Chapter 94 Everyone was shocked
Cui Li was originally studying very seriously, but after hearing what Xu Yuan said, he was completely stunned. He stared at Xu Yuan with wide eyes in shock.

I can't understand how Xu Yuan, a beginner who taught himself hash functions, could say this confidently.

The project of cracking MD5 passwords has been started for some time, and some progress has been made thanks to the calculations of several professors.

There are at least hundreds of equations involved in the deduction alone.

In this situation, if you are suddenly told that a method is not suitable, then all previous efforts will be in vain.

If it is said by Professor Biham, an internationally renowned cryptography expert, it may still be authoritative.

But this does not apply to Xu Yuan.

After all, the hash functions involved in cryptography are not number theory.

Not only Cui Li, who is a doctor, but three professors including Wang Xiaoyun all looked a little unnatural, especially Wang Changxin and Ding Jian, who had smiles on their faces just now but now could not see anything.

I originally wanted to help Xu Yuan solve the problem in hash function, but in the end I was given the guidance instead.

Even if Xu Yuan didn't care about ordinary students, as a professor of cryptography, he would inevitably feel embarrassed.

The main thing is that this is equivalent to directly overturning their current progress, and all previous efforts have become useless.

It would be difficult for any professor to accept such a thing.

It is like someone who knows that he has taken the wrong road, but because he has walked too far, he is unwilling to turn back, thus deceiving himself into thinking that he will definitely be able to reach the right road ahead.

In the end, all that's left is regret when faced with the cruel reality.

Because if you choose to turn back halfway, at least you don’t have to waste time trying again later.

Of course, as a professor, he is in front of students, so his qualities must be great.

After listening to Xu Yuan's words, their mood quickly adjusted. Among them, Wang Xiaoyun took over the conversation and asked curiously.

"Student Xu Yuan."

"You are familiar with the collision method used to crack hash function passwords?"

"I wonder what the basis of your conclusion is?"

"Hash functions are more complicated than you think. Compared with brute force enumeration and dictionary methods to try collisions, rainbow tables are the most widely accepted method."

Without waiting for Xu Yuan to answer, Wang Changxin added a sentence in line with Wang Xiaoyun's words.

Obviously he did not agree with what Xu Yuan said.

It took them so long to crack the MD5 algorithm. Who knows how long it would take if they were to overturn it and start over again.

No one really wants to start from scratch.

Xu Yuan was not at all surprised by the reactions of the professors in front of him.

On the contrary, it is considered normal.

It would be surprising if the entire project team overturned their previous approach based on just one sentence from him.

However, since he decided to come in person to persuade Wang Xiaoyun, he must have been fully prepared.

Faced with the doubts, he organized his words a little and began to explain in a deep voice.

"The rainbow table method is indeed balanced in cracking hash function passwords, and the effect is higher than the brute force enumeration method and the dictionary method, but I think it is impossible to crack MD5 with these alone."

"Even if it really works, the time it takes is unacceptable."

At this time, Ding Jian raised his eyes and looked at Xu Yuan, seeming to agree with what he said.

"What you said is indeed correct, but for now, the rainbow table method is indeed the most suitable."

Finally, I couldn't help but reply.

Xu Yuan still maintained his own rhythm, and then once again said something that shocked everyone present.

"The rainbow table collision method was indeed the most balanced before, but it is no longer the case."

"In fact, during the winter vacation at home, I found a more efficient collision method."

“That is, given an original text and adding a specific constant to get a new plaintext, the cryptographic algorithm is attacked by comparing and analyzing the plaintext with specific differences and then propagating the changes after encryption.”

"Although this requires many sufficient conditions, as long as we find the strings that meet these conditions step by step, there will be a high probability of collision."

After hearing Xu Yuan's words, Wang Xiaoyun, Wang Changxin and Ding Jian's faces suddenly changed.

I never expected that Xu Yuan is so familiar with hash functions. He definitely doesn't look like a beginner to be able to talk about these things.

But it would have been fine if Xu Yuan had just pointed out that their collision method was wrong, but now he actually said that he had developed a new method. The confidence he displayed in his every move made him seem like an expert in the field of cryptography.

If they had not taken Xu Yuan's words to heart before, they thought it was just a sudden idea that came to them during the process of learning hash functions.

Well, now we have to take it seriously.

It should be noted that there are precedents for the improved method. The successful proof of the Carmichael number interval problem in the field of number theory was based on the innovative optimization of Chen Jingrun's weighted sieve method.

Just as Xu Yuan said, if a new collision method is designed, it might really improve efficiency.

Just as they were thinking about it, they saw Xu Yuan take out a specially compiled calculation manuscript from his backpack.

He turned around, picked up the eraser, wiped off the contents on the writing board, picked up the pen and started writing quickly on it again.

"M = (m..., m) and M' = (m'..., m') represent two different 512-bit messages."

