Patent Application
20070076887
This invention relates generally to the field of network communications, and more particularly to cryptology.
Public key encryption is currently a popular technique for secure network communications. Public key encryption utilizes “one-way functions” that are relatively simple for computers to calculate, but difficult to reverse calculate. In particular, a one way function f(x) is relatively easy for a computer to calculate given the variable x, but calculating x given f(x) is difficult for the computer, although not necessarily impossible. Some one way functions can be much more easily reverse calculated with the assistance of particular “trap door” information, i.e., a key. Public key cryptography utilizes such one-way functions in a two-key system in which one key is used for encryption and the other key is used for decryption. In particular, the one-way function is a “public key” which is openly advertised by Node A for the purposes of sending encrypted messages to Node A. The trap door key is a “private key” which is held in confidence by Node A for decrypting the messages sent to Node A. For two-way encrypted communications each node utilizes a different public key and a different private key. One advantage of this system is that secure key distribution is not required. However, advances in the capabilities of computers tend to erode the level of security provided by public key encryption because the difficulty of reverse calculating the one-way function decreases as computing capabilities increase.
It is generally accepted in the field of cryptology that the most secure encryption technique is the Vernam cipher, i.e., one-time pad. A Vernam cipher employs a key to encrypt a message that the intended recipient decrypts with an identical key. The encrypted message is secure provided that the key is random, at least equal to the message in length, used for only a single message, and known only to the sender and intended receiver. However, in modern communication networks the distribution of Vernam cipher keys is often impractical, e.g., because the keys can be quite long and key distribution itself is subject to eavesdropping.
One technique for secure key distribution is known as Quantum Key Distribution (“QKD”). Quantum Key Distribution transmits an individual photon for each bit of the key being distributed to an intended recipient. The photons may be polarization modulated in order to differentiate logic 1 from logic 0. Distribution of the quantum key is secure because of the laws of quantum physics. In particular, it is not possible to measure an unknown quantum state of a photon without modifying it. Hence, an eavesdropper attempting to intercept the key would introduce detectable errors into the key. Unfortunately, photon-per-bit key distribution is so inefficient with current technology as to be impractical. This is due in-part to the attenuation technique and equipment used to generate a single photon pulse. In particular, in order to avoid transmitting more than one photon the attenuator must be set such that about 91% of the attempted pulses generate zero photons.
In accordance with the invention, a method of Quantum Key Distribution to a target device, comprises the steps of: generating a pulse having multiple photons, the pulse representing at least one bit indicative of the Quantum Key; splitting the pulse into first and second sub-pulses; modulating the first sub-pulse on a first basis with a secret key; transmitting both the first and second sub-pulses to the target device; receiving the first and second sub-pulses back from the target device; modulating the second sub-pulse on the first basis with the secret key; and comparing the first and second sub-pulses to detect modulation mismatch of the first basis.
A network architecture operable to distribute a Quantum Key in accordance with the invention comprises: a first device including: a laser operable to generate a pulse having multiple photons, the pulse representing at least one bit indicative of the Quantum Key; a coupler operable to split the pulse into first and second sub-pulses; a first modulator operable to modulate the first sub-pulse on a first basis with a secret key; a port operable to transmit both the first and second sub-pulses to the target device; a port operable to receive the first and second sub-pulses back from a target device; logic operable to prompt modulation of the second sub-pulse on the first basis with the secret key; and logic operable to compare the first and second sub-pulses to detect modulation mismatch of the first basis.
A general advantage of the invention is more efficient and practical key distribution. Efficiency is enhanced because multiple photons can be used to represent each bit of the key. Using multiple photons enable use of attenuator setting that are less likely to result in zero photons (complete attenuation). Security is maintained using multiple pulses per bit because attempted eavesdropping can be detected from phase mismatches in the key pulses returned to the sender. Another advantage of the invention is that the need for active polarization compensation is obviated. In particular, since the initial pulse is split into two pulses which traverse the same round-trip path there is no need for polarization compensation. Further, the same laser can be employed for both synchronization and key distribution. Other advantages will be apparent in view of the following detailed description.
A series of short laser pulses is employed for quantum key distribution between Bob and Alice. The short laser pulses are generated by the laser diode (118). Considering now the case of a single pulse from the laser diode, coupler C1 (116) splits the pulse into two pulses: P1 and P2. Pulse P1 is transmitted via the long loop and P2 is transmitted via the short loop.
Referring now to only
Alice is operable to receive pulse P1 from the optical fiber and enable phase modulator PMa (104) to modulate a phase shift ø1, selected randomly from bases B1 and B2 based on Alice's key bit. Faraday Mirror (“FM”) (106) then reflects P1 back and flips its polarization, i.e., a change of π/2. The resulting pulse P1 is then transmitted back to node Bob (102).
Node Bob (102) is operable to receive pulse P1 from node Alice (100). The PBS (114) is operable to transmit the returning pulse P1 to the short loop due to the polarization flip by FM (106) and subsequently to the coupler (116) where a combination is made with returning pulse P2.
Referring now to
Referring again to both
pulse P1: ø1+Φs=Φ1
pulse P2: ø2+Φs=Φ2
Phase differences at Bob's coupler are then detected as follows:
ΔΦ=Φ1−Φ2=ø1−ø2
ΔΦ=0: constructive interference→detector 0
ΔΦ=π: destructive interference→detector 1
ΔΦ: [0,π]→randomly detected.
After measuring the photon pulses, Bob publicly tells Alice his measurement types. Alice then tells Bob which are correct. If correct measurements are recorded in one detector, so-called one-click, there is no “intercept-resend” attack. Bob will continue BB84's error correction and privacy amplification and find the final shared secret key.
A photon-split attack is the most dangerous attack against a multi-photon quantum key distribution because each individual photon in a pulse has 100% of the information of the encoded key bit value. The current technique uses a secret phase key Φs modulated into pulse P1 on the way out and into pulse P2 after returning back. Because of its randomization of Φs, the attacker EVE can not correctly guess the secret phase key applied by Bob. Suppose that Eve uses a “photon-split” attack technique, i.e., Eve splits a single photon portion p1 from P1 and p2 from P2. Eve needs to combine p1 and p2 together to create an original photon which carries quantum key information. Also suppose that Eve learns the measurement information from the public communication between Bob and Alice and successfully guesses phase shifts ø1 and ø2. The difference of p1 and p2 will be ø1−ø2+φs. From here, we know that even if EVE knows ø1 and ø2, Eve still can not get a definitely constructive or destructive interference from p1 and p2 due to the unknown secret phase key φs. Therefore, the invention is an absolutely secure key distribution technique, even for multi-photon pulses.
While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.
