Ch.82Looking Back to Speak (2)

The three of them sat around a circular table. Kain pointed to a disc with six concentric circles drawn on it like a target.

“Encryption is ultimately about replacing letters with different symbols. Sometimes you can use musical notes, or there might be codes that can only be solved in very specific ways.

Ancient barbarian tribes used leather straps with letters written on them, which would only reveal the message when wrapped diagonally around a stick of an agreed-upon thickness.

But the most famous is probably the Caesar cipher.”

Kain pointed to the outermost circle and the next one. The letters were arranged in alphabetical order.

A B C D E (…)

A B C D E (…)

“The Caesar cipher works by slightly ‘rotating’ this disc. If we shift by one letter, it would look like this:

A B C D E (…)

B C D E F (…)

“Like this. A becomes B, and C becomes D. Do you understand?”

Maria scratched her chin.

“Even neighborhood kids do this. You call this encryption?”

Lily took over from Kain.

“It’s fundamental and basic, that’s why. The Caesar cipher can be quickly solved with just this disc. Since all letters shift by the same ‘distance,’ you can simply rotate the disc one position at a time until you get a sentence that makes sense.”

Maria looked at Lily with apparent surprise.

“You know about this too?”

“Ah, yes. It’s one of the first things Security Bureau agents learn. But cryptography requires experience and skill, so we have a dedicated department for it.”

“Because the amount of encrypted messages coming in from across the Empire is substantial,” Kain chimed in.

“I suppose so. The amount of text to read must be enormous. But didn’t you say our cipher is Vigenère or something? How is that different from this?”

“It’s a kind of advanced version. The problem with the Caesar cipher is that it’s too easy to decode.

As Lily mentioned, you can just take a short sentence, place it next to the disc, rotate it, and the pattern is quickly broken. That would allow foreign intelligence services to read documents too easily.

But what if, instead of applying the same shift to the entire document, you rotate the disc differently for each letter?”

Maria blinked at this sudden leap.

“What do you mean? Explain it more clearly.”

“The biggest problem with the Caesar cipher is that the entire document is transformed using a single rule. If we shift by one position, A becomes B, and B becomes C.

But what if we have a sentence like ABCDE, and we rotate the disc by one position for the first letter, two positions for the second, four for the third, six for the fourth, and so on?”

Maria tried rotating the disc one by one.

“Let me see. So A shifts by one position to become B, B shifts by two positions to become D, and C… What is this? Isn’t this madness?”

“This is the Vigenère method. Most countries use it because the transformation itself is simple. Not just the Empire, but also the East, South, West, and even the Northern pagans.”

Maria stared intently at the complex letter.

“You said William’s diary was written this way too? He’s not just any madman. Not only did he paste the hair of women he… you know… seduced, but he also wrote the contents in such a complicated cipher?”

“Well, the world is wide and full of strange people,” Kain shook his head.

“But how do you solve this?”

“As you can see, it’s extremely difficult to decode if you don’t know the pattern. But at least the sender and receiver need to know what the rule is.

The problem is this: how does the sender tell the receiver, ‘I’m sending you an encrypted letter, and here’s the rule’? It’s not like saying, ‘The key is under the flowerpot by the front door.'”

“Ha, they would make an arrangement in advance.”

Kain nodded. Maria seemed slightly proud of herself.

“That’s right. But it’s not like saying, ‘Rotate the first letter three times, the second letter twelve times!’ They use a much simpler method.”

“What kind of method?”

“By designating specific letters. We call this the cipher key or keyword. Simple things like ‘Apple’ or ‘King.’ What this means is… let’s say the keyword is KING.”

Kain realigned all six discs in alphabetical order.

A B C D E F (…)

A B C D E F (…)

A B C D E F (…)

A B C D E F (…)

A B C D E F (…)

A B C D E F (…)

“Here, the top row is the original letter. The second row of characters is rotated so that A becomes K. Then it would change like this.”

A B C D E F (…)

– – – – – – – – – – – –

K L M N O P (…)

A B C D E F (…)

A B C D E F (…)

A B C D E F (…)

A B C D E F (…)

“The rest follows the same pattern. The third row of characters is rotated so A becomes I, the fourth so A becomes N, and the fifth so A becomes G.”

A B C D E F (…)

– – – – – – – – – – – –

K L M N O P (…)

I J K L M N (…)

N O P Q R S (…)

G H I J K L (…)

A B C D E F (…)

Kain covered the last character row with his finger. The disc now looked like this:

A B C D E F (…)

– – – – – – – – – – – –

K L M N O P (…)

I J K L M N (…)

N O P Q R S (…)

G H I J K L (…)

“This is how you transform it. The first letter is changed using the row where A becomes K, the second letter uses the next row where A becomes I, the third uses the N row, and the fourth letter uses the G row.”

“What about the fifth letter and beyond?”

“Then you start again from K. You keep changing like this. For example, if you have a sentence ABCDE, it would change like this:

A -> K L M N O

B -> I J K L M

C -> N O P Q R

D -> G H I J K

E -> K L M N O

It becomes KJPJO, an unintelligible sentence.”

Maria took a deep breath. Her eyes seemed to be spinning.

“Why do people do these crazy things? My head would explode trying to decode secrets.”

Lily stuck out the tip of her tongue and smiled sheepishly.

