|a new key every time|
"It may be best to let them settle slowly, Watson," my friend replied. "What else have you been wondering about?"
"It may seem a small thing, Holmes," I said, "and it may be a small thing, too. But since you've asked, I'm curious about the arrangements you made for meeting Bucky. Things were happening so fast, I may have missed something, but it seemed to me you set up a secret way for him to summon you, but no way for you to contact him."
"Sometimes you are more observant than I give you credit for, Watson," he said. "Yes, that's exactly right."
"I was wondering why you did it that way," I continued, "if you don't mind my asking."
"I don't mind at all," he said. "First and foremost, he's the client. In general, a client may consult me whenever he wishes, within reason. Usually they find me in our Baker Street flat, as you well know; but sometimes I arrange for them to find me elsewhere. If I have to find them, well, let's just say I'd be a poor detective if I couldn't find my own clients."
"Indeed, Holmes, but in this case --"
"In this case, Watson, the client is a senior Scotland Yard detective who's come to me in disguise! and who was followed! Surely his life is in more danger than my own."
"Do you really think so, Holmes?" I asked.
"Yes indeed," he answered. "The situation is very serious, and Bucky is cutting across the plans of some very powerful people, Watson.
"As you say, things were happening very fast," he continued. "That was because I wanted them to. The longer we had kept those two across the street waiting, the more time they would have had to see that we were onto them, and the more likely they would have been to follow Bucky and Mrs. Hudson. Or so I thought.
"I may have been wrong about that -- perhaps if we'd left them standing there long enough, they would have fallen asleep! But it wasn't the sort of chance I was willing to take. In any event, I was pushing the pace and wanted to keep things simple for Bucky.
"I set up a one-time cipher so he can call me the next time he needs me," Holmes carried on, "and I hope he'll come to our next meeting alone, which would give us more time. Regardless, when I see him again, I will give him another one-time cipher and -- what's that, Watson?"
"What exactly is a 'one-time cipher'?" I asked.
"'Cipher' means 'code', Watson," he said, "and a 'one-time cipher' is a code that is only meant to be used once."
"How absurdly simple, Holmes," I said.
"It is simple, and yet again it's not," he replied. "The main drawback with one-time ciphers is the very fact that they can only be used once. You need a new key for each message."
"And the key is --?"
"The key is the information that unlocks the message," he explained. "In some cases the key might be a password. But there are other sorts of keys as well -- it might be a pass-phrase, or a series of numbers, or even something more unusual."
"And you need a new key every time?" I asked. "That must be inconvenient."
"It is," said Holmes, "and even more inconvenient, the key must be at least as long as the message itself. But on the other hand, a properly constructed one-time cipher is an unbreakable code. Do it right and it's absolutely impossible to crack -- the only means of encryption that can make this claim. For some applications, the security is well worth the expense.
"The current situation," he continued, "calls for the highest security possible -- encryption so secure that our message can actually be broadcast!"
"Broadcast, Holmes?" I asked.
"Slate will put an ad in the paper, Watson. Tens of thousands of people will see it, maybe hundreds of thousands -- but they won't understand it. Only Bucky and I will know what it means."
"How can you be so sure of that, Holmes?"
"Because it's a one-time cipher, Watson," he reminded me with a chuckle. "If you don't have the key, you can't read it!"
"Forgive me, Holmes," I said, "but I still don't understand. Can you give me an example?"
Holmes scribbled on a scrap of paper and handed it to me. "It could be as simple as this, Watson," he said.
I looked at the paper and read it aloud:
Ernie,"What is this, a joke? Characters from a show for children?" I asked.
I will be away until Tuesday.
"It could be taken as a joke, and it would suit me if it were, but it really doesn't make any difference," he replied. "As long as both Bucky and I have the same key, and nobody else does, we could use any message at all. It's the key that matters. In a one-time cipher, the key is virtually everything."
"And in this case --?"
"In this case," he said, "'Ernie' would mean 'Dear Mr. Holmes'; 'Bert' would mean 'Buckingham Slate'; 'I will be away until' would mean 'Please meet me at Harrington's pub at eight o'clock on'; and now you can see, can't you, Watson, that the only actual information contained in this message is the word 'Tuesday'."
I whistled softly. "It is very clever, Holmes," I said.
"It is very ancient, Watson," he replied. "But it still works!"