APSIL -Chapter 111 The Method of Encoding

Deep in the night, the Shuntian Prefecture Office in the northern inner city of the capital was brightly lit.

At the same time, the Ministry of War’s offices in Qianbu Corridor on the southern side of the inner city were also illuminated.

However, in the southern Ministry of War, when they learned who was burning the midnight oil at the Shuntian Prefecture Office, Minister of War Lu Wan, for the first time in many years, lost his composure and dropped the brush in his hand.

Then, a trusted clerk who had followed him for more than a decade and had watched him rise from a minor official to Minister of War tried to offer a few words of comfort.

The result?

He was immediately kicked out of the room.

After that, no one else dared to speak.

“He, Wang Datou, can invite any stray cat or dog into his Shuntian Prefecture Office to help him. But my Ministry of War is an important government office. It does not allow outsiders to casually enter!”

Although Lu Wan spoke these righteous words, his heart was burning with frustration.

Earlier, after Zhang Shou pointed out the key clue at Zhigong Tower, Lu Wan had not failed to consider bringing his son Lu Sanlang over to help decode the letters.

But the Ministry of War was a critical government department. Even as Minister, he could not simply bring his own son inside whenever he wished.

And now?

Because he hesitated and failed to order his family to confine Lu Sanlang at home, Wang Datou had actually intercepted him and taken the opportunity first!

That unfilial son had even happily gone to help Wang Datou!

Lu Wan took a deep breath and spoke in a cold yet stern voice:

“Doctor Zhang has already revealed the key to deciphering these secret letters. All of the most skilled clerks in the Ministry of War who are proficient in numbers and calculations have gathered here.”

“If you can decode them before dawn, everyone can atone for their mistakes through merit.”

“If you cannot…”

“I will naturally have to report my failure to His Majesty, but every one of you will share the blame!”

“This time, the Ministry of War has truly lost face!”

The Ministry of War needed to decipher thirteen secret letters at once.

However, the Shuntian Prefecture Office had Prefect Wang Datou, whose mathematical skills were extremely impressive.

Therefore, Zhang Shou and his three students only needed to solve one remaining letter.

Yet this one letter had already been examined countless times by Qi Liang and Lu Sanlang, who arrived earlier.

They turned it over.

They studied it repeatedly.

They analyzed it again and again.

But they remained completely lost.

After Zhang Shou took over, however, he first examined the twelve letters that had supposedly already been decoded.

Only after that did he look at the final undeciphered letter.

Then he fell into deep thought.

Perhaps because the secret letters needed to pass inspections, or perhaps because the sender wanted to avoid suspicion, the contents of the twelve decoded letters were all ordinary family correspondence.

The encryption method was simple:

The presence or absence of horizontal strokes between characters represented one and zero.

Punctuation marks were used as separators.

This created binary numbers.

Then, using the Thousand Character Classic as the codebook, the letters could be easily decoded.

But this final letter was different.

It was only a single sheet of paper.

A few dozen characters.

Yet the words were completely disconnected, unable to form proper sentences.

The handwriting looked like childish scribbles.

At the end, there was even a line written in elegant handwriting:

“The child copied characters from the Thousand Character Classic randomly while practicing writing. Although somewhat childish, it looks interesting, so I am sending it to my husband in the army for him to see.”

Seeing the three words “Thousand Character Classic,” Zhang Shou immediately smiled.

He suspected this was another letter using the Thousand Character Classic as the cipher book.

Only the encoding method was different from the previous twelve.

Although the letter used many chaotic characters, if the method followed the same logic as the previous letters—which appeared complicated but were actually simple—then this secret message should not be too difficult.

After all…

Even though the encryption was simple, the exchange of these secret letters had already caused a huge mistake.

The receiving party had actually calculated the wrong date.

It was probably the most ridiculous and tragic incident.

No wonder both the sender and receiver ended up committing suicide.

If the encryption was simple, then a substitution cipher was the most likely possibility.

More specifically—

A shift cipher.

That meant replacing each original character with another character located at a certain interval.

According to the logic of simple ciphers, the interval should follow an obvious pattern.

Replacing every character by the same fixed distance would be the easiest method.

But it would also be the easiest to crack.

Therefore, the shifting formula could perhaps be:

y = ax + b

If it became more complicated…

Perhaps a quadratic equation.

But since the shifting distance must be an integer, and x must also be an integer, it actually was not that difficult.

He would start by testing the simplest possibility.

According to the logic of a simple cipher:

The first character in the secret letter corresponded to the original character shifted by:

A+b positions.

The second character:

2a+b positions.

The third: 3a+b and so on.

The letter only contained several dozen characters.

The amount of calculation was manageable.

However, because the Thousand Character Classic only contained one thousand characters, he also had to consider the possibility of looping.

