Cryptography is an important secret means for communication parties to carry out special information transformation according to agreed rules. According to these laws, changing plaintext into ciphertext is called encryption transformation; Changing ciphertext into plaintext is called decryption transformation. In the early days, passwords only changed the encryption and decryption of characters or numbers. With the development of communication technology, encryption and decryption of voice, image and data can be implemented.
Cryptography is gradually developed in the practice of the struggle between coding and decoding, and with the application of advanced science and technology, it has become a comprehensive cutting-edge technical science. It is closely related to linguistics, mathematics, electronics, acoustics, information theory and computer science. Its practical research results, especially the encryption and decoding methods used by governments all over the world, are highly confidential.
the law of secret transformation is called the cipher system. The parameter indicating this transformation is called the key. They are an important part of cryptography. The basic types of cryptographic systems can be divided into four types: confusion-changing the position of plaintext letters or numbers into ciphertext according to the prescribed graphics and lines; Substitution-replacing plaintext letters or numbers with ciphertext by one or more substitution tables; Codebook-changing plaintext into ciphertext by replacing certain phrases and words with pre-programmed alphabetic or numeric cipher groups; Scrambling-using a series of finite elements as random numbers, and combining them with plaintext sequences according to the specified algorithm to become ciphertext. The above four cryptosystems can be used separately or mixed to compile various practical passwords with high complexity.
since 197s, some scholars have proposed a public key system, that is, using the mathematical principle of one-way function to realize the separation of encryption and decryption keys. The encryption key is public, and the decryption key is confidential. This new cryptosystem has attracted extensive attention and discussion in the field of cryptography.
Based on the laws of characters and passwords, under certain conditions, various technical means are adopted, and through the analysis of intercepted ciphertext, the plaintext can be obtained, and the ciphertext can be restored, that is, the ciphertext can be deciphered. Deciphering passwords with different strengths requires different conditions, even very different.
In ancient China, there were some rudiments of secret communication methods that were close to passwords. Ceng Gongliang and Ding Du in Song Dynasty recorded that in the early Northern Song Dynasty, 4 Chinese characters of a five-character rhythmic poem were used to represent 4 situations or requirements, which has the characteristics of the secret edition system.
in p>1871, 6,899 Chinese characters were selected by Shanghai Dabei Waterline Telegraph Company and replaced by four-code numbers, which became the first commercial plain code in China. At the same time, the method of adapting plain code into secret code and scrambling it was designed. On this basis, it gradually developed into various more complex passwords.
In Europe, in 45 BC, Lysandros, the general of Sparta, used the original disordered code; In the first century BC, the Roman emperor Caesar used an ordered single table instead of passwords; After that, it gradually developed into various cryptographic systems such as cipher book, multi-table substitution and scrambling.
At the beginning of the 2th century, the first practical mechanical and electric cipher machines appeared, and at the same time, commercial cipher machine companies and markets appeared. After 6' s, the electronic cipher machine got rapid development and wide application, which made the development of cipher enter a new stage.
password cracking is gradually produced and developed with the use of passwords. In 1412, the encyclopedia compiled by the Persian Kalekashandi contained the method of deciphering simple instead of password. By the end of 16th century, some European countries had full-time decipherers to decipher intercepted secret messages. Cryptographic decoding technology has made considerable progress. Cryptography and Decoding Techniques written by Kasinsky, a Prussian in 1863, and Military Cryptography written by Kerckhoff, a Frenchman in 1883, all discussed and discussed the theory and methods of cryptography. Shannon, an American, published the article Communication Theory of Secret System in 1949, and analyzed some basic problems in cryptography by applying the principle of information theory.
Since the 19th century, the widespread use of telegrams, especially wireless telegrams, has provided extremely favorable conditions for cryptographic communication and interception by third parties. Communication secrecy and detection and decoding have formed a hidden front with fierce struggle.
In p>1917, Britain deciphered the telegram of German Foreign Minister zimmermann, which prompted the United States to declare war on Germany. In 1942, the United States learned from deciphering the Japanese Navy's secret report that the Japanese army's operational intention and deployment of troops in the Midway area could break the main force of the Japanese navy with inferior forces and reverse the war situation in the Pacific region. In the defense of the British Isles and many other famous historical events, the success of password cracking has played an extremely important role, and these examples also illustrate the important position and significance of password secrecy from the opposite side.
