license.txt

古典密码学的经典例子。通过该程序可以实现古典密码学的加密过程

“Why? What, no… should I have?” He had given up being surprised, there didn’t seem to be any point any longer.
Ford clicked his tongue in irritation.
“Drink up,” he urged.
At that moment the dull sound of a rumbling crash from outside filtered through the low murmur of the pub, through the sound of the jukebox, through the sound of the man next to Ford hiccupping over the whisky Ford had eventually bought him.
Arthur choked on his beer, leapt to his feet.
“What’s that?” he yelped.
“Don’t worry,” said Ford, “they haven’t started yet.”
“Thank God for that,” said Arthur and relaxed.
“It’s probably just your house being knocked down,” said Ford, drowning his last pint.
“What?” shouted Arthur. Suddenly Ford’s spell was broken. Arthur looked wildly around him and ran to the window.
“My God they are! They’re knocking my house down. What the hell am I doing in the pub, Ford?”
“It hardly makes any difference at this stage,” said Ford, “let them have their fun.”
“Fun?” yelped Arthur. “Fun!” He quickly checked out of the window again that they were talking about the same thing.
“Damn their fun!” he hooted and ran out of the pub furiously waving a nearly empty beer glass. He made no friends at all in the pub that lunchtime.
“Stop, you vandals! You home wreckers!” bawled Arthur. “You half crazed Visigoths, stop will you!”
Ford would have to go after him. Turning quickly to the barman he asked for four packets of peanuts.
“There you are sir,” said the barman, slapping the packets on the bar, “twenty-eight pence if you’d be so kind.”
Ford was very kind – he gave the barman another five-pound note and told him to keep the change. The barman looked at it and then looked at Ford. He suddenly shivered: he experienced a momentary sensation that he didn’t understand because no one on Earth had ever experienced it before. In moments of great stress, every life form that exists gives out a tiny sublimal signal. This signal simply communicates an exact and almost pathetic sense of how far that being is from the place of his birth. On Earth it is never possible to be further than sixteen thousand miles from your birthplace, which really isn’t very far, so such signals are too minute to be noticed. Ford Prefect was at this moment under great stress, and he was born 600 light years away in the near vicinity of Betelgeuse.
The barman reeled for a moment, hit by a shocking, incomprehensible sense of distance. He didn’t know what it meant, but he looked at Ford Prefect with a new sense of respect, almost awe.
“Are you serious, sir?” he said in a small whisper which had the effect of silencing the pub. “You think the world’s going to end?”
“Yes,” said Ford.
“But, this afternoon?”
Ford had recovered himself. He was at his flippest.
“Yes,” he said gaily, “in less than two minutes I would estimate.”
The barman couldn’t believe the conversation he was having, but he couldn’t believe the sensation he had just had either.
“Isn’t there anything we can do about it then?” he said.
“No, nothing,” said Ford, stuffing the peanuts into his pockets.
Someone in the hushed bar suddenly laughed raucously at how stupid everyone had become.
The man sitting next to Ford was a bit sozzled by now. His eyes waved their way up to Ford.
“I thought,” he said, “that if the world was going to end we were meant to lie down or put a paper bag over our head or something.”
“If you like, yes,” said Ford.
“That’s what they told us in the army,” said the man, and his eyes began the long trek back down to his whisky.
“Will that help?” asked the barman.
“No,” said Ford and gave him a friendly smile. “Excuse me,” he said, “I’ve got to go.” With a wave, he left.
The pub was silent for a moment longer, and then, embarrassingly enough, the man with the raucous laugh did it again. The girl he had dragged along to the pub with him had grown to loathe him dearly over the last hour or so, and it would probably have been a great satisfaction to her to know that in a minute and a half or so he would suddenly evaporate into a whiff of hydrogen, ozone and carbon monoxide. However, when the moment came she would be too busy evaporating herself to notice it.
The barman cleared his throat. He heard himself say:
“Last orders, please.”
The huge yellow machines began to sink downward and to move faster.
Ford knew they were there. This wasn’t the way he had wanted it.
Running up the lane, Arthur had nearly reached his house. He didn’t notice how cold it had suddenly become, he didn’t notice the wind, he didn’t notice the sudden irrational squall of rain. He didn’t notice anything but the caterpillar bulldozers crawling over the rubble that had been his home.
“You barbarians!” he yelled. “I’ll sue the council for every penny it’s got! I’ll have you hung, drawn and quartered! And whipped! And boiled… until… until… until you’ve had enough.”
Ford was running after him very fast. Very very fast.
“And then I’ll do it again!” yelled Arthur. “And when I’ve finished I will take all the little bits, and I will jump on them!”
Arthur didn’t notice that the men were running from the bulldozers; he didn’t notice that Mr. Prosser was staring hectically into the sky. What Mr. Prosser had noticed was that huge yellow somethings were screaming through the clouds. Impossibly huge yellow somethings.
“And I will carry on jumping on them,” yelled Arthur, still running, “until I get blisters, or I can think of anything even more unpleasant to do, and then…”
Arthur tripped, and fell headlong, rolled and landed flat on his back. At last he noticed that something was going on. His finger shot upwards.
“What the hell’s that?” he shrieked.
Whatever it was raced across the sky in monstrous yellowness, tore the sky apart with mind-buggering noise and leapt off into the distance leaving the gaping air to shut behind it with a bang that drove your ears six feet into your skull.
Another one followed and did the same thing only louder.
