applied cryptography - protocols, algorithms, and source code in c

Some interesting new ideas along these lines are in

Info icon This preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: le would pick different primitive polynomials. Since, if p(x) is primitive, then so is xnp(1/x); each entry on the table is actually two primitive polynomials. For example, if (a, b, 0) is primitive, then (a, a - b, 0) is also primitive. If (a, b, c, d, 0) is primitive, then (a, a - d, a - c, a - b, 0) is also primitive. Mathematically: if xa + xb + 1 is primitive, so is xa + xa-b + 1 if xa + xb + xc + xd + 1 is primitive, so is xa + xa-d + xa-c + xa-b + 1 Primitive trinomials are fastest in software, because only two bits of the shift register have to be XORed to generate each new bit. Actually, all the feedback polynomials listed in Table 16.2 are sparse, meaning that they only have a few coefficients. Sparseness is always a source of weakness, sometimes enough to break the algorithm. It is far better to use dense primitive polynomials, those with a lot of coefficients, for cryptographic applications. If you use dense polynomials, and especially if you make them part of the key, you can live with much shorter LFSRs. Generating dense primitive polynomials modulo 2 is not easy. In general, to generate primitive polynomials of degree k you need to know the factorization of 2k - 1. Three good references for finding primitive polynomials are [652,1285,1287]. LFSRs are competent pseudo-random-sequence generators all by themselves, but they have some annoying nonrandom properties. Sequential bits are linear, which makes them useless for encryption. For an LFSR of length n, the internal state is the next n output bits of the generator. Even if the feedback scheme is unknown, it can be determined from only 2n output bits of the generator, by using the highly efficient Berlekamp-Massey algorithm [1082,1083]: see Section 16.3. Also, large random numbers generated from sequential bits of this sequence are highly correlated and, for certain types of applications, not very random at all. Even so, LFSRs are often used as building blocks in encryption algorithms. LFSRs in Software LFSRs are slow in software, but they’re faster in assembly language than in C. One solution is to run 16 LFSRs (or 32, depending on your...
View Full Document

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern