Quantum cryptographyHow could you

transmit information safety?

Thanks to codes

Cryptography is everywhere

Credit card

Military secret

Internet communication (shopping, mails, …)

(…)

The Enigma machine

Hackers too!!

Every short of information

could be captured

Contents :

Why ‘quantum’ ?

How does it works?

And the hacker?

Conclusion

What is quantum cryptography ?

Some rules of quantum mechanic : • every energy variation is quantified (e.g. allows

production of single photon by exciting diamond with laser pulse);

• a measure (every type of measure!) on a system always disrupt it or destruct it (e.g. seeing is absorbing a photon);

• in quantum physic everything is probability, nothing is perfectly predictable.

Why ‘quantum’ ?

Looking a cylinder is like observing a quantum object.

Where is this electron? (simulation)

How does it works ?

A is sending to B a sequence of some thousand random bits coded on single photons by random polarization

BA

Polarization bases convention (‘code’ convention): circular (C) or linear (L)

1 0 1 0

LC RC V H

Key code

B detection

B randomize the polarization base to detect each photon and attribute a value to the bit thanks to the ‘code’ convention.

BA

User Sequence

Bits A 0 1 1 0 0 1 1 0 1 Random key

Polarization A C C L C L C C L L Random basesB L L L C C C C L C

Bits B 1 1 1 0 1 1 0 0 0 Rough key

User Sequence

Bits A 0 1 1 0 0 1 1 0 1 Random key

Polarization A C C L C L C C L L Random basesB L L L C C C C L C

Bits B 1 1 1 0 1 1 0 0 0 Rough key

User Sequence

Bits A 0 1 1 0 0 1 1 0 1 Random key

Polarization A C C L C L C C L L Random basesB L L L C C C C L C

Bits B 1 1 1 0 1 1 0 0 0 Rough key

Measure errors detection and correction

After transmission it subsists mistakes User Sequence

Bits A 0 1 1 0 0 1 1 0 1 Random key

Polarization A C C L C L C C L L Random basesB L L L C C C C L C

Bits B 1 1 1 0 1 1 0 0 0 Rough key

User Sequence

Bits A 0 1 1 0 0 1 1 0 1 Random key

Polarization A C C L C L C C L L Random basesB L L L C C C C L C

Bits B 1 1 1 0 1 1 0 0 0 Rough key

A and B compare their base choices sequence using a classical network and eliminate mistakes : reconciliation step.

User Sequence

Bits A 1 0 1 1 0 Filtered key

Polarization A L C C C L SamebasesB L C C C L

Bits B 1 0 1 0 0 Filtered key

Transmission errors detection and correction

A and B sacrifice a part of their filtered key to evaluate the transmission error ratio due to physical causes or to a hacker (or more!).

User Sequence

Bits A 1 0 1 1 0 Filtered key

Polarization A L C C C L SamebasesB L C C C L

Bits B 1 0 1 0 0 Filtered key

1

C

C

0

0

C

C

0

Ok ERROR

Confirmation step : if too much errors => start the process again

Exchanged part

Last step : KEY CONSTRUCTION

A and B use separately corrective algorithms to correct transmission errors and to amplify the confidentiality thanks to jamming the key .After that : only ONE key for A and B but WITHOUT TRANSMISSION of this one!

Users Keys

A 1 1 0 The same

keysB 1 1 0

And the hacker ?

BA

The hacker could make many things…SO MANY!!....

The hacker could take information during transmissions. He could :

During the first exchange :

• steal some photons during the first exchange;• and/or (re)emit photons to mistake B;

During the reconciliation step :

• read all the base choices of A and B;

During the confirmation step :

• read all the sacrificed sequence;

Hacker’s actions have manyconsequences :

• he must detect photons as the same way as B => he makes errors;• he introduces more errors;• he cannot control error rate;

• he haven't necessarily the same choices => he loses information whereas A and B no;

• he can’t do anything from this time!

…BUT there is a price!!!

To get enough information, the hacker had introduced a non-negligible amount of errors and so he will be detected and…

EVERYTHING WILL RESTART!!!

And it’s even worst!!!

And so, hacker is THE great looser!

??

Quantum cryptography is a good solution

Allows secure communication against hackers because

everything is transmitted before creating the key.

Whatever the hacker want to do is against him.

Thanks for your attention