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A comment on Proof emerges that a quantum computer can outperform a classical one. Parent comment

Re:Proof emerges that a quantum computer can outperform a cl... by atq2119 on Sunday September 29, 2019 @11:53AM (#21107722)
This doesn't work for encryption, though. You'd need a cryptosystem where it's possible to encrypt a message with H(x) such that it can only (efficiently) be decrypted given x, for a hash function H. To the best of my knowledge, no such hash function is known.
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Re:Proof emerges that a quantum computer can outperform a cl... by EGreg on Sunday September 29, 2019 @12:01PM (#21107776)
It’s an open question whether hash functions can be used for encryption using XOR:

https://crypto.stackexchange.com/questions/35809/whats-wrong...

Here is a basic explanation:

https://cryptography.fandom.com/wiki/Snuffle

Note however that you would need a nonce / salt transmitted with each message because otherwise decrypting messages encrypted with the same key would just be as easy as XORing them!

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Re:Proof emerges that a quantum computer can outperform a cl... by atq2119 on Sunday September 29, 2019 @12:38PM (#21108023)
Snuffle is symmetric key encryption, and yes, it's a well-known construction. However, you previously suggested that an asymmetric encryption scheme (with public and private keys) could be constructed using a hash.
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Re:Proof emerges that a quantum computer can outperform a cl... by EGreg on Sunday September 29, 2019 @01:41PM (#21108418)
oh, right. It’s only good for signing and random number generation, not encryption.

Still that’s plenty useful! It can be used for blockchains and crypto currency.

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