Method and apparatus for generating encryption stream ciphers

[0002] I. Field of the Invention

[0003] The present invention relates to encryption. More particularly, the present invention relates to a method and apparatus for generating encryption stream ciphers.

[0004] II. Description of the Related Art

[0005] Encryption is a process whereby data is manipulated by a random process such that the data is made unintelligible by all but the targeted recipient. One method of encryption for digitized data is through the use of stream ciphers. Stream ciphers work by taking the data to be encrypted and a stream of pseudo-random bits (or encryption bit stream) generated by an encryption algorithm and combining them, usually with the exclusive-or (XOR) operation. Decryption is simply the process of generating the same encryption bit stream and removing the encryption bit stream with the corresponding operation from the encrypted data. If the XOR operation was performed at the encryption side, the same XOR operation is also performed at the decryption side. For a secured encryption, the encryption bit stream must be computationally difficult to predict.

[0006] Many of the techniques used for generating the stream of pseudo-random numbers are based on linear feedback shift register (LFSR) over the Galois finite field of order 2. This is a special case of the Galois Finite field of order ₂ⁿwhere n is a positive integer. For n=1, the elements of the Galois field comprise bit values zero and one. The register is updated by shifting the bits over by one bit position and calculating a new output bit. The new bit is shifted into the register. For a Fibonacci register, the output bit is a linear function of the bits in the register. For a Galois register, many bits are updated in accordance with the output bit just shifted out from the register. Mathematically, the Fibonacci and Galois register architectures are equivalent.

[0007] The operations involved in generating the stream of pseudo-random numbers, namely the shifting and bit extraction, are efficient in hardware but inefficient in software or other implementations employing a general purpose processor or microprocessor. The inefficiency increases as the length of the shift register exceeds the length of the registers in the processor used to generate the stream. In addition, for n=0, only one output bit is generated for each set of operations which, again, results in a very inefficient use of the processor.

[0008] An exemplary application which utilizes stream ciphers is wireless telephony. An exemplary wireless telephony communication system is a code division multiple access (CDMA) system. The operation of CDMA system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention, and incorporated by reference herein. The CDMA system is further disclosed in U.S. Pat. No. 5,103,459, entitled SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM, assigned to the assignee of the present invention, and incorporated by reference herein. Another CDMA system includes the GLOBALSTAR communication system for world wide communication utilizing low earth orbiting satellites. Other wireless telephony systems include time division multiple access (TDMA) systems and frequency division multiple access (FDMA) systems. The CDMA systems can be designed to conform to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as the IS-95 standard. Similarly, the TDMA systems can be designed to conform to the TIA/EIA/IS-54 (TDMA) standard or to the European Global System for Mobile Communication (GSM) standard.

[0009] Encryption of digitized voice data in wireless telephony has been hampered by the lack of computational power in the remote station. This has led to weak encryption processes such as the Voice Privacy Mask used in the TDMA standard or to hardware generated stream ciphers such as the A5 cipher used in the GSM standard. The disadvantages of hardware based stream ciphers are the additional manufacturing cost of the hardware and the longer time and larger cost involved in the event the encryption process needs to be changed. Since many remote stations in wireless telephony systems and digital telephones comprise a microprocessor and memory, a stream cipher which is fast and uses little memory is well suited for these applications.

[0010] The present invention is a novel and improved method and apparatus for generating encryption stream ciphers. In accordance with the present invention, the recurrence relation is designed to operate over finite fields larger than GF(2). The linear feedback shift register used to implement the recurrence relation can be implemented using a circular buffer or sliding a window. In the exemplary embodiment, multiplications of the elements of the finite field are implemented using lookup tables. A non-linear output can be obtained by using one or a combination of non-linear processes. The stream ciphers can be designed to support multi-tier keying to suit the requirements of the applications for which the stream ciphers are used.

[0011] It is an object of the present invention to generate encryption stream ciphers using architectures which are simple to implement in a processor. In particular, more efficient implementations can be achieved by selecting a finite field which is more suited for the processor. The elements and coefficients of the recurrence relation can be selected to match the byte or word size of the processor. This allows for efficient manipulation of the elements by the processor. In the exemplary embodiment, the finite field selected is the Galois field with 256 elements (GF(2⁸)). This results in elements and coefficients of the recurrence relation occupying one byte of memory which can be efficiently manipulated. In addition, the use of a larger finite field reduces the order of the recurrence relation. For a finite field GF(2ⁿ), the order k of the recurrence relation which encodes the same amount of states is reduced by a factor of n (or a factor of 8 for the exemplary GF(2⁸)).

