A Journey Through the Flames of War in the Republican Era

Yu Dawei clearly realized that China, which needed to investigate the secret communications of its great enemy, the Eastern Kingdom, at that time, simply did not have the technical capability to crack the enemy's cipher machine.

Conversely, the other side recklessly infiltrated our territory to gather intelligence, using its embassies and consulates in major and medium-sized cities to send and receive classified messages extremely frequently. Its embassy in Nanjing even secretly established a high-powered radio station to directly communicate with the Tokyo Ministry of Foreign Affairs.

In the recent espionage war between China and Japan, although the Military Commission's coded telegram group intercepted a large number of coded telegrams from the other side, the decryption experts were at a loss. The few breakthroughs they made were only partial cracking of the lowest level diplomatic manual code starting with LA, commonly known as "LA code".

On the contrary, because China's encryption technology was lagging behind, major powers such as the United States, Britain, Japan and the Soviet Union were all intercepting its communications, leaving China virtually unprotected and making it almost "one-way transparent".

Yu Dawei sighed inwardly. The Nationalist government's top brass hadn't even realized that the great powers' coded telegraph technology had advanced to such a high level of automation and mechanization. No, he had to raise sufficient awareness among the higher-ups.

Seeing Yu Dawei's solemn expression, Song Hongfei knew that he had grasped the key point.

Song Hongfei was well aware that many high-ranking officers in the Nationalist Army at the time did not value coded telegram technology and had a poor sense of secrecy, resulting in numerous intelligence leaks and causing heavy losses in the war. He felt he had issued a warning in advance, but he was unsure of its effectiveness.

He sighed inwardly, "I just don't know much about Japanese cipher machines. I only remember that they use more than 50 Japanese Romanized phonetic symbols, which makes them more difficult. Otherwise, I could have used this opportunity to get started, and maybe it would have been a breakthrough."

There was silence.

Both were lost in their own thoughts, unaware that the other was actually lost in their own.

Yu Dawei stared at the blackboard filled with writing and pondered, then said, "We now know that the Japanese cipher machine was also developed based on this rotary principle. In any case, this is a good start for us."

He seemed to remember something and added, "Espionage is a constantly escalating process for both sides. If the Enigma machine is improved, the resulting characteristic values ​​will change. Will this method still be applicable then?"

Song Hongfei said, "This method mainly takes advantage of the flaw of 'using the same key to encrypt three information keys twice.' As long as this point remains unchanged, it will still be applicable. However, such manual work is ultimately a clumsy and inefficient method. To deal with machines, the best way is of course to use machine-to-machine communication."

Yu Dawei asked, "Use a machine?"

Song Hongfei nodded and said, "Now that we have the structural data of the Enigma machine, we can design a machine that can automatically verify all the initial positions of the three rotors. Since we need to test all possible arrangements of the three rotors at the same time, we need to have six machines running in cooperation at the same time. This way, we can test all 6 possibilities. It is also much more efficient."

The method Song Hongfei mentioned is the same one used by Poland to crack the code; this method relies heavily on flaws in its operational procedures.

Historically, the German military implemented numerous measures to enhance the security of the Enigma machine before and after World War II, but the countermeasures were always more sophisticated. The Polish methods only became completely ineffective after the German military stipulated in May 1940 that the information key only needed to be entered once.

Yu Dawei pondered for a moment, then said, "What if the German army also improved this operational flaw?"

Song Hongfei thought to himself, "No wonder he has two doctoral degrees. He really has the spirit of exploration to get to the bottom of things!"

He said, "Then this method fails. But the Enigma machine has a design flaw that the Germans may not have realized yet. Due to the design of the reflectors, the Enigma machine has a characteristic that a letter can never be replaced by itself!"

Yu Dawei exclaimed in surprise, "It can't be replaced by itself? Is this a design flaw?"

Song Hongfei nodded and said, "On the surface, replacing a letter with itself seems to make it appear unencrypted. But in reality, the fact that 'it will never be a certain result' becomes a definite piece of information, which is a serious flaw in cryptography."

"Among the intercepted telegrams, some content is fixed and repetitive, which can be deduced by combining it with other intelligence: for example, the recipient's name, position and rank, or the name of the sending unit, etc."

"For example, the Germans liked to send a weather forecast at 6 a.m., and the messages intercepted at 6 a.m. would begin with the word 'wetter,' and they also liked to end with 'Heil Hitler.' Fixed patterns are a major taboo in cryptography. Furthermore, you can set a trap to lure the other party into sending a telegram containing certain information that you can confirm."

Song Hongfei continued, "Using the plaintext word 'wetter' as an example, move the letters of this word back and forth above the ciphertext you're guessing. If two letters above and below are the same, it's definitely not that position. By eliminating all impossible cases, you're very likely to get the ciphertext that corresponds to the plaintext."

Yu Dawei exclaimed in admiration, "Very ingenious, and makes perfect sense!"

Song Hongfei picked up the chalk and wrote on the blackboard, saying, "Suppose we have the following plaintext-ciphertext correspondence table:"

Plaintext WETTER

Ciphertext E R K M G W

"

He suddenly smiled and said, "If we get enough to form a letter ring, then it will be very easy, just like the previous method."

Song Hongfei added, "Even if we can only deduce a small amount of the plaintext, there are still ways!"

Yu Dawei smiled and said, "Hmm, let me guess, you mean cracking it with a machine?"

Song Hongfei was also somewhat impressed and said, "Commander Yu has truly keen insight! That's right, it's about using machines!"