In the night, Angel's voice continued to speak weakly, the more wrong it became.

"Next...... Keep walking and come across a tavern called 'Fire and Water'...... Turn right and ...... again."

Richard listened to Angel's words, suddenly interrupted, and asked seriously, "Do you know where you are now?"

"Huh?" Angel was slightly stunned and looked down at the ground, "On the ground?" looked up at the starry sky: "Under the stars?"

"......" Richard was silent for a while, exhaled helplessly, and said to Angel, "Okay." ”

"Huh?" Angel didn't know.

"I'll take you back to the White Rock Tower Academy. Richard said. In Richard's current opinion, according to the degree of Angel's road blindness, even if she is pointed out a hundred times, she may not be able to reach it smoothly. He still sent the other party back, it was better to get rid of this matter, and the other party had some accidents on the way back, causing unnecessary trouble.

Richard stepped forward, glanced at Angel, who was standing there and hadn't reacted, and said, "What's the matter?

"Uh...... No...... No. Angel hurriedly said.

"Then keep up, keep up. ”

"Well, yes. Angel hurriedly caught up and followed.

Richard and Angel, one after the other, quickly arrived in White Rock City.

Walking through White Rock and walking to the more familiar "Salt and Fire" tavern, Richard found that the tavern was closed. However, there was still a faint singing voice floating out of it, it was a tune he had heard before, and it was somewhat familiar. Maybe the meaning is not understood, but the rhythm of the song is constant:

First there is a 2/4 beat of "strong and weak, strong and weak", then a 3/4 time of "strong and weak", and then a 6/8 time of "strong and weak, weak and weak".

"Click, click......"

Richard listened to the tune as he took Angel around the tavern and turned left and continued on his way......

Eventually, the singing of the "Salt and Fire" tavern died down, and Richard stopped at the entrance to the White Rock Tower Academy. Stretching out his hand to move forward, Richard said to Angel behind him, "You should know how to get here, right?

"Well, no. Angel shook her head.

"Alright, let's go in, I'm leaving. ”

"Forehead......" Angel lowered her head and walked into the academy. Turning around and secretly glancing at Richard, he found that Richard had disappeared into the darkness, biting his lip, and walking into the depths of the academy with a somewhat complicated expression.

......

Laboratory, main laboratory.

Richard returned.

Because of solving all the inconsequential little things, Richard relaxed a little at this time, strode to the experimental table in the room, took out the five newly obtained Black Spirit Empire books in the space iron ring, and began to study.

Placing the five books on the table, Richard quickly flipped to the last page, swept over the numbers on the Black Spirit Empire's emblem, his eyes flashing slightly.

The five numbers are 3, 7, 24, 27, and 29, and the number of palindromes obtained is 33, 55, 66, 99, and 121 according to the current world version of the Lickrel number.

The books that have been owned before are:

"The Soul Wails", palindromic number 11;

Blood and Fire, palindrome 22;

"Towards Nobility", palindrome 77;

My Autobiography – Moratos, palindrome 99.

That is, of the five newly acquired books, only one book is duplicated, and all four are new:

The first new book, "Dark Spirits", ghost legends;

the second new book, "Thirty-six Days", a collection of fables;

The third new book, "The Secret of Coppola", is a relatively simple book of wizarding spells.

The fourth new book, "I Was Born from Fire - Prometheus", is the autobiography of a wizard, similar to "My Autobiography - Moratos", but it does not contain any strange content about training male slaves.

After figuring out the books, Richard's eyes lit up, and I have to say that this night was a big harvest, and the harvest was worth all the previous ones.

It was previously guessed that the number behind the Black Spirit Empire's book logo should be within 40, so the number of palindromes obtained through the Likrel number verification process was only 11.

11 pcs!

Now I have got 8 palindromic books, and only 3 palindromic books have not been obtained.

In this way, even if you can't unlock the secrets hidden in the book, you should be able to see the general framework.

Richard's eyes were sharp.

What is the secret hidden in the book?

He didn't know the secret yet, but he had some guesses about how the mystery was hidden.

The way books are hidden in mysteries should not be hidden texts - special conditions such as water immersion and fire are used to reveal the text. In doing so, first, it could not be preserved for too long, and secondly, it was not related to the number on the symbol of the Black Spirit Empire on the last page of the book, and the palindrome number.

The numbers on the emblem of the Black Spirit Empire, as well as the number of Likrel number verifying palindromes, are some very special numbers, which should not be confusing information, but related to secrets.

So......

When it comes to numbers, Richard can't help but think of a very common lookup table replacement method in cryptography in the early days.

The so-called lookup table substitution method, in simple terms, is the codebook method: to transmit information, not to compose normal words, but to replace them with some seemingly meaningless numbers.

The person who receives the message must have the corresponding codebook. According to the content of the codebook, the numbers can be matched one by one, so as to get the correct information and prevent being intercepted and leaked in the middle.

This method can be considered flexible, be careful, in order to prevent the codebook from falling into the hands of the enemy and causing heavy losses, you can change the codebook periodically. Be more careful, you can use a different book as a new codebook every time you pass a message.

For example, if the sender gives a message of "137, 826", then it may represent the 137th and 826th words in a certain book.

The person who received the information checked the books, and the results obtained were different from the books, which could be "success" or "failure" messages, or "attack", "retreat", "hide" and other instructions, and there was almost no possibility of leakage. Of course, there is still a gap between it and the encryption methods developed based on computer technology on the modern earth, such as DES, 3DES, AES encryption with symmetric encryption, and RSA and ellipse encryption with asymmetric encryption.

Anyway, what are the results of the current book using the lookup table substitution method?

Richard first picked up the book "Soul Wailing" with serial number 1 and palindromic number 11, and found the 11th character in the book, and found that it was the word "gram".

Then pick up the book "Blood and Fire" with serial number 2 and palindromic number 22, and find the 22nd word in the book, which is "fruit".

The third serial number, the palindromic number 33 of "Dark Spirit", the 33rd character, "li".

The book with serial number 4 and palindromic number 44 is missing, omitted.

The 5th character of the "Thirty-six Days" with the 5th serial number and the palindromic number 55 is "ancient".

The 66th character of "The Secret of Coppola" with the number 66 and the palindromic number 66, the partition symbol.

The 7th serial number, the palindromic number 77, "To the Noble", the 77th character, "Du".

The book with serial number 8 and palindromic number 88 is not available, omitted.

Serial number 9, palindromic number 99 "My Autobiography - Moratos", the 99th word, "Er".

Serial number 10, palindromic number 121 "I was born from the fire - Prometheus", the 121st word, "Meng".

Serial number 11, the palindromic number 363 is missing, omitted.

The end result is ......