Cryptographic method and apparatus with electronically redefinable algorithm

CRYPTOGRAPHIC METHOD AND APPARATUS WITH ELECTRONICALLY REDEFINABLE ALGORITHM

0002Background of the Invention

0003This invention is generally directed to the field of digital cryptographic devices and is more specifically directed to the architecture and method by which a cipher algorithm is stored in the cryptographic apparatus. This invention also addresses a method for manufacturing encrypted communications equipment in an unsecured environment as a result of the cipher algorithm being loaded into the encryption device after the manufacture of the equipment.

0004In a typical encryption device, a secret key is utilized in conjunction with a cipher algorithm to encrypt and decrypt messages. The keys may be changed as frequently as desired in order to enhance security.

0005The cipher algorithm which consists of the steps by which the message is encrypted and decrypted using a particular key is frequently protected in order to maintain high levels of security. If the algorithm is known, it becomes easier to decipher a coded message since only the message and key are then unknowns. Thus it is important that the algorithm itself be protected for maximum security applications. It is also important that the electronic circuits in which the algorithm resides be designed not to fail in a way which would compromise the message being communicated. Algorithms which are stored in logic hardware are easier to be made to "fail safe" than algorithms stored in software. The manufacture of logic hardware containing the cipher algorithms and equipment containing same is often strictly controlled. The manufacture of communications equipment containing such fixed encryption logic requires substantial additional procedures as compared with communications equipment that does not utilize encryption. This results in the need for strict controls and substantially increases manufacturing costs.

0006Objects of the Invention

0007It is an object of the present invention to provide an encryption apparatus having an architecture which permits the cipher algorithm to be electronically entered subsequent to the manufacture of the apparatus and subsequent to the manufacture of communications equipment utilizing the encryption apparatus.

0008Another object of this invention is to provide an improved method for the manufacture of encrypted communications equipment in which security control measures need not be utilized during the manufacture of such equipment.

0009Brief Description of the Drawings

0010Figure 1 is a block diagram of an embodiment of an encryption device according to the present invention. Figure 2 is a block diagram of communications equipment incorporating the encryption apparatus in accordance with the present invention. Detailed Description

0011Figure 1 illustrates an embodiment 10 of a cryptographic apparatus in which the cipher algorithm can be electronically defined after its manufacture in accordance with the present invention. A sequence controller 12 shown to the left of the dashed line includes a program counter 14 having an output coupled by bus 16 to multiplexer 18. The output of multiplexer 18 is coupled to an electronically erasable, programmable array logic device (EEPAL) 20. Such EEPAL devices are commercially available and consist of a number of logic gates which can be electronically programmed to interconnect to each other to produce an output determined by a Boolean algebraic formula operating on the input data. As used herein, array logic devices do not include microprocessors. It is important that the array logic devices be electronically programmable and are preferably erasable to permit reprogramming. The output of device 20 is coupled by latch 22 to buffer 24 and to latch 26. An output of device 20 also provides a clock input to program counter 14 which causes the counter to increment periodically and step through a preprogrammed number of steps. The general purpose of sequence controller 12 is to control the cyclic operation sequence of the remainder of the circuitry shown in Figure 1, i.e. implement the cipher algorithm on a step by step basis. A common address bus 28 with communication port 29 and a data bus 30 with communication port 31 establish digital communication paths with EEPAL 20, random access memory (RAM) device 32, and EEPAL's 34 and 36. RAM 32 is addressed by multiplexer 38 which selects between the output 16 of program counter 14 or address bus 28. Data is input to RAM 32 by data bus 30. The data information stored in a particular address is coupled from RAM 32 by latch 40 and buffer 42 to data bus 30. EEPAL 34 has address and data inputs from the respective buses. The output from EEPAL 34 is coupled by latch 44 and buffer 46 to data bus 30. Similarly, EEPAL 36 receives address and data inputs from the respective bus lines. Its output as determined by its particular gate configuration is coupled to data bus 30 by latch 48 and buffer 50. Its output is also coupled to latch 26 and defines the encrypted digital output 52 at the end of the program sequence which is communicated to latch 26 via the output of latch 22.

0012The digital information to be encrypted is coupled by input 54 to buffer 56 which in turn couples the information to data bus 30. The digital information consists of digital words having one or more bits. It will be understood that the encryption of the input data by the encryption apparatus 10 must be accomplished prior to the input of the next information to be encrypted in order to maintain a real time message flow. Thus it will be apparent that the clock (not shown) which provides clock information to the elements shown in Figure 1 must be of a sufficient rate to allow the number of steps required by the cipher algorithm to be completed within the input time frame.

