Modification Exercise M01 In Modification Exercises You Will

Modification Exercise M 01in Modification Exercises You Will Be Given

In this assignment, you are given a program that implements a monoalphabetic substitution cipher, specifically a Caesar cipher. Your task involves understanding the code, modifying it to perform decryption, and applying the cipher to new ciphertexts. Additionally, you will create a new substitution cipher based on a sentence, analyze the role of dictionaries in this context, and demonstrate your understanding through testing and explanations.

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The exercise begins with understanding a Python program, monosubenc.py, which encrypts plaintext messages using a simple Caesar (shift) cipher. This cipher shifts each letter by a fixed number of positions within the alphabet, producing ciphertext that obscures the original message. The core of the program relies on a Python dictionary, dEncrypt, that maps each plaintext character to its corresponding ciphertext character, facilitating quick and efficient substitution. The use of dictionaries in this context exemplifies how key-value pairs enable reversible transformations central to substitution ciphers.

Following the initial understanding, the first practical task is to modify the given encryption program to create a decryption program, monosubdec.py. This program will take a known ciphertext, such as "NKRRU CUXRJ," and correctly decrypt it back to "HELLO WORLD." Instead of merely switching print statements, you will need to invert the encryption process by either creating a decryption dictionary or reversing the existing one. Proper implementation is crucial to ensure accurate decryption. After the modification, you will test the program with the provided ciphertext and confirm the plaintext output matches expectations.

The next phase involves applying the cipher to a new ciphertext "MUUJ PUH". Using your decryption program, decode this message to reveal the original plaintext. This exercise demonstrates understanding of the cipher mechanics and your ability to adapt code for different ciphertext inputs.

In Part II, the assignment moves to constructing a substitution cipher based on a specific sentence, "THE QUICK BROWN FOX JUMPED OVER LAZY DOGS." This sentence contains every letter of the alphabet, and using it, you build a substitution alphabet, alphaCiphr. As you process each character in the sentence, any first-time letter is appended to alphaCiphr, ensuring that the sequence of unique letters reflects the order of first appearance. Subsequent appearances of the same letter are ignored. This method creates a dynamic substitution alphabet that is unique to the sentence, thus shaping a new cipher key.

In the code, you will comment out the original alphaCiphr used in Part I and replace it with this sentence-derived sequence. By doing this, your encryption process now depends on a cipher alphabet generated from the sentence, producing a different ciphertext for the plaintext "HELLO WORLD," which should now encrypt as "KUWWX ZXPWQ." You will then decrypt a given ciphertext "GXL IXLFQ BV" using your revised decryption program to retrieve the original message.

Part III asks for an explanation of the role of the Python dictionary, dict, in these programs. Your response should describe how the dictionary acts as a crucial data structure that enables direct and efficient mapping between plaintext and ciphertext characters. It allows for quick lookup, reversible transformations, and flexibility in handling different cipher keys based on various alphabet arrangements, highlighting its importance in implementing substitution ciphers.

Throughout the assignment, you are expected to test your code thoroughly, provide correct decryption outputs, and articulate the conceptual role of dictionaries in substitution cipher algorithms. Proper coding practices, such as inverting mappings for decryption and dynamically generating cipher keys, will demonstrate your understanding of fundamental cryptographic principles and Python programming techniques.

References

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