[0012] It is another object of the present invention to implement field multiplications using lookup tables. In the exemplary embodiment, a multiplication (of non-zero elements) in the field can be performed by taking the logarithm of each of the two operands, adding the logarithmic values, and exponentiating the combined logarithmic value. The logarithmic and exponential tables can be created using an irreducible polynomial. In the exemplary embodiment, the tables are pre-computed and stored in memory. Similarly, a field multiplication with a constant coefficient can be performed using a simple lookup table. Again, the table can be pre-computed using the irreducible polynomial and stored in memory.

[0013] It is yet another object of the present invention to remove linearity in the output of a linear feedback shift register by the use of one or a combination of the following processes: irregular stuttering (sometimes referred to as decimation), non-linear function, multiple shift registers and combining outputs from the registers, variable feedback polynomial on one register, and other non-linear processes. In the exemplary embodiment, the non-linear output can be use to randomly control the stuttering of the shift register. Additionally, a non-linear output can be derived by performing a non-linear operation on selected elements of the shift register. Furthermore, the output from the non-linear function can be XORed with a set of constants such that the non-linear output bits are unpredictably inverted.

[0014] It is yet another object of the present invention to implement the linear feedback shift register using a circular buffer or a sliding window. With the circular buffer or sliding window implementation, the elements are not shifted within the buffer. Instead, a pointer or index is used to indicate the location of the most recently computed element. The pointer is moved as new elements are computed and shifted into the circular buffer or sliding window. The pointer wraps around when it reaches an edge.

[0015] It is yet another object of the present invention to provide stream ciphers having multi-tier keying capability. In the exemplary embodiment, the state of the shift register is first initialized with a secret key. For some communication system wherein data are transmitted over frames, a stream cipher can be generated for each frame such that erased or out of sequence frames do not disrupt the operation of the encryption process. A second tier keying process can be initialized for each frame using a frame key initialization process.

[0016] The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

[0017] FIG. 1 is a block diagram of an exemplary implementation of a recurrence relation;

[0018] FIG. 2 is a exemplary block diagram of an stream cipher generator utilizing a processor;

[0019] FIGS. 3A and 3B are diagrams showing the contents of a circular buffer at time n and time n+1, respectively;

[0020] FIG. 3C is a diagram showing the content of a sliding window;

[0021] FIG. 4 is a block diagram of an exemplary stream cipher generator of the present invention;

[0022] FIG. 5 is a flow diagram of an exemplary secret key initialization process of the present invention; and

[0023] FIG. 6 is a flow diagram of an exemplary per frame initialization process of the present invention.

[0024] Linear feedback shift register (LFSR) is based on a recurrence relation over the Galois field, where the output sequence is defined by the following recurrence relation:

[0025] S_n+k=C_n+k−1S_n+k−1+C_k−2S_n+k−2+ . . . +C₁S_n+1+C₀S_n,   (1)

[0026] where S_n+kis the output element, C_jare constant coefficients, k is the order of the recurrence relation, and n is an index in time. The state variables S and coefficients C are elements of the underlying finite field. Equation (1) is sometimes expressed with a constant term which is ignored in this specification.

[0027] A block diagram of an exemplary implementation of the recurrence relation in equation (1) is illustrated in FIG. 1. For a recurrence relation of order k, register 12 comprises k elements S_nto S_n+k−1. The elements are provided to Galois field multipliers 14 which multiplies the elements with the constants C_j. The resultant products from multipliers 14 are provided Galois field adders 16 which sum the products to provide the output element.

[0028] For n=1, the elements of GF(2) comprise a single bit (having a value of 0 or 1) and implementation of equation (1) requires many bit-wise operations. In this case, the implementation of the recurrence relation using a general purpose processor is inefficient because a processor which is designed to manipulate byte or word sized objects is utilized to perform many operations on single bits.

[0029] In the present invention, the linear feedback shift register is designed to operate over finite fields larger than GF(2). In particular, more efficient implementations can be achieved by selecting a finite field which is more suited for a processor. In the exemplary embodiment, the finite field selected is the Galois field with 256 elements (GF(2⁸)) or other Galois fields with 2ⁿelements, where n is the word size of the processor.

