"Can you read this?"
Mira held out a paper. Meaningless character strings lined up.
"Is it code?" Yuki was intrigued.
"Encrypted message," Mira nodded.
Riku peered over. "Wow, totally don't understand. What's 'KHOOR ZRUOG'?"
Aoi smiled. "Classical cipher. Might be Caesar cipher."
"Caesar cipher?"
"Shift each letter by a fixed number in alphabetical order. For example, shifting 3 letters makes A become D."
Yuki tried. "If I shift K in KHOOR back 3 letters... H?"
"Right. It's H. Continue."
"HELLO...?"
"Correct! 'KHOOR ZRUOG' is 'HELLO WORLD'."
Riku was impressed. "So that's how ciphers work!"
Aoi drew a diagram on the whiteboard. "Encryption is technology to hide information. Convert plaintext to ciphertext."
"But if you know the rule, it's easy to solve," Yuki pointed out.
"That's where the concept of keys becomes important. Encryption and decryption require keys."
Mira showed a new note. "Public key, Private key"
"Public key cryptography. Sender encrypts with public key. Only receiver with private key can decrypt."
Riku was confused. "Even though it's public, it's safe?"
"Mathematically, inferring private key from public key is extremely difficult. Based on computational complexity theory like prime factorization."
"Sounds difficult..."
Aoi explained simply. "For example, 15 is easily factored as 3×5. But with huge numbers, factorization takes years."
"So that maintains security," Yuki understood.
"Right. At the intersection of information theory and computational theory lies cryptography."
Mira showed an even more complex cipher. This time mixed numbers and letters.
"What's this?" Riku felt like challenging it.
"Let me give a hint. Think information theoretically. What letters are commonly used?"
"In English, E?" Yuki answered.
"Accurate. There's a technique called frequency analysis. The most common character in ciphertext might be E in plaintext."
Riku counted. "Most common in this cipher is '7'."
"Then let's assume 7 is E."
The three cooperated, gradually solving the cipher. Frequency, patterns, context. Using information theory principles.
"Solved!" Yuki exclaimed.
"What did it say?"
"'Information is power'"
Mira smiled quietly.
Aoi continued explaining. "Ciphers maintain information confidentiality. But perfect ciphers don't exist. Only one-time pad proven by Shannon is theoretically unbreakable."
"One-time pad?"
"Key is same length as plaintext, random, and used only once. Meeting these conditions makes it information-theoretically secure."
Riku thought. "But how do you safely send the key?"
"That's the key distribution problem. Quantum communication and public key cryptography are solutions."
Mira showed a final message. This time a QR code.
"Is this also a cipher?" Yuki asked.
"In a sense, a type of encoding. Converting information into two-dimensional pattern."
Riku scanned with his phone. "'Thank you for learning' came up!"
"Hiding, protecting, and conveying information. That's the world of cryptography and encoding," Aoi said.
Yuki was impressed. "Messaging apps we use daily are also encrypted, right?"
"Yes. Modern society can't function without cryptography. Credit cards, passwords, communications. All protected by encryption."
Mira stood up. "Secrets everywhere. Information is valuable."
"Protecting secrets is also an important role of information theory."
Riku laughed. "Then should we encrypt today's club activities?"
"That's not necessary," Aoi laughed.
But the three certainly learned the value of information and importance of secrets.