Comment on With public key cryptography, why can't someone decrypt a message using the public key?
rufus@discuss.tchncs.de 8 months ago
It think the common analogy is a padlock and a key. One party gets the padlock and the other one the key. Now with the padlock you can just lock the box but not open it. And with the key you can just unlock and open the box.
The maths behind that is a bit difficult to explain but not that difficult. I think it’s about one-way functions that are easy to calculate in one direction and impossible to solve the other way around. Like the old approaches with prime numbers. It’s easy to multiply two prime numbers. But given an arbitrary number, it’s difficult to tell which prime numbers it consists of.
OmegaMouse@pawb.social 8 months ago
So using the formula in that guide, you get a numerical value for O. But surely someone else could follow the same process and also get the same answer? Unless the primes change each time? But then how would the sender and receiver know the way in which the values change?
bobburger@fedia.io 8 months ago
This post on Stack Overflow does a good job of explaining the issue.
rufus@discuss.tchncs.de 8 months ago
I’m not sure I get your question… Sure other people can also follow the same process and encrypt stuff to you. They can also do the calculations with your private key and arrive at the same result, sure. But the calculation involves your private key. Your secret. If that’s known to someone, they can do the calculations.
OmegaMouse@pawb.social 8 months ago
I explained it poorly - what I mean to say is, two people trying to send the message ‘Hello’ for example both using the same public key would get the same output. So if you had a simple message like that, someone could work out by checking every word in the dictionary what your message was by checking if the output matched.
But I guess it’s a bit of a moot point - it’s unlikely that an encrypted message would ever be so simple. It could just as easily be much longer, and therefore basically impossible to guess the plaintext.
mumblerfish@lemmy.world 8 months ago
No, it can be very important. As I answered in another comment, its called padding en.m.wikipedia.org/wiki/Padding_(cryptography). And to see that it is imortant say you encrypt your easy to remember password in an encrypted file. Now if your attack was possible, having your public key, you could just generate the passwords and encrypt them to figure out your password. Much easier than trying to find your key. Using forms of padding, that does not work.
rufus@discuss.tchncs.de 8 months ago
Ah, that is a really good question. These things happen. People have entire harddisks filled with “rainbow tables” which do these kind of attacks against hash-functions which are supposed to be one-way functions. This way they have terabytes worth of pre-computed hashes for the most common passwords and can immediately tell if one of those passwords is in a database leak.
For this it needs additional measures. Passwords are augmented with additional random data so people can’t pre-compute the hashes. So it wouldn’t be just ‘Hello’, but ‘Hello’ plus an additional “salt” that gets fed into the one-way function so it can’t be brute forced.
PGP for example uses both symmetric cryptography and asymmetric cryptography. The actual message is encrypted with symmetric encryption and the key to that is encrypted with asymmetric encryption. Unfortunately it’s been a while since I last read a book on cryptography. I think they did that because symmetric cryptography is way faster. But things like that could also prevent such attacks.
It’s not always obvious to the layman what kinds of attacks are possible with the crypto algorithms. They definitely need to protect against such scenarios or they’re worthless for that kind of use. There are “known plaintext attacks”. Usually people don’t want anyone even able to prove that you send a certain message. And an algorithm also isn’t good if you can learn something about the secret key if you have access to a ciphertext and plaintext. I think this was part of how they cracked the supposedly secure enigma machines of the Nazis.