[0030] In the preferred embodiment, a Galois field with 256 elements (GF(2⁸)) is utilized. This results in each element and coefficient of the recurrence relation occupying one byte of memory. Byte manipulations can be performed efficiently by the processor. In addition, the order k of the recurrence relation which encodes the same amount of states is reduced by a factor of n, or 8 for GF(2⁸).

[0031] In the present invention, a maximal length recurrence relation is utilized for optimal results. Maximal length refers to the length of the output sequence (or the number of states of the register) before repeating. For a recurrence relation of order k, the maximal length is N^k−1, where N is the number of elements in the underlying finite field, and N=256 in the preferred embodiment. The state of all zeros is not allowed.

[0032] An exemplary block diagram of a stream cipher generator utilizing a processor is shown in FIG. 2. Controller 20 connects to processor 22 and comprises the set of instructions which directs the operation of processor 22. Thus, controller 20 can comprise a software program or a set of microcodes. Processor 22 is the hardware which performs the manipulation required by the generator. Processor 22 can be implemented as a microcontroller, a microprocessor, or a digital signal processor designed to perform the functions described herein. Memory element 24 connects to processor 22 and is used to implement the linear feedback shift register and to store pre-computed tables and instructions which are described below. Memory element 24 can be implemented with random-access-memory or other memory devices designed to perform the functions described herein. The instructions and tables can be stored in read-only memory, only the memory for the register itself needs to be modified during the execution of the algorithm.

[0033] I. Generating Non-linear Output Stream

[0034] The use of linear feedback shift register for stream ciphers can be difficult to implement properly. This is because any linearity remaining in the output stream can be exploited to derive the state of the register at a point in time. The register can then be driven forward or backward as desired to recover the output stream. A number of techniques can be used to generate non-linear stream ciphers using linear feedback shift register. In the exemplary embodiment, these non-linear techniques comprise stuttering (or unpredictable decimation) of the register, the use of a non-linear function on the state of the register, the use of multiple registers and non-linear combination of the outputs of the registers, the use of variable feedback polynomials on one register, and other non-linear processes. These techniques are each described below. Some of the techniques are illustrated by the example below. Other techniques to generate non-linear stream ciphers can be utilized and are within the scope of the present invention.

[0035] Stuttering is the process whereby the register is clocked at a variable and unpredictable manner. Stuttering is simple to implement and provides good results. With stuttering, the output associated with some states of the register are not provided at the stream cipher, thus making it more difficult to reconstruct the state of the register from the stream cipher.

[0036] Using a non-linear function on the state of the shift register can also provide good results. For a recurrence relation, the output element is generated from a linear function of the state of the register and the coefficients, as defined by equation (1). To provide non-linearity, the output element can be generated from a non-linear function of the state of the register. In particular, non-linear functions which operate on byte or word sized data on general purpose processors can be utilized.

[0037] Using multiple shift registers and combining the outputs from the registers in a non-linear fashion can provide good results. Multiple shift registers can be easily implemented in hardware where additional cost is minimal and operating the shift registers in parallel to maintain the same operating speed is possible. For implementations on a general purpose processor, a single larger shift register which implements the multiple shift registers can be utilized since the larger shift register can be updated in a constant time (without reducing the overall speed).

[0038] Using a variable feedback polynomial which changes in an unpredictable manner on one register can also provide good results. Different polynomials can be interchanged in a random order or the polynomial can be altered in a random manner. The implementation of this technique can be simple if properly designed.

[0039] II. Operations on Elements of Larger Order Finite Fields

[0040] The Galois field GF(2⁸) comprises 256 elements. The elements of Galois field GF(2⁸) can be represented in one of several different ways. A common and standard representation is to form the field from the coefficients modulo 2 of all polynomials with degree less than 8. That is, the element α of the field can be represented by a byte with bits (a₇, a₆, . . . , a₀) which represent the polynomial:

[0041] a₇x⁷+a₆x⁶+ . . . +a₁x+a₀.   (2)

[0042] The bits are also referred to as the coefficients of the polynomial. The addition operation on two polynomials represented by equation (2) can be performed by addition modulo two for each of the corresponding coefficients (a₇, a₆, . . . a₀). Stated differently, the addition operation on two bytes can be achieved by performing the exclusive-OR on the two bytes. The additive identity is the polynomial with all zero coefficients (0, 0, . . . , 0).

