THE EXTENSION OF COLLISIONAND AVALANCHE EFFECT TO

k-ARY SEQUENCES

Viktória Tóth

Eötvös Loránd University, BudapestDepartment of Algebra and Number

Theory, Department of Computer Algebra

9-12th June, 2010, Bedlewo

Pseudorandom sequences

• They have many applications

Cryptography:keystream in the Vernam cipher

• The notion of pseudorandomness can be defined in different ways

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Motivation

• The standard approach:– based on computational complexity– limitations and difficulties

• New, constructive approach: Mauduit, Sárközy

• about 50 papers in the last 10-15 years

The standard approach

Notions:

• PRBG seed, PR sequence

• next bit test unpredictable

• cryptographically secure PRBG

Problems• „probability significantly greater than ½”

• The non-existence of a polynomial time algorithm has not been shown unconditionally

yet

–There is no PRBG whose cryptographycal sequrity has beenproved unconditionally.

• These definitions measure only the quality of PRBG’s, not the output sequences

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Goals• More constructive

• We do not want to use unproved hypothesis

• We describe the single sequences

• Apriori testing

• Characterizing with real-valued function» comparable

Historical background

• Infinity sequences:normality (Borel)

• Finite sequences:– Golomb– Knuth– Kolmogorov– Linear complexity

Advantages

• Normality

• Well-distribution

• Low correlation of low order

• characterizing with real-valued function

comperable

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Measures

• mmm

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Measures

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Previous results

• „good” sequence:

If both and (at least for

small k) are „small” in terms of N

• This terminology is justified:

Theorem: for truly random sequences

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Further properties

● collision free: two different choice of the parameters should not lead to the same sequence;

● avalanche effect: changing only one bit on the input leads to the change about half of the bits on the output.

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• In the applications one usually needs LARGE FAMILIES of sequences with strong pseudorandom properties.

• I have tested two of the most important

constructions:

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1.construction:Generalized Legendre symbol

2. construction:

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My results

• These constructions are ideal of this point of view as well:

– both possess the strong avalanche effect

AND

– they are collision free

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Extension to k symbol

• Mauduit and Sárközy studied k-ary sequences instead of binary ones

• They extended the notion of well-distribution measure and correlation measure

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The construction

• They generated the sequences with a character of order k:

• Mauduit and Sárközy proved that both the correlation measure and the well-distribution measure are „small”

• So we can say that this is a good construction of pseudorandom k-ary sequences

A good family of pseudorandom sequences of k symbols

• Ahlswede, Mauduit and Sárközy extended:

• They proved that both measures are small

New results

• I extended the notion of collisions and avalanche effect to k symbol

• I studied the previous family of k-ary sequences with strong pseudorandom properties.

• Let Hd be the set of polynomials of degree d which do not have multiple zeroes

• Theorem: If f is an element of Hd , then the family of k-ary sequences constructed above is collision free and it also possesses the avalanche effect.

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Conclusion

• If we have a large family of sequences

with strong pseudorandom properties,

then it worth studying it from other point of view

In this way we can get further beneficial properties, which can be profitable, especially in applications

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Thank you for your attention!