Cryptography and the Classification of Ciphers: Types and Their Characteristics

Cryptographic Methods of Information Protection

Cryptographic methods of information protection are a powerful weapon in the fight for information security.

Cryptography (from Ancient Greek κρυπτος – hidden and γραϕω – I write) – the science of methods for ensuring the confidentiality and authenticity of information.

Cryptography is a set of methods for transforming data aimed at making that data useless to an attacker. Such transformations make it possible to solve two main problems concerning information security:

• protecting confidentiality;
• protecting integrity.

The problems of protecting the confidentiality and integrity of information are closely related, so methods for solving one of them are often applicable to solving the other.

Various approaches to classifying methods of cryptographic information transformation are known. By the type of effect on the original information, methods of cryptographic information transformation can be divided into four groups:

Fig. 2.1. Classification of methods of cryptographic information transformation

The process of encryption consists in performing reversible mathematical, logical, combinatorial and other transformations of the original information, as a result of which the encrypted information becomes a chaotic set of letters, digits, other characters and binary codes.

Encrypting information requires a transformation algorithm and a key. As a rule, the algorithm for a given encryption method remains unchanged. The input data for the encryption algorithm are the information to be encrypted and the encryption key. The key contains control information that determines the choice of transformation at certain steps of the algorithm and the values of the operands used in implementing the encryption algorithm. An operand – is a constant, variable, function, expression or other programming-language object on which operations are performed.

Unlike other methods of cryptographic information transformation, steganography methods make it possible to hide not only the meaning of the stored or transmitted information but also the very fact that closed information is being stored or transmitted. All steganography methods are based on masking closed information among open files, i.e. secret data are hidden while realistic data are created that cannot be distinguished from genuine data. The processing of multimedia files in information systems has opened up practically unlimited possibilities for steganography.

Graphic and audio information are represented in numeric form. Thus, in graphic objects the smallest picture element can be coded by a single byte. Bits of the hidden file are placed in the low-order bits of certain image bytes in accordance with the cryptographic transformation algorithm. If the transformation algorithm and the image in which the hidden file is placed are chosen correctly, it is practically impossible for the human eye to distinguish the resulting image from the original one. Steganographic tools can be used to mask text, images, speech, digital signatures, and encrypted messages.

The hidden file can also be encrypted. If someone happens to discover the hidden file, the encrypted information will be perceived as a system malfunction. The combined use of steganography and encryption multiplies the difficulty of detecting and uncovering confidential information.

The content of the coding process is replacing the original meaning of a message (words, sentences) with codes. Combinations of letters, digits, and symbols can be used as codes. Coding and the reverse transformation use special tables or dictionaries. In information networks, coding of the original message (or signal) by hardware and software means is used to increase the reliability of the transmitted information.

Coding and encryption are often mistakenly treated as one and the same thing, forgetting that recovering a coded message requires only knowledge of the replacement rule, whereas decrypting a message requires, besides knowledge of the encryption rules, a key to the cipher.

Compression of information can be classed among the methods of cryptographic information transformation, with certain reservations. The purpose of compression is to reduce the volume of information. At the same time, compressed information cannot be read or used without the reverse transformation. Given the availability of compression and decompression tools, these methods cannot be regarded as reliable means of cryptographic information transformation. Even if the algorithms are kept secret, they can be comparatively easily uncovered by statistical processing methods. Therefore, compressed files of confidential information are subjected to subsequent encryption. To reduce data transmission time, it is advisable to combine the processes of compression and encryption of information.

The main type of cryptographic information transformation in computer networks is encryption. Encryption is understood as the process of transforming open information into encrypted information (ciphertext), or the reverse process of transforming encrypted information into open information. The process of transforming open information into closed information is called encipherment, and the process of transforming closed information into open information – decipherment.

Over the centuries-long history of using encryption of information, humanity has devised a great many encryption methods, or ciphers. A method of encryption (a cipher) is a set of reversible transformations of open information into closed information in accordance with an encryption algorithm. Most encryption methods have not stood the test of time, while some are still used today. The advent of computers and computer networks initiated the development of new ciphers that take into account the possibilities of using computer technology both for encrypting/decrypting information and for attacking a cipher. An attack on a cipher (cryptanalysis, a crypto-attack) – is the process of recovering closed information without knowledge of the key and, possibly, without any information about the encryption algorithm.

Modern encryption methods must satisfy the following requirements:

• the strength of a cipher to withstand cryptanalysis (cryptographic strength) must be such that breaking it can be accomplished only by solving the exhaustive key-search problem;
• cryptographic strength is ensured not by the secrecy of the encryption algorithm, but by the secrecy of the key;
• the ciphertext must not significantly exceed the original information in volume;
• errors occurring during encryption must not lead to distortion or loss of information;
• the encryption time must not be long;
• the cost of encryption must be commensurate with the value of the information being protected.

