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Comment by newpavlov | Hacker News Reader

Try to run a non-toy RSA by hand using pen and paper, it will be quite painful and impractical. Practicality of RSA largely depends on existence of computers. Even in the presence of all required theoretical knowledge, an application may not be practical, thus it does not get "invented" until the practicality issue gets resolved.

I would say it's more interesting that one-time pad encryption was not invented and used much earlier than 19th century. It has obvious military and intelligence uses and is simple enough to be executed by hand.

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RSA on pen and paper isn’t that bad compared to some other calculations people used to do by hand. By comparison in 1800 multiple people had already calculated the first 100 digits of pie. https://en.wikipedia.org/wiki/Chronology_of_computation_of_π

People can get shockingly good at that kind of thing with enough practice.

Well book ciphers go some ways back and they are almost OTP (in the sense of how they are performed, not in the sense of security properties). It may have been prohibitively expensive to print a custom pad used for encryption.

Expensive but spy agencies did it.

A one-time pad seems totally feasible to me without a computer, and sufficient motivation. RSA does not. They're different algorithms of course! OTP is a very simple one, as you point out.

For centuries people have been encrypting things "by hand" and "inconveniently" (Cesar's cipher is an example, I can make many more).

RSA doesn't largely depends on the existence of computer, it could've been used "by hand" using the math behind it. The reason why was not is because, prior to computer, there was not a problem of "sharing" the key (and btw Nazi knew that a single key was weak, that's why they'd change it everyday to encrypt the communication with enigma)

> I would say it's more interesting that one-time pad encryption was not invented and used much earlier than 19th century

That's not true, as I mentioned Cesar's cipher is an example of that, though of course uses a simple key. Radio communications (and of course internet later) is the reason why encryption started to boom on the early 19th century onwards.

Ceasar’s cipher is neither complex nor an OTP. What are you talking about?

What are you talking about. That was the first cipher.

RSA is OTP?

The point originally made was that RSA couldn't practically be used because of how complex the math is to do by hand. So, the parent is pointing out that the Caesar cipher isn't a counterargument.

In other words, the "inconvenience" of doing a Caesar cipher by hand is nowhere near comparable as doing an RSA cipher by hand.

Is not complex at all, if you can't multiply two prime number and do subtraction probably you should reconsider your skills.

If people were able to do that in 15th century by hand I don't see how who really needs it couldn't.

Is just the needs were different, but is not complicated

Otp

> The reason why was not is because, prior to computer, there was not a problem of "sharing" the key

Lol. There is a new dude sitting in a castle one sea over. You want to communicate with them. Let’s say you want to agree to attack a third neighbour at the same time. If your message gets intercepted you will be in trouble.

How do you share your secret keys with them? Mind you, you don’t really trust any of your man, and the sea route is treacherous so you don’t fancy risking going in person.

Sharing keys is a problem old as time.

That's the point. Still though is very old is not a 19th century problem. Yet RSA was used 400 years after the theory behind

Why didn't people came up with a manual "RSA" before. Since the math is 400 years old.

I recommend you try to do it -- it's a fun exercise, and will very quickly answer any remaining doubt why it can't be done manually.

We actually did that in university; the public key we used was something like 91 (not hard to factor). Wikipedia has an example with 3233, and that's already very cumbersome: https://en.wikipedia.org/wiki/RSA_(cryptosystem)#Example

I did that in university before you born

It can be done manually easily

I tried generating an Ethereum address by hand starting by creating a random private key. It quickly got away from me.

I wish there was some way to simplify the math to allow doing it by hand. It would be a nice way to check for correctness while computing it offline, and while ensuring dependencies hadn't been compromised to produce a fake public address.

I feel like making your own circuits that could do this would be a more feasible approach. Or at least a circuit that uses small purpose build ICs that are easy to verify.

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