[0043] Multiplication in the field can be performed by normal polynomial multiplication with modulo two coefficients. However, multiplication of two polynomials of order n produces a resultant polynomial of order (2n-1) which needs to be reduced to a polynomial of order n. In the exemplary embodiment, the reduction is achieved by dividing the resultant polynomial by an irreducible polynomial, discarding the quotient, and retaining the remainder as the reduced polynomial. The selection of the irreducible polynomial alters the mapping of the elements of the group into encoded bytes in memory, but does not otherwise affect the actual group operation. In the exemplary embodiment, the irreducible polynomial of degree 8 is selected to be:

[0044] x⁸+x⁶+x³+x²+1.   (3)

[0045] Other irreducible monic polynomial of degree 8 can also be used and are within the scope of the present invention. The multiplicative identity element is (a₇, a₆, . . . , a₀)=(0, 0, . . . , 1).

[0046] Polynomial multiplication and the subsequent reduction are complicated operations on a general purpose processor. However, for Galois fields having a moderate number of elements, these operations can be performed by lookup tables and more simple operations. In the exemplary embodiment, a multiplication (of non-zero elements) in the field can be performed by taking the logarithm of each of the two operands, adding the logarithmic values modulo 255, and exponentiating the combined logarithmic value. The reduction can be incorporated within the lookup tables.

[0047] The exponential and logarithm tables can be generated as follows. First, a generator g of the multiplicative subgroup GF(2⁸) is determined. In this case, the byte value g=2 (representing the polynomial x) is a generator. The exponential table, shown in Table 1, is a 256-byte table of the values gⁱ, for i=0, 1, . . . 2⁸−1. For gⁱ(considered as an integer) of less than 256, the value of the exponential is as expected as evidenced by the first eight entries in the first row of Table 1. Since g=2, each entry in the table is twice the value of the entry to the immediate left (taking into account the fact that Table 1 wraps to the next row). However, for each gⁱgreater than 255, the exponential is reduced by the irreducible polynomial shown in equation (3). 8 For example, the exponential x⁸(first row, ninth column) is reduced by the irreducible polynomial x⁸+x⁶+x³+x²+1 to produce the remainder −x⁶−x³−x²−1. This remainder is equivalent to x⁶+x³+x²+1 for modulo two operations and is represented as 77 (2⁶+2³+2²+1) in Table 1. The process is repeated until gⁱfor all index i=0 to 255 are computed.

[0048] Having defined the exponential table, the logarithm table can be computed as the inverse of the exponential table. In Table 1, there is a unique one to one mapping of the exponential value gⁱfor each index i which results from using an irreducible polynomial. For Table 1, the mapping is i2ⁱ, or the value stored in the i-th location is 2 . Taking log₂of both sides results in the following : log₂(i)i. These two mappings indicate that if the content of the i-th location in the exponential table is used as the index of the logarithm table, the log of this index is the index of the exponential table. For example, for i=254, the exponential value 2ⁱ=2²⁵⁴=166 as shown in the last row, fifth column in Table 1. Taking log₂of both sides yields 254=log₂(166). Thus, the entry for the index i=166 in the logarithmic table is set to 254. The process is repeated until all entries in the logarithmic table have been mapped. The log of 0 is an undefined number. In the exemplary embodiment, a zero is used as a place holder.

[0049] Having defined the exponential and logarithmic tables, a multiplication (of non-zero elements) in the field can be performed by looking up the logarithmic of each of the two operands in the logarithmic table, adding the logarithmic values using modulo 255, and exponentiating the combined logarithmic value by looking up the exponential table. Thus, the multiplication operation in the field can be performed with three lookup operations and a truncated addition. In the exemplary Galois field GF(2⁸), each table is 255 bytes long and can be pre-computed and stored in memory. In the exemplary embodiment, the logarithm table has an unused entry in position 0 to avoid the need to subtract 1 from the indexes. Note that when either operand is a zero, the corresponding entry in the logarithmic table does not represent a real value. To provide the correct result, each operand needs to be tested to see if it is zero, in which case the result is 0, before performing the multiplication operation as described.

