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來自 University of Colorado System 的課程

Symmetric Cryptography

25 個評分

University of Colorado System

Symmetric Cryptography

25 個評分

課程 2（共 4 門，Specialization Applied Cryptography）

Welcome to Symmetric Cryptography! Symmetric cryptography relies on shared secret key to ensure message confidentiality, so that the unauthorized attackers cannot retrieve the message. The course describes substitution and transposition techniques, which were the bases for classical cryptography when the message is encoded in natural language such as English. Then, we build on product ciphers (using both substitution and transposition/permutation) to describe modern block ciphers and review the widely used cipher algorithms in DES, 3-DES, and AES. Lastly, we enable the use of block ciphers to support variable data length by introducing different modes of block cipher operations in ECB, CBC, CFB, OFB, and CTR modes. This course is cross-listed and is a part of the two specializations, the Applied Cryptography specialization and the Introduction to Applied Cryptography specialization.

從本節課中

Classical Cipher: Substitution

This module defines substitution cipher technique and describes multiple examples for substitution-based classical algorithms: Caesar Cipher, Monoalphabetic Cipher, and Vigenere Cipher (which is a type of Polyalphabetic Cipher). We will also discuss the mathematical concepts in Modulo Operations to use them to describe the cipher algorithms.

與講師見面

Sang-Yoon Chang
Assistant Professor
Computer Science

The history of cryptography is estimated to be at least 4000 years long,

and it's longer than any natural language that we use.

The earliest evidence of cryptography comes from ancient Egypt in ciphering

their hieroglyphic or symbol-based writings on monuments and walls.

Since then, cryptography has been used to protect communications which

may use different languages or different ways to represent and convey information.

That is, cryptography is used for

many different information systems which use

different ways to code or represent information.

Because of that reason,

cipher algorithms are designed to support communications

regardless of the language or the methods for representing information.

To support that, cryptography uses the notion of alphabet to

express the minimal information unit used by the communication system.

Let's first look at the term alphabet in a non-technical context.

Merriam-Webster Dictionary defines alphabet as a set of

letters or other characters with which one or more languages are written,

especially if arranged in a customary order.

The alternative definition of alphabet is a system of signs or signals that

serve as equivalents for letters.

We will use the term alphabet to encompass both definitions from Merriam-Webster.

That is, alphabet in cryptography is the minimal unit for

information where a collection of alphabets are used to convey information or a message.

Different information coding system has different alphabets.

For example, English contains 26 alphabet letters from the letter A

to the letter Z. Morse Code has two alphabets, dots or dashes.

Digital computer is also used for alphabets in bits,

bit one or bit zero.

Decimal system has 10 numbers from zero to nine,

and hexadecimal system has 16 numbers from zero to nine and then from A to F,

where A to F corresponds to numbers from 10 to 15 in decimals.

Let's review the alphabet size or the number of elements in the alphabet set.

The alphabet size is 26 for English,

two for Morse Code,

two for computer bits, and so on.

In cryptography, the information representation system provides the alphabet set,

which will be used for the plaintext construction.

Before the plaintext construction,

there can be encoding which maps the information from one alphabet set to another,

possibly a string of alphabets in a different alphabet set.

However, our focus here will be the mapping between the plaintext and the ciphertext.

The alphabet size, or the number of elements in the alphabet set,

is important in cryptography since it determines

the number of options for encryption per alphabet,

which affects the security strength of the cipher or

how difficult it would be for an attacker to break the cipher.

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