when its possible to decode info without knowing key then how is quantum cryptography unconditionally secure?

Question

since in quantum cryptography the care is taken only while distributing the key and the info is sent over an unsecure channel then by applying various key combinations we can decode the info as how we expect things to be decoded by the use of quantum computers. then how is quantum cryptography unconditionally secure when it is not computationally secure.

Answer 1

Keep in mind that right now there is no good way to predict how a quantum computer could decipher a generic code.

The real strength of quantum computers to decipher codes involves use with PGP-like codes that use public and private keys. If a public key is broken into its prime factors, it would be possible to construct the private key and thus it would be possible to decipher encrypted data. It is very difficult to factor a very large number. However, quantum computers have been shown to be able to perform one particular operation very quickly that allows for a major simplification to the process of factoring numbers. (see the "Shor's algorithm" reference below, the first source) If a quantum computer can make it "easy" to factor numbers, then public-private key encryption schemes will be vulnerable.

In the case where a cipher has to be known by BOTH sides, then quantum computers (at the present time) have no way to greatly improve how quickly these codes can be broken. Thus, quantum encryption's ability to transfer a super-secret and super-long and super-complex cipher would make encryption very secure, even if the actual encrypted data was sent over a very public channel.

Quantum computers are not silver bullets. They have limitations as well. Algorithms have to be developed that make proper use of them. Thus, encryption algorithms can be built that give you no advantage to using quantum computers.

I recommend you take a look at the "See also" in the "Quantum computers" source below (the second source). It lists some of the algorithms that currently exist for doing computations on quantum computers. As you can see, they are intended for for very specific operations. Either additional algorithms need to be generated or other problems need to be framed into a context that allows the existing algorithms to be used to solve those problems.

(note that often the quantum algorithms don't do all the work; their results often have to then go through traditional computers to do additional crunching before getting a real result. This is in fact the case with public-private key encryption; quantum computing is inserted in a special spot in the solution process to convert the problem from being intractable to being tractable (but still requiring a powerful traditional computer))

Finally, take a look at Simon Singh's popular book _The Code Book_, which I list as the third source. There are two versions of this book, one that is more advanced than the other. I think it will answer many of your questions (about quantum cryptography and cryptography in general).

Answer 2

The term unconditionally secure is a tricky. When security of a algorithm or mechanism is not dependent on the complexity of the algorithm or the mechanism except guessing then it is unconditionally secure. The unconditional secrecy of the key is guaranteed by the uncertainty relation of quantum mechanics: No eavesdropper can observe the photons without disturbing their quantum-mechanical state. Thus, any measurement of the adversary can be detected immediately.

So as soon as you had breaked the algorithm but the sharing party is also come to know that you have done it and they can change their state.
So u are againg in the loop. You can have look on shamir's secret sharing algorithm , that is also unconditionally secure.

For any further, please feel free to contact me.

Answer 3

I don't know what quantum cryptography, but there is a type of cypher that uses something called a one time pad that is totally impossible to decrypt without the pad. The encrypted signal is indistinguishable form random noise and even an infinite amount of time and computer power cannot decrypt it.

Answer 4

there is a difference between statistically secure and realistically secure the chances of someone breaking a 32 character code is not impossible but is quite a bit more difficult than a 4 digit code---- it all depends on the amount of time and computing power available to decipher