[0050] For the generation of the output element from a linear feedback shift register using a recurrence relation, the situation is simpler since the coefficients C_jare constant as shown in equation (1). For efficient implementation, these coefficients are selected to be 0 or 1 whenever possible. Where C_jhave values other than 0 or 1, a table can be pre-computed for the multiplication t_i=C_j·i, where i=0, 1, 2, . . . , 2⁸−1. In this case, the multiplication operation can be performed with a single table lookup and no tests. Such a table is fixed and can be stored in read-only memory.

[0051] II. Memory Implementation

[0052] When implemented in hardware, shifting bits is a simple and efficient operation. Using a processor and for a shift register larger than the registers of the processor, shifting bits is an iterative procedure which is very inefficient. When the units to be shifted are bytes or words, shifting becomes simpler because there is no carry between bytes. However, the shifting process is still iterative and inefficient.

[0053] In the exemplary embodiment, the linear feedback shift register is implemented with circular buffer or sliding window. The diagrams showing the contents of circular buffer 24aat time n at time n+1 are shown in FIGS. 3A and 3B, respectively. For circular buffer 24a, each element of the shift register is stored in a corresponding location in memory. A single index, or pointer 30, maintains the memory location of the most recent element stored in memory, which is S_k−1in FIG. 3A. At time n+1, the new element S_kis computed and stored over the oldest element S_oin memory, as shown in FIG. 3B. Thus, instead of shifting all elements in memory, pointer 30 is moved to the memory location of the new element S_k. When pointer 30 reaches the end of circular buffer 24a, it is reset to the beginning (as shown in FIGS. 3A and 3B). Thus, circular buffer 24aacts as if it is a circle and not a straight line.

[0054] Circular buffer 24acan be shifted from left-to-right, or right-to-left as shown in FIGS. 3A and 3B. Correspondingly, pointer 30 can move left-to-right, or right-to-left as shown in FIGS. 3A and 3B. The choice in the direction of the shift is a matter of implementation style and does not affect the output result.

[0055] To generate an output element in accordance with a recurrence relation, more than one elements are typically required from memory. The memory location associated with each required element can be indicated by a separate pointer which is updated when the register is shifted. Alternatively, the memory location associated with each required element can be computed from pointer 30 as necessary. Since there is a one-to-one mapping of each element to a memory location, a particular element can be obtained by determining the offset of that element from the newest element (in accordance with the recurrence relation), adding that offset to pointer 30, and addressing the memory location indicated by the updated pointer. Because of the circular nature of the memory, the calculation of the updated pointer is determined by an addition modulo k of the offset to pointer 30. Addition modulo k is simple when k is a power of two but is otherwise an inefficient operation on a processor.

[0056] In the preferred embodiment, the shift register is implemented with sliding window 24bas shown in FIG. 3C. Sliding window 24bis at least twice as long as circular buffer 24aand comprises two circular buffers 32aand 32barranged adjacent to each other. Each of circular buffers 32aand 32bbehaves like circular 24adescribed above. Circular buffer 32bis an exact replica of circular buffer 32a. Thus, each element of the shift register is stored in two corresponding locations in memory, one each for circular buffers 32aand 32b. Pointer 34 maintains the memory location of the most recent element stored in circular buffer 32a, which is S_k−1in FIG. 3C. In the exemplary embodiment, pointer 34 starts at the middle of sliding window 24b, moves right-to-left, and resets to the middle again when it reaches the end on the left side.

[0057] From FIG. 3C, it can be observed that no matter where in circular buffer 32apointer 34 appears, the previous k−1 elements can be addressed to the right of pointer 34. Thus, to address an element in the shift register in accordance with the recurrence relation, an offset of k−1 or less is added to pointer 34. Addition modulo k is not required since the updated pointer is always to the right of pointer 34 and computational efficiency is obtained. For this implementation, sliding window 24bcan be of any length at least twice as long as circular buffer 24a, with any excess bytes being ignored. Furthermore, the update time is constant and short.

[0058] IV. Exemplary Stream Cipher Based on LFSR Over GF(2⁸)

[0059] The present invention can be best illustrated by an exemplary generator for a stream cipher based on a linear feedback shift register over GF(2⁸). The stream cipher described below uses the byte operations described above over the Galois field of order 8 with the representation of ⊕ and {circle around (X)} for operations of addition and multiplication, respectively, over the Galois field. In the exemplary embodiment, table lookup is utilized for the required multiplication with constants C_j. In the exemplary embodiment, a sliding window is used to allow fast updating of the shift register.