At present, the governments of major countries in the world attach great importance to cryptography. Some of them have set up huge institutions, allocated huge funds, concentrated tens of thousands of experts and scientific and technological personnel, and invested a lot of high-speed computers and other advanced equipment in their work. At the same time, private enterprises and academic circles have paid more and more attention to cryptography, and many mathematicians, computer scientists and experts in other related disciplines have also devoted themselves to the research of cryptography, which has accelerated the development of cryptography.
Now cryptography has become a separate discipline. In the traditional sense, cryptography is to study how to transform information into a hidden way and prevent others from getting it.
Cryptography is an interdisciplinary subject, which is derived from many fields: it can be regarded as information theory, but it uses a lot of tools in the field of mathematics, such as number theory and finite mathematics.
the original information, that is, the information that needs to be protected by password, is called plaintext. Encryption is the process of transforming the original information into an unreadable form, which is a password. Decryption is the reverse process of encryption, and the original information is obtained from the encrypted information. Cipher is an algorithm used in encryption and decryption.
The earliest steganography only needed pen and paper, and now it is called classical cryptography. Its two categories are permutation encryption, which rearranges the order of letters; Replace encryption, replacing a group of letters with other letters or symbols. The information of classical encryption method is easily broken by statistics. The more information, the easier it is to crack. Using analysis frequency is a good way. Classical cryptography has not disappeared yet, and it often appears in intelligence games. In the early 2th century, some mechanical devices, including rotary wheel machines, were invented for encryption, the most famous of which was Enigma, a cipher machine used in World War II. The passwords generated by these machines greatly increase the difficulty of password analysis. For example, various attacks against Enigma were successful only after considerable efforts.
traditional cryptography
Autokey password
replacement password
by Charles Wheatstone.
multi-letter replacement password
hill password
Virginia password
replacement password
Caesar password
ROT13
affine password
Atbash password
transposition password
Scytale
Grille password
VIC password (a complicated manual password, It was very safe at that time)
attack on traditional cryptography
frequency analysis
coincidence index
modern algorithm, Methods
the federal information processing standards publication program (run by NIST to produce standards in many areas to guide operations of the United States) was evaluated and selected. many FIPS Pubs are cryptography related, ongoing)
the ANSI standardization process (produces many standards in many areas; some are cryptography related, ongoing)
ISO standardization process (produces many standards in many areas; some are cryptography related, ongoing)
IEEE standardization process (produces many standards in many areas; some are cryptography related, ongoing)
IETF standardization process (produces many standards (called RFCs) in many areas; Some are cryptography related, onging)
see cryptography standards
encryption organization
NSA internal evaluation/selections (surely extensive, nothing is publicly known of the process or its results for internal use; NSA is charged with assisting NIST in its cryptographic responsibilities)
GCHQ internal evaluation/selections (surely extensive, nothing is publicly known of the process or its results for GCHQ use; a division of GCHQ is charged with developing and recommending cryptographic standards for the UK government)
DSD Australian SIGINT agency - part of ECHELON
Communications Security Establishment (CSE) -Canadian intelligence agency.
published efforts
the des selection (nbs selection process, ended 1976)
the RIPE division of the RACE project (sponsored by the European Union, ended mid-'8s)
the AES competition (a 'break-off' sponsored by NIST; ended 21)
the NESSIE Project (evaluation/selection program sponsored by the European Union; ended 22)
the CRYPTREC program (Japanese government sponsored evaluation/recommendation project; draft recommendations published 23)
the Internet Engineering Task Force (technical body responsible for Internet standards -- the Request for Comment series: ongoing)
the CrypTool project (eLearning programme in English and German; freeware; Exhaustive educational tool about cryptography and cryptanalysis)
cryptographic hash function (message digest algorithm, MD algorithm)
encryption hash function
message authentication code
keyed-hash message authentication code
emac (Nessie selection MAC)
hmac (Nessie selection MAC; Iso/iec 9797-1, FIPS and IETF RFC)
ttmac is also called two-track-MAC (nessie selection MAC; K.U.Leuven (Belgium) & debis AG (Germany))
UMAC (NESSIE selection MAC; Intel, UNevada Reno, IBM, Technion, & UCal Davis)
MD5 (one of a series of message digest algorithms, proposed by Professor Ron Rivest of MIT; 128-bit abstract)
sha-1 (16-bit abstract developed by NSA, one of FIPS standards; The first release version was issued.