Lecture 03: Cryptanalysis on Substitution Cipher (Frequency Analysis ).

Introduction

In this tutorial, we will explore the process of cryptanalysis specifically focused on substitution ciphers through frequency analysis. This technique is essential in understanding how to break ciphers by analyzing the frequency of letters and patterns in the ciphertext. Mastering this skill can enhance your knowledge of cryptography and its applications in security.

Step 1: Understand Substitution Ciphers

• A substitution cipher replaces each letter in the plaintext with a letter from a fixed set of substitutions.
• Familiarize yourself with the difference between monoalphabetic and polygraphic substitution ciphers.
• Monoalphabetic substitution is where a single alphabet is used throughout, while polygraphic involves multiple alphabets.

Step 2: Gather Frequency Data

• Collect a sample of the ciphertext you wish to analyze.

Count the frequency of each letter in the ciphertext. You can do this by

• Manual counting for small samples.
• Using a frequency analysis tool or writing a simple program to automate the counting.

Step 3: Analyze Letter Frequencies

Compare the frequencies you've collected with the typical frequency of letters in the English language

• The most common letters in English are E, T, A, O, I, N.
• Less common letters include Q, J, Z, X.
• Create a frequency table to visualize and compare the data.

Step 4: Identify Patterns and Common Words

• Look for common short words in the ciphertext, such as "the," "and," "is," or "to."
• Identify patterns in the letter frequencies that match common letter combinations in English, like "TH," "HE," "IN," and "ER."
• Use these patterns to hypothesize potential substitutions.

Step 5: Create a Substitution Key

• Start building a substitution key based on your frequency analysis.
  • Replace the most frequent letters in the ciphertext with the most common letters in the English language.
  • Adjust your key as more patterns and substitutions are identified.

Step 6: Decode the Ciphertext

• Use your substitution key to decode the ciphertext.
• Continuously check for errors or inconsistencies.
• Refine your key as needed. If something does not make sense, revisit your analysis and adjust accordingly.

Step 7: Validate and Cross-Check

• Once you believe you have the correct plaintext, cross-check the decoded message for logical consistency.
• Ensure that the decoded message makes sense contextually and grammatically.

Conclusion

Through the steps outlined above, you can effectively use frequency analysis to break substitution ciphers. By understanding the structure of substitution ciphers and analyzing letter frequencies, you can decode messages that seem secure at first glance. As a next step, consider practicing with different ciphers and analyzing more complex encryption techniques to further enhance your cryptanalysis skills.