C/C#/Java/Python/MATLAB 语言辅导、Encryption/Decryption Algorithms 代码辅导讲解辅导留学生

Information Systems Security and Protection

Beijing Jiao Tong University

Homework 2 (10% of final grade)

Coding for Encryption/Decryption Algorithms

Submission Deadline: 23:59:59, Sept 28.

You can write in C/C#/Java/Python/MATLAB, etc.

1. Substitution Ciphers

In Substitution cipher, each letter in the plaintext is replaced by a letter having some fixed

number of positions down/or up the alphabet.

• Example:

Please implement Encrypt(Letter, Key) and Decrypt(Letter, Key). Key is the input

parameter, which can be defined by users. For example, when Key=3,

Encrypt(“HELLO”,3) = “KHOOR”, and Decrypt(“KHOOR”,3) = “HELLO”. Key can be

other values.

2. Transposition Cipher

In transposition cipher, the message is encrypted by rearranging the letter order, i.e., write

letters out diagonally over a number of rows, then read off cipher row by row. For

example, the plain text “meet after the toga party”, can be written out as:

• m e m a t r h t g p r y

• e t e f e t e o a a t

if the cipher text is written in two rows.

Please implement Encrypt(Letter, Key) and Decrypt(Letter, Key), with Key as how many

rows are there in cipher text.

3. Product Ciphers

2

We define Product Cipher as the Function of Boolean algebra: two bits are compared. If

two bits are identical, the result is binary 0; If two bits are not identical, the result is

binary 1, shown as in the following figure.

Please implement Encrypt(Letter, Key) and Decrypt(Letter, Key), with Key as defined by

users.

Submission:

Zip all C codes, submit it to Class Representative before deadline.