"M is a 32-bit string." ...

"MD5(m,m)=MD5(m',m')"

At this moment, Wang Xiaoyun and others' attention has been completely attracted. They are staring at the formula written by Xu Yuan, and then they do not forget to pick up Xu Yuan's draft on the table and study it carefully.

"Are these all equations you worked out by hand?"

"Yes."

In response to Wang Xiaoyun's question, Xu Yuan nodded and admitted: "I think computers can't handle the logic between equations very well."

This answer made Wang Xiaoyun's eyes light up, and her impression of Xu Yuan continued to improve.

In contrast, Wang Changxin and Ding Jian were somewhat ashamed, especially when they thought about using computers to automate the derivation of equations.

At this point they naturally did not dare to underestimate Xu Yuan, let alone think that the current method was nonsense.

The main reason is that Xu Yuan's understanding and mastery of hash functions is really amazing.

It is no exaggeration to say that just looking at these contents, even if you tell them that they are internationally renowned cryptography experts, they will not doubt anything.

It is simply unimaginable that Xu Yuan could learn to this level through self-study alone.

In the following time, Wang Xiaoyun and others no longer hesitated and immediately entered into their normal working state.

Calculation is performed based on the method proposed by Xu Yuan.

It has to be said that Professor Wang Xiaoyun is indeed outstanding in the field of cryptography. Not long after, she put down the pen in her hand, and when she looked at Xu Yuan again, her eyes were full of excitement and excitement.

"This is an innovative optimization of the traditional differential attack by constructing a prefix collision attack."

“It’s just so exquisite.”

"Using this method, we only need to find a message pair (x, y) such that MD5(y)=MD5(x), but this series of sufficient conditions is difficult to meet at the same time."

After he finished speaking, Wang Changxin and Ding Jian beside him also put smiles on their faces again, without any doubts from before.

"Differential attacks were proven to be effective against one cycle a few years ago, but they were ineffective against four cycles of MD5. Now Xu Yuan's innovative method may change this situation."

"Xu Yuan's method is indeed more efficient than the rainbow table method. His suggestion just now is correct."

After listening to the other two professors, Wang Xiaoyun seemed to have made a decision in her heart.

He raised his eyes and exchanged glances with the two people beside him, then fixed his gaze on Xu Yuan.

"Xiao Xu."

"You know, you gave us so many surprises today."

"It turns out that inviting you to join this project was the best decision I made."

"The collision method you designed is indeed much better than the rainbow table method, so we decided to abandon the previous progress."

"Use your method to crack the MD5 password again."

Xu Yuan listened to every word Wang Xiaoyun said. When he learned that the other party approved of his method, he finally breathed a sigh of relief, knowing that his trip was not in vain.

He was really worried that the other party would continue to stick to the original method, as the sunk cost would be too high.

"Professor Wang, you are too kind. Since I am a member of the project team, I should contribute to the project."

At this time, Xu Yuan responded modestly, and then planned to leave here and return to Qinghua University.

Anyway, we have successfully convinced Professor Wang Xiaoyun's team, so we can use the new method to try to crack it later. His presence at Qinghua University will not delay the progress of the cracking.

Besides, Qinghua University has already started its term, so it wouldn’t be a good idea for him to stay there.

However, just after he told Professor Wang Xiaoyun about his plan, he was forcibly retained by the other party.

"It's too late today to travel. I'll arrange for someone to take you to the station and back to school tomorrow."

"Listen to me on this matter."

Seeing Wang Xiaoyun's firm tone, Xu Yuan couldn't continue to refuse.

Looking outside through the window, it was indeed getting dark. I didn't expect that I had spent so much time on this matter.

"Okay then, Professor Wang, I'll go back to school tomorrow."

"I'll have someone arrange a place for you to stay right away. We can discuss your new method in detail during the evening self-study." Wang Xiaoyun said this to Xu Yuan happily, and then quickly looked at Cui Li and said, "Xiao Cui, arrange a doctoral student dormitory for Xu Yuan tonight."

Cui Li was so surprised by Xu Yuan that he was speechless at this moment. Although he was standing there, his thoughts had already drifted away.

Witnessing Xu Yuan's shocking remarks in front of several professors, and then making the professors applaud excitedly, it can be said that this was completely beyond his understanding.

It was not until this time that he fully understood why Xu Yuan, as a college freshman, was able to publish a paper in the Annals of Mathematics.

It is also so favored by Professor Wang Xiaoyun.

I just felt that I was deeply stimulated, and it felt like all my years of study had been wasted.

After Professor Wang Xiaoyun called him that two or three times, he came to his senses and quickly agreed to the request.

"Okay, Professor Wang, I'll go arrange a dormitory for Junior Xu Yuan right away."

After saying this, he turned around and left quickly without caring about anything else, fearing that if he stayed any longer he would be hit by Xu Yuan again.

(End of this chapter)