"Mira, can you read this cipher?"
Yuki showed Mira a mysterious string of characters written in the notebook.
Mira gazed quietly and nodded slightly. Then she wrote something on another paper and handed it over.
"'Tomorrow, at the library'... How did you decode it?"
Mira put a finger to her lips. A gesture saying it's a secret.
Aoi approached. "Practicing encrypted communication?"
"Only Mira could decode it," Yuki explained.
"There's a concept called perfect secrecy," Aoi said with interest. "A state where the ciphertext reveals no information about the plaintext."
"Nothing at all?"
"Yes. Shannon proved it. The only way to achieve perfect secrecy is the one-time pad."
Mira wrote in her notebook. "Key length ≥ Message length"
"As Mira says. The key must be at least as long as the message, random, and used only once. These are the conditions."
Yuki was surprised. "So Mira and I share a secret key of the same length?"
Mira smiled and nodded. When did she share the key?
Aoi continued explaining. "H(M|C) = H(M). Even observing the ciphertext C, the plaintext M's entropy doesn't decrease. You learn nothing."
"But," Yuki thought, "how do you securely share the key?"
"The key distribution problem. This is modern cryptography's biggest challenge."
Mira wrote another note. "Quantum key distribution: solve key problem"
"Quantum key distribution?" Yuki tilted her head.
"A method using quantum mechanics properties to securely share keys. But that's another story," Aoi said.
Yuki looked at Mira. "How did you give me the key?"
Mira didn't answer. She just pointed out the window.
"That time!" Yuki remembered. "When we watched stars together, Mira murmured some number sequence."
Mira nodded.
Aoi was impressed. "You shared the key visually or audibly. A method resistant to eavesdropping."
"But someone might have heard."
"In that case," Aoi explained, "perfect secrecy isn't guaranteed. One-time pad security depends on key secrecy."
Mira wrote. "Trust = secure channel"
"Trust creates a secure communication channel," Aoi translated. "The intersection of information theory and human relationships."
Yuki stared at the notebook. "Between Mira and me, there's a secret channel."
"Yes. A key only you two share, an encoding method only you two know. That's a secret channel."
Mira stood up and handed Yuki a new note.
"New key for tomorrow. Memorize and destroy."
Yuki memorized the number sequence and tore the paper into small pieces.
Aoi said, "The cost of perfect secrecy is key management effort. But some communications are worth it."
"The communication between Mira and me has value," Yuki smiled.
Mira nodded slightly.
"Information theory is also the mathematics of keeping secrets," Aoi said quietly. "But the strongest encryption might be trust."
Yuki and Mira made eye contact. Even without words, the mutual information was high.
Aoi continued, "In practice, most secure systems combine cryptography with human trust. Perfect secrecy is theoretically beautiful, but practically demanding. That's why modern systems use computational security instead."
"Computational security?" Yuki asked.
"Making breaking the code so difficult it would take millions of years with current computers. Not perfect, but good enough."
In the club room, there was a silence only the two could decode.
It was a cipher of the heart, stronger than perfect secrecy.