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periodicity in the discrete log algorithm modulo a prime Watch

    • Thread Starter

    so I was playing around with the discrete log algorithm and found that if we compute

    10^{k}\mod  53 for k  \epsilon \mathbb{Z} _{53}, then this is periodic with period 13 - wolfram alpha has the values here.

    My question is whether this is a coincidence, or is there anything special about 10 and 53?

    It also turns out that 4,6,7,9 and 11 are periodic with period 26. obviously periods will divide p-1. I haven't checked any higher bases.

    Is something going on here?

    (Also considering encryption with Diffie–Hellman is it a weakness if the base you choose is periodic modulo your prime? Is there a way of determining whether it is periodic or not, apart from actually checking?)

    I think I might be missing something extremely obvious. anyway. any info would be appreciated, thanks in advance.
    • Thread Starter

    I've been doing a few more calculations, and it keeps on getting stranger, and it feels like I keep on understanding less.

    However I now know after a bit of research that if we choose our prime to be a 'safe prime' i.e. p, s.t. p= 2q+1, where q is prime, then the only possible periods are 1,2,q and p. Ideally we want a period of p, so since periods of 1 and 2 are trivial (bases 1 and (p-1) respectively) we only need to check the value of b^q mod p. if this is congurent to 1, then our base b has periodicity q.

    So, experimenting with large primes of this form, for example 1000667=2.500333+1, we have for example, choosing a base of 37997 (which happens to be prime), that 37997^500333 (mod 1000667) is congruent to 1. hence this base may not be appropriate.

    then I chose as another example 40009 (also prime) we have 40009^500333 (mod 1000667) is 1000666 or -1, hence 40009 will have period p. or will it? is the value of -1 significant?

    i might now take a look at primes of this form in general, then try to look at all bases, and find their periods, and try to see if some patterns emerge.

    One thing I was also wondering was whether the discrete log is suitable for pseudorandom number generation? my instinct says no, since each number is exactly dependent on the one before. however given a large enough modulus and, staying with in periodicity, exactly how random are the numbers? i have looked at some statistical tests to determine randomness, but it is complicated. i'm sure there is an answer somewhere.
    • Thread Starter

    So i've narrowed it down. conjecture: working modulo p, if p-1 =mn, then a^m will be congruent to exactly n different values. (if a is not a multiple of p)

    eg if m= p-1, then it will take one value (1) via FLT.

    eg working mod 7

    7-1 =6 = 2.3

    so x^2 will take 3 different values: wolfram alpha (1,2,4)

    similarly x^3 will take 2 different values: wolfram alpha (1,-1)

    is this a basic corollary of some theorem?

    EDIT: p is prime, as before
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