For example, shifting the nine-hundredth character forward by two or three hundred places would return to the beginning.

At present, he only needed to test the first four numbers.

The calculation workload was completely acceptable.

Thinking this way, Zhang Shou casually began writing calculations on paper.

He did not notice that Shuntian Prefect Wang Jie had quietly walked behind him.

He himself did not know how much time had passed.

Eventually, he calculated the simplest encryption equation:

y = 2x + 3

Then he worked backward to determine the first four characters of the original text.

And he was immediately speechless.

Because once again…

It was Kong University Scholar.

It seems Kong University Scholar really is the unluckiest person this time…

The shift variable:

x = 1, 2, 3…

The encrypted results:

y = 5, 7, 9…

Meaning:

Take each character in the secret letter.

Subtract 5, 7, 9 respectively.

Then find the corresponding character in the Thousand Character Classic.

Following the decoded numbers, Zhang Shou compared them with the Thousand Character Classic index table copied personally by Prefect Wang.

Then he wrote a line of text:

“On the day Kong University Scholar passed through Linhai Camp, commander Du Heng ordered the complete deployment of rebel troops to surround Kong. If the capital becomes fearful, then punish the unjust and overthrow the current order, changing heaven and earth.”

“Absurd!”

The moment Zhang Shou finished writing all the characters, an angry shout came from behind.

He turned his head and saw Shuntian Prefect Wang Jie standing there, furious.

Seeing Lu Sanlang, who had been struggling with calculations nearby, suddenly jump up and rush over, Zhang Shou raised a hand to stop him.

He picked up the paper and said to Wang Jie: “Although the characters have now been connected into sentences, does Prefect Wang not find something strange?”

After his anger faded, Wang Jie immediately calmed down.

He took the paper from Zhang Shou and carefully examined it for a long time.

Then he said solemnly: “Could it be…”

“The person who wrote this letter was clearly a traitor, yet he called himself part of the rebel army?”

“That is one point.”

Zhang Shou nodded.

“I also think that if they were true rebels, they might actually call themselves a righteous army instead.”

Wang Jie’s eyebrows moved slightly.

“Oh? You mean there is another point?”

“This is only my intuition.”

Zhang Shou smiled lightly.

“The twelve other letters all used the same cipher method.”

“Yet this one letter suddenly uses another method.”

“And it deliberately adds an explanation saying that a child copied the Thousand Character Classic.”

“But think about it—what child copying the Thousand Character Classic would randomly write one character here and another character there instead of copying complete sentences?”

Lu Sanlang, who had quietly approached, immediately understood.

“Exactly!”

“This behavior is practically telling people directly that there is something wrong with this letter!”

On the other side, Qi Liang exchanged a glance with Deng Xiaodai.

Qi Liang also thoughtfully said: “I also felt that the writing on this letter, which appears like a child’s scribbles, was too deliberate.”

“It feels like an adult imitating a child’s handwriting.”

Deng Xiaodai suddenly had a flash of inspiration.

“Could it be that the person who wrote this letter never understood the secret behind the previous twelve letters?”

“But they knew the cipher reference was the Thousand Character Classic, so they desperately created another encryption method and wrote this letter?”

Hearing everyone’s analysis, Wang Jie narrowed his eyes.

His heart became heavy.

“If I only report this decoded letter to His Majesty, then commander Du Heng of Linhai Camp will become the primary traitor.”

“But based only on these suspicions you mentioned, it is still insufficient to clear his name.”

“But it is also insufficient to prove his guilt.”

Zhang Shou finally yawned.

Then he lazily said: “Perhaps the people who supposedly committed suicide on both sides were merely scapegoats.”

“Perhaps someone wanted to remove that troublesome commander and replace him.”

“Perhaps…”

“Anything is possible.”

“But in my opinion, the biggest suspicious point is that the encryption method suddenly changed.”

“This letter was not the final letter chronologically. There is no reason that later communications would abandon the original encryption method.”

“In short, this is only my analysis from the perspective of mathematics. Whether it is right or wrong depends on Prefect Wang’s judgment.”

“Excellent.”

Wang Jie’s stern face finally revealed a faint smile.

“Very excellent.”

“At least I did not waste my efforts helping Duke Zhao Manor deal with those two traitors. Otherwise, the Old Madam might not have agreed to let you come here.”

He looked at Zhang Shou.

“Doctor Zhang, you have worked hard tonight.”

“But the night is already deep.”

“I have prepared guest rooms in the Shuntian Prefecture Office. Hot water and supper have also been arranged.”

“Please do not misunderstand this as confinement.”

“This matter is extremely important. When I report the details at tomorrow’s court session, I will not forget the contributions of all of you.”