In this chapter, we introduce basic coding terms and concepts. Our goal is to make our show known
In some simplicity, and to provide the largest possible general picture of the subject.
(2) (Basic terms
The idea of any encryption system is to hide confidential information in such a way that it becomes meaningless
Incomprehensible to any unauthorized person. The two uses are
her via the Gree channel
Encryption is most commonly used to securely store data in a computer file or file
Being encrypted does not prevent other people from
As safe as the Internet. Either way, really
It allows them to access it, but it ensures that they cannot understand what they are seeing.
The information to be hidden is often called the “original text”, while the
The process of hiding it is called "encryption". The original ciphertext is called the “ciphertext” or
The cipher statement, as it is called the set of rules used to encode text information
The original "Cryptographic Algorithm". Typically, this algorithm relies on a cipher key; and he
It is an introduction to it in addition to the message. In order for the recipient to retrieve the message from
During the ciphertext, there must be a “decryption algorithm” which, when used with
The appropriate “decryption key” retrieves the original text from the ciphertext.
In general, the set of rules that compose one of these "cryptographic algorithms" is considered
Very complex and needs to be designed carefully. However, within the framework of the objectives of this book, it is possible to
The reader may view these algorithms as “magic formulas” that transform data into an image
Unreadable with the help of encryption keys.
. The following figure provides a schematic description of the use of a “cryptosystem” to protect a message
movable:
Message Algorithm Message
encryption
Text A (see Algorithm
code extraction
Encryption key $ Decryption key $
Anyone who intercepts a message during its transmission is called an “interceptor”. This, is used
Other authors give other names, such as "eavesdropper", "adversary", "grim" and "bad person".
However, it should be noted that objectors can be “good people” in some cases
Sometimes, which we will talk about in more detail later. Even if the objectors knew
With the decryption algorithm, they generally do not know the decryption key. It is hoped
This lack of knowledge prevents objectors from knowing the original text. Cryptography is a science
The design of ciphering systems, while “ciphertext parsing” refers to the process by which
During which information about the original text is elicited without knowing the appropriate cipher key. As for
Cryptography is an umbrella term that includes both cryptography and ciphertext analysis.
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It is important to know that ciphertext analysis may not be the only method that
The objecting party can see the original text.
Say — for example — that someone is storing encrypted data on a device
This person needs some way to retrieve the unlock key
laptop computer; a priori
Encryption to read the stored information. If this includes writing this key on a piece of paper to stick it on
On the cover of the laptop, anyone who steals the computer will get
R. This example is a simple example
The key without the need to perform a ciphertext analysis
Indicates the need for more than just using a good cryptographic algorithm to protect
data. In fact, as we often emphasize, ensuring the security of cryptographic keys is important
It is very important to ensure that protection for encryption systems is achieved.
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Define a decoder key
In practice, most ciphertext parsing attacks involve an attempt to
cipher. If the objecting party succeeds, it will have the same knowledge that it possesses
It cannot decode all other correspondence until I change the code
intended recipient; and from then
cipher keys. There may be cases in which it is not the sole objective of the objecting party
Only read a specific message. Yet when the authors describe an algorithm as “broken,”
They usually mean by this that the objecting party has discovered a practical way to be able to
During which you select the decryption key.
Of course, the intercepting party can only decipher an encryption algorithm if it is available
He has enough information to enable him to know the correct key, or — more so
Common — select incorrect keys. It is important to know that this information is
The additional ones are likely to be very important to the objecting party. e b that party
The objector knows that the original text was in English, and that the decoding process for some
Using a guessed key that does not result in a meaningful original text
Transcoded parts
in english; In this case, the guessed key will be considered incorrect.
There is an important fact that should have become clear through this introduction; namely that
Knowing the encryption key is not necessary to obtain the message through the ciphertext.
This simple observation is the basis of Davy-Hellman's highly influential paper. Lost
It had a great impact on modern cryptography, as it resulted in a natural division between the two species
of coding systems; They are the symmetrical system and the asymmetric system.
The coding system is called a “traditional” or “symmetric” system if it is easy to elicit
Decryption key through the decoder key. In fact, these two keys are often
identical for symmetric coding systems. Therefore, these systems are usually called
“Secret Key” or “One Key” systems. On the other hand, if it is not possible in terms of
The process is to derive the decryption key through the decoder key, the cipher system is called
He is completely aware of the discrimination between me
"Asymmetrical" or "with a public key". There is a reason we must
These two types of systems are; To prevent an interceptor with knowledge of the algorithm from obtaining the original text via an intercepted ciphertext
It is necessary to keep a secret
Decoder key. Whereas in the case of a symmetric system, it is required to keep a secret
Also, in the case of an asymmetric system, the knowledge of this key is incorrect
of practical benefit to the objecting party. In fact, this key can be declared, and usually
that happens. It follows that the sender and receiver do not need the ciphertext to exchange any
secrets between them. In fact, there is probably no need for one to trust the other.