“That’s why the cryptography department requires skill and experience. Those who are familiar with it can do it very quickly, but it takes a very long time to become familiar.”

“No, I think it’s just an endurance test. When I get back to Magdeburg, I should suggest adding this to the Inquisitor training program. Anyone with a short temper would run away.”

Maria still seemed bewildered. Kain smiled as he arranged the disc.

“There’s actually a reason for doing this. I said the Caesar cipher is easy to decode, right? I also mentioned that one decoding method is to just keep rotating the disc until something makes sense. But there’s an even easier way to break it.”

“What is it?”

“Some letters in the alphabet appear more frequently than others. Especially vowels. E appears most frequently, followed by A. So you can examine which letter appears most often in the entire ciphertext, and then assume it’s E.”

“But there would still be many undecoded parts?”

“The next thing to look for are words. Common words that inevitably appear in any lengthy text. Words like A or The, We or They, You. If something at the beginning of a sentence is decoded as ??E, wouldn’t that already help you identify T and H? This is precisely the problem with the Caesar cipher.”

“What about Vigenère?”

“Remember how ABCDE was transformed into KJPJO? Notice that the letter J appears twice already. You can’t solve it using that method. In short, it’s for security.

That’s why the Vigenère method is more difficult. Even the same word can be transformed completely differently. If you transform ABCDE ABCDE with the four-letter keyword ‘king,’ you get KJPJO IOINM.”

“Phew. Damn. It’s like a complete word puzzle. The kind my uncle used to enjoy,” Maria stuck out her tongue.

“We were able to solve it relatively easily because Father Haspel’s note told us it was the ‘Vigenère’ method. If we hadn’t known what kind of cipher it was, it would have taken much longer.”

When Haspel was mentioned, Maria’s expression became somewhat gloomy. But she soon bit her lip and clapped her hands softly.

“Alright. So, what do we do now? I hope the end of this long explanation isn’t ‘this cipher can’t be solved.’ That would be really disappointing. As you know, we only have half of this encrypted letter. The part with the translation has no original text, and the part with the original text has no translation.”

“I was worried about that too,” Lily asked with a concerned voice. “If we can’t compare them, is there another way to verify?”

“Of course there is. That’s actually why I mentioned ‘the’ earlier. The word ‘the’ is a common and essential word that appears in any text, even if it’s just half a page long. But there are also words that are the opposite—rare words that don’t appear in ordinary sentences, which makes them identifiable.”

“Are there such words?”

“People’s names.”

“Ah!” Lily covered her mouth in surprise.

Kain pointed to the first line of the translated portion.

“Take this letter for example. ‘Arianne. Only now do I say this…’ The proper noun here is Arianne. So we…”

Kain pointed to the remaining encrypted part of the letter.

“First, let’s find seven-letter words in this letter. Fortunately, William didn’t ‘join all the words together.’ A truly proper cipher would eliminate spaces or include spaces in the encryption table, but this one doesn’t go that far.

Of course, not all seven-letter words will be ‘Arianne.’ But let’s assume they are and approach it that way. If we can track how the seven letters of ‘Arianne’ were transformed, we can also figure out the keyword.”

“Wait a minute. This is a letter. Most letters typically end with the sender’s name… Oh. The ending is written in plain text. ‘In the name of true love and friendship, your friend William.'”

Maria couldn’t hide her disappointment. But Kain was quite impressed.

“You’re incredibly fast.”

“Can’t be an investigator if you can’t read between the lines. So, are the three of us going to play with this one disc?”

“Of course not.”

Kain gently removed the disc. The six discs easily came apart. There might have been a cracking sound like wood breaking, but he didn’t seem to mind.

“Let’s divide them, two each. Seven-letter words… there are three of them.”

The three of them each took paper, ink, and quill pens. They wrote down the seven-letter words and worked backward through the discs to trace the keyword.

Maria wondered why Kain had devoted half a day to this seemingly simple task, and in less than 5 minutes, she understood why.

If the word “Arianne” had appeared at the very beginning, they could have worked down from the top row of the conversion table. But the seven-letter words they were looking for now appear in the middle to latter part of the letter.

This means the sequence might not be KING but INGK or NGKI or GKIN. Worse yet, the keyword might not be four letters. It could be three or four, or five or nine.

One hour. Two hours.

‘Ugh, this is so tedious!’

Maria, who had been grumbling, reached her limit. She was thinking of tearing up the paper once she finished it and going out for a drink. But at the last moment, Maria suddenly stood up without realizing it.

“I found it.”

Her hands were trembling. “That’s amazing!” Lily hugged Maria tightly and jumped up and down. Kain looked down at Maria’s scribbled letters in disbelief.

“H-how did you find it?”

Maria was breathing heavily, as if she had been running around, and needed a moment to catch her breath.

“Well, you know… the name Arianne has repeated letters A and N. So I suddenly thought, if the keyword is three letters long, then A would appear as the same letter because they’re three letters apart. Like Ari / ann / e.

But that wasn’t the case. So the keyword isn’t three letters. Then is it four? I looked at how A was transformed, and how the adjacent n’s were transformed, and tried to piece it together… and found it pretty quickly.”

“Would you like to join the cryptography department!”

“I’d rather burn down the Security Bureau!”

Kain looked at the letters Maria had scribbled. She had drawn a large circle around what must be the keyword, as if she wanted to emphasize it. It was four letters.

HOPE.