Basics of Cryptography: Plain Text and Cipher Text

What is Cryptography? (In Simple Words)

The word Cryptography comes from two Greek words:

• Kryptos = “Hidden” or “Secret”
• Graphein = “To Write”

So, Cryptography literally means “Secret Writing”. It is the science and art of converting a normal, readable message into a secret, unreadable code so that only the intended person can read it.

💡 Real-Life Example:
When you send a message on WhatsApp, you see the notice: “Messages and calls are end-to-end encrypted.” This means if you type "Meet me at 5 PM", WhatsApp uses Cryptography to scramble it into "X9#kP!2m@Q" before sending it over the internet. Even if a hacker or telecom company intercepts it, they cannot read it!

1. Fundamental Terms in Cryptography

To master Unit 1 and Unit 2, you must know these 6 essential terms:

1. Plain Text (or Clear Text)

• The original, normal, human-readable message before it is locked or after it is unlocked.
• Anyone who sees Plain Text can easily understand it.
• Example: "TRANSFER 5000 RUPEES" or "MEET AT COLLEGE"

2. Cipher Text

• The scrambled, unreadable, secret message produced as the output of encryption.
• It looks like random garbage characters to anyone who does not have the secret key.
• Example: "WUDQVIHU 5000 UXSHHV"

3. Cipher (Cryptographic Algorithm)

• The mathematical formula, rule, or step-by-step algorithm used to transform Plain Text into Cipher Text and vice versa.
• Examples: Caesar Cipher, Playfair Cipher, DES, AES, RSA.

4. Key (K)

• A secret value (a number, word, or string of bits) that works together with the Cipher algorithm to lock and unlock the message.
• Think of the Algorithm as a standard lock design, and the Key as the unique metal key that opens only your specific lock.

5. Encryption (Enciphering)

• The process of converting Plain Text into Cipher Text using an algorithm and a key:

  Formula: C = EK(P)

6. Decryption (Deciphering)

• The reverse process of converting Cipher Text back into the original Plain Text using a decryption algorithm and key:

  Formula: P = DK(C)

2. Difference Between Plain Text and Cipher Text

This is one of the most frequently asked short-answer questions in DAVV BCA exams:

3. Cryptography vs. Cryptanalysis vs. Cryptology

Students often confuse these three terms. Let’s make them crystal clear:

• Cryptography (Code Making): Designing secure encryption systems to hide messages from attackers (Done by security defenders).
• Cryptanalysis (Code Breaking): The science and art of breaking Cipher Text to recover the Plain Text or Key without knowing the secret key beforehand (Done by ethical researchers or hackers).
• Cryptology: The parent branch of mathematics that includes both Cryptography and Cryptanalysis.

4. Classification of Classical Cryptographic Techniques

In Unit 1, classical encryption techniques are divided into two main families:

1. Substitution Techniques:
   The letters of Plain Text are replaced by other letters, numbers, or symbols.
   (Examples: Caesar Cipher, Mono-alphabetic Cipher, Playfair Cipher, Hill Cipher, Polyalphabetic Cipher).
2. Transposition Techniques:
   The letters of Plain Text are not changed, but their positions/order are rearranged (shuffled like an anagram).
   (Examples: Rail Fence Cipher, Columnar Transposition Cipher).

💡 DAVV Exam Preparation Tips

5-Mark Question: “Differentiate between Plain Text and Cipher Text. Explain the components of a basic Cryptosystem.”

• Draw the 5-step Cryptosystem flow diagram (Plain Text → Encryption → Cipher Text → Decryption → Plain Text).
• Write the 6-point comparison table between Plain Text and Cipher Text.
• Write the mathematical equations C = EK(P) and P = DK(C).