[0060] A block diagram of the exemplary generator is shown in FIG. 4. In the exemplary embodiment, linear feedback shift register 52 is 17 octets (or 136 bits) long which allows shift register 52 to be in 2¹³⁶−1 (or approximately 8.7×10⁴⁰) states. The state where the entire register is 0 is not a valid state and does not occur from any other state. The time to update register 52 with a particular number of non-zero elements in the recurrence relation is constant irrespective of the length of register 52. Thus, additional length for register 52 (for higher order recurrence relation) can be implemented at a nominal cost of extra bytes in memory.

[0061] In the exemplary embodiment, linear feedback shift register 52 is updated in accordance with the following recurrence relation:

[0062] S_n+17=(100⊕S_n+9)⊕S_n+4⊕(141{circle around (X)}S_n),   (4)

[0063] where the operations are defined over GF(2⁸), δ is the exclusive-OR operation on two bytes represented by Galois adders 58, and {circle around (X)} is a polynomial modular multiplication represented by Galois multipliers 54 (see FIG. 4). In the exemplary embodiment, the modular multiplications on coefficients 56 are implemented using byte table lookups on pre-computed tables as described above. In the exemplary embodiment, the polynomial modular multiplication table is computed using the irreducible polynomial defined by equation (3). The recurrence relation in equation (4) was chosen to be maximal length, to have few non-zero coefficients, and so that the shift register elements used were distinct from ones used for the non-linear functions below.

[0064] In the exemplary embodiment, to disguise the linearity of shift register 52, two of the techniques described above are used, namely stuttering and using a non-linear function. Additional non-linearity techniques are utilized and are described below.

[0065] In the exemplary embodiment, non-linearity is introduced by performing a non-linear operation on multiple elements of shift register 52. In the exemplary embodiment, four of the elements of shift register 52 are combined using a function which is non-linear. An exemplary non-linear function is the following:

[0066] V_n=(S_n+S_n+5)×(S_n+2+S_n+12),   (5)

[0067] where V_nis the non-linear output (or the generator output), + is the addition truncated modulo 256 represented by arithmetic adders 60, and × is the multiplication modulo 257 represented by modular multiplier 62 and described below. In the exemplary embodiment, the four bytes used are S_n, S_n+2, S_n+5and S_n+12, where S_nis the oldest calculated element in the sequence according to the recurrence relation in equation (4). These elements are selected such that, as the register shifts, no two elements are used in the computation of two of the generator outputs. The pairwise distances between these elements are distinct values. For example, S_n+12is not combined with S_n+5, S_n+2, nor S_nagain as it is shifted through register 52.

[0068] Simple byte addition, with the result truncated modulo 256, is made non-linear in GF(2⁸) by the carry between bits. In the exemplary embodiment, two pairs of elements in the register {(S_nand S_n+5) and (S_n+2and S_n+12)} are combined using addition modulo 256 to yield two intermediate results. However, addition modulo 256 is not ideal since the least significant bits have no carry input and are still combined linearly.

[0069] Another non-linear function which can be computed conveniently on a processor is multiplication. However, truncation of a normal multiplication into a single byte may not yield good result because multiplication modulo 256 does not form a group since the results are not well distributed within the field. A multiplicative group of the field of integers modulo the prime number 257 can be used. This group consists of integers in the range of 1 to 256 with the group operation being integer multiplication reduced modulo 257. Note that the value 0 does not appear in the group but the value 256 does. In the exemplary embodiment, the value of 256 can be represented by a byte value of 0.

[0070] Typically, processors can perform multiplication instructions efficiently but many have no capability to perform, nor to perform efficiently, divide or modulus instructions. Thus, the modulo reduction by 257 can represent a performance bottleneck. However, reduction modulo 257 can be computed using other computational modulo 2ⁿ, which in the case of n=8 are efficient on common processors. It can be shown that for a value X in the range of 1 to 2¹⁶−1 (where X is the result of a multiplication of two 8th order operands), reduction modulo 257 can be computed as: $\begin{array}{cc}{X}_{257}={\left\{X_{256}-\frac{X}{256}\right\}}_{257},& \left(6\right)\end{array}$

[0071] where X₂₅₇is the reduction modulo 257 of X and X₂₅₆is the reduction modulo 256 of X. Equation (6) indicates that reduction modulo 257 of a 16-bit number can be obtained by subtracting the 8 most significant bits (X/256) from the 8 least significant bits (X₂₅₆). The result of the subtraction is in the range of −255 and 255 and may be negative. If the result is negative, it can be adjusted to the correct range by adding 257. In the alternative embodiment, reduction modulo 257 can be performed with a lookup table comprising 65,536 elements, each 8 bits wide.

