Cryptography – Video cryptography – Video electric signal masking
Reexamination Certificate
1995-07-03
2001-12-11
Peeso, Thomas R. (Department: 2132)
Cryptography
Video cryptography
Video electric signal masking
C380S264000, C380S205000, C380S206000, C380S210000
Reexamination Certificate
active
06330333
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to the field of communications. More particularly, this invention relates to a cryptographic system for wireless communications.
BACKGROUND OF THE INVENTION
Every day, millions of users communicate with wireless systems. Such communications include voice and data transmissions. Most, if not all, users of these systems do not want the content of the communication to be publicly available. Rather, users generally desire to keep the content of the communication private. Unfortunately, without proper cryptographic precautions, interlopers can easily eavesdrop on communications in some wireless systems. For example, most analog wireless systems do not protect communications from interception. An eavesdropper can access the substance of the transmission by simply tuning a radio to the proper frequency.
Some current digital wireless communication systems take precautions to protect the privacy of users. For example, the Telecommunications Industry Association is drafting a standard for a spread spectrum wireless communications system. The current version of the draft standard was published in December of 1994 as
Mobile Station—Base Station Compatibility Standard for Dual—Mode Spread Spectrum Cellular System
marked PN-3421 (to be published as IS-95a) (hereinafter the “Draft Standard”) the teachings of which are incorporated herein by reference. The spread spectrum system described in the Draft Standard is referred to colloquially as Code Division Multiple Access, or CDMA. The Draft Standard includes plans for encrypting voice or data signals prior to transmission for added privacy. Thus, only the true recipient of the voice or data transmission should obtain the content of the transmission.
SUMMARY OF THE INVENTION
A previously unrecognized problem with the cryptographic system that is specified by the Draft Standard is that it permits an eavesdropper to easily and quickly cryptanalyze transmissions encrypted according to the Draft Standard and thereby gain access to the substance of the transmission. The forward traffic channel described in the Draft Standard calls for encrypting an input voice or data signal prior to transmission with a key signal. The Draft Standard also specifies that the input signal be combined with the long code sequence in an Exclusive-OR (i.e. mod
2
addition) function to produce an encrypted output signal.
The Draft Standard calls for generating the long code sequence from a publicly known sequence and a private 42-bit pattern, known as the long code mask. The publicly available sequence is placed in what can be conceptualized as a linear shift register. The output of the linear shift register is combined with the bits of the long code mask. The linear nature of the combination causes the long code sequence to depend linearly on the bits of the private long code mask. This enables an eavesdropper to decrypt a wireless communication with access to 42 bits of the long code sequence. The eavesdropper could use the bits from the long code sequence to create 42 linear equations that depend on the 42 unknown bits of the long code mask. However, the Draft Standard does not call for direct transmission of the bits of the long code sequence. Rather, an Exclusive-OR function combines the bits of the long code sequence with the unknown input signal thus corrupting the bits of the long code, sequence. This should diminish the chances that an eavesdropper will successfully cryptanalyze a transmission. This is not the case with the Draft Standard because of the way that the input signal is processed to form frames of 384 bits for error correction.
An eavesdropper can cryptanalyze a transmission by recognizing relationships among the last sixteen bits in each frame of the input signal. Specifically, the eavesdropper can combine selected bits of the input signal from the last sixteen bits in each frame so as to produce modulo-2 sums of zero. By adding (mod
2
) the bits of the output signal such that the sum of the corresponding input bits is zero, the eavesdropper can obtain data that represent combinations of bits of the long code sequence. Essentially, the eavesdropper can cancel the effect of the input signal on the output signal. Each bit of the long code sequence is linearly dependent on the 42 bits of the long code mask. Thus, the eavesdropper can combine known bits of the output signal to create equations that are linearly dependent on the bits of the long code mask. Successive frames of data yield 42 equations so as to allow decryption of the communication within less than a second after the communication commences.
Embodiments of the present invention substantially eliminate or reduce the ability of eavesdroppers to cryptanalyze encrypted transmissions. Specifically, exemplary embodiments of the present invention include a long code mask generator that creates a rolling long code mask that changes,with a predetermined frequency. This removes the linearity from the decimated long code sequence and makes it more difficult to determine the long code mask and thus cryptanalyze the encrypted output signal.
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“Mobile Station—Base Station Compatibility Standard for Dual—Mode Spread Spectrum Cellular System”, PN-3421 (to be published as IS-95a), 12/94, pp. 6-1 through 6-18 and 7-1 through 7-12.
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Mizikovsky Semyon B.
Reeds, III James Alexander
DeMont & Breyer LLC
Lucent Technologies - Inc.
Peeso Thomas R.
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