Although the statements in the previous paragraph may seem simple and intuitive, the results
The consequences are far-reaching. . The above figure assumes that the sender and receiver have
A matching pair of keys. In fact, it may be very difficult for them to achieve this
the situation. For example, if the coding system is symmetric, it may be necessary to
Distribute the value of the secret key before exchanging secret messages. A problem should not be underestimated
Provide adequate protection for these keys. In fact, the problem of key management is mainly
General—which includes creating, distributing, storing, altering, and destroying—the most difficult aspects
Get a secure system. Usually the problems associated with a key management problem are different
The coding system differs between symmetric and asymmetric. As we have seen, if the coding system
symmetrically, it may be necessary to have the ability to allocate keys while maintaining secrecy
Value Maha. In the case of an asymmetric coding system, this problem can be overcome by
Distribute only cryptographic keys that do not need to be kept secret. However, a problem appears
Another is to ensure that each participant's encryption key is verified; No guarantee of knowing the person
The user value of the decryption key declared to the identity of the “owner” of the corresponding decryption key.
When we presented the difference between symmetric and asymmetric coding systems, we were assuming
Knowledge of the objecting party of the algorithm. Of course, this is not always true. and with
It would have been better for the designer of the coding system to assume that the interceptor had
The potential for as much general intelligence and knowledge as possible. over there
A well-known principle in cryptography says that the security of any cryptographic system should not depend on retention
confidentiality of the cipher algorithm; Which makes the integrity of the system accordingly dependent on retention
Only secret decryption key.
One of the goals of studying cryptography is to enable anyone who wants to design or…
Implementation of a cryptographic system assesses whether that system is secure enough to achieve the purpose of
its implementation. To assess the security of the cipher system we make the following three assumptions, which we make
It has “worst-case circumstances”:
Worst-case circumstance 1: The party performing the parsing of the ciphertext has knowledge of
Complete coding system.
Worst-case circumstance 2: The party performing the parsing of the ciphertext gets as much
greater than this text.
Worst-case circumstance 3: The party performing the parsing of the ciphertext knows the text
The original equivalent of a specified amount of ciphertext.
In any of these cases, an attempt should be made to determine what the words “greater” and “destiny” mean
specified” realistically, depending on the coding system in mind.
Worst-case circumstance 1 implies that we believe we should not rely on
Keeping the details of the encryption system confidential. But this does not mean that we allow the coding system
For everyone. Of course, the task of the objecting party will be more difficult if they do not know
The encryption system used, which can be hidden to some degree at present. For example, for
To modern electronic systems, the encryption algorithm can be hidden in the devices themselves from
by using microelectronic components; The entire algorithm can be hidden
Inside a small "chip". In order to obtain the algorithm, the objecting party must “open”
One of these slides, which is a delicate and probably very time-consuming process, though
could have been implemented; We must not assume the lack of ability and patience of the opposing party
To do that. Similarly, it is possible to hide any part of the algorithm that is included
As a program in the machine through a carefully written program. We emphasize again, perhaps it can
With patience and skill, this can be revealed, and perhaps even a particular algorithm becomes available
to the objecting party in some cases. From the point of view of any manufacturer or designer of a coding system,
remove a large amount
Worst-case condition 1 is a basic assumption; Since it is also hypothetical
It is their ultimate responsibility to maintain the confidentiality of any encryption system.
Worst case condition 2 is a reasonable assumption. If there is no possibility of
interception process, there is no need to use an encryption system. However, if objection is possible, then it is assumed that the continuing parties will not be able to determine when operations will occur
The safest option is to assume the possibility
exact objection; and from then
Intercept all correspondence.
Worst-case condition 3 is also a realistic assumption. The objecting party may be able to
. The ability to obtain this type of information by following the transmission of messages and conducting
Smart guesses. Rather, the objecting party may also be able to choose the original text that
Knows its cipher. One of the historical “classic” examples of this happened during
World War II when a light buoy was subjected to an explosive attack only to ensure
Appearance of the German word Leuchttonne in original text messages was to be encoded
Using Enigma ciphers (see The Secret War by B Johnson
published by the British Broadcasting Corporation).
An objection process that takes advantage of the presence of a pair of an original and a text is called an objection process
Maaroufni cipher “the process of intercepting a known source text”. If the objecting party selects the text
The original, as was the case with the example of exploding buoys