[0072] Multiplication of the two intermediate results is one of many non-linear functions which can be utilized. Other non-linear functions, such as bent functions or permuting byte values before combining them, can also be implemented using lookup tables. The present invention is directed at the use of these various non-linear functions for producing non-linear output.

[0073] In the exemplary embodiment, stuttering is also utilized to inject additional non-linearity. The non-linear output derived from the state of the linear feedback shift register as described above may be used to reconstruct the state of the shift register. This reconstruction can be made more difficult by not representing some of the states at the output of the generator, and choosing which in an unpredictable manner. In the exemplary embodiment, the non-linear output is used to determine what subsequent bytes of non-linear output appear in the output stream. When the generator is started, the first output byte is used as the stutter control byte. In the exemplary embodiment, each stutter control byte is divided into four pairs of bits, with the least significant pair being used first. When all four pairs have been used, the next non-linear output byte from the generator is used as the next stutter control byte, and so on.

[0074] Each pair of stutter control bits can take on one of four values. In the exemplary embodiment, the action performed for each pair value is tabulated in Table 3.

[0075] As shown in Table 3, in the exemplary embodiment, when the pair value is (0, 0), the register is cycled once but no output is produced. Cycling of the register denotes the calculation of the next sequence output in accordance with equation (4) and the shifting this new element into the register. The next stutter control pair is then used to determine the action to be taken next.

[0076] In the exemplary embodiment, when the pair value is (0, 1) the register is cycled and the non-linear output generated in accordance with equation (5). The non-linear output is XORed with the constant (0 1 1 0 1 0 0 1)₂and the result is provided as the generator output. The register is then cycled again. In FIG. 4, the XORed function is performed by XOR gate 66 and the constant is selected by multiplexer (MUX) 64 using the stutter control pair from buffer 70. The output from XOR gate 66 is provided to switch 68 which provides the generator output and the output byte for stutter control in accordance with the value of the stutter control pair. The output byte for stutter control is provided to buffer 70.

[0077] In the exemplary embodiment, when the pair value is (1, 0) the register is cycled twice and the non-linear output generated in accordance with equation (5) is provided as the generator output.

[0078] In the exemplary embodiment, when the pair value is (1, 1) the register is cycled and the non-linear output generated in accordance with equation (5). The non-linear output is then XORed with the constant (1 1 0 0 0 1 0 1)₂and the result is provided as the generator output.

[0079] In the exemplary embodiment, the constants which are used in the above steps are selected such that when a generator output is produced, half of the bits in the output are inverted with respect to the outputs produced by the other stutter control pairs. For stutter control pair (1, 0), the non-linear output can be viewed as being XORed with the constant (0 0 0 0 0 0 0 0)₂. Thus, the Hamming distance between any of the three constants is four. The bit inversion further masks the linearity of the generator and frustrates any attempt to reconstruct the state based on the generator output.

[0080] The present invention supports a multi-tier keying structure. A stream cipher which supports multi-tier keying structure is especially useful for wireless communication system wherein data are transmitted in frames which may be received in error or out-of-sequence. An exemplary two-tier keying structure is described below.

[0081] In the exemplary embodiment, one secret key is used to initialized the generator. The secret key is used to cause the generator to take an unpredictable leap in the sequence. In the exemplary embodiment, the secret key has a length of four to k−1 bytes (or 32 to 128 bits for the exemplary recurrence relation of order 17). Secret keys of less than 4 bytes are not preferred because the initial randomization may not be adequate. Secret keys of greater than k−1 bytes can also be utilized but are redundant, and care should be taken so that a value for the key does not cause the register state to be set to all 0, a state which cannot happen with the current limitation.

