Cryptography – Communication system using cryptography – Circuit switched network
Reexamination Certificate
1999-10-28
2001-07-24
Barron, Jr., Gilberto (Department: 2131)
Cryptography
Communication system using cryptography
Circuit switched network
C380S269000, C380S275000, C379S442000
Reexamination Certificate
active
06266418
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to encryption devices. In particular, the invention relates to methods and apparatus for securing telephone communications by encrypting audio signals between the handset and base unit of a host telephone.
BACKGROUND OF THE INVENTION
Historically, non-governmental voice communications over telephone networks have rarely been secured with an encryption product. This is largely due to the high expense typically associated with such a product, and the administrative burden of managing encryption keys among the devices. Yet the value of the information conveyed over telephone networks is increasing steadily. Telephone security products would see widespread use if their costs were reduced to the point where the corporate, financial, legal, medical, and industrial communities could afford them, and if the administrative tasks associated with the set-up and control of these products was minimized.
Existing telephone security products typically connect between the telephone and the telephone network. This typically permitted their application only on public-switched telephone networks (PSTNs), however, since they frequently interfere with proprietary services offered over private branch exchange (PBX) based telephone networks. In addition, these products usually cannot be applied to networks where proprietary digital PBXs or Integrated Services Digital Network (ISDN) protocols are employed since these interfaces are not standardized.
Thus, there is a need in the art for a small, inexpensive encryption device that can be connected between the handset and base unit of any of a variety of ordinary telephones to provide secure, full-duplex telephone conversations that are immune from eavesdropping with no degradation in speech quality.
SUMMARY OF THE INVENTION
According to the present invention, an encryption device for a telephone having a handset and a base unit comprises a handset interface, a first converter, an encryption processor, a second converter, and a host interface. The handset interface receives analog output signals from the handset, and the first converter converts the analog output signals into digital output signals.
The encryption processor comprises a compressor, a key manager, an encryptor, and a modulator. The key manager generates key material for encrypting the digital output signals. The compressor compresses the digital output signals, the encryptor encrypts the digital output signals based on the key material, and the modulator modulates the encrypted digital output signals.
The second converter converts the encrypted digital output signals into encrypted analog output signals, and the host interface receives the encrypted analog output signals from the encryption processor, and forwards the encrypted analog output signals to the base unit.
The encryption device can also include a human-machine interface coupled to the encryption processor via which a user of the encryption device can communicate with the encryption processor.
According to one aspect of the invention, the encryption device can include a gain adjustment circuit coupled to the base unit interface that adjusts a signal level of the encrypted analog output signals. A user of the device can use the human-machine interface to enter a code that corresponds to the telephone to which the device is coupled. The gain adjustment circuit can then adjust the signal level of the encrypted analog output signals based on the received code.
According to another aspect of the invention, the encryption device can include a bypass control circuit that is coupled to the handset interface and to the base unit interface, via which the analog output signals can bypass the encryption processor. A user of the device can use the human-machine interface to cause the analog output signals to selectively bypass the encryption processor.
According to still another aspect of the invention, the encryption device can include a bias detect circuit coupled to the base unit interface, and a microphone bias circuit coupled to the bias detect circuit and to the handset interface. The bias detect circuit detects a bias voltage polarity provided by the base unit interface, and directs the microphone bias circuit to provide the bias voltage polarity to the handset.
According to another aspect of the invention, the encryption processor encrypts the output signals by generating a cryptographic session key, defining a state vector, encrypting the state vector to produce a keystream. The state vector is encrypted using the cryptographic session key and a cryptographic block transformation. Then, the keystream is combined with the output signals to produce encrypted output signals. The encryption processor can define the state vector, at least in part, by incrementing a value of the variable field.
According to still another aspect of the invention, the encryption device can include a processor having a memory for storing a set of security parameters. The processor transmits to a far-end telephone a message containing a representation of the set of security parameters. The processor then receives from the far-end telephone a message containing a selected security parameter selected from the set of security parameters. The encryption device then establishes a secure session with the far-end telephone based on the selected security parameter.
A decryption device according to the invention comprises a host interface, a first converter, a decryption processor, a second converter, and a handset interface. The host interface receives analog input signals from the base unit, and the first converter converts the analog input signals into digital input signals. The decryption processor comprises a demodulator that demodulates the digital input signals, a key manager that generates key material for decrypting the digital input signals, a decryptor that decrypts the digital input signals based on the key material, and a decompressor that decompresses the decrypted digital input signals. The second converter converts the decrypted digital input signals into decrypted analog input signals, and the handset interface receives the decrypted analog input signals from the decryption processor, and forwards the decrypted analog input signals to the handset.
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Carter Matthew
Costantini Frank
Kleidermacher Moishe
McGrogan Ellwood
Paraggio Ronald
Barron Jr. Gilberto
Di Lorenzo Anthony
L3-Communications Corporation
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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