0013One important aspect of the present invention is the ability to have the cipher algorithm defined after the manufacture of the hardware which will carry out the algorithm. The preferred embodiment shown in Figure 1 accomplishes this goal by the use of electronically erasable, programmable array logic devices and random access memory. After the manufacture of the encryption device 10 and its insertion into the communications equipment, an external device such as a microprocessor controlled computer is coupled to the address and data bus ports and is utilized to program the internal gate configurations of each EEPAL with a test algorithm and define initial test data stored in RAM 32. After testing is complete the communications equipment can be delivered to the customer with the test program still in the encryption device so that the completed unit can be handled without the use of security measures. The customer would then load the encryption device with the cipher algorithm by use of a similar external computer. Since the cipher algorithm can not be determined based on the test algorithm, it is not necessary to have strict security control of such equipment prior to the actual cipher algorithm being externally programmed. This permits the encryption hardware as well as equipment containing the hardware to be manufactured without a strict security control environment.

0014In order to better understand the operation of the encryption apparatus 10, the following illustrative example is provided. An external computer which has been preprogrammed to define the EEPAL's and load initial data into RAM is coupled to the address and data bus. The EEPAL*s are each sequentially programmed to perform a specific Boolean algebra function. A key and initialization data are stored in RAM 32. The apparatus is now ready to process incoming data on input 54 since it contains the desired cipher algorithm.

0015The first input data byte or word is stored in RAM 32. The key and the input data stored in RAM 32 are transferred as input data to EEPAL 34 which acts upon such data to produce a resulting output data Rl. The resulting data Rl is stored in RAM 32. The initialization data and the resulting data Rl are transferred from RAM 32 to EEPAL 36 which acts upon these inputs to produce output data F. This output data F is stored in RAM 32 and is substituted in the place of the initialization data so that it will be used in the place of same for the next input data to be encrypted. Simultaneously, the data F is transferred to latch 26 and is clocked out at line 52 as the encrypted output data. A repetitive process occurs for each new input data byte to produce a corresponding output encrypted data. The above example illustrates a possible encryption application. It will be apparent that since RAM 32 is available, intermediate products can be stored and later used for subsequent calculations in a variety of ways depending upon the complexity of the mathematical functions used. Depending upon the level of security desired, the number of iterations required, and the speed at.which the encryption processed must occur, more or f wer EEPAL's can be used. Another advantage of the present invention resides in the use of the RAM 32 instead of an alternative storage device such as a shift register. The RAM can function as a programmable register thereby allowing the number of bytes which can be stored during any process to be varied within the ultimate capacity of the RAM. This adds a degree of security to such an apparatus since it increases the difficulty of determining the number of bytes or length of the calculations being used.

0016The encryption apparatus of the present invention can also be contrasted with the use of a general purpose microprocessor or a more specialized digital signalling processor to accomplish a similar encryption technique. Although such alternatives could be used, the relatively complicated internal structure of the microprocessor makes such an alternative difficult to design to "fail safe"; that is, fail in an acceptable manner which protects the security of the message without compromising the algorithm being utilized. The more direct functioning apparatus in accordance with the present invention can be more easily protected and diagnosed in the event of a failure.

0017Figure 2 illustrates communication equipment consisting of a two-way radio which incorporates the encryption apparatus 10 in accordance with the present invention. An antenna 60 couples signals carrying voice information to a duplexer and filter 62 which couples a received signal to mixer 64. The other signal to mixer 64 from oscillator 66 is mixed to produce a resulting intermediate frequency output 68. Conventional receiver circuits between 68 and demodulator 70 are not shown. The demodulator provides an output to multiplexer 72 which routes the received encrypted signal as digital input 54 to the encryption device 10. The decrypted output 52 from the device 10 is coupled by multiplexer 74 to digital to analog converter 76. The analog output from converter 76 is amplified by audio amplifier 78 before being coupled to speaker 80.

0018The signal received will have been encrypted by a corresponding encryption device 10 in a remote radio utilizing a corresponding cipher algorithm and key. In this example the recovered data is decrypted by device 10 and converted to an analog signal to provide a clear text voice message to the listener.

0019To transmit an encrypted message, a voice signal is coupled by microphone 82 to audio amplifier 84 whose output is converted into a digital signal by analog to digital converter 86. The output of this converter is coupled by multiplexer 72 to encryption device 10 which encrypts the input data and provides an output 52 coupled by multiplexer 74 to the modulator 88 in a transmitter. The conventional transmitter stages normally found between modulator 88 and power amplifier 90 are not shown. The amplified signal from amplifier 90 is coupled through filter 62 to antenna 60 where it is transmitted to a corresponding receiver having a similar encryption device 10. As previously explained, the manufacture of such a radio would require security and accounting procedures if the encryption device 10 contained the cipher algorithm during the manufacture of the radio. Since the present invention allows the cipher algorithm to be loaded at a later time, the radio including the encryption device 10 can now be manufactured in a factory environment not subject to strict security measures. Although a radio transceiver is shown in Figure 2 as incorporating the encryption device, it will be apparent that other communications equipment designed to carry voice or data, such as modems and secure telephones can equally use the encryption apparatus according to the present invention.

0020Although an embodiment of the present invention has been described and illustrated herein, the scope of the invention is defined by the claims which follow.