Introduction to Cryptography

- December 14, 2020

What is cryptography

As human beings, we do not prefer to reveal our secret and private communication to outsiders. Because of that we usually send our letters inside an envelope to make sure that the contents of those letters should not disclose to unwanted people until the right persons get them. Likewise, we use the methodology of "cryptography" to ensure the secrecy of our data through online communication. This cryptography method ensures the basic features of secure data communication such as confidentiality, integrity, and authorization. Normally we define cryptography as the study and practice of hiding information from unauthorized persons. This ensures the confidentiality of data by restricting unauthorized persons to our data, ensure the integrity by using the hash functions which can detect whether our data was modified by someone or not, and it ensures authorization and non-repudiation. these are the benefits of cryptography.

Before centuries ago, people felt the need to have a secure communication method to send their messages securely without allowing other persons to read their messages. Then they tested lots of methods to use for secure communication. Among those methods, few were not strong to do secure communication, but some of those methods can be considered as the foundation of modern cryptography.

Why it never ends

Time to time people find new cryptographic techniques to secure the information, but this is a war between hackers and cyber security people since when someone finds a way to break that method, then there is no use in exploiting that method to do secure communication. so, we need to find a new method to do secure communication. Likewise, this never ends with finding new methods and breaking those methods. And this process continues from ancient times to the future also.

What are the terms related to cryptography

Sender - the person who sends the message
Receiver - the person who received the message
Encryption - turn the plain text into an unreadable format
Decryption - turn an unreadable message into a readable format
plain text - an original message without encryption
Cipher text - the message after the encryption
Encryption algorithm - the algorithm uses to encrypt the data
Decryption algorithm - the algorithm uses to decrypt the data
cryptanalysis - study and practice of breaking secret data
Symmetric key encryption - use the same key to encrypt and decrypt
Asymmetric key encryption - use two different key to encrypt and decrypt
Secret key - provide uniqueness to the cipher and create a link between plain text and cipher text

What are the two types

All the cryptographic methods can be divided into two types.

Transposition method
Substitution method

Transition method - it is the method which is used in the ancient time. Here we rearrange the words in the plain text into a different order and produce the cipher. But in the modern world, this kind of cryptographic methods never use, because it can break with less effort by using modern computer power.

e.x - Scytale

Substitution method - Here we substitute one letter from another. This is the modern world cryptographic method used in different kinds of cryptographic algorithms with certain updates.

e.x - Caesar's cipher

Now there are different kinds of substitution methods some are works with the transposition method as a combination.

*Simple substitution - here cipher operates on single letters.

*Mono-alphabetic substitution - here each letter in the source is mapped to a fixed symbol.

*Polygraphic substitution - here cipher operates on a large set of letters.

*Poly-alphabetic substitution - here use multiple substitutions and mapped the message several times. MjAyMQ==