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Comment by TeMPOraL | Hacker News Reader

The problem is arguably the symmetry in asymmetrical encryption - namely both "private" and "public" key are the same thing; the labeling "public" vs. "private" reflects your arbitrary choice of which of one you give away, and which one you keep for yourself.

It's surprising that we didn't invent any kind of physical padlock that accepts two keys, with the lock mechanism such that, when locked with one of the two keys, can only be unlocked by the other key. I can imagine obvious use cases for that, e.g. in shipping, but I guess this won't adopted because it makes the key management problem immediately obvious. But should such a thing existed, that would be the best (edit: just better - see my other comment explaining why "lock" is the dumb part here) analogy to draw terminology from.

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> It's surprising that we didn't invent any kind of physical padlock that accepts two keys, with the lock mechanism such that, when locked with one of the two keys, can only be unlocked by the other key.

Key exchange blocks are basically this, except one of the two keys is perpetually locked inside the keyblock, and you must use the other to retrieve it (whereupon the key you just used is now locked in the block).

So not quite the same thing, but the closest example I know of.

> It's surprising that we didn't invent any kind of physical padlock that accepts two keys, with the lock mechanism such that, when locked with one of the two keys, can only be unlocked by the other key.

They do:

* https://www.youtube.com/watch?v=VAriLDgpnY8

As mentioned in the video they're usually used in commercial settings. Also:

> One Way Cylinder Keying: Allows for the issuance of one key that can ONLY lock, one key that can ONLY unlock and one key that can BOTH lock and unlock the cylinder. Perfect for applications where one key holder should only have authorization to lock, while another should only have authorization to open and yet another can have the authorization to perform both functions. One way keyed products are supplied with 2 nylon head cut keys per product and 1 key order card per product.

* https://mangionelocksmiths.com/wp-content/uploads/2017/02/Mu...

> namely both "private" and "public" key are the same thing; the labeling "public" vs. "private" reflects your arbitrary choice of which of one you give away, and which one you keep for yourself.

It is not quite arbitrary. With RSA, you could potentially store only the modulus and the exponents, publishing one exponent and keeping the other one private (or making each privately known to different people, with both having the modulus). However, the way it is commonly stored is with the private key file includes both exponents and several other numbers, and the public exponent is usually 65537 which makes it easy to guess so you cannot effectively keep it secret. With some other kinds of cryptography (other than RSA), you can figure out the public key from the private key even without doing things like this.

We did/do have that kind of lock: A safe deposit box at a bank. To open it, you need two keys, one of which you have and one of which the bank has.

But that's the "you need two keys at the same time" case, which is different. I'm talking about "each of them can lock alone, but once locked, only the other one can unlock".

The real problem is that lock and key is a fundamentally dumb analogy for a process that scrambles something.

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