The cryptographic strength of a cipher is its main performance indicator. It is measured by the time or the cost of the resources a cryptanalyst needs to obtain the original information from the ciphertext, given that the key is unknown to them.

It is practically impossible to keep a widely used encryption algorithm secret. Therefore, the algorithm must not have hidden weaknesses that cryptanalysts could exploit. If this condition is met, the cryptographic strength of the cipher is determined by the key length, since the only way to break the encrypted information – is to search through key combinations and run the decryption algorithm. Thus, the time and resources spent on cryptanalysis depend on the key length and the complexity of the encryption algorithm.

The operation of a simple cryptosystem is illustrated in Fig. 2.2.

Fig. 2.2. Generalized diagram of a cryptographic system

The sender generates the plaintext of the original message M, which must be transmitted to the legitimate recipient over an unsecured channel. The channel is monitored by an interceptor whose aim is to intercept and disclose the transmitted message. So that the interceptor cannot learn the content of message M, the sender encrypts it using the reversible transformation Ek and obtains the ciphertext (or cryptogram) C=Ek(M) , which is sent to the recipient.

The legitimate recipient, upon receiving the ciphertext C, decrypts it using the inverse transformation Dk(C) and obtains the original message in the form of plaintext M.

The transformation Ek is chosen from a family of cryptographic transformations called crypto-algorithms. The parameter used to select a particular transformation is called the cryptographic key K.

A cryptosystem can be implemented in various ways: as a set of instructions, hardware, or a suite of programs that allow plaintext to be encrypted and ciphertext to be decrypted in various ways, one of which is selected by means of a specific key K.

The encryption transformation can be symmetric or asymmetric with respect to the decryption transformation. This important property defines two classes of cryptosystems:

• symmetric (single-key) cryptosystems;
• asymmetric (two-key) cryptosystems (with a public key).

Classification of Ciphers

I. By field of application, ciphers are divided into limited-use and general-use ciphers.

The strength of limited-use ciphers is based on keeping the cryptographic transformation algorithm secret, owing to its vulnerability, the small number of keys, or the absence of keys altogether (secret code systems).

The strength of general-use ciphers is based on the secrecy of the key and the difficulty of guessing it faced by a potential adversary.

II. By the features of the encryption algorithm, general-use ciphers can be divided into the following types.

In single-key systems, the same key is used for encryption and decryption.

In transposition ciphers, all the letters of the plaintext remain in the cryptogram but change their positions. In substitution ciphers, conversely, the positions of the letters in the cryptogram remain the same as in the plaintext, but the plaintext characters are replaced with characters from another alphabet.

In additive ciphers, the letters of the alphabet are replaced with numbers, to which numbers from a secret random (pseudorandom) numeric sequence (the keystream) are then added, after which the remainder of division modulo a number is taken (the mod operation). If the original message and the keystream are represented in bit form, the logical operation “Exclusive OR” (XOR, addition modulo 2) is used for encryption and decryption.

Quantum cryptography introduces natural uncertainty of the quantum world into the encryption process. The sending and receiving of information is carried out by means of objects of quantum mechanics (for example, using electrons in an electric current or photons in fiber-optic communication lines). The most valuable property of this type of encryption is that, when a message is sent, the sending and receiving parties can, with fairly high probability, establish the fact that an adversary has intercepted the encrypted message.

In two-key systems, two completely different keys are used for encryption and decryption. In deterministic ciphers, encrypting the same message with the same key will always produce the same ciphertext. In probabilistic ciphers, an additional random quantity (number) is used in the encryption procedure - as a result, encrypting the same original message with the same key can produce different ciphertexts, which upon decryption yield the same result (the original message).

Combined (composite) methods involve using several methods at once to encrypt a message (for example, first replacing symbols and then permuting them).

III. By the number of message symbols (or their code replacement) encrypted or decrypted by a single-type transformation procedure, a distinction is made between:

- stream ciphers – the transformation procedure is applied to an individual symbol of the message;

- block ciphers – the transformation procedure is applied to a set (block) of message symbols.

A stream cipher can be distinguished from a block cipher by the following criterion - if, as a result of splitting the original message into individual elementary symbols and applying a single-type transformation procedure to them, the resulting cryptogram is equivalent to the one obtained by applying the transformation “as if to the entire original message at once”, then the cipher is a stream cipher; otherwise it is a block cipher.