[0082] A flow diagram of an exemplary secret key initialization process is shown in FIG. 5. The process starts at block 110. In the exemplary embodiment, at block 112, the state of the shift register is first initialized with the Fibonacci numbers modulo 256. Thus, elements S₀, S₁, S₂, S₃, S₄, S₅, and so on, are initialized with 1, 1, 2, 3, 5, 8, and so on, respectively. Although Fibonacci numbers are used, any set of non-zero numbers which are not linearly related in the Galois field can be used to initialize the register. These numbers should not have exploitable linear relationships which can be used to reconstruct the state of the register.

[0083] Next, the loop index n is set to zero, at block 114. The secret key initialization process then enters a loop. In the first step within the loop, at block 116, the first unused byte of the key material is added to S_n. Addition of the key material causes the generator to take an unpredictable leap in the sequence. The key is then shifted by one byte, at block 118, such that the byte used in block 116 is deleted. The register is then cycled, at block 120. The combination of blocks 116 and 120 effectively performs the following calculation:

[0084] S_n+17=(100{circle around (X)}S_n+9) ⊕S_n+4⊕(141{circle around (X)}(S_n⊕K)),   (7)

[0085] where K is the first unused byte of the key material. The loop index n is incremented, at block 122. A determination is then made whether all key material has been used, at block 124. If the answer is no, the process returns to block 116. Otherwise, the process continues to block 126.

[0086] In the exemplary embodiment, the length of the key is added to S_n, at block 126. Addition of the length of the key causes the generator to take an additional leap in the sequence. The process then enters a second loop. In the first step within the second loop, at block 128, the register is cycled. The loop index n is incremented, at block 130, and compared against the order k of the generator, at block 132. If n is not equal to k, the process returns to block 128. Otherwise, if n is equal to k, the process continues to block 134 where the state of the generator is saved. The process then terminates at block 136.

[0087] In addition to the secret key, a secondary key can also be used in the present invention. The secondary key is not considered secret but is used in an exemplary wireless telephony system to generate a unique cipher for each frame of data. This ensures that erased or out-of-sequence frames do not disrupt the flow of information. In the exemplary embodiment, the stream cipher accepts a per-frame key, called a frame key, in the form of a 4-octet unsigned integer. The per-frame initialization is similar to the secret key initialization above but is performed for each frame of data. If the use of the stream cipher is such that it is unnecessary to utilize per-frame key information, for example for file transfer over a reliable link, the per-frame initialization process can be omitted.

[0088] A flow diagram of an exemplary per-frame initialization process with the frame key is shown in FIG. 6. The process starts at block 210. In the exemplary embodiment, at block 212, the state of the generator is initialized with the state saved from the secret key initialization process as described above. Next, the loop index n is set to zero, at block 214. The per-frame initialization process then enters a loop. In the first step within the loop, at block 216, the least significant byte of the frame key is added modulo 256 to S_n. The frame key is then shifted by three bits, at block 218, such that the three least significant bits used in block 216 are deleted. The register is then cycled, at block 220. In the exemplary embodiment, the loop index n is incremented at block 222 and compared against 11 at block 224. The value of 11, as used in block 224, corresponds to the 32 bits used as the frame key and the fact that the frame key is shifted three bits at a time. Different selections of the frame key and different numbers of bits shifted at a time can result in different comparison values used in block 224. If n is not equal to 11, the process returns to block 216. Otherwise, if n is equal to 11, the process continues to block 226 and the register is cycled again. The loop index n is incremented, at block 228, and compared against 2k, at block 230. If n is not equal to 2k, the process returns to block 226. Otherwise, if n is equal to 2k, the process terminates at block 232.

[0089] The present invention has been described for the exemplary Galois finite field having 256 elements. Different finite fields can also be utilized such that the size of the elements matches the byte or word size of the processor used to manipulate the elements and/or the memory used to implement the shift register, or having other advantages. Thus, various finite fields having more than two elements can by utilized and are within the scope of the present invention.

[0090] The example shown above utilizes a variety of non-linear processes to mask the linearity of the recurrence relation. Other generators can be design utilizing different non-linear processes, or different combinations of the above described non-linear processes and other non-linear processes. Thus, the use of various non-linear processes to generate non-linear outputs can be contemplated and is within the scope of the present invention.

[0091] The example shown above utilizes a recurrence relation having an order of 17 and defined by equation (4). Recurrence relation having other orders can also be generated and are within the scope of the present invention. Furthermore, for a given order, various recurrence relations can be generated and are within the scope of the present invention. In the present invention, a maximal length recurrence relation is preferred for optimal results.

[0092] The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.