文章目录

1 Basic Introduction
- 1.1 Definition
- 1.2 Main Branches
2 History of cryptography and cryptanalysis
- 2.1 Classic cryptography
- - 2.1.1 main classical cipher types
  - 2.1.2 This fundamental principle of Cryptography
  - 2.1.3devices and aids
- 2.2 Computer era
- 2.3 Advent of modern cryptography
- 2.3 Modern cryptography
- - 2.3.1 Public-key cryptography
  - 2.3.2 Public-key cryptography
  - 2.3.3 Cryptanalysis
3 Legal Issues

1 Basic Introduction

1.1 Definition

Cryptography is the practice and study of techniques for secure communication in the presence of third parties called adversaries.

More generally, cryptography cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages

Alice →{\rightarrow}→ sender
Bob →{\rightarrow}→ recipient
Eve →{\rightarrow}→ adversary

1.2 Main Branches

Aspects of cryptography in information security:
→{\rightarrow}→ data confidentiality
→{\rightarrow}→data integrity
→{\rightarrow}→authentication
→{\rightarrow}→non-repudiation
Modern cryptography exists at the intersection of the disciplines:
→{\rightarrow}→mathematics
→{\rightarrow}→computer science
→{\rightarrow}→electrical engineering, communication science
→{\rightarrow}→physics
Applications of cryptography
→{\rightarrow}→electronic commerce
→{\rightarrow}→chip-based payment cards
→{\rightarrow}→digital currencies
→{\rightarrow}→computer passwords
→{\rightarrow}→military communications

2 History of cryptography and cryptanalysis

                             Brief Introduction

Classic Cryptography

Computer era

Advent of modern cryptography

2.1 Classic cryptography

2.1.1 main classical cipher types

Security of the key used should alone be sufficient for a good cipher to maintain confidentiality under an attack. This fundamental principle was first explicitly stated in 1883 by Auguste Kerckhoffs and is generally called Kerckhoffs’s Principle; alternatively and more bluntly, it was restated by Claude Shannon, the inventor of information theory and the fundamentals of theoretical cryptography, as Shannon’s Maxim—‘the enemy knows the system’.

transposition ciphers, which rearrange the order of letters in a message
- e.g. Caesar cipher. Suetonius reports that Julius Caesar used it with a shift of three to communicate with his generals.
substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters
Steganography, to hide even the existence of a message so as to keep it confidential
- e.g. An early example, from Herodotus, was a message tattooed on a slave’s shaved head and concealed under the regrown hair.
- e.g. include the use of invisible ink, microdots, and digital watermarks to conceal information.
frequency analysis cryptanalysis techniques: After the discovery of frequency analysis about in the 9th century, nearly all such ciphers could be broken by an informed attacker. Such classical ciphers still enjoy popularity today, though mostly as puzzles.
- e.g. Language letter frequencies may offer little help for some extended historical encryption techniques
polyalphabetic cipher with automatic cipher device
- e.g. In the Vigenère cipher, encryption uses a key word, which controls letter substitution depending on which letter of the key word is used.

2.1.2 This fundamental principle of Cryptography

Security of the key used should alone be sufficient for a good cipher to maintain confidentiality under an attack. This fundamental principle was first explicitly stated in 1883 by Auguste Kerckhoffs and is generally called Kerckhoffs’s Principle; alternatively and more bluntly, it was restated by Claude Shannon, the inventor of information theory and the fundamentals of theoretical cryptography, as Shannon’s Maxim—‘the enemy knows the system’.

2.1.3devices and aids

Different physical devices and aids have been used to assist with ciphers.

the scytale of ancient Greece
cipher grille in medieval times
cipher disk, Johannes Trithemius’ tabula recta scheme, and Thomas Jefferson’s wheel cypher
rotor machines—famously including the Enigma machine used by the German government and military from the late 1920s and during World War II.

2.2 Computer era

Prior to the early 20th century, cryptography was chiefly concerned with linguistic and lexicographic patterns. Since then the emphasis has shifted, and cryptography now makes extensive use of mathematics, including aspects of information theory, computational complexity, statistics, combinatorics, abstract algebra, number theory, and finite mathematics generally.

Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much more complex ciphers.
computers allowed for the encryption of any kind of data representable in any binary format, unlike classical ciphers ;
Many computer ciphers can be characterized by their operation on binary bit sequences
computers have also assisted cryptanalysis, which has compensated to some extent for increased cipher complexity.

Nonetheless, good modern ciphers have stayed ahead of cryptanalysis; it is typically the case that use of a quality cipher is very efficient (i.e., fast and requiring few resources, such as memory or CPU capability)

2.3 Advent of modern cryptography

Colossus was made as cryptanalysis of the new mechanical devices proved to be both difficult and laborious.
- e.g. assisted in the decryption of ciphers generated by the German Army’s Lorenz SZ40/42 machine.

Extensive open academic research into cryptography is relatively recent; it began only in the mid-1970s.

In recent times, IBM personnel designed the algorithm that became the Federal (i.e., US) Data Encryption Standard; Whitfield Diffie and Martin Hellman published their key agreement algorithm;[24] and the RSA algorithm was published in Martin Gardner’s Scientific American column.
Following their work in 1976, it became popular to consider cryptography systems based on mathematical problems that are easy to state but have been found difficult to solve.
There are very few cryptosystems that are proven to be unconditionally secure
- e.g. one-time pad
There are a few important algorithms that have been proven secure under certain assumptions.
- e.g. For example, the infeasibility of factoring extremely large integers is the basis for believing that RSA is secure, and some other systems, but even so proof of unbreakability is unavailable since the underlying mathematical problem remains open.

The discrete logarithm problem is the basis for believing some other cryptosystems are secure, and again, there are related, less practical systems that are provably secure relative to the solvability or insolvability discrete log problem.

2.3 Modern cryptography

To Be Continue…

2.3.1 Public-key cryptography

2.3.2 Public-key cryptography

2.3.3 Cryptanalysis

3 Legal Issues