Question

Write a function that accepts a stream of ASCII characters and encrypts this input using a substitution cipher with the Cipher Block Chaining mode. The block size should be 8 bytes. The program should take plaintext from the standard input and print the ciphertext on the standard output. For this problem, you are allowed to select any reasonable system to determine that the end of the input is reached, and/or when padding should be applied to complete the block. You may select any output format, as long as it is unambiguous. The program should receive two parameters: 1\. A pointer to the initializing vector; and 2\. A number, \(k\), representing the substitution cipher shift, such that each ASCII character would be encrypted by the \(k\) th character ahead of it in the alphabet. For example, if \(x=3\), then " \(\mathrm{A} "\) is encoded by " \(\mathrm{D} ", " \mathrm{~B} "\) is encoded by "E" etc. Make reasonable assumptions with respect to reaching the last character in the ASCII set. Make sure to document clearly in your code any assumptions you make about the input and encryption algorithm.

Short answer

Encrypt input in 8-byte blocks using XOR and a substitution cipher.

Step-by-step solution

Define the Substitution Cipher Function

Create a function that accepts a character and a shift value, \( k \). This function will find the ASCII value of the input character, add \( k \) to it, and then return the character corresponding to the new ASCII value. If the value surpasses the ASCII limit, wrap it around to stay within the valid range of printable ASCII characters.

Initialize Cipher Block Chaining (CBC) Requirements

Define the block size as 8 bytes (characters). Use the initializing vector (IV) as the starting block of text. Assume that the IV is already of size 8 bytes and contains characters from the ASCII set. This vector will XOR with the plaintext to start the encryption process.

Define the Padding Method

Ensure that the input stream is a multiple of 8 bytes by adding padding. Choose a standard padding scheme, such as PKCS#5, where padding bytes are added to the end of the input to meet the block size requirement.

Process the Plaintext Input

Read the plaintext stream in blocks of 8 characters. For each block, perform an XOR operation with the current block of ciphertext (starting with the IV for the first block). This is the Cipher Block Chaining step that ensures encryption is applied in a sequence based on previous data states.

Encrypt Each Block

For each resulting block from Step 4, apply the substitution cipher function defined in Step 1 to create the final encrypted block. This block is transformed character by character by shifting each character using the provided substitution cipher method.

Output the Ciphertext

Continue reading, processing, and encrypting all blocks until reaching the end of the input stream. Print each encrypted block of text as it is processed. Choose a clear output format, such as hexadecimal, to represent the encrypted bytes unambiguously.

Key concepts

Cipher Block Chaining

Cipher Block Chaining (CBC) is a method used in cryptography to ensure a higher level of security during the encryption of data. This method involves processing data in blocks, typically fixed sizes like 8 bytes, as mentioned in our exercise. Rather than encrypting each block independently, CBC uses the previous block's output to influence the encryption of the current block. This chaining effect greatly enhances security by ensuring that identical plaintext blocks will produce different ciphertexts when they occur at different places in the data stream.

In the CBC mode, an Initializing Vector (IV) is typically used at the beginning of the process. This vector is combined with the initial block of plaintext data to start the encryption. By performing an XOR operation between the IV and the plaintext, a level of randomness is introduced in the process, making it more robust against certain types of attacks. The resulting ciphertext from one block feeds into the next block, substituting the IV for subsequent rounds. This creates a chain of dependency across all blocks.

• Encrypts data in block sequences
• Utilizes an Initializing Vector (IV) for the first block
• Ensures security by chaining blocks together
• Uses XOR operation between blocks

ASCII characters

ASCII stands for American Standard Code for Information Interchange, and it's a character encoding standard used extensively in computers and digital systems. It represents text in computers, telecommunications equipment, and other devices that use text. Each letter, number, and symbol in the ASCII set, which consists of 128 characters, can be represented by an 8-bit binary number or, in simpler terms, a byte.

In the context of a substitution cipher, each character from the plaintext is substituted with another character based on a shift value, denoted as "k". For example, if the letter “A” (represented by the ASCII value 65) is shifted by 3 positions, it becomes "D" (which has the ASCII value 68). This transformation involves wrapping around the end of the ASCII set to avoid surpassing 127, ensuring all characters remain valid ASCII characters.

• Standard encoding with 128 characters
• Represents text with 8-bit values
• Basis for substitution ciphers by shifting characters
• Ensures shifts remain within the ASCII range

encryption algorithm

Encryption algorithms are procedures used to encode data, ensuring its confidentiality and protection from unauthorized access. An encryption algorithm transforms readable information into an unreadable format, often referred to as ciphertext. The substitution cipher discussed here is a simple encryption algorithm where each character in the plaintext is shifted by a certain number of positions down the ASCII character set. The shift value, denoted by "k", is crucial, as it dictates the pattern of transformation.

To implement a secure encryption process, it is paired with the Cipher Block Chaining (CBC) mode. This combination ensures that altering a single character will affect subsequent blocks due to the chaining nature of CBC. When encrypting, consistency in applying the algorithm across all data is key, as is choosing an output format that properly conveys the encrypted data. In many cases, hexadecimal representation is preferred to maintain clarity and avoid ambiguity.

• Ensures data confidentiality via transformation
• Utilizes shift-based substitution mechanism
• Combines with CBC for improved security
• Produces clear, unambiguous output formats