IV. By strength, ciphers are divided into three groups:

- perfect (unconditionally secure, theoretically secure) – ciphers that are known in advance to be unbreakable (when used correctly). Cryptanalysis of the secret message leads to several equally probable meaningful plaintext messages;

- practically (computationally, sufficiently) secure – ciphers that cannot be broken in an acceptable time given the current or foreseeable level of computing technology. The practical strength of such systems is based on complexity theory and is assessed exclusively at a given point in time from two standpoints:

- the computational complexity of exhaustive search;

- currently known weaknesses (vulnerabilities) and their effect on computational complexity;

- weak ciphers.

There are several classifications of ciphers. Let us consider some of them.

Fig. 3.1. Classification of ciphers

The figure shows, against a gray background, subgroups of ciphers that do not form a complete overlying group of ciphers but have found wide practical application and are described in this course of lectures.

The following table gives a brief description of the types of ciphers.

Table 3.1. Types of ciphers

^*) Some ciphers (asymmetric ones in particular) allow the transformation procedures to be carried out in either stream or block mode.

Terminology

• Plaintext (open text) — data (not necessarily textual) transmitted without the use of cryptography, or, in other words, unencrypted data.
• Ciphertext, encrypted (closed) text — data obtained after applying a cryptosystem (usually — with some specified key). Another name: cryptogram.
• Cipher, cryptosystem — a family of reversible transformations of plaintext into ciphertext.
• Key — a cipher parameter that determines the choice of a specific transformation for a given text. In modern ciphers, the cryptographic strength of the cipher is determined entirely by the secrecy of the key (the Kerckhoffs principle). A distinction is also made between the encryption key (encryption key) and the decryption key (decryption key)
• Encryption — the process of the normal application of a cryptographic transformation to plaintext on the basis of an algorithm and a key, as a result of which encrypted text arises.
• Decryption — the process of the normal application of a cryptographic transformation of encrypted text into plaintext.
• Asymmetric cipher, two-key cipher, public-key cipher — a cipher in which two keys are used, an encrypting key and a decrypting key. Moreover, knowing only the encryption key, it is impossible to decrypt the message, and vice versa.
• Public key — the one of the two keys of an asymmetric system that is freely distributed. It is the encrypting key for secret correspondence and the decrypting key — for a digital signature.
• Secret key, private key — the one of the two keys of an asymmetric system that is kept secret.

• Cryptanalysis — the science that studies mathematical methods for breaching the confidentiality and integrity of information.
• Cryptanalyst — a scientist who devises and applies methods of cryptanalysis.
• Cryptography and cryptanalysis together make up cryptology, a unified science of creating and breaking ciphers (this division was introduced from the West; before that, no special division was used in the USSR and Russia).
• Cryptographic attack — an attempt by a cryptanalyst to induce deviations in the protected information-exchange system under attack. A successful cryptographic attack is called cracking or breaking.
• Cryptanalytic decryption (deciphering) — the process of extracting plaintext without knowledge of the cryptographic key, on the basis of the known encrypted text. The term cryptanalytic decryption is usually applied to the process of cryptanalyzing ciphertext (cryptanalysis itself, generally speaking, can also consist in analyzing the cryptosystem, not only the plaintext message it encrypts).
• Cryptographic strength — the ability of a cryptographic algorithm to withstand cryptanalysis.

• Authentication protection (imitation protection) — protection against the imposition of false information. In other words, the text remains open, but it becomes possible to verify that it has not been altered, either accidentally or deliberately. Authentication protection is usually achieved by including a message authentication tag in the transmitted data packet.
• Message authentication tag — a block of information used for authentication protection, dependent on the key and the data.
• Electronic digital signature, or digital signature — an asymmetric authentication tag (the protection key differs from the verification key). In other words, an authentication tag that the verifier cannot forge.
• Certificate authority — a party whose honesty is beyond question and whose public key is widely known. The digital signature of the certificate authority confirms the authenticity of a public key.

• Hash function — a function that transforms a message of arbitrary length into a number (a “digest”) of fixed length. For a cryptographic hash function (unlike a general-purpose hash function), it is difficult to compute the inverse or even to find two messages with the same hash value.
• Hybrid cryptosystem — an encryption system that combines the advantages of a public-key cryptosystem with the performance of symmetric cryptosystems.

Self-Check Questions

• 1. Define the concepts of “cipher”, “key”, “cryptanalytic decryption”.
• 2. List the main requirements imposed on cryptosystems.
• 3. Give a classification of ciphers by the features of the message transformation procedures.
• 4. Give a classification of ciphers by the strength of the cipher.

See Also

• [[b4325]]
• [[b5755]]
• [[b6435]]
• Information Security
• [[b5824]]
• Neurocryptography

See also

• Cryptographic algorithms.
• 5.2. Means of cryptographic protection of connections in computer networks
• SSL, HTTPS
• Steganography, classification of types of steganography