It’s difficult to say exactly what the people on the surface of the planet were doing now, because they didn’t really know what they were doing themselves. None of it made a lot of sense – running into houses, running out of houses, howling noiselessly at the noise. All around the world city streets exploded with people, cars slewed into each other as the noise fell on them and then rolled off like a tidal wave over hills and valleys, deserts and oceans, seeming to flatten everything it hit.
Only one man stood and watched the sky, stood with terrible sadness in his eyes and rubber bungs in his ears. He knew exactly what was happening and had known ever since his Sub-Etha Sens-O-Matic had started winking in the dead of night beside his pillar and woken him with a start. It was what he had waited for all these years, but when he had deciphered the signal pattern sitting alone in his small dark room a coldness had gripped him and squeezed his heart. Of all the races in all of the Galaxy who could have come and said a big hello to planet Earth, he thought, didn’t it just have to be the Vogons.
Still he knew what he had to do. As the Vogon craft screamed through the air high above him he opened his satchel. He threw away a copy of Joseph and the Amazing Technicolor Dreamcoat, he threw away a copy of Godspell: He wouldn’t need them where he was going. Everything was ready, everything was prepared.
He knew where his towel was.
A sudden silence hit the Earth. If anything it was worse than the noise. For a while nothing happened.
The great ships hung motionless in the air, over every nation on Earth. Motionless they hung, huge, heavy, steady in the sky, a blasphemy against nature. Many people went straight into shock as their minds tried to encompass what they were looking at. The ships hung in the sky in much the same way that bricks don’t.
And still nothing happened.
Then there was a slight whisper, a sudden spacious whisper of open ambient sound. Every hi fi set in the world, every radio, every television, every cassette recorder, every woofer, every tweeter, every mid-range driver in the world quietly turned itself on.
Every tin can, every dust bin, every window, every car, every wine glass, every sheet of rusty metal became activated as an acoustically perfect sounding board.
Before the Earth passed away it was going to be treated to the very ultimate in sound reproduction, the greatest public address system ever built. But there was no concert, no music, no fanfare, just a simple message.
“People of Earth, your attention please,” a voice said, and it was wonderful. Wonderful perfect quadrophonic sound with distortion levels so low as to make a brave man weep.
“This is Prostetnic Vogon Jeltz of the Galactic Hyperspace Planning Council,” the voice continued. “As you will no doubt be aware, the plans for development of the outlying regions of the Galaxy require the building of a hyperspatial express route through your star system, and regrettably your planet is one of those scheduled for demolition. The process will take slightly less that two of your Earth minutes. Thank you.”
The PA died away.
Uncomprehending terror settled on the watching people of Earth. The terror moved slowly through the gathered crowds as if they were iron fillings on a sheet of board and a magnet was moving beneath them. Panic sprouted again, desperate fleeing panic, but there was nowhere to flee to.
Observing this, the Vogons turned on their PA again. It said:
“There’s no point in acting all surprised about it. All the planning charts and demolition orders have been on display in your local planning department on Alpha Centauri for fifty of your Earth years, so you’ve had plenty of time to lodge any formal complaint and it’s far too late to start making a fuss about it now.”
The PA fell silent again and its echo drifted off across the land. The huge ships turned slowly in the sky with easy power. On the underside of each a hatchway opened, an empty black space.
By this time somebody somewhere must have manned a radio transmitter, located a wavelength and broadcasted a message back to the Vogon ships, to plead on behalf of the planet. Nobody ever heard what they said, they only heard the reply. The PA slammed back into life again. The voice was annoyed. It said:
“What do you mean you’ve never been to Alpha Centauri? For heaven’s sake mankind, it’s only four light years away you know. I’m sorry, but if you can’t be bothered to take an interest in local affairs that’s your own lookout.
“Energize the demolition beams.”
Light poured out into the hatchways.
“I don’t know,” said the voice on the PA, “apathetic bloody planet, I’ve no sympathy at all.” It cut off.
There was a terrible ghastly silence.
There was a terrible ghastly noise.
There was a terrible ghastly silence.
The Vogon Constructor fleet coasted away into the inky starry void.


Chapter 4

Far away on the opposite spiral arm of the Galaxy, five hundred thousand light years from the star Sol, Zaphod Beeblebrox, President of the Imperial Galactic Government, sped across the seas of Damogran, his ion drive delta boat winking and flashing in the Damogran sun.
Damogran the hot; Damogran the remote; Damogran the almost totally unheard of.
Damogran, secret home of the Heart of Gold.
The boat sped on across the water. It would be some time before it reached its destination because Damogran is such an inconveniently arranged planet. It consists of nothing but middling to large desert islands separated by very pretty but annoyingly wide stretches of ocean.
The boat sped on.
Because of this topological awkwardness Damogran has always remained a deserted planet. This is why the Imperial Galactic Government chose Damogran for the Heart of Gold project, because it was so deserted and the Heart of Gold was so secret.
The boat zipped and skipped across the sea, the sea that lay between the main islands of the only archipelago of any useful size on the whole planet. Zaphod Beeblebrox was on his way from the tiny spaceport on Easter Island (the name was an entirely meaningless coincidence – in Galacticspeke, easter means small flat and light brown) to the Heart of Gold island, which by another meaningless coincidence was called France.