Method of Protecting a Linear Feedback Shift Register (LFSR) Output Signal

The present invention relates generally to a cryptographic method and system. In so doing, it addresses the use of a linear feedback shift register (LFSR) as a pseudorandom signal generator. Specifically, however, the present invention pertains to a cryptographic method and apparatus wherein a LFSR output signal is made non-linear and thereby protected from cryptographic attack.

LFSRs are well known in the art. Typically, these devices are utilized in operations where the generation of a pseudonoise/pseudorandom (PN) signal is required. A PN signal is a binary signal, which appears to be random. In reality, a PN signal is not random at all; it is a deterministic, periodic signal whose periodicity is dependent upon the number of stages within the LFSR, the feedback taps, and the LFSR's initial conditions. Typical operations employing LFSR's as PN signal generators are: spread spectrum systems, noise generators, and cryptographic systems hereinafter referred to as cryptosystems.

FIG. 1 depicts a simplistic representation of an LFSR 100. Mathematically, the LFSR defines an Nth degree polynomial (where N is the length of the LFSR) with one coefficient for each "tap" (output bit) used to form the feed back signal. Accordingly, the LFSR 100 is definable as a forth degree polynomial, comprising the four stages 11-14, with feedback signal .sup.T 1, which performs an exclusive-or operation on the output of stage 13 and 14 and feeds that operation back to the input of stage 11.

It will be appreciated by those skilled in the art that the LFSR 100 is a simplistic model of a PN signal generator, for use as, for example, the encryption key, which dictates a specific encryption transformation for a cryptosystem. This particular example was selected primarily to show that an LFSR Of the Nth degree is ultimately periodic in 2.sup.n -1 bits (see Table. 1).

In the preferred embodiment, LFSR 100 comprises a 64 stage shift register, providing a 64th degree polynomial. Approximately 32 taps are used to create the desired PN signal. Accordingly, the 64 bit maximal length LFSR will produce a digital sequence having approximately 1.84.times.10.sup.19 bits. At 12 KHz, it would take nearly 50 million years for this sequence to repeat. As PN signal generators increase in sophistication, typical values reach 80 to 100 stages with anywhere from 40 to 50 taps, in which case the sequences can be expected to repeat every 4.times.10.sup.16 years.

The primary goal of any cryptosystem is to prevent the unauthorized introduction (spoofing) or extraction (eavesdropping) of information from the communication channel. Since the previously discussed encryption keys repeats so infrequently, one might suppose that a cryptosystem utilizing these keys would be unconditionally secure. Unfortunately, any cryptosystem that uses an LFSR to generate the encryption key is extremely vulnerable to attack.

The Cryptosystems weakness is caused by the LFSR's linearity. Since the PN signal is generated by an algorithm, knowledge of the algorithm reveals the entire sequence. While it takes 2.sup.n -1 bits for a PN sequence to repeat, a cryptanalyst needs only 2 n bits of plaintext and its corresponding ciphertext to determine the feedback taps, the initial state of the register, and ultimately the entire PN signal. This vulnerability represents a major drawback to the continued use of LFSRs in modern cryptosystems.

Accordingly, it would be extremely advantageous to provide a cryptographic method and apparatus wherein the PN signal of an LFSR is made non-linear, thereby rendering the cryptosystem virtually indecipherable.

Accordingly, a general object of the present invention is to provide a cryptographic method and apparatus.

It is another object of the present invention to provide a cryptographic method and apparatus utilizing a LFSR to generate a PN signal.

It is the ultimate object of the present invention, however, to provide a cryptographic method and apparatus wherein the PN signal is made non-linear, thereby rendering the cryptosystem virtually indecipherable.

These and other objects are achieved by the present invention which is briefly described as a method of protecting a Linear Feedback Shift Register (LFSR) output signal. This invention is based on the recognition that after the capture of 2 n bits of ciphertext and its plaintext equivalent, a cryptanalyst, using known techniques, can easily decipher the algorithm that generates the entire PN sequence. Accordingly, the present invention discloses a method and apparatus for protecting the pseudorandom (PN) signal generated by a Linear Feedback Shift Register (LFSR) from cryptographic attack. This is accomplished by introducing non-linearity into the PN signal generated by an LFSR.

In one embodiment non-linearity is introduced by altering the state of at least one bit of the PN signal in order to produce a deterministic bit pattern. This bit pattern is then used as the cryptosystem's PN signal instead of the LFSR PN signal. In an alternative embodiment, the location of at least one bit of the PN signal is repositioned in order to once again introduce non-linearity into the otherwise linear output of an LFSR. In this fashion, the cryptanalyst's task is made increasingly more difficult, thereby rendering the cryptosystem virtually impervious to attack.

FIGS. 1a and 1b depict a simple LFSR representation;

FIG. 2 depicts a model cryptographic channel; and

FIG. 3 depicts a portion of the cryptosystem according to the present invention.

