There’s a server, a client, and a hacker in a network. For encryption, the client and the server need to share their private keys. Wouldn’t the hacker be able to grab those during their transmission and decrypt further messages as they please?
this post was submitted on 28 Apr 2024
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You've missed a key detail in how asymmetric encryption works:
But how does the encryption work if you have the public key? Since your computer knows how to encrypt the data with the public key, couldn't you use that same public key to run that algorithm in reverse? If not, since the public and private keys are not the same, how does the private key go about decrypting that data?
The actual math is way beyond me, but the algorithm is "one way" - it exploits the fact that given two prime numbers (ie, the private key) it is trivial to multiply them together, but if you only know the result (ie, the public key) it is computationally very expensive to determine the original prime factors. If you pick big enough numbers, it becomes effectively impossible to undo the multiplication
No. The encryption methods are designed in a way that using a public key will not decrypt the message.
I didn't believe this is true... In public/private key crypto there is no such thing as "private" and "public". They're just a key pair. You choose to make one public by sharing it and the other private by not sharing it.
It depends on the cryptosystem. The private and public halves of the pair are often not symmetrical and often have overlap.
The parent is likely confused because in most situations the "private key file" will also contain all of the public key. Whether by necessity or for convenience.
Just a nitpick:
The public key and the private key are just two big prime numbers. The "trivial to compute" part only works once more information has been shared over the network, like it happens during key exchange. If you were to swap the prime number before initiating any contact it would work the same way.
In RSA, the private key is a pair of big semi-primes, and the public key is derived from those numbers. I think you are confusing DHKE and RSA with your other points, the private key is never transmitted over the network. For TLS you typically use an asymmetric crypto system to validate identities and encrypt the key exchange to prevent person-in-the-middle, but the key that is agreed using that process is a symmetric key for AES or similar, but that is specific to TLS.
Also, there are other asymmetric systems that don't use primes at all - eliptic curve crypto is based on completely different math
I used to know that and still struggle to understand how a handshake wouldn't allow MitM. Later I found out that it requires a third party with a trusted and known certificate for signing handshake exchange messages in order to ensure there's no man in the middle: https://stackoverflow.com/a/10496684
Yes, that's why https needs certificates (and sometimes shows a broken lock) and why you need to accept the fingerprint when first connecting to a server via ssh.
Accepting ssh key fingerprints on first ssh is a bad practice. Ssh ca’s and or sshfp are around and have been for decades. Accepting random host keys is like trusting random self signed ssl certificates.
Use ssh ca’s for user and host keys so you can revoke and rekey hosts without having to update authorized keys. And then you can revoke access to hosts for users as well and much more.
A big "It Depends" on that - plenty of applications of asymmetric crypto where you just hard-code the servers public key into the client and call it a day, and GPG has its own PKI scheme that is just kinda weird.
You also don't have to use Diffie-Hellman - early versions of SSL just sent the ephemeral key (the symmetric key used for the actual AES session) directly. This works, but using DH also gives you "forward secrecy" - even if a malicious third party has captured the entire encrypted session, then later steals (or factors) your private key they still won't be able to read the encrypted traffic because they can't recover the ephemeral key because it wasn't sent over the wire in the first place