*oblivious transfer protocols can now be implemented.

*The protocols in the BQSM presented by Damgård, Fehr, Salvail, and Schaffner[18] do not assume that honest protocol participants

*store any quantum information; the technical requirements are similar to those in QKD protocols. These protocols can thus, at least in

*principle, be realized with today's technology. The communication complexity is onlyaconstant factor larger than the bound Q the adversary's quantum memory.

*Post-quantum cryptography

*Quantum computers may become a technological reality; it is therefore important to study cryptographic schemes used against

*adversaries with access to a quantum computer. The study of such schemes is often referred to as post-quantum cryptography. The

*need for post-quantum cryptography arises from the fact that many popular encryption and signature schemes (such as RSA and its

*variants, and schemes based on elliptic curves) can be broken using Shor's algorithm for factoring and computing discrete logarithms

*on a quantum computer. Examples for schemes that are, as of today's knowledge, secure against quantum adversaries are McEliece

*and lattice-based schemes. Surveys of post-quantum cryptography are available[3.6][37]

*There is also research into how existing cryptographic techniques have to be modified to be able to cope with quantum adversaries.

*For example, when trying to develop zero-knowledge proof systems that are secure against quantum adversaries, new techniques

*need to be used: In a classical setting, the analysis of a zero-knowledge proof system usually involves "rewinding", a technique that

*makes it necessary to copy the internal state of the adversary. In a quantum setting, copying a state is not always possible (no-cloning

*theorem); a variant of the rewinding technique has to be used[3. 8]

*Post quantum algorithms are also called "quantum resistant", because – unlike QKD – it is not known or provable that there will not

*be potential future quantum attacks against them. Even though they are not vulnerable to Shor's algorithm, the NSA is announcing

*plans to transition to quantum resistant algorithms.[39] The National Institute of Security and Technology (NIST) believes that it is

*time to think of quantum-safe primitives[.40]