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Writing a Cipher

Writing a Cipher

Computer Networking: A Top-Down Approach (7th Edition)
Computer Networking: A Top-Down Approach (7th Edition)
7th Edition
ISBN:9780133594140
Author:James Kurose, Keith Ross
Publisher:James Kurose, Keith Ross
Chapter1: Computer Networks And The Internet
Section: Chapter Questions
Problem R1RQ: What is the difference between a host and an end system? List several different types of end...
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Writing a Cipher

Hiding secrets from unwanted eyes has fascinated the humankind since ancient times. In the digital age, this is usually done by cipher algorithms. A cipher algorithm takes a string message called the plaintext and returns another string called the ciphertext that is (a) apparently hard to read, but (b) can be converted back into the original plaintext message. Turning the plaintext into the ciphertext is called encryption; reversing this process is called decryption. In this task, you'll learn about a classic cipher called the transposition cipher and implement it. Transposition Cipher. As the name suggests, the basic idea of the transposition cipher is to take the message's symbols and jumble them up so that they become unreadable. The cipher decides how to move the symbols around using what's known as the key, denoted by k. To encrypt a text of length n, the key k is usually chosen to be a number between 2 and k – 1 (inclusive). The exact encryption process is best described using an example. Consider encrypting the text Common_sense_in_an_uncommon_degree_is_what the_world_calls_wisdom which contains 65 symbols, including spaces. The spaces are indicated by the symbol. We'll use k = 10. First, we'll draw k boxes in a row-hence, drawing 10 boxes in our case. Following that, we'll start filling the boxes from left to right using the symbols (letters and punctuation marks) from the input text. When you run out of boxes, you create a new row of k boxes and continue filling them until you accommodate the text entirely: m m S e n e i co gr|e S n a u n m m d e e i S h the a W r 1 d a 1 1 i S d m We include the x's in the last row only to remind ourselves that they aren't part of the input text and should be ignored in later processing. To complete the encryption, we'll read the symbol in the first (leftmost) column from top to bottom, then the second column from top to bottom, so on so forth. Notice that when we hit x, we simply skip over it onto the next column. The resulting encrypted string is Csngalioecrtdsm_oedmimetcoonm hamn,_oiel_ans_lsnwse_dwo_nuehrw
Transcribed Image Text:Hiding secrets from unwanted eyes has fascinated the humankind since ancient times. In the digital age, this is usually done by cipher algorithms. A cipher algorithm takes a string message called the plaintext and returns another string called the ciphertext that is (a) apparently hard to read, but (b) can be converted back into the original plaintext message. Turning the plaintext into the ciphertext is called encryption; reversing this process is called decryption. In this task, you'll learn about a classic cipher called the transposition cipher and implement it. Transposition Cipher. As the name suggests, the basic idea of the transposition cipher is to take the message's symbols and jumble them up so that they become unreadable. The cipher decides how to move the symbols around using what's known as the key, denoted by k. To encrypt a text of length n, the key k is usually chosen to be a number between 2 and k – 1 (inclusive). The exact encryption process is best described using an example. Consider encrypting the text Common_sense_in_an_uncommon_degree_is_what the_world_calls_wisdom which contains 65 symbols, including spaces. The spaces are indicated by the symbol. We'll use k = 10. First, we'll draw k boxes in a row-hence, drawing 10 boxes in our case. Following that, we'll start filling the boxes from left to right using the symbols (letters and punctuation marks) from the input text. When you run out of boxes, you create a new row of k boxes and continue filling them until you accommodate the text entirely: m m S e n e i co gr|e S n a u n m m d e e i S h the a W r 1 d a 1 1 i S d m We include the x's in the last row only to remind ourselves that they aren't part of the input text and should be ignored in later processing. To complete the encryption, we'll read the symbol in the first (leftmost) column from top to bottom, then the second column from top to bottom, so on so forth. Notice that when we hit x, we simply skip over it onto the next column. The resulting encrypted string is Csngalioecrtdsm_oedmimetcoonm hamn,_oiel_ans_lsnwse_dwo_nuehrw
YOUR TASK: Implement a function enc (msg: str, key: int) -> str that takes a message string msg and a key key, 1<key < len(msg), and returns a string representing the message encrypted using the transposition cipher (as explained above). Here are some more examples: enc ("abc",2) • enc("monosodium glutamate", 7)=='mitouanmmo asgtoledu' enc ("polylogarithmicsubexponential", 3)=='pygimseonaolatiuxntllorhcbpei' 'acb' =%3=
Transcribed Image Text:YOUR TASK: Implement a function enc (msg: str, key: int) -> str that takes a message string msg and a key key, 1<key < len(msg), and returns a string representing the message encrypted using the transposition cipher (as explained above). Here are some more examples: enc ("abc",2) • enc("monosodium glutamate", 7)=='mitouanmmo asgtoledu' enc ("polylogarithmicsubexponential", 3)=='pygimseonaolatiuxntllorhcbpei' 'acb' =%3=
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