# Tag Info

## Hot answers tagged cryptography

### Can a public key be used to decrypt a message encrypted by the corresponding private key?

Q: If you pedal backwards on a fish, does it go backwards? A: ??? A fish is not a bicycle. Similarly, you cannot use a private key to encrypt a message or a public key to decrypt a message. They ...

### Meaning of: "'If factoring large integers is hard, then breaking RSA is hard,' is unproven"

The easiest way to think about it is to think of the contrapositive. The statement: if factoring large integers is hard, then breaking RSA is hard is equivalent to the following: if breaking ...
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### How does an operating system create entropy for random seeds?

The title and the body of your question ask two different questions: how the OS creates entropy (this should really be obtains entropy), and how it generates pseudo-randomness from this entropy. I'll ...
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### Given RSA, why do we not know if public-key cryptography is possible?

We don't know for sure that RSA is safe. It could be that RSA can be broken in polynomial time, for example if factoring can be done efficiently. What is open is the existence of a a provably secure ...
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### Meaning of: "'If factoring large integers is hard, then breaking RSA is hard,' is unproven"

The existence of a hard way does not imply there is no easy way. There may be a number of ways to break RSA and we only need to find one of them. One of these ways is factoring a large integer, so ...
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### What is an extremely basic asymmetric cipher that I can present at the pub?

If you want to present public key cryptography to your parents or friends, then I suggest you follow some guidelines. First, don't talk about specific functions, nobody cares about SHAxxx, keep your ...
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### Problem with the pseudo random number generator One-Time-Pad

You seem to have misunderstood what the key is. In the context of symmetric encryption, the key is a shared secret: something that is known to both the sender and receiver. For OTP, the key is the ...
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### What are the flaws in this encryption algorithm?

This is not a secure encryption scheme. It is similar to a Hill cipher, and vulnerable to similar attacks. For instance, it is vulnerable to known-plaintext attacks: an attacker who observes a ...
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### Does there exist an equivalent arithmetic circuit for each computable function?

Any computable boolean function with a fixed-length input can be computed by an arithmetic circuit. Consider any boolean function $f:\{0,1\}^n \to \{0,1\}$. Then there exists a multivariate ...
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### What is an extremely basic asymmetric cipher that I can present at the pub?

A common metaphor I hear used is manufacturing a bunch of padlocks, keeping all the keys, and sending out open padlocks to anyone who wants one. Then anyone with such a padlock can send you secret ...
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### Can you prevent a man in the middle from reading the message?

Can the man in the middle not just take the keys swapped by the opponents, change the keys and then decrypt and encrypt the message again? Yes, they can. A key exchange protocol like (the "textbook" ...
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### Running an algorithm on data remotely and ensuring answer has not been tampered with

In the crypto community, this task is known as delegated computation, or verifiable delegation. You wish to let the server (the "cloud") to do the work for you, but you also want the cloud to give you ...
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