A principal application of the present invention is for a PN signal generator like those employed in the field of cryptography. FIG. 2. depicts a model of a cryptographic channel 200. In operation, a message, or plaintext M, is encrypted via transformation, .sup.E k, to produce ciphertext, C=E.sub.k (M). The ciphertext is then transmitted over lo an insecure or public channel. When an authorized listener receives the ciphertext C, she deciphers it with the inverse transformation, D.sub.k =E.sub.k.sup.-1, to obtain the original plaintext message as follows:

The parameter K refers to a set of characters or symbols called a key, which dictates the encryption transformations .sup.E k and .sup.D k. In the cryptosystem of FIG. 2, the key K is manipulated by PN signal generators 210 and 220 to create a key stream. This key stream is ultimately exclusiveored with the plaintext M for encryption and with the ciphertext C for decryption. Anyone having access to the key therefore, can encrypt and decrypt plaintext messages. Accordingly, the key is transmitted between authorized users via secure channels only. In a cryptographic attack, the goal of the cryptanalyst is to produce an estimate of the plaintext message, M, by analyzing the ciphertext obtained from the public channel, without benefit of the key.

As previously discussed, when the PN signal generators 210 and 220 employ LFSRs, the cryptosystem becomes extremely vulnerable to attack. This attack, known as a plaintext attack, involves knowledge of the plaintext and knowledge of its ciphertext counterpart. Armed with this information the competent cryptanalyst can successfully decrypt the message intercepted from the public channel. While knowledge of the plaintext will not always be readily available to the cryptanalyst, plaintext attacks occur with sufficient frequency that no cryptosystem is considered secure unless it is designed to defeat this threat.

In order to secure the system against such cryptographic attacks, the present invention suggests the introduction of non-linearity into the PN signal sequence. To aid in this discussion, refer to FIG. 3. FIG. 3 depicts a portion of the cryptosystem according to the present invention. In operation, LFSR 300, which is capable of being implemented via flip flops, latches, shift registers, working registers, memory device addresses, or as a software variable is identical to the LFSR 100 previously discussed.

According to the present invention, the LFSR 300 content is modulo-two added to the content of one of a plurality of selectively addressed registers 320-327 via exclusive-or gate 310. This summation generates an address signal which can be any function of the LFSR and register contents. The address is then transmitted along address bus 315 to address a preprogrammed RAM look-up table 350. Thereafter, a RAM look-up table entry corresponding to the address signal is sent to the comparator circuit 330. The comparator 330 compares the RAM look-up table entry with a reference bit pattern, in this case, the contents of the register selectively added to the LFSR's content. It will be appreciated that other reference bit patterns are available, as for example, the LFSR contents, flip flop contents, latch contents, shift register contents, working register contents, memory device address contents, or the contents of a software variable.

Assuming the look-up table entry and the reference bit pattern values do not compare, the LFSR 300 output signal will not be altered. Upon a comparison, however, the comparator 330 will direct switch S2, via control line 332, to open. According to this embodiment, each comparison will cause at least one bit to be dropped from the PN sequence generated by the LFSR300. This process of ignoring at least one bit of the PN sequence is but one way of deterministically introducing non-linearity into the PN sequence of an LFSR.

Another alternative suggests simply masking at least one bit of the PN sequence. In such an embodiment, switch S2 would be replaced by a signal invertor. Accordingly, upon each comparison by the comparator 330, the state of at least one bit of the PN sequence would be inverted. Again, this process of masking at least one bit of the PN sequence is an effective means of deterministically introducing non-linearity into the PN sequence, thereby rendering the original PN sequence virtually impervious to attack.

Yet another embodiment suggests that non-linearity may be deterministically introduced into the PN sequence of an LFSR output by altering the relative position of at least one bit of the original PN sequence upon the occurrence of some specified trigger condition. Accordingly, unique to the present invention is the deterministic introduction of non-linearity into the PN sequence of an LFSR output signal in order to protect the LFSR output from cryptographic attack. In this fashion, the cryptanalyst's task is made increasingly more difficult, thereby rendering the cryptosystem virtually indecipherable. While particular embodiments of the invention have been described herein, it will be obvious that additional modifications may be made without departing from the spirit of this disclosure.

In summary, the present invention discloses a a method of protecting a pseudorandom (PN) signal generated by a Linear Feedback Shift Register (LFSR) from cryptographic attack. This is accomplished by first receiving a PN output signal generated by an LFSR, or by clocking an LFSR to produce a PN output signal. Thereafter, non-linearity is deterministically introduced into the PN signal by masking or ignoring the state of at least one bit of the PN signal, or repositioning the location of at least one bit of the PN signal to produce a deterministic bit pattern. Next, the deterministic bit pattern is substituted in place of the LFSR PN signal, thereby protecting the